Merge pull request #743 from 0xProject/feature/contracts/abi-encoded

ABI encode assetData fields

35 files changed, 478 insertions, 361 deletions

diff --git a/packages/contracts/compiler.json b/packages/contracts/compiler.json
index 6b7b69caa..e1ea401b3 100644
--- a/packages/contracts/compiler.json
+++ b/packages/contracts/compiler.json
@@ -27,6 +27,7 @@
         "ERC721Proxy",
         "Exchange",
         "ExchangeWrapper",
+        "IAssetData",
         "MixinAuthorizable",
         "MultiSigWallet",
         "MultiSigWalletWithTimeLock",
diff --git a/packages/contracts/package.json b/packages/contracts/package.json
index 8249b9e0c..95f2dbb4e 100644
--- a/packages/contracts/package.json
+++ b/packages/contracts/package.json
@@ -34,7 +34,7 @@
     },
     "config": {
         "abis":
-            "../migrations/artifacts/2.0.0/@(AssetProxyOwner|DummyERC20Token|DummyERC721Receiver|DummyERC721Token|ERC20Proxy|ERC721Proxy|Exchange|ExchangeWrapper|MixinAuthorizable|MultiSigWallet|MultiSigWalletWithTimeLock|TestAssetProxyOwner|TestAssetDataDecoders|TestAssetProxyDispatcher|TestLibBytes|TestLibs|TestSignatureValidator|TestValidator|TestWallet|TokenRegistry|Whitelist|WETH9|ZRXToken).json"
+            "../migrations/artifacts/2.0.0/@(AssetProxyOwner|DummyERC20Token|DummyERC721Receiver|DummyERC721Token|ERC20Proxy|ERC721Proxy|Exchange|ExchangeWrapper|IAssetData|MixinAuthorizable|MultiSigWallet|MultiSigWalletWithTimeLock|TestAssetProxyOwner|TestAssetDataDecoders|TestAssetProxyDispatcher|TestLibBytes|TestLibs|TestSignatureValidator|TestValidator|TestWallet|TokenRegistry|Whitelist|WETH9|ZRXToken).json"
     },
     "repository": {
         "type": "git",
diff --git a/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC20Proxy.sol b/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC20Proxy.sol
index 7ca823d1f..b74d2c231 100644
--- a/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC20Proxy.sol
+++ b/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC20Proxy.sol
@@ -30,14 +30,14 @@ contract ERC20Proxy is
     MixinERC20Transfer
 {
     // Id of this proxy.
-    uint8 constant PROXY_ID = 1;
+    bytes4 constant PROXY_ID = bytes4(keccak256("ERC20Token(address)"));
 
     /// @dev Gets the proxy id associated with the proxy address.
     /// @return Proxy id.
     function getProxyId()
         external
         view
-        returns (uint8)
+        returns (bytes4)
     {
         return PROXY_ID;
     }
diff --git a/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC721Proxy.sol b/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC721Proxy.sol
index 7ff25aea3..f6293f97d 100644
--- a/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC721Proxy.sol
+++ b/packages/contracts/src/contracts/current/protocol/AssetProxy/ERC721Proxy.sol
@@ -30,14 +30,14 @@ contract ERC721Proxy is
     MixinERC721Transfer
 {
     // Id of this proxy.
-    uint8 constant PROXY_ID = 2;
+    bytes4 constant PROXY_ID = bytes4(keccak256("ERC721Token(address,uint256,bytes)"));
 
     /// @dev Gets the proxy id associated with the proxy address.
     /// @return Proxy id.
     function getProxyId()
         external
         view
-        returns (uint8)
+        returns (bytes4)
     {
         return PROXY_ID;
     }
diff --git a/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinAuthorizable.sol b/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinAuthorizable.sol
index 37c12f861..3b9584a44 100644
--- a/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinAuthorizable.sol
+++ b/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinAuthorizable.sol
@@ -19,12 +19,10 @@
 pragma solidity ^0.4.24;
 pragma experimental ABIEncoderV2;
 
-import "./libs/LibAssetProxyErrors.sol";
 import "../../utils/Ownable/Ownable.sol";
 import "./mixins/MAuthorizable.sol";
 
 contract MixinAuthorizable is
-    LibAssetProxyErrors,
     Ownable,
     MAuthorizable
 {
@@ -33,7 +31,7 @@ contract MixinAuthorizable is
     modifier onlyAuthorized {
         require(
             authorized[msg.sender],
-            SENDER_NOT_AUTHORIZED
+            "SENDER_NOT_AUTHORIZED"
         );
         _;
     }
@@ -49,7 +47,7 @@ contract MixinAuthorizable is
     {
         require(
             !authorized[target],
-            TARGET_ALREADY_AUTHORIZED
+            "TARGET_ALREADY_AUTHORIZED"
         );
 
         authorized[target] = true;
@@ -65,7 +63,7 @@ contract MixinAuthorizable is
     {
         require(
             authorized[target],
-            TARGET_NOT_AUTHORIZED
+            "TARGET_NOT_AUTHORIZED"
         );
 
         delete authorized[target];
@@ -91,15 +89,15 @@ contract MixinAuthorizable is
     {
         require(
             authorized[target],
-            TARGET_NOT_AUTHORIZED
+            "TARGET_NOT_AUTHORIZED"
         );
         require(
             index < authorities.length,
-            INDEX_OUT_OF_BOUNDS
+            "INDEX_OUT_OF_BOUNDS"
         );
         require(
             authorities[index] == target,
-            AUTHORIZED_ADDRESS_MISMATCH
+            "AUTHORIZED_ADDRESS_MISMATCH"
         );
 
         delete authorized[target];
diff --git a/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinERC20Transfer.sol b/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinERC20Transfer.sol
index 0e7f3fc89..a09db43bd 100644
--- a/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinERC20Transfer.sol
+++ b/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinERC20Transfer.sol
@@ -21,11 +21,9 @@ pragma experimental ABIEncoderV2;
 
 import "../../utils/LibBytes/LibBytes.sol";
 import "../../tokens/ERC20Token/IERC20Token.sol";
-import "./libs/LibTransferErrors.sol";
 
-contract MixinERC20Transfer is
-    LibTransferErrors
-{
+contract MixinERC20Transfer {
+
     using LibBytes for bytes;
 
     /// @dev Internal version of `transferFrom`.
@@ -42,41 +40,74 @@ contract MixinERC20Transfer is
         internal
     {
         // Decode asset data.
-        address token = assetData.readAddress(0);
-
+        address token = assetData.readAddress(16);
+        
         // Transfer tokens.
         // We do a raw call so we can check the success separate
         // from the return data.
-        bool success = token.call(abi.encodeWithSelector(
-            IERC20Token(token).transferFrom.selector,
-            from,
-            to,
-            amount
-        ));
-        require(
-            success,
-            TRANSFER_FAILED
-        );
+        // We construct calldata for the `token.transferFrom` ABI.
+        // The layout of this calldata is in the table below.
+        // 
+        // | Area     | Offset | Length  | Contents                            |
+        // |----------|--------|---------|-------------------------------------|
+        // | Header   | 0      | 4       | function selector                   |
+        // | Params   |        | 3 * 32  | function parameters:                |
+        // |          | 4      |         |   1. from                           |
+        // |          | 36     |         |   2. to                             |
+        // |          | 68     |         |   3. amount                         |
         
-        // Check return data.
-        // If there is no return data, we assume the token incorrectly
-        // does not return a bool. In this case we expect it to revert
-        // on failure, which was handled above.
-        // If the token does return data, we require that it is a single
-        // value that evaluates to true.
+        bytes4 transferFromSelector = IERC20Token(token).transferFrom.selector;
+        bool success;
         assembly {
-            if returndatasize {
-                success := 0
-                if eq(returndatasize, 32) {
-                    // First 64 bytes of memory are reserved scratch space
-                    returndatacopy(0, 0, 32)
-                    success := mload(0)
-                }
-            }
+            /////// Setup State ///////
+            // `cdStart` is the start of the calldata for `token.transferFrom` (equal to free memory ptr).
+            let cdStart := mload(64)
+
+            /////// Setup Header Area ///////
+            // This area holds the 4-byte `transferFromSelector`.
+            // Any trailing data in transferFromSelector will be
+            // overwritten in the next `mstore` call.
+            mstore(cdStart, transferFromSelector)
+
+            /////// Setup Params Area ///////
+            // Each parameter is padded to 32-bytes.
+            // The entire Params Area is 96 bytes.
+            // A 20-byte mask is applied to addresses to
+            // zero-out the unused bytes.
+            mstore(add(cdStart, 4), and(from, 0xffffffffffffffffffffffffffffffffffffffff))
+            mstore(add(cdStart, 36), and(to, 0xffffffffffffffffffffffffffffffffffffffff))
+            mstore(add(cdStart, 68), amount)
+
+            /////// Call `token.transferFrom` using the calldata ///////
+            success := call(
+                gas,            // forward all gas
+                token,          // call address of token contract
+                0,              // don't send any ETH
+                cdStart,        // pointer to start of input
+                100,            // length of input
+                cdStart,        // write output over input
+                32              // output size should be 32 bytes
+            )
+
+            /////// Check return data. ///////
+            // If there is no return data, we assume the token incorrectly
+            // does not return a bool. In this case we expect it to revert
+            // on failure, which was handled above.
+            // If the token does return data, we require that it is a single
+            // nonzero 32 bytes value.
+            // So the transfer succeeded if the call succeeded and either
+            // returned nothing, or returned a non-zero 32 byte value. 
+            success := and(success, or(
+                iszero(returndatasize),
+                and(
+                    eq(returndatasize, 32),
+                    gt(mload(cdStart), 0)
+                )
+            ))
         }
         require(
             success,
-            TRANSFER_FAILED
+            "TRANSFER_FAILED"
         );
     }
 }
diff --git a/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinERC721Transfer.sol b/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinERC721Transfer.sol
index 944068bbb..c170917ea 100644
--- a/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinERC721Transfer.sol
+++ b/packages/contracts/src/contracts/current/protocol/AssetProxy/MixinERC721Transfer.sol
@@ -28,6 +28,8 @@ contract MixinERC721Transfer is
 {
     using LibBytes for bytes;
     
+    bytes4 constant SAFE_TRANSFER_FROM_SELECTOR = bytes4(keccak256("safeTransferFrom(address,address,uint256,bytes)"));
+    
     /// @dev Internal version of `transferFrom`.
     /// @param assetData Encoded byte array.
     /// @param from Address to transfer asset from.
@@ -54,11 +56,85 @@ contract MixinERC721Transfer is
             bytes memory receiverData
         ) = decodeERC721AssetData(assetData);
 
-        ERC721Token(token).safeTransferFrom(
-            from,
-            to,
-            tokenId,
-            receiverData
+        // We construct calldata for the `token.safeTransferFrom` ABI.
+        // The layout of this calldata is in the table below.
+        // 
+        // | Area     | Offset | Length  | Contents                            |
+        // |----------|--------|---------|-------------------------------------|
+        // | Header   | 0      | 4       | function selector                   |
+        // | Params   |        | 4 * 32  | function parameters:                |
+        // |          | 4      |         |   1. from                           |
+        // |          | 36     |         |   2. to                             |
+        // |          | 68     |         |   3. tokenId                        |
+        // |          | 100    |         |   4. offset to receiverData (*)     |
+        // | Data     |        |         | receiverData:                       |
+        // |          | 132    | 32      | receiverData Length                 |
+        // |          | 164    | **      | receiverData Contents               |
+
+        bytes4 safeTransferFromSelector = SAFE_TRANSFER_FROM_SELECTOR;
+        bool success;
+        assembly {
+            /////// Setup State ///////
+            // `cdStart` is the start of the calldata for
+            // `token.safeTransferFrom` (equal to free memory ptr).
+            let cdStart := mload(64)
+            // `dataAreaLength` is the total number of words
+            // needed to store `receiverData`
+            // As-per the ABI spec, this value is padded up to
+            // the nearest multiple of 32,
+            // and includes 32-bytes for length.
+            // It's calculated as folows:
+            //      - Unpadded length in bytes = `mload(receiverData) + 32`
+            //      - Add 31 to convert rounding down to rounding up.
+            //        Combined with the previous and this is `63`.
+            //      - Round down to nearest multiple of 32 by clearing
+            //        bits 0x1F. This is done with `and` and a mask.
+            let dataAreaLength := and(add(mload(receiverData), 63), 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFE0)
+            // `cdEnd` is the end of the calldata for `token.safeTransferFrom`.
+            let cdEnd := add(cdStart, add(132, dataAreaLength))
+
+            /////// Setup Header Area ///////
+            // This area holds the 4-byte `transferFromSelector`.
+            // Any trailing data in transferFromSelector will be
+            // overwritten in the next `mstore` call.
+            mstore(cdStart, safeTransferFromSelector)
+            
+            /////// Setup Params Area ///////
+            // Each parameter is padded to 32-bytes.
+            // The entire Params Area is 128 bytes.
+            // Notes:
+            //   1. A 20-byte mask is applied to addresses
+            //      to zero-out the unused bytes.
+            //   2. The offset to `receiverData` is the length
+            //      of the Params Area (128 bytes).
+            mstore(add(cdStart, 4), and(from, 0xffffffffffffffffffffffffffffffffffffffff))
+            mstore(add(cdStart, 36), and(to, 0xffffffffffffffffffffffffffffffffffffffff))
+            mstore(add(cdStart, 68), tokenId)
+            mstore(add(cdStart, 100), 128)
+
+            /////// Setup Data Area ///////
+            // This area holds `receiverData`.
+            let dataArea := add(cdStart, 132)
+            for {} lt(dataArea, cdEnd) {} {
+                mstore(dataArea, mload(receiverData))
+                dataArea := add(dataArea, 32)
+                receiverData := add(receiverData, 32)
+            }
+            
+            /////// Call `token.safeTransferFrom` using the calldata ///////
+            success := call(
+                gas,                    // forward all gas
+                token,                  // call address of token contract
+                0,                      // don't send any ETH
+                cdStart,                // pointer to start of input
+                sub(cdEnd, cdStart),    // length of input
+                cdStart,                // write output over input
+                0                       // output size is 0 bytes
+            )
+        }
+        require(
+            success,
+            TRANSFER_FAILED
         );
     }
 
@@ -79,11 +155,9 @@ contract MixinERC721Transfer is
         )
     {
         // Decode asset data.
-        token = assetData.readAddress(0);
-        tokenId = assetData.readUint256(20);
-        if (assetData.length > 52) {
-            receiverData = assetData.readBytesWithLength(52);
-        }
+        token = assetData.readAddress(16);
+        tokenId = assetData.readUint256(36);
+        receiverData = assetData.readBytesWithLength(100);
 
         return (
             token,
diff --git a/packages/contracts/src/contracts/current/protocol/AssetProxy/interfaces/IAssetData.sol b/packages/contracts/src/contracts/current/protocol/AssetProxy/interfaces/IAssetData.sol
new file mode 100644
index 000000000..8c78ee8c4
--- /dev/null
+++ b/packages/contracts/src/contracts/current/protocol/AssetProxy/interfaces/IAssetData.sol
@@ -0,0 +1,36 @@
+/*
+
+  Copyright 2018 ZeroEx Intl.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+*/
+
+pragma solidity ^0.4.23;
+
+// @dev Interface of the asset proxy's assetData.
+// The asset proxies take an ABI encoded `bytes assetData` as argument.
+// This argument is ABI encoded as one of the methods of this interface.
+interface IAssetData {
+    
+    function ERC20Token(
+        address tokenContract)
+        external pure;
+    
+    function ERC721Token(
+        address tokenContract,
+        uint256 tokenId,
+        bytes receiverData)
+        external pure;
+    
+}
diff --git a/packages/contracts/src/contracts/current/protocol/AssetProxy/interfaces/IAssetProxy.sol b/packages/contracts/src/contracts/current/protocol/AssetProxy/interfaces/IAssetProxy.sol
index 22f43b12f..d12d23610 100644
--- a/packages/contracts/src/contracts/current/protocol/AssetProxy/interfaces/IAssetProxy.sol
+++ b/packages/contracts/src/contracts/current/protocol/AssetProxy/interfaces/IAssetProxy.sol
@@ -56,6 +56,5 @@ contract IAssetProxy is
     function getProxyId()
         external
         view
-        returns (uint8);
+        returns (bytes4);
 }
-
diff --git a/packages/contracts/src/contracts/current/protocol/AssetProxy/libs/LibAssetProxyErrors.sol b/packages/contracts/src/contracts/current/protocol/AssetProxy/libs/LibAssetProxyErrors.sol
index dca4f400f..b0b20c044 100644
--- a/packages/contracts/src/contracts/current/protocol/AssetProxy/libs/LibAssetProxyErrors.sol
+++ b/packages/contracts/src/contracts/current/protocol/AssetProxy/libs/LibAssetProxyErrors.sol
@@ -18,7 +18,10 @@
 
 pragma solidity ^0.4.24;
 
+/// @dev This contract documents the revert reasons used in the AssetProxy contracts.
+/// This contract is intended to serve as a reference, but is not actually used for efficiency reasons.
 contract LibAssetProxyErrors {
+
     /// Authorizable errors ///
     string constant SENDER_NOT_AUTHORIZED = "SENDER_NOT_AUTHORIZED";              // Sender not authorized to call this method.
     string constant TARGET_NOT_AUTHORIZED = "TARGET_NOT_AUTHORIZED";              // Target address not authorized to call this method.
diff --git a/packages/contracts/src/contracts/current/protocol/AssetProxy/libs/LibTransferErrors.sol b/packages/contracts/src/contracts/current/protocol/AssetProxy/libs/LibTransferErrors.sol
index ba784ab22..88187f196 100644
--- a/packages/contracts/src/contracts/current/protocol/AssetProxy/libs/LibTransferErrors.sol
+++ b/packages/contracts/src/contracts/current/protocol/AssetProxy/libs/LibTransferErrors.sol
@@ -18,7 +18,10 @@
 
 pragma solidity ^0.4.24;
 
+/// @dev This contract documents the revert reasons used in the `transferFrom` methods of different AssetProxy contracts.
+/// This contract is intended to serve as a reference, but is not actually used for efficiency reasons.
 contract LibTransferErrors {
+
     /// Transfer errors ///
     string constant INVALID_AMOUNT = "INVALID_AMOUNT";                            // Transfer amount must equal 1.
     string constant TRANSFER_FAILED = "TRANSFER_FAILED";                          // Transfer failed.
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/MixinAssetProxyDispatcher.sol b/packages/contracts/src/contracts/current/protocol/Exchange/MixinAssetProxyDispatcher.sol
index b8d6c0722..3203322aa 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/MixinAssetProxyDispatcher.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/MixinAssetProxyDispatcher.sol
@@ -19,17 +19,18 @@
 pragma solidity ^0.4.24;
 
 import "../../utils/Ownable/Ownable.sol";
-import "./libs/LibExchangeErrors.sol";
+import "../../utils/LibBytes/LibBytes.sol";
 import "./mixins/MAssetProxyDispatcher.sol";
 import "../AssetProxy/interfaces/IAssetProxy.sol";
 
 contract MixinAssetProxyDispatcher is
     Ownable,
-    LibExchangeErrors,
     MAssetProxyDispatcher
 {
+    using LibBytes for bytes;
+    
     // Mapping from Asset Proxy Id's to their respective Asset Proxy
-    mapping (uint8 => IAssetProxy) public assetProxies;
+    mapping (bytes4 => IAssetProxy) public assetProxies;
 
     /// @dev Registers an asset proxy to an asset proxy id.
     ///      An id can only be assigned to a single proxy at a given time.
@@ -37,7 +38,7 @@ contract MixinAssetProxyDispatcher is
     /// @param newAssetProxy Address of new asset proxy to register, or 0x0 to unset assetProxyId.
     /// @param oldAssetProxy Existing asset proxy to overwrite, or 0x0 if assetProxyId is currently unused.
     function registerAssetProxy(
-        uint8 assetProxyId,
+        bytes4 assetProxyId,
         address newAssetProxy,
         address oldAssetProxy
     )
@@ -48,17 +49,17 @@ contract MixinAssetProxyDispatcher is
         address currentAssetProxy = assetProxies[assetProxyId];
         require(
             oldAssetProxy == currentAssetProxy,
-            ASSET_PROXY_MISMATCH
+            "ASSET_PROXY_MISMATCH"
         );
 
         IAssetProxy assetProxy = IAssetProxy(newAssetProxy);
 
         // Ensure that the id of newAssetProxy matches the passed in assetProxyId, unless it is being reset to 0.
         if (newAssetProxy != address(0)) {
-            uint8 newAssetProxyId = assetProxy.getProxyId();
+            bytes4 newAssetProxyId = assetProxy.getProxyId();
             require(
                 newAssetProxyId == assetProxyId,
-                ASSET_PROXY_ID_MISMATCH
+                "ASSET_PROXY_ID_MISMATCH"
             );
         }
 
@@ -74,7 +75,7 @@ contract MixinAssetProxyDispatcher is
     /// @dev Gets an asset proxy.
     /// @param assetProxyId Id of the asset proxy.
     /// @return The asset proxy registered to assetProxyId. Returns 0x0 if no proxy is registered.
-    function getAssetProxy(uint8 assetProxyId)
+    function getAssetProxy(bytes4 assetProxyId)
         external
         view
         returns (address)
@@ -83,14 +84,12 @@ contract MixinAssetProxyDispatcher is
     }
 
     /// @dev Forwards arguments to assetProxy and calls `transferFrom`. Either succeeds or throws.
-    /// @param assetData Byte array encoded for the respective asset proxy.
-    /// @param assetProxyId Id of assetProxy to dispach to.
+    /// @param assetData Byte array encoded for the asset.
     /// @param from Address to transfer token from.
     /// @param to Address to transfer token to.
     /// @param amount Amount of token to transfer.
     function dispatchTransferFrom(
         bytes memory assetData,
-        uint8 assetProxyId,
         address from,
         address to,
         uint256 amount
@@ -99,19 +98,94 @@ contract MixinAssetProxyDispatcher is
     {
         // Do nothing if no amount should be transferred.
         if (amount > 0) {
+            // Ensure assetData length is valid
+            require(
+                assetData.length > 3,
+                "LENGTH_GREATER_THAN_3_REQUIRED"
+            );
+            
             // Lookup assetProxy
+            bytes4 assetProxyId;
+            assembly {
+                assetProxyId := and(mload(
+                    add(assetData, 32)),
+                    0xFFFFFFFF00000000000000000000000000000000000000000000000000000000
+                )
+            }
             IAssetProxy assetProxy = assetProxies[assetProxyId];
+
             // Ensure that assetProxy exists
             require(
                 assetProxy != address(0),
-                ASSET_PROXY_DOES_NOT_EXIST
+                "ASSET_PROXY_DOES_NOT_EXIST"
             );
-            // transferFrom will either succeed or throw.
-            assetProxy.transferFrom(
-                assetData,
-                from,
-                to,
-                amount
+            
+            // We construct calldata for the `assetProxy.transferFrom` ABI.
+            // The layout of this calldata is in the table below.
+            // 
+            // | Area     | Offset | Length  | Contents                                    |
+            // | -------- |--------|---------|-------------------------------------------- |
+            // | Header   | 0      | 4       | function selector                           |
+            // | Params   |        | 4 * 32  | function parameters:                        |
+            // |          | 4      |         |   1. offset to assetData (*)                |
+            // |          | 36     |         |   2. from                                   |
+            // |          | 68     |         |   3. to                                     |
+            // |          | 100    |         |   4. amount                                 |
+            // | Data     |        |         | assetData:                                  |
+            // |          | 132    | 32      | assetData Length                            |
+            // |          | 164    | **      | assetData Contents                          |
+
+            bytes4 transferFromSelector = IAssetProxy(assetProxy).transferFrom.selector;
+            bool success;
+            assembly {
+                /////// Setup State ///////
+                // `cdStart` is the start of the calldata for `assetProxy.transferFrom` (equal to free memory ptr).
+                let cdStart := mload(64)
+                // `dataAreaLength` is the total number of words needed to store `assetData`
+                //  As-per the ABI spec, this value is padded up to the nearest multiple of 32,
+                //  and includes 32-bytes for length.
+                let dataAreaLength := and(add(mload(assetData), 63), 0xFFFFFFFFFFFE0)
+                // `cdEnd` is the end of the calldata for `assetProxy.transferFrom`.
+                let cdEnd := add(cdStart, add(132, dataAreaLength))
+
+                
+                /////// Setup Header Area ///////
+                // This area holds the 4-byte `transferFromSelector`.
+                mstore(cdStart, transferFromSelector)
+                
+                /////// Setup Params Area ///////
+                // Each parameter is padded to 32-bytes. The entire Params Area is 128 bytes.
+                // Notes:
+                //   1. The offset to `assetData` is the length of the Params Area (128 bytes).
+                //   2. A 20-byte mask is applied to addresses to zero-out the unused bytes.
+                mstore(add(cdStart, 4), 128)
+                mstore(add(cdStart, 36), and(from, 0xffffffffffffffffffffffffffffffffffffffff))
+                mstore(add(cdStart, 68), and(to, 0xffffffffffffffffffffffffffffffffffffffff))
+                mstore(add(cdStart, 100), amount)
+                
+                /////// Setup Data Area ///////
+                // This area holds `assetData`.
+                let dataArea := add(cdStart, 132)
+                for {} lt(dataArea, cdEnd) {} {
+                    mstore(dataArea, mload(assetData))
+                    dataArea := add(dataArea, 32)
+                    assetData := add(assetData, 32)
+                }
+
+                /////// Call `assetProxy.transferFrom` using the constructed calldata ///////
+                success := call(
+                    gas,                    // forward all gas
+                    assetProxy,             // call address of asset proxy
+                    0,                      // don't send any ETH
+                    cdStart,                // pointer to start of input
+                    sub(cdEnd, cdStart),    // length of input  
+                    cdStart,                // write output over input
+                    0                       // output size is 0 bytes
+                )
+            }
+            require(
+                success,
+                "TRANSFER_FAILED"
             );
         }
     }
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/MixinExchangeCore.sol b/packages/contracts/src/contracts/current/protocol/Exchange/MixinExchangeCore.sol
index b207b3e57..c0ed023ac 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/MixinExchangeCore.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/MixinExchangeCore.sol
@@ -20,11 +20,9 @@ pragma solidity ^0.4.24;
 pragma experimental ABIEncoderV2;
 
 import "./libs/LibConstants.sol";
-import "../../utils/LibBytes/LibBytes.sol";
 import "./libs/LibFillResults.sol";
 import "./libs/LibOrder.sol";
 import "./libs/LibMath.sol";
-import "./libs/LibExchangeErrors.sol";
 import "./mixins/MExchangeCore.sol";
 import "./mixins/MSignatureValidator.sol";
 import "./mixins/MTransactions.sol";
@@ -35,14 +33,11 @@ contract MixinExchangeCore is
     LibMath,
     LibOrder,
     LibFillResults,
-    LibExchangeErrors,
     MAssetProxyDispatcher,
     MExchangeCore,
     MSignatureValidator,
     MTransactions
 {
-    using LibBytes for bytes;
-    
     // Mapping of orderHash => amount of takerAsset already bought by maker
     mapping (bytes32 => uint256) public filled;
 
@@ -73,7 +68,7 @@ contract MixinExchangeCore is
         // Ensure orderEpoch is monotonically increasing
         require(
             newOrderEpoch > oldOrderEpoch, 
-            INVALID_NEW_ORDER_EPOCH
+            "INVALID_NEW_ORDER_EPOCH"
         );
 
         // Update orderEpoch
@@ -285,20 +280,20 @@ contract MixinExchangeCore is
         // An order can only be filled if its status is FILLABLE.
         require(
             orderInfo.orderStatus == uint8(OrderStatus.FILLABLE),
-            ORDER_UNFILLABLE
+            "ORDER_UNFILLABLE"
         );
 
         // Revert if fill amount is invalid
         require(
             takerAssetFillAmount != 0,
-            INVALID_TAKER_AMOUNT
+            "INVALID_TAKER_AMOUNT"
         );
 
         // Validate sender is allowed to fill this order
         if (order.senderAddress != address(0)) {
             require(
                 order.senderAddress == msg.sender,
-                INVALID_SENDER
+                "INVALID_SENDER"
             );
         }
 
@@ -306,7 +301,7 @@ contract MixinExchangeCore is
         if (order.takerAddress != address(0)) {
             require(
                 order.takerAddress == takerAddress,
-                INVALID_TAKER
+                "INVALID_TAKER"
             );
         }
 
@@ -318,7 +313,7 @@ contract MixinExchangeCore is
                     order.makerAddress,
                     signature
                 ),
-                INVALID_ORDER_SIGNATURE
+                "INVALID_ORDER_SIGNATURE"
             );
         }
 
@@ -329,7 +324,7 @@ contract MixinExchangeCore is
                 order.takerAssetAmount,
                 order.makerAssetAmount
             ),
-            ROUNDING_ERROR
+            "ROUNDING_ERROR"
         );
     }
 
@@ -347,14 +342,14 @@ contract MixinExchangeCore is
         // An order can only be cancelled if its status is FILLABLE.
         require(
             orderInfo.orderStatus == uint8(OrderStatus.FILLABLE),
-            ORDER_UNFILLABLE
+            "ORDER_UNFILLABLE"
         );
 
         // Validate sender is allowed to cancel this order
         if (order.senderAddress != address(0)) {
             require(
                 order.senderAddress == msg.sender,
-                INVALID_SENDER
+                "INVALID_SENDER"
             );
         }
 
@@ -362,7 +357,7 @@ contract MixinExchangeCore is
         address makerAddress = getCurrentContextAddress();
         require(
             order.makerAddress == makerAddress,
-            INVALID_MAKER
+            "INVALID_MAKER"
         );
     }
 
@@ -412,33 +407,27 @@ contract MixinExchangeCore is
     )
         private
     {
-        uint8 makerAssetProxyId = uint8(order.makerAssetData.popLastByte());
-        uint8 takerAssetProxyId = uint8(order.takerAssetData.popLastByte());
         bytes memory zrxAssetData = ZRX_ASSET_DATA;
         dispatchTransferFrom(
             order.makerAssetData,
-            makerAssetProxyId,
             order.makerAddress,
             takerAddress,
             fillResults.makerAssetFilledAmount
         );
         dispatchTransferFrom(
             order.takerAssetData,
-            takerAssetProxyId,
             takerAddress,
             order.makerAddress,
             fillResults.takerAssetFilledAmount
         );
         dispatchTransferFrom(
             zrxAssetData,
-            ZRX_PROXY_ID,
             order.makerAddress,
             order.feeRecipientAddress,
             fillResults.makerFeePaid
         );
         dispatchTransferFrom(
             zrxAssetData,
-            ZRX_PROXY_ID,
             takerAddress,
             order.feeRecipientAddress,
             fillResults.takerFeePaid
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/MixinMatchOrders.sol b/packages/contracts/src/contracts/current/protocol/Exchange/MixinMatchOrders.sol
index e36fcc2c5..1a43eec79 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/MixinMatchOrders.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/MixinMatchOrders.sol
@@ -15,11 +15,9 @@ pragma solidity ^0.4.24;
 pragma experimental ABIEncoderV2;
 
 import "./libs/LibConstants.sol";
-import "../../utils/LibBytes/LibBytes.sol";
 import "./libs/LibMath.sol";
 import "./libs/LibOrder.sol";
 import "./libs/LibFillResults.sol";
-import "./libs/LibExchangeErrors.sol";
 import "./mixins/MExchangeCore.sol";
 import "./mixins/MMatchOrders.sol";
 import "./mixins/MTransactions.sol";
@@ -28,14 +26,11 @@ import "./mixins/MAssetProxyDispatcher.sol";
 contract MixinMatchOrders is
     LibConstants,
     LibMath,
-    LibExchangeErrors,
     MAssetProxyDispatcher,
     MExchangeCore,
     MMatchOrders,
     MTransactions
 {
-    using LibBytes for bytes;
-
     /// @dev Match two complementary orders that have a profitable spread.
     ///      Each order is filled at their respective price point. However, the calculations are
     ///      carried out as though the orders are both being filled at the right order's price point.
@@ -144,7 +139,7 @@ contract MixinMatchOrders is
         require(
             safeMul(leftOrder.makerAssetAmount, rightOrder.makerAssetAmount) >=
             safeMul(leftOrder.takerAssetAmount, rightOrder.takerAssetAmount),
-            NEGATIVE_SPREAD_REQUIRED
+            "NEGATIVE_SPREAD_REQUIRED"
         );
     }
 
@@ -242,27 +237,22 @@ contract MixinMatchOrders is
     )
         private
     {
-        uint8 leftMakerAssetProxyId = uint8(leftOrder.makerAssetData.popLastByte());
-        uint8 rightMakerAssetProxyId = uint8(rightOrder.makerAssetData.popLastByte());
         bytes memory zrxAssetData = ZRX_ASSET_DATA;
         // Order makers and taker
         dispatchTransferFrom(
             leftOrder.makerAssetData,
-            leftMakerAssetProxyId,
             leftOrder.makerAddress,
             rightOrder.makerAddress,
             matchedFillResults.right.takerAssetFilledAmount
         );
         dispatchTransferFrom(
             rightOrder.makerAssetData,
-            rightMakerAssetProxyId,
             rightOrder.makerAddress,
             leftOrder.makerAddress,
             matchedFillResults.left.takerAssetFilledAmount
         );
         dispatchTransferFrom(
             leftOrder.makerAssetData,
-            leftMakerAssetProxyId,
             leftOrder.makerAddress,
             takerAddress,
             matchedFillResults.leftMakerAssetSpreadAmount
@@ -271,14 +261,12 @@ contract MixinMatchOrders is
         // Maker fees
         dispatchTransferFrom(
             zrxAssetData,
-            ZRX_PROXY_ID,
             leftOrder.makerAddress,
             leftOrder.feeRecipientAddress,
             matchedFillResults.left.makerFeePaid
         );
         dispatchTransferFrom(
             zrxAssetData,
-            ZRX_PROXY_ID,
             rightOrder.makerAddress,
             rightOrder.feeRecipientAddress,
             matchedFillResults.right.makerFeePaid
@@ -288,7 +276,6 @@ contract MixinMatchOrders is
         if (leftOrder.feeRecipientAddress == rightOrder.feeRecipientAddress) {
             dispatchTransferFrom(
                 zrxAssetData,
-                ZRX_PROXY_ID,
                 takerAddress,
                 leftOrder.feeRecipientAddress,
                 safeAdd(
@@ -299,14 +286,12 @@ contract MixinMatchOrders is
         } else {
             dispatchTransferFrom(
                 zrxAssetData,
-                ZRX_PROXY_ID,
                 takerAddress,
                 leftOrder.feeRecipientAddress,
                 matchedFillResults.left.takerFeePaid
             );
             dispatchTransferFrom(
                 zrxAssetData,
-                ZRX_PROXY_ID,
                 takerAddress,
                 rightOrder.feeRecipientAddress,
                 matchedFillResults.right.takerFeePaid
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/MixinSignatureValidator.sol b/packages/contracts/src/contracts/current/protocol/Exchange/MixinSignatureValidator.sol
index cbb55bfce..29172057a 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/MixinSignatureValidator.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/MixinSignatureValidator.sol
@@ -19,23 +19,17 @@
 pragma solidity ^0.4.24;
 
 import "../../utils/LibBytes/LibBytes.sol";
-import "./libs/LibExchangeErrors.sol";
 import "./mixins/MSignatureValidator.sol";
 import "./mixins/MTransactions.sol";
 import "./interfaces/IWallet.sol";
 import "./interfaces/IValidator.sol";
 
 contract MixinSignatureValidator is
-    LibExchangeErrors,
     MSignatureValidator,
     MTransactions
 {
     using LibBytes for bytes;
     
-    // Personal message headers
-    string constant ETH_PERSONAL_MESSAGE = "\x19Ethereum Signed Message:\n32";
-    string constant TREZOR_PERSONAL_MESSAGE = "\x19Ethereum Signed Message:\n\x20";
-
     // Mapping of hash => signer => signed
     mapping (bytes32 => mapping (address => bool)) public preSigned;
 
@@ -59,7 +53,7 @@ contract MixinSignatureValidator is
                 signerAddress,
                 signature
             ),
-            INVALID_SIGNATURE
+            "INVALID_SIGNATURE"
         );
         preSigned[hash][signerAddress] = true;
     }
@@ -99,14 +93,14 @@ contract MixinSignatureValidator is
         // TODO: Domain separation: make hash depend on role. (Taker sig should not be valid as maker sig, etc.)
         require(
             signature.length > 0,
-            LENGTH_GREATER_THAN_0_REQUIRED
+            "LENGTH_GREATER_THAN_0_REQUIRED"
         );
 
         // Ensure signature is supported
         uint8 signatureTypeRaw = uint8(signature.popLastByte());
         require(
             signatureTypeRaw < uint8(SignatureType.NSignatureTypes),
-            SIGNATURE_UNSUPPORTED
+            "SIGNATURE_UNSUPPORTED"
         );
 
         // Pop last byte off of signature byte array.
@@ -124,7 +118,7 @@ contract MixinSignatureValidator is
         // it an explicit option. This aids testing and analysis. It is
         // also the initialization value for the enum type.
         if (signatureType == SignatureType.Illegal) {
-            revert(SIGNATURE_ILLEGAL);
+            revert("SIGNATURE_ILLEGAL");
 
         // Always invalid signature.
         // Like Illegal, this is always implicitly available and therefore
@@ -133,7 +127,7 @@ contract MixinSignatureValidator is
         } else if (signatureType == SignatureType.Invalid) {
             require(
                 signature.length == 0,
-                LENGTH_0_REQUIRED
+                "LENGTH_0_REQUIRED"
             );
             isValid = false;
             return isValid;
@@ -142,7 +136,7 @@ contract MixinSignatureValidator is
         } else if (signatureType == SignatureType.EIP712) {
             require(
                 signature.length == 65,
-                LENGTH_65_REQUIRED
+                "LENGTH_65_REQUIRED"
             );
             v = uint8(signature[0]);
             r = signature.readBytes32(1);
@@ -155,13 +149,16 @@ contract MixinSignatureValidator is
         } else if (signatureType == SignatureType.EthSign) {
             require(
                 signature.length == 65,
-                LENGTH_65_REQUIRED
+                "LENGTH_65_REQUIRED"
             );
             v = uint8(signature[0]);
             r = signature.readBytes32(1);
             s = signature.readBytes32(33);
             recovered = ecrecover(
-                keccak256(abi.encodePacked(ETH_PERSONAL_MESSAGE, hash)),
+                keccak256(abi.encodePacked(
+                    "\x19Ethereum Signed Message:\n32",
+                    hash
+                )),
                 v,
                 r,
                 s
@@ -180,7 +177,7 @@ contract MixinSignatureValidator is
         } else if (signatureType == SignatureType.Caller) {
             require(
                 signature.length == 0,
-                LENGTH_0_REQUIRED
+                "LENGTH_0_REQUIRED"
             );
             isValid = signerAddress == msg.sender;
             return isValid;
@@ -230,13 +227,16 @@ contract MixinSignatureValidator is
         } else if (signatureType == SignatureType.Trezor) {
             require(
                 signature.length == 65,
-                LENGTH_65_REQUIRED
+                "LENGTH_65_REQUIRED"
             );
             v = uint8(signature[0]);
             r = signature.readBytes32(1);
             s = signature.readBytes32(33);
             recovered = ecrecover(
-                keccak256(abi.encodePacked(TREZOR_PERSONAL_MESSAGE, hash)),
+                keccak256(abi.encodePacked(
+                    "\x19Ethereum Signed Message:\n\x20",
+                    hash
+                )),
                 v,
                 r,
                 s
@@ -250,6 +250,6 @@ contract MixinSignatureValidator is
         // that we currently support. In this case returning false
         // may lead the caller to incorrectly believe that the
         // signature was invalid.)
-        revert(SIGNATURE_UNSUPPORTED);
+        revert("SIGNATURE_UNSUPPORTED");
     }
 }
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/MixinTransactions.sol b/packages/contracts/src/contracts/current/protocol/Exchange/MixinTransactions.sol
index 20a4a12df..e0f450d0a 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/MixinTransactions.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/MixinTransactions.sol
@@ -20,12 +20,10 @@ pragma solidity ^0.4.24;
 import "./libs/LibExchangeErrors.sol";
 import "./mixins/MSignatureValidator.sol";
 import "./mixins/MTransactions.sol";
-import "./libs/LibExchangeErrors.sol";
 import "./libs/LibEIP712.sol";
 
 contract MixinTransactions is
     LibEIP712,
-    LibExchangeErrors,
     MSignatureValidator,
     MTransactions
 {
@@ -90,7 +88,7 @@ contract MixinTransactions is
         // Prevent reentrancy
         require(
             currentContextAddress == address(0),
-            REENTRANCY_ILLEGAL
+            "REENTRANCY_ILLEGAL"
         );
 
         bytes32 transactionHash = hashEIP712Message(hashZeroExTransaction(
@@ -102,7 +100,7 @@ contract MixinTransactions is
         // Validate transaction has not been executed
         require(
             !transactions[transactionHash],
-            INVALID_TX_HASH
+            "INVALID_TX_HASH"
         );
 
         // Transaction always valid if signer is sender of transaction
@@ -114,7 +112,7 @@ contract MixinTransactions is
                     signerAddress,
                     signature
                 ),
-                INVALID_TX_SIGNATURE
+                "INVALID_TX_SIGNATURE"
             );
 
             // Set the current transaction signer
@@ -125,7 +123,7 @@ contract MixinTransactions is
         transactions[transactionHash] = true;
         require(
             address(this).delegatecall(data),
-            FAILED_EXECUTION
+            "FAILED_EXECUTION"
         );
 
         // Reset current transaction signer
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/MixinWrapperFunctions.sol b/packages/contracts/src/contracts/current/protocol/Exchange/MixinWrapperFunctions.sol
index 724f95518..00668ca43 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/MixinWrapperFunctions.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/MixinWrapperFunctions.sol
@@ -22,13 +22,11 @@ pragma experimental ABIEncoderV2;
 import "./libs/LibMath.sol";
 import "./libs/LibOrder.sol";
 import "./libs/LibFillResults.sol";
-import "./libs/LibExchangeErrors.sol";
 import "./mixins/MExchangeCore.sol";
 
 contract MixinWrapperFunctions is
     LibMath,
     LibFillResults,
-    LibExchangeErrors,
     MExchangeCore
 {
     /// @dev Fills the input order. Reverts if exact takerAssetFillAmount not filled.
@@ -50,7 +48,7 @@ contract MixinWrapperFunctions is
         );
         require(
             fillResults.takerAssetFilledAmount == takerAssetFillAmount,
-            COMPLETE_FILL_FAILED
+            "COMPLETE_FILL_FAILED"
         );
         return fillResults;
     }
@@ -366,14 +364,6 @@ contract MixinWrapperFunctions is
                 signatures[i]
             );
 
-            // HACK: the proxyId is "popped" from the byte array before a fill is settled
-            // by subtracting from the length of the array. Since the popped byte is 
-            // still in memory, we can "unpop" it by incrementing the length of the byte array.
-            assembly {
-                let len := mload(takerAssetData)
-                mstore(takerAssetData, add(len, 1))
-            }
-
             // Update amounts filled and fees paid by maker and taker
             addFillResults(totalFillResults, singleFillResults);
 
@@ -467,14 +457,6 @@ contract MixinWrapperFunctions is
                 signatures[i]
             );
 
-            // HACK: the proxyId is "popped" from the byte array before a fill is settled
-            // by subtracting from the length of the array. Since the popped byte is 
-            // still in memory, we can "unpop" it by incrementing the length of the byte array.
-            assembly {
-                let len := mload(makerAssetData)
-                mstore(makerAssetData, add(len, 1))
-            }
-
             // Update amounts filled and fees paid by maker and taker
             addFillResults(totalFillResults, singleFillResults);
 
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/interfaces/IAssetProxyDispatcher.sol b/packages/contracts/src/contracts/current/protocol/Exchange/interfaces/IAssetProxyDispatcher.sol
index 2c331dc34..fa55dff00 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/interfaces/IAssetProxyDispatcher.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/interfaces/IAssetProxyDispatcher.sol
@@ -26,7 +26,7 @@ contract IAssetProxyDispatcher {
     /// @param newAssetProxy Address of new asset proxy to register, or 0x0 to unset assetProxyId.
     /// @param oldAssetProxy Existing asset proxy to overwrite, or 0x0 if assetProxyId is currently unused.
     function registerAssetProxy(
-        uint8 assetProxyId,
+        bytes4 assetProxyId,
         address newAssetProxy,
         address oldAssetProxy
     )
@@ -35,7 +35,7 @@ contract IAssetProxyDispatcher {
     /// @dev Gets an asset proxy.
     /// @param assetProxyId Id of the asset proxy.
     /// @return The asset proxy registered to assetProxyId. Returns 0x0 if no proxy is registered.
-    function getAssetProxy(uint8 assetProxyId)
+    function getAssetProxy(bytes4 assetProxyId)
         external
         view
         returns (address);
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibConstants.sol b/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibConstants.sol
index 4a9452448..488ca956c 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibConstants.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibConstants.sol
@@ -25,9 +25,6 @@ contract LibConstants {
     //        not constant to make testing easier.
     bytes public ZRX_ASSET_DATA;
 
-    // Proxy Id for ZRX token.
-    uint8 constant ZRX_PROXY_ID = 1;
-
     // @TODO: Remove when we deploy.
     constructor (bytes memory zrxAssetData)
         public
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibExchangeErrors.sol b/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibExchangeErrors.sol
index a43f0f927..e37f41ada 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibExchangeErrors.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibExchangeErrors.sol
@@ -18,7 +18,10 @@
 
 pragma solidity ^0.4.24;
 
+/// @dev This contract documents the revert reasons used in the Exchange contract.
+/// This contract is intended to serve as a reference, but is not actually used for efficiency reasons.
 contract LibExchangeErrors {
+
     /// Order validation errors ///
     string constant ORDER_UNFILLABLE = "ORDER_UNFILLABLE";                              // Order cannot be filled.
     string constant INVALID_MAKER = "INVALID_MAKER";                                    // Invalid makerAddress.
@@ -56,9 +59,11 @@ contract LibExchangeErrors {
 
     /// dispatchTransferFrom errors ///
     string constant ASSET_PROXY_DOES_NOT_EXIST = "ASSET_PROXY_DOES_NOT_EXIST";          // No assetProxy registered at given id.
+    string constant TRANSFER_FAILED = "TRANSFER_FAILED";                                // Asset transfer unsuccesful.
 
     /// Length validation errors ///
     string constant LENGTH_GREATER_THAN_0_REQUIRED = "LENGTH_GREATER_THAN_0_REQUIRED";  // Byte array must have a length greater than 0.
+    string constant LENGTH_GREATER_THAN_3_REQUIRED = "LENGTH_GREATER_THAN_3_REQUIRED";  // Byte array must have a length greater than 3.
     string constant LENGTH_0_REQUIRED = "LENGTH_0_REQUIRED";                            // Byte array must have a length of 0.
     string constant LENGTH_65_REQUIRED = "LENGTH_65_REQUIRED";                          // Byte array must have a length of 65.
 }
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibMath.sol b/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibMath.sol
index 233547b9f..bfe2fd33f 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibMath.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/libs/LibMath.sol
@@ -23,7 +23,6 @@ import "../../../utils/SafeMath/SafeMath.sol";
 contract LibMath is
     SafeMath
 {
-    string constant ROUNDING_ERROR_ON_PARTIAL_AMOUNT = "A rounding error occurred when calculating partial transfer amounts.";
 
     /// @dev Calculates partial value given a numerator and denominator.
     /// @param numerator Numerator.
diff --git a/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MAssetProxyDispatcher.sol b/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MAssetProxyDispatcher.sol
index 788f42c60..c2b506dcf 100644
--- a/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MAssetProxyDispatcher.sol
+++ b/packages/contracts/src/contracts/current/protocol/Exchange/mixins/MAssetProxyDispatcher.sol
@@ -27,20 +27,18 @@ contract MAssetProxyDispatcher is
 
     // Logs registration of new asset proxy
     event AssetProxySet(
-        uint8 id,               // Id of new registered AssetProxy.
+        bytes4 id,              // Id of new registered AssetProxy.
         address newAssetProxy,  // Address of new registered AssetProxy.
         address oldAssetProxy   // Address of AssetProxy that was overwritten at given id (or null address).
     );
 
     /// @dev Forwards arguments to assetProxy and calls `transferFrom`. Either succeeds or throws.
-    /// @param assetData Byte array encoded for the respective asset proxy.
-    /// @param assetProxyId Id of assetProxy to dispach to.
+    /// @param assetData Byte array encoded for the asset.
     /// @param from Address to transfer token from.
     /// @param to Address to transfer token to.
     /// @param amount Amount of token to transfer.
     function dispatchTransferFrom(
         bytes memory assetData,
-        uint8 assetProxyId,
         address from,
         address to,
         uint256 amount
diff --git a/packages/contracts/src/contracts/current/test/TestAssetProxyDispatcher/TestAssetProxyDispatcher.sol b/packages/contracts/src/contracts/current/test/TestAssetProxyDispatcher/TestAssetProxyDispatcher.sol
index d469a07f0..2ae69e0ef 100644
--- a/packages/contracts/src/contracts/current/test/TestAssetProxyDispatcher/TestAssetProxyDispatcher.sol
+++ b/packages/contracts/src/contracts/current/test/TestAssetProxyDispatcher/TestAssetProxyDispatcher.sol
@@ -24,12 +24,11 @@ import "../../protocol/Exchange/MixinAssetProxyDispatcher.sol";
 contract TestAssetProxyDispatcher is MixinAssetProxyDispatcher {
     function publicDispatchTransferFrom(
         bytes memory assetData,
-        uint8 assetProxyId,
         address from,
         address to,
         uint256 amount)
         public
     {
-        dispatchTransferFrom(assetData, assetProxyId, from, to, amount);
+        dispatchTransferFrom(assetData, from, to, amount);
     }
 }
diff --git a/packages/contracts/src/contracts/current/utils/LibBytes/LibBytes.sol b/packages/contracts/src/contracts/current/utils/LibBytes/LibBytes.sol
index e4cbf318b..78b1ddf7c 100644
--- a/packages/contracts/src/contracts/current/utils/LibBytes/LibBytes.sol
+++ b/packages/contracts/src/contracts/current/utils/LibBytes/LibBytes.sol
@@ -19,17 +19,8 @@
 pragma solidity ^0.4.24;
 
 library LibBytes {
-    using LibBytes for bytes;
 
-    // Revert reasons
-    string constant GREATER_THAN_ZERO_LENGTH_REQUIRED = "GREATER_THAN_ZERO_LENGTH_REQUIRED";
-    string constant GREATER_OR_EQUAL_TO_4_LENGTH_REQUIRED = "GREATER_OR_EQUAL_TO_4_LENGTH_REQUIRED";
-    string constant GREATER_OR_EQUAL_TO_20_LENGTH_REQUIRED = "GREATER_OR_EQUAL_TO_20_LENGTH_REQUIRED";
-    string constant GREATER_OR_EQUAL_TO_32_LENGTH_REQUIRED = "GREATER_OR_EQUAL_TO_32_LENGTH_REQUIRED";
-    string constant GREATER_OR_EQUAL_TO_NESTED_BYTES_LENGTH_REQUIRED = "GREATER_OR_EQUAL_TO_NESTED_BYTES_LENGTH_REQUIRED";
-    string constant GREATER_OR_EQUAL_TO_SOURCE_BYTES_LENGTH_REQUIRED = "GREATER_OR_EQUAL_TO_SOURCE_BYTES_LENGTH_REQUIRED";
-    string constant FROM_LESS_THAN_TO_REQUIRED = "FROM_LESS_THAN_TO_REQUIRED";
-    string constant TO_LESS_THAN_LENGTH_REQUIRED = "TO_LESS_THAN_LENGTH_REQUIRED";
+    using LibBytes for bytes;
 
     /// @dev Gets the memory address for a byte array.
     /// @param input Byte array to lookup.
@@ -176,8 +167,14 @@ library LibBytes {
         pure
         returns (bytes memory result)
     {
-        require(from <= to, FROM_LESS_THAN_TO_REQUIRED);
-        require(to < b.length, TO_LESS_THAN_LENGTH_REQUIRED);
+        require(
+            from <= to,
+            "FROM_LESS_THAN_TO_REQUIRED"
+        );
+        require(
+            to < b.length,
+            "TO_LESS_THAN_LENGTH_REQUIRED"
+        );
         
         // Create a new bytes structure and copy contents
         result = new bytes(to - from);
@@ -199,8 +196,14 @@ library LibBytes {
         pure
         returns (bytes memory result)
     {
-        require(from <= to, FROM_LESS_THAN_TO_REQUIRED);
-        require(to < b.length, TO_LESS_THAN_LENGTH_REQUIRED);
+        require(
+            from <= to,
+            "FROM_LESS_THAN_TO_REQUIRED"
+        );
+        require(
+            to < b.length,
+            "TO_LESS_THAN_LENGTH_REQUIRED"
+        );
         
         // Create a new bytes structure around [from, to) in-place.
         assembly {
@@ -220,7 +223,7 @@ library LibBytes {
     {
         require(
             b.length > 0,
-            GREATER_THAN_ZERO_LENGTH_REQUIRED
+            "GREATER_THAN_ZERO_LENGTH_REQUIRED"
         );
 
         // Store last byte.
@@ -244,7 +247,7 @@ library LibBytes {
     {
         require(
             b.length >= 20,
-            GREATER_OR_EQUAL_TO_20_LENGTH_REQUIRED
+            "GREATER_OR_EQUAL_TO_20_LENGTH_REQUIRED"
         );
 
         // Store last 20 bytes.
@@ -290,7 +293,7 @@ library LibBytes {
     {
         require(
             b.length >= index + 20,  // 20 is length of address
-            GREATER_OR_EQUAL_TO_20_LENGTH_REQUIRED
+            "GREATER_OR_EQUAL_TO_20_LENGTH_REQUIRED"
         );
 
         // Add offset to index:
@@ -322,7 +325,7 @@ library LibBytes {
     {
         require(
             b.length >= index + 20,  // 20 is length of address
-            GREATER_OR_EQUAL_TO_20_LENGTH_REQUIRED
+            "GREATER_OR_EQUAL_TO_20_LENGTH_REQUIRED"
         );
 
         // Add offset to index:
@@ -365,7 +368,7 @@ library LibBytes {
     {
         require(
             b.length >= index + 32,
-            GREATER_OR_EQUAL_TO_32_LENGTH_REQUIRED
+            "GREATER_OR_EQUAL_TO_32_LENGTH_REQUIRED"
         );
 
         // Arrays are prefixed by a 256 bit length parameter
@@ -392,7 +395,7 @@ library LibBytes {
     {
         require(
             b.length >= index + 32,
-            GREATER_OR_EQUAL_TO_32_LENGTH_REQUIRED
+            "GREATER_OR_EQUAL_TO_32_LENGTH_REQUIRED"
         );
 
         // Arrays are prefixed by a 256 bit length parameter
@@ -447,7 +450,7 @@ library LibBytes {
     {
         require(
             b.length >= index + 4,
-            GREATER_OR_EQUAL_TO_4_LENGTH_REQUIRED
+            "GREATER_OR_EQUAL_TO_4_LENGTH_REQUIRED"
         );
         assembly {
             result := mload(add(b, 32))
@@ -459,6 +462,8 @@ library LibBytes {
     }
 
     /// @dev Reads nested bytes from a specific position.
+    /// @dev NOTE: the returned value overlaps with the input value.
+    ///            Both should be treated as immutable.
     /// @param b Byte array containing nested bytes.
     /// @param index Index of nested bytes.
     /// @return result Nested bytes.
@@ -478,17 +483,13 @@ library LibBytes {
         // length of nested bytes
         require(
             b.length >= index + nestedBytesLength,
-            GREATER_OR_EQUAL_TO_NESTED_BYTES_LENGTH_REQUIRED
+            "GREATER_OR_EQUAL_TO_NESTED_BYTES_LENGTH_REQUIRED"
         );
-
-        // Allocate memory and copy value to result
-        result = new bytes(nestedBytesLength);
-        memCopy(
-            result.contentAddress(),
-            b.contentAddress() + index,
-            nestedBytesLength
-        );
-
+        
+        // Return a pointer to the byte array as it exists inside `b`
+        assembly {
+            result := add(b, index)
+        }
         return result;
     }
 
@@ -508,7 +509,7 @@ library LibBytes {
         // length of input
         require(
             b.length >= index + 32 /* 32 bytes to store length */ + input.length,
-            GREATER_OR_EQUAL_TO_NESTED_BYTES_LENGTH_REQUIRED
+            "GREATER_OR_EQUAL_TO_NESTED_BYTES_LENGTH_REQUIRED"
         );
 
         // Copy <input> into <b>
@@ -533,7 +534,7 @@ library LibBytes {
         // Dest length must be >= source length, or some bytes would not be copied.
         require(
             dest.length >= sourceLen,
-            GREATER_OR_EQUAL_TO_SOURCE_BYTES_LENGTH_REQUIRED
+            "GREATER_OR_EQUAL_TO_SOURCE_BYTES_LENGTH_REQUIRED"
         );
         memCopy(
             dest.contentAddress(),
diff --git a/packages/contracts/src/contracts/current/utils/Ownable/Ownable.sol b/packages/contracts/src/contracts/current/utils/Ownable/Ownable.sol
index 99b93d0d2..6f5761cc7 100644
--- a/packages/contracts/src/contracts/current/utils/Ownable/Ownable.sol
+++ b/packages/contracts/src/contracts/current/utils/Ownable/Ownable.sol
@@ -13,9 +13,6 @@ import "./IOwnable.sol";
 contract Ownable is IOwnable {
     address public owner;
 
-    // Revert reasons
-    string constant ONLY_CONTRACT_OWNER = "ONLY_CONTRACT_OWNER";
-
     constructor ()
         public
     {
@@ -25,7 +22,7 @@ contract Ownable is IOwnable {
     modifier onlyOwner() {
         require(
             msg.sender == owner,
-            ONLY_CONTRACT_OWNER
+            "ONLY_CONTRACT_OWNER"
         );
         _;
     }
diff --git a/packages/contracts/test/asset_proxy/decoder.ts b/packages/contracts/test/asset_proxy/decoder.ts
index 98d18aa38..902255caf 100644
--- a/packages/contracts/test/asset_proxy/decoder.ts
+++ b/packages/contracts/test/asset_proxy/decoder.ts
@@ -44,7 +44,6 @@ describe('TestAssetDataDecoders', () => {
         it('should correctly decode ERC721 asset data', async () => {
             const tokenId = generatePseudoRandomSalt();
             const encodedAssetData = assetProxyUtils.encodeERC721AssetData(testAddress, tokenId);
-            const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
             const expectedDecodedAssetData = assetProxyUtils.decodeERC721AssetData(encodedAssetData);
             let decodedTokenAddress: string;
             let decodedTokenId: BigNumber;
@@ -53,7 +52,7 @@ describe('TestAssetDataDecoders', () => {
                 decodedTokenAddress,
                 decodedTokenId,
                 decodedData,
-            ] = await testAssetProxyDecoder.publicDecodeERC721Data.callAsync(encodedAssetDataWithoutProxyId);
+            ] = await testAssetProxyDecoder.publicDecodeERC721Data.callAsync(encodedAssetData);
             expect(decodedTokenAddress).to.be.equal(expectedDecodedAssetData.tokenAddress);
             expect(decodedTokenId).to.be.bignumber.equal(expectedDecodedAssetData.tokenId);
             expect(decodedData).to.be.equal(expectedDecodedAssetData.receiverData);
diff --git a/packages/contracts/test/asset_proxy/proxies.ts b/packages/contracts/test/asset_proxy/proxies.ts
index 2c27f7382..2313c0b0e 100644
--- a/packages/contracts/test/asset_proxy/proxies.ts
+++ b/packages/contracts/test/asset_proxy/proxies.ts
@@ -96,13 +96,12 @@ describe('Asset Transfer Proxies', () => {
             it('should successfully transfer tokens', async () => {
                 // Construct ERC20 asset data
                 const encodedAssetData = assetProxyUtils.encodeERC20AssetData(zrxToken.address);
-                const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
                 // Perform a transfer from makerAddress to takerAddress
                 const erc20Balances = await erc20Wrapper.getBalancesAsync();
                 const amount = new BigNumber(10);
                 await web3Wrapper.awaitTransactionSuccessAsync(
                     await erc20Proxy.transferFrom.sendTransactionAsync(
-                        encodedAssetDataWithoutProxyId,
+                        encodedAssetData,
                         makerAddress,
                         takerAddress,
                         amount,
@@ -123,13 +122,12 @@ describe('Asset Transfer Proxies', () => {
             it('should do nothing if transferring 0 amount of a token', async () => {
                 // Construct ERC20 asset data
                 const encodedAssetData = assetProxyUtils.encodeERC20AssetData(zrxToken.address);
-                const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
                 // Perform a transfer from makerAddress to takerAddress
                 const erc20Balances = await erc20Wrapper.getBalancesAsync();
                 const amount = new BigNumber(0);
                 await web3Wrapper.awaitTransactionSuccessAsync(
                     await erc20Proxy.transferFrom.sendTransactionAsync(
-                        encodedAssetDataWithoutProxyId,
+                        encodedAssetData,
                         makerAddress,
                         takerAddress,
                         amount,
@@ -174,19 +172,13 @@ describe('Asset Transfer Proxies', () => {
             it('should throw if requesting address is not authorized', async () => {
                 // Construct ERC20 asset data
                 const encodedAssetData = assetProxyUtils.encodeERC20AssetData(zrxToken.address);
-                const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
+
                 // Perform a transfer from makerAddress to takerAddress
                 const amount = new BigNumber(10);
                 return expectRevertOrAlwaysFailingTransactionAsync(
-                    erc20Proxy.transferFrom.sendTransactionAsync(
-                        encodedAssetDataWithoutProxyId,
-                        makerAddress,
-                        takerAddress,
-                        amount,
-                        {
-                            from: notAuthorized,
-                        },
-                    ),
+                    erc20Proxy.transferFrom.sendTransactionAsync(encodedAssetData, makerAddress, takerAddress, amount, {
+                        from: notAuthorized,
+                    }),
                 );
             });
         });
@@ -196,10 +188,9 @@ describe('Asset Transfer Proxies', () => {
                 const erc20Balances = await erc20Wrapper.getBalancesAsync();
 
                 const encodedAssetData = assetProxyUtils.encodeERC20AssetData(zrxToken.address);
-                const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
                 const amount = new BigNumber(10);
                 const numTransfers = 2;
-                const assetData = _.times(numTransfers, () => encodedAssetDataWithoutProxyId);
+                const assetData = _.times(numTransfers, () => encodedAssetData);
                 const fromAddresses = _.times(numTransfers, () => makerAddress);
                 const toAddresses = _.times(numTransfers, () => takerAddress);
                 const amounts = _.times(numTransfers, () => amount);
@@ -228,10 +219,9 @@ describe('Asset Transfer Proxies', () => {
 
             it('should throw if not called by an authorized address', async () => {
                 const encodedAssetData = assetProxyUtils.encodeERC20AssetData(zrxToken.address);
-                const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
                 const amount = new BigNumber(10);
                 const numTransfers = 2;
-                const assetData = _.times(numTransfers, () => encodedAssetDataWithoutProxyId);
+                const assetData = _.times(numTransfers, () => encodedAssetData);
                 const fromAddresses = _.times(numTransfers, () => makerAddress);
                 const toAddresses = _.times(numTransfers, () => takerAddress);
                 const amounts = _.times(numTransfers, () => amount);
@@ -244,9 +234,10 @@ describe('Asset Transfer Proxies', () => {
             });
         });
 
-        it('should have an id of 1', async () => {
+        it('should have an id of 0xf47261b0', async () => {
             const proxyId = await erc20Proxy.getProxyId.callAsync();
-            expect(proxyId).to.equal(1);
+            const expectedProxyId = '0xf47261b0';
+            expect(proxyId).to.equal(expectedProxyId);
         });
     });
 
@@ -255,7 +246,6 @@ describe('Asset Transfer Proxies', () => {
             it('should successfully transfer tokens', async () => {
                 // Construct ERC721 asset data
                 const encodedAssetData = assetProxyUtils.encodeERC721AssetData(erc721Token.address, erc721MakerTokenId);
-                const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
                 // Verify pre-condition
                 const ownerMakerAsset = await erc721Token.ownerOf.callAsync(erc721MakerTokenId);
                 expect(ownerMakerAsset).to.be.bignumber.equal(makerAddress);
@@ -263,7 +253,7 @@ describe('Asset Transfer Proxies', () => {
                 const amount = new BigNumber(1);
                 await web3Wrapper.awaitTransactionSuccessAsync(
                     await erc721Proxy.transferFrom.sendTransactionAsync(
-                        encodedAssetDataWithoutProxyId,
+                        encodedAssetData,
                         makerAddress,
                         takerAddress,
                         amount,
@@ -279,14 +269,13 @@ describe('Asset Transfer Proxies', () => {
             it('should call onERC721Received when transferring to a smart contract without receiver data', async () => {
                 // Construct ERC721 asset data
                 const encodedAssetData = assetProxyUtils.encodeERC721AssetData(erc721Token.address, erc721MakerTokenId);
-                const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
                 // Verify pre-condition
                 const ownerMakerAsset = await erc721Token.ownerOf.callAsync(erc721MakerTokenId);
                 expect(ownerMakerAsset).to.be.bignumber.equal(makerAddress);
                 // Perform a transfer from makerAddress to takerAddress
                 const amount = new BigNumber(1);
                 const txHash = await erc721Proxy.transferFrom.sendTransactionAsync(
-                    encodedAssetDataWithoutProxyId,
+                    encodedAssetData,
                     makerAddress,
                     erc721Receiver.address,
                     amount,
@@ -315,14 +304,13 @@ describe('Asset Transfer Proxies', () => {
                     erc721MakerTokenId,
                     receiverData,
                 );
-                const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
                 // Verify pre-condition
                 const ownerMakerAsset = await erc721Token.ownerOf.callAsync(erc721MakerTokenId);
                 expect(ownerMakerAsset).to.be.bignumber.equal(makerAddress);
                 // Perform a transfer from makerAddress to takerAddress
                 const amount = new BigNumber(1);
                 const txHash = await erc721Proxy.transferFrom.sendTransactionAsync(
-                    encodedAssetDataWithoutProxyId,
+                    encodedAssetData,
                     makerAddress,
                     erc721Receiver.address,
                     amount,
@@ -351,7 +339,6 @@ describe('Asset Transfer Proxies', () => {
                     erc721MakerTokenId,
                     receiverData,
                 );
-                const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
                 // Verify pre-condition
                 const ownerMakerAsset = await erc721Token.ownerOf.callAsync(erc721MakerTokenId);
                 expect(ownerMakerAsset).to.be.bignumber.equal(makerAddress);
@@ -359,7 +346,7 @@ describe('Asset Transfer Proxies', () => {
                 const amount = new BigNumber(1);
                 return expectRevertOrAlwaysFailingTransactionAsync(
                     erc721Proxy.transferFrom.sendTransactionAsync(
-                        encodedAssetDataWithoutProxyId,
+                        encodedAssetData,
                         makerAddress,
                         erc20Proxy.address, // the ERC20 proxy does not have an ERC721 receiver
                         amount,
@@ -371,7 +358,6 @@ describe('Asset Transfer Proxies', () => {
             it('should throw if transferring 0 amount of a token', async () => {
                 // Construct ERC721 asset data
                 const encodedAssetData = assetProxyUtils.encodeERC721AssetData(erc721Token.address, erc721MakerTokenId);
-                const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
                 // Verify pre-condition
                 const ownerMakerAsset = await erc721Token.ownerOf.callAsync(erc721MakerTokenId);
                 expect(ownerMakerAsset).to.be.bignumber.equal(makerAddress);
@@ -379,7 +365,7 @@ describe('Asset Transfer Proxies', () => {
                 const amount = new BigNumber(0);
                 return expectRevertOrAlwaysFailingTransactionAsync(
                     erc721Proxy.transferFrom.sendTransactionAsync(
-                        encodedAssetDataWithoutProxyId,
+                        encodedAssetData,
                         makerAddress,
                         takerAddress,
                         amount,
@@ -391,7 +377,6 @@ describe('Asset Transfer Proxies', () => {
             it('should throw if transferring > 1 amount of a token', async () => {
                 // Construct ERC721 asset data
                 const encodedAssetData = assetProxyUtils.encodeERC721AssetData(erc721Token.address, erc721MakerTokenId);
-                const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
                 // Verify pre-condition
                 const ownerMakerAsset = await erc721Token.ownerOf.callAsync(erc721MakerTokenId);
                 expect(ownerMakerAsset).to.be.bignumber.equal(makerAddress);
@@ -399,7 +384,7 @@ describe('Asset Transfer Proxies', () => {
                 const amount = new BigNumber(500);
                 return expectRevertOrAlwaysFailingTransactionAsync(
                     erc721Proxy.transferFrom.sendTransactionAsync(
-                        encodedAssetDataWithoutProxyId,
+                        encodedAssetData,
                         makerAddress,
                         takerAddress,
                         amount,
@@ -411,7 +396,6 @@ describe('Asset Transfer Proxies', () => {
             it('should throw if allowances are too low', async () => {
                 // Construct ERC721 asset data
                 const encodedAssetData = assetProxyUtils.encodeERC721AssetData(erc721Token.address, erc721MakerTokenId);
-                const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
                 // Remove transfer approval for makerAddress.
                 await web3Wrapper.awaitTransactionSuccessAsync(
                     await erc721Token.setApprovalForAll.sendTransactionAsync(erc721Proxy.address, false, {
@@ -422,27 +406,20 @@ describe('Asset Transfer Proxies', () => {
                 // Perform a transfer; expect this to fail.
                 const amount = new BigNumber(1);
                 return expectRevertOrAlwaysFailingTransactionAsync(
-                    erc20Proxy.transferFrom.sendTransactionAsync(
-                        encodedAssetDataWithoutProxyId,
-                        makerAddress,
-                        takerAddress,
-                        amount,
-                        {
-                            from: notAuthorized,
-                        },
-                    ),
+                    erc20Proxy.transferFrom.sendTransactionAsync(encodedAssetData, makerAddress, takerAddress, amount, {
+                        from: notAuthorized,
+                    }),
                 );
             });
 
             it('should throw if requesting address is not authorized', async () => {
                 // Construct ERC721 asset data
                 const encodedAssetData = assetProxyUtils.encodeERC721AssetData(erc721Token.address, erc721MakerTokenId);
-                const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
                 // Perform a transfer from makerAddress to takerAddress
                 const amount = new BigNumber(1);
                 return expectRevertOrAlwaysFailingTransactionAsync(
                     erc721Proxy.transferFrom.sendTransactionAsync(
-                        encodedAssetDataWithoutProxyId,
+                        encodedAssetData,
                         makerAddress,
                         takerAddress,
                         amount,
@@ -459,8 +436,8 @@ describe('Asset Transfer Proxies', () => {
 
                 const numTransfers = 2;
                 const assetData = [
-                    assetProxyUtils.encodeERC721AssetData(erc721Token.address, makerTokenIdA).slice(0, -2),
-                    assetProxyUtils.encodeERC721AssetData(erc721Token.address, makerTokenIdB).slice(0, -2),
+                    assetProxyUtils.encodeERC721AssetData(erc721Token.address, makerTokenIdA),
+                    assetProxyUtils.encodeERC721AssetData(erc721Token.address, makerTokenIdB),
                 ];
                 const fromAddresses = _.times(numTransfers, () => makerAddress);
                 const toAddresses = _.times(numTransfers, () => takerAddress);
@@ -506,9 +483,10 @@ describe('Asset Transfer Proxies', () => {
             });
         });
 
-        it('should have an id of 2', async () => {
+        it('should have an id of 0x08e937fa', async () => {
             const proxyId = await erc721Proxy.getProxyId.callAsync();
-            expect(proxyId).to.equal(2);
+            const expectedProxyId = '0x08e937fa';
+            expect(proxyId).to.equal(expectedProxyId);
         });
     });
 });
diff --git a/packages/contracts/test/exchange/core.ts b/packages/contracts/test/exchange/core.ts
index ff652d3aa..5b2d4590f 100644
--- a/packages/contracts/test/exchange/core.ts
+++ b/packages/contracts/test/exchange/core.ts
@@ -86,7 +86,7 @@ describe('Exchange core', () => {
             artifacts.Exchange,
             provider,
             txDefaults,
-            zrxToken.address,
+            assetProxyUtils.encodeERC20AssetData(zrxToken.address),
         );
         exchangeWrapper = new ExchangeWrapper(exchange, provider);
         await exchangeWrapper.registerAssetProxyAsync(AssetProxyId.ERC20, erc20Proxy.address, owner);
diff --git a/packages/contracts/test/exchange/dispatcher.ts b/packages/contracts/test/exchange/dispatcher.ts
index abbfd7ac7..f96f15f16 100644
--- a/packages/contracts/test/exchange/dispatcher.ts
+++ b/packages/contracts/test/exchange/dispatcher.ts
@@ -276,14 +276,13 @@ describe('AssetProxyDispatcher', () => {
             );
             // Construct metadata for ERC20 proxy
             const encodedAssetData = assetProxyUtils.encodeERC20AssetData(zrxToken.address);
-            const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
+
             // Perform a transfer from makerAddress to takerAddress
             const erc20Balances = await erc20Wrapper.getBalancesAsync();
             const amount = new BigNumber(10);
             await web3Wrapper.awaitTransactionSuccessAsync(
                 await assetProxyDispatcher.publicDispatchTransferFrom.sendTransactionAsync(
-                    encodedAssetDataWithoutProxyId,
-                    AssetProxyId.ERC20,
+                    encodedAssetData,
                     makerAddress,
                     takerAddress,
                     amount,
@@ -304,13 +303,11 @@ describe('AssetProxyDispatcher', () => {
         it('should throw if dispatching to unregistered proxy', async () => {
             // Construct metadata for ERC20 proxy
             const encodedAssetData = assetProxyUtils.encodeERC20AssetData(zrxToken.address);
-            const encodedAssetDataWithoutProxyId = encodedAssetData.slice(0, -2);
             // Perform a transfer from makerAddress to takerAddress
             const amount = new BigNumber(10);
             return expectRevertOrAlwaysFailingTransactionAsync(
                 assetProxyDispatcher.publicDispatchTransferFrom.sendTransactionAsync(
-                    encodedAssetDataWithoutProxyId,
-                    AssetProxyId.ERC20,
+                    encodedAssetData,
                     makerAddress,
                     takerAddress,
                     amount,
diff --git a/packages/contracts/test/exchange/match_orders.ts b/packages/contracts/test/exchange/match_orders.ts
index b8dca04fd..18a46187f 100644
--- a/packages/contracts/test/exchange/match_orders.ts
+++ b/packages/contracts/test/exchange/match_orders.ts
@@ -96,7 +96,7 @@ describe('matchOrders', () => {
             artifacts.Exchange,
             provider,
             txDefaults,
-            zrxToken.address,
+            assetProxyUtils.encodeERC20AssetData(zrxToken.address),
         );
         exchangeWrapper = new ExchangeWrapper(exchange, provider);
         await exchangeWrapper.registerAssetProxyAsync(AssetProxyId.ERC20, erc20Proxy.address, owner);
diff --git a/packages/contracts/test/exchange/transactions.ts b/packages/contracts/test/exchange/transactions.ts
index 9a625880c..43b9f46b2 100644
--- a/packages/contracts/test/exchange/transactions.ts
+++ b/packages/contracts/test/exchange/transactions.ts
@@ -73,7 +73,7 @@ describe('Exchange transactions', () => {
             artifacts.Exchange,
             provider,
             txDefaults,
-            zrxToken.address,
+            assetProxyUtils.encodeERC20AssetData(zrxToken.address),
         );
         exchangeWrapper = new ExchangeWrapper(exchange, provider);
         await exchangeWrapper.registerAssetProxyAsync(AssetProxyId.ERC20, erc20Proxy.address, owner);
diff --git a/packages/contracts/test/exchange/wrapper.ts b/packages/contracts/test/exchange/wrapper.ts
index 703f644b8..a39a81057 100644
--- a/packages/contracts/test/exchange/wrapper.ts
+++ b/packages/contracts/test/exchange/wrapper.ts
@@ -80,7 +80,7 @@ describe('Exchange wrappers', () => {
             artifacts.Exchange,
             provider,
             txDefaults,
-            zrxToken.address,
+            assetProxyUtils.encodeERC20AssetData(zrxToken.address),
         );
         exchangeWrapper = new ExchangeWrapper(exchange, provider);
         await exchangeWrapper.registerAssetProxyAsync(AssetProxyId.ERC20, erc20Proxy.address, owner);
diff --git a/packages/order-utils/src/asset_proxy_utils.ts b/packages/order-utils/src/asset_proxy_utils.ts
index 915ee5032..8140ad89d 100644
--- a/packages/order-utils/src/asset_proxy_utils.ts
+++ b/packages/order-utils/src/asset_proxy_utils.ts
@@ -1,32 +1,43 @@
 import { AssetProxyId, ERC20AssetData, ERC721AssetData } from '@0xproject/types';
-import { BigNumber, NULL_BYTES } from '@0xproject/utils';
+import { BigNumber } from '@0xproject/utils';
 import BN = require('bn.js');
 import ethUtil = require('ethereumjs-util');
-import * as _ from 'lodash';
 
-const ERC20_ASSET_DATA_BYTE_LENGTH = 21;
-const ERC721_ASSET_DATA_MINIMUM_BYTE_LENGTH = 53;
-const ASSET_DATA_ADDRESS_OFFSET = 0;
-const ERC721_ASSET_DATA_TOKEN_ID_OFFSET = 20;
-const ERC721_ASSET_DATA_RECEIVER_DATA_LENGTH_OFFSET = 52;
-const ERC721_ASSET_DATA_RECEIVER_DATA_OFFSET = 84;
+import { constants } from './constants';
+
+// TODO: Push upstream to DefinitelyTyped
+interface EthAbi {
+    simpleEncode(signature: string, ...args: any[]): Buffer;
+    rawDecode(signature: string[], data: Buffer): any[];
+}
+// tslint:disable:no-var-requires
+const ethAbi = require('ethereumjs-abi') as EthAbi;
 
 export const assetProxyUtils = {
     encodeAssetProxyId(assetProxyId: AssetProxyId): Buffer {
         return ethUtil.toBuffer(assetProxyId);
     },
     decodeAssetProxyId(encodedAssetProxyId: Buffer): AssetProxyId {
-        return ethUtil.bufferToInt(encodedAssetProxyId);
+        const hexString = ethUtil.bufferToHex(encodedAssetProxyId);
+        if (hexString === AssetProxyId.ERC20) {
+            return AssetProxyId.ERC20;
+        }
+        if (hexString === AssetProxyId.ERC721) {
+            return AssetProxyId.ERC721;
+        }
+        throw new Error(`Invalid ProxyId: ${hexString}`);
     },
     encodeAddress(address: string): Buffer {
         if (!ethUtil.isValidAddress(address)) {
             throw new Error(`Invalid Address: ${address}`);
         }
         const encodedAddress = ethUtil.toBuffer(address);
-        return encodedAddress;
+        const padded = ethUtil.setLengthLeft(encodedAddress, constants.WORD_LENGTH);
+        return padded;
     },
     decodeAddress(encodedAddress: Buffer): string {
-        const address = ethUtil.bufferToHex(encodedAddress);
+        const unpadded = ethUtil.setLengthLeft(encodedAddress, constants.ADDRESS_LENGTH);
+        const address = ethUtil.bufferToHex(unpadded);
         if (!ethUtil.isValidAddress(address)) {
             throw new Error(`Invalid Address: ${address}`);
         }
@@ -37,33 +48,27 @@ export const assetProxyUtils = {
         const formattedValue = new BN(value.toString(base));
         const encodedValue = ethUtil.toBuffer(formattedValue);
         // tslint:disable-next-line:custom-no-magic-numbers
-        const paddedValue = ethUtil.setLengthLeft(encodedValue, 32);
+        const paddedValue = ethUtil.setLengthLeft(encodedValue, constants.WORD_LENGTH);
         return paddedValue;
     },
     decodeUint256(encodedValue: Buffer): BigNumber {
         const formattedValue = ethUtil.bufferToHex(encodedValue);
-        const value = new BigNumber(formattedValue, 16);
+        const value = new BigNumber(formattedValue, constants.BASE_16);
         return value;
     },
     encodeERC20AssetData(tokenAddress: string): string {
-        const encodedAssetProxyId = assetProxyUtils.encodeAssetProxyId(AssetProxyId.ERC20);
-        const encodedAddress = assetProxyUtils.encodeAddress(tokenAddress);
-        const encodedAssetData = Buffer.concat([encodedAddress, encodedAssetProxyId]);
-        const encodedAssetDataHex = ethUtil.bufferToHex(encodedAssetData);
-        return encodedAssetDataHex;
+        return ethUtil.bufferToHex(ethAbi.simpleEncode('ERC20Token(address)', tokenAddress));
     },
-    decodeERC20AssetData(proxyData: string): ERC20AssetData {
-        const encodedAssetData = ethUtil.toBuffer(proxyData);
-        if (encodedAssetData.byteLength !== ERC20_ASSET_DATA_BYTE_LENGTH) {
+    decodeERC20AssetData(assetData: string): ERC20AssetData {
+        const data = ethUtil.toBuffer(assetData);
+        if (data.byteLength < constants.ERC20_ASSET_DATA_BYTE_LENGTH) {
             throw new Error(
-                `Could not decode ERC20 Proxy Data. Expected length of encoded data to be 21. Got ${
-                    encodedAssetData.byteLength
-                }`,
+                `Could not decode ERC20 Proxy Data. Expected length of encoded data to be at least ${
+                    constants.ERC20_ASSET_DATA_BYTE_LENGTH
+                }. Got ${data.byteLength}`,
             );
         }
-        const assetProxyIdOffset = encodedAssetData.byteLength - 1;
-        const encodedAssetProxyId = encodedAssetData.slice(assetProxyIdOffset);
-        const assetProxyId = assetProxyUtils.decodeAssetProxyId(encodedAssetProxyId);
+        const assetProxyId = ethUtil.bufferToHex(data.slice(0, constants.SELECTOR_LENGTH));
         if (assetProxyId !== AssetProxyId.ERC20) {
             throw new Error(
                 `Could not decode ERC20 Proxy Data. Expected Asset Proxy Id to be ERC20 (${
@@ -71,98 +76,61 @@ export const assetProxyUtils = {
                 }), but got ${assetProxyId}`,
             );
         }
-        const encodedTokenAddress = encodedAssetData.slice(ASSET_DATA_ADDRESS_OFFSET, assetProxyIdOffset);
-        const tokenAddress = assetProxyUtils.decodeAddress(encodedTokenAddress);
-        const erc20AssetData = {
+        const [tokenAddress] = ethAbi.rawDecode(['address'], data.slice(constants.SELECTOR_LENGTH));
+        return {
             assetProxyId,
-            tokenAddress,
+            tokenAddress: ethUtil.addHexPrefix(tokenAddress),
         };
-        return erc20AssetData;
     },
     encodeERC721AssetData(tokenAddress: string, tokenId: BigNumber, receiverData?: string): string {
-        const encodedAssetProxyId = assetProxyUtils.encodeAssetProxyId(AssetProxyId.ERC721);
-        const encodedAddress = assetProxyUtils.encodeAddress(tokenAddress);
-        const encodedTokenId = assetProxyUtils.encodeUint256(tokenId);
-        let encodedAssetData = Buffer.concat([encodedAddress, encodedTokenId]);
-        if (!_.isUndefined(receiverData)) {
-            const encodedReceiverData = ethUtil.toBuffer(receiverData);
-            const receiverDataLength = new BigNumber(encodedReceiverData.byteLength);
-            const encodedReceiverDataLength = assetProxyUtils.encodeUint256(receiverDataLength);
-            encodedAssetData = Buffer.concat([encodedAssetData, encodedReceiverDataLength, encodedReceiverData]);
-        }
-        encodedAssetData = Buffer.concat([encodedAssetData, encodedAssetProxyId]);
-        const encodedAssetDataHex = ethUtil.bufferToHex(encodedAssetData);
-        return encodedAssetDataHex;
+        // TODO: Pass `tokendId` as a BigNumber.
+        return ethUtil.bufferToHex(
+            ethAbi.simpleEncode(
+                'ERC721Token(address,uint256,bytes)',
+                tokenAddress,
+                `0x${tokenId.toString(constants.BASE_16)}`,
+                ethUtil.toBuffer(receiverData || '0x'),
+            ),
+        );
     },
     decodeERC721AssetData(assetData: string): ERC721AssetData {
-        const encodedAssetData = ethUtil.toBuffer(assetData);
-        if (encodedAssetData.byteLength < ERC721_ASSET_DATA_MINIMUM_BYTE_LENGTH) {
+        const data = ethUtil.toBuffer(assetData);
+        if (data.byteLength < constants.ERC721_ASSET_DATA_MINIMUM_BYTE_LENGTH) {
             throw new Error(
-                `Could not decode ERC20 Proxy Data. Expected length of encoded data to be at least 53. Got ${
-                    encodedAssetData.byteLength
-                }`,
+                `Could not decode ERC721 Asset Data. Expected length of encoded data to be at least ${
+                    constants.ERC721_ASSET_DATA_MINIMUM_BYTE_LENGTH
+                }. Got ${data.byteLength}`,
             );
         }
-
-        const encodedTokenAddress = encodedAssetData.slice(
-            ASSET_DATA_ADDRESS_OFFSET,
-            ERC721_ASSET_DATA_TOKEN_ID_OFFSET,
-        );
-        const proxyIdOffset = encodedAssetData.byteLength - 1;
-        const encodedAssetProxyId = encodedAssetData.slice(proxyIdOffset);
-        const assetProxyId = assetProxyUtils.decodeAssetProxyId(encodedAssetProxyId);
+        const assetProxyId = ethUtil.bufferToHex(data.slice(0, constants.SELECTOR_LENGTH));
         if (assetProxyId !== AssetProxyId.ERC721) {
             throw new Error(
-                `Could not decode ERC721 Proxy Data. Expected Asset Proxy Id to be ERC721 (${
+                `Could not decode ERC721 Asset Data. Expected Asset Proxy Id to be ERC721 (${
                     AssetProxyId.ERC721
                 }), but got ${assetProxyId}`,
             );
         }
-        const tokenAddress = assetProxyUtils.decodeAddress(encodedTokenAddress);
-        const encodedTokenId = encodedAssetData.slice(
-            ERC721_ASSET_DATA_TOKEN_ID_OFFSET,
-            ERC721_ASSET_DATA_RECEIVER_DATA_LENGTH_OFFSET,
+        const [tokenAddress, tokenId, receiverData] = ethAbi.rawDecode(
+            ['address', 'uint256', 'bytes'],
+            data.slice(constants.SELECTOR_LENGTH),
         );
-        const tokenId = assetProxyUtils.decodeUint256(encodedTokenId);
-        let receiverData = NULL_BYTES;
-        const lengthUpToReceiverDataLength = ERC721_ASSET_DATA_RECEIVER_DATA_LENGTH_OFFSET + 1;
-        if (encodedAssetData.byteLength > lengthUpToReceiverDataLength) {
-            const encodedReceiverDataLength = encodedAssetData.slice(
-                ERC721_ASSET_DATA_RECEIVER_DATA_LENGTH_OFFSET,
-                ERC721_ASSET_DATA_RECEIVER_DATA_OFFSET,
-            );
-            const receiverDataLength = assetProxyUtils.decodeUint256(encodedReceiverDataLength);
-            const lengthUpToReceiverData = ERC721_ASSET_DATA_RECEIVER_DATA_OFFSET + 1;
-            const expectedReceiverDataLength = new BigNumber(encodedAssetData.byteLength - lengthUpToReceiverData);
-            if (!receiverDataLength.equals(expectedReceiverDataLength)) {
-                throw new Error(
-                    `Data length (${receiverDataLength}) does not match actual length of data (${expectedReceiverDataLength})`,
-                );
-            }
-            const encodedReceiverData = encodedAssetData.slice(
-                ERC721_ASSET_DATA_RECEIVER_DATA_OFFSET,
-                receiverDataLength.add(ERC721_ASSET_DATA_RECEIVER_DATA_OFFSET).toNumber(),
-            );
-            receiverData = ethUtil.bufferToHex(encodedReceiverData);
-        }
-        const erc721AssetData: ERC721AssetData = {
+        return {
             assetProxyId,
-            tokenAddress,
-            tokenId,
-            receiverData,
+            tokenAddress: ethUtil.addHexPrefix(tokenAddress),
+            tokenId: new BigNumber(tokenId.toString()),
+            receiverData: ethUtil.bufferToHex(receiverData),
         };
-        return erc721AssetData;
     },
     decodeAssetDataId(assetData: string): AssetProxyId {
         const encodedAssetData = ethUtil.toBuffer(assetData);
-        if (encodedAssetData.byteLength < 1) {
+        if (encodedAssetData.byteLength < constants.SELECTOR_LENGTH) {
             throw new Error(
-                `Could not decode Proxy Data. Expected length of encoded data to be at least 1. Got ${
+                `Could not decode Proxy Data. Expected length of encoded data to be at least 4. Got ${
                     encodedAssetData.byteLength
                 }`,
             );
         }
-        const encodedAssetProxyId = encodedAssetData.slice(-1);
+        const encodedAssetProxyId = encodedAssetData.slice(0, constants.SELECTOR_LENGTH);
         const assetProxyId = assetProxyUtils.decodeAssetProxyId(encodedAssetProxyId);
         return assetProxyId;
     },
diff --git a/packages/order-utils/src/constants.ts b/packages/order-utils/src/constants.ts
index ed5bd8101..383a657b8 100644
--- a/packages/order-utils/src/constants.ts
+++ b/packages/order-utils/src/constants.ts
@@ -5,4 +5,10 @@ export const constants = {
     // tslint:disable-next-line:custom-no-magic-numbers
     UNLIMITED_ALLOWANCE_IN_BASE_UNITS: new BigNumber(2).pow(256).minus(1),
     TESTRPC_NETWORK_ID: 50,
+    ADDRESS_LENGTH: 20,
+    WORD_LENGTH: 32,
+    ERC20_ASSET_DATA_BYTE_LENGTH: 36,
+    ERC721_ASSET_DATA_MINIMUM_BYTE_LENGTH: 53,
+    SELECTOR_LENGTH: 4,
+    BASE_16: 16,
 };
diff --git a/packages/types/src/index.ts b/packages/types/src/index.ts
index e5a0f41a9..88abdc1e7 100644
--- a/packages/types/src/index.ts
+++ b/packages/types/src/index.ts
@@ -152,18 +152,18 @@ export interface ECSignature {
 }
 
 export enum AssetProxyId {
-    INVALID,
-    ERC20,
-    ERC721,
+    INVALID = '0x00000000',
+    ERC20 = '0xf47261b0',
+    ERC721 = '0x08e937fa',
 }
 
 export interface ERC20AssetData {
-    assetProxyId: AssetProxyId;
+    assetProxyId: string;
     tokenAddress: string;
 }
 
 export interface ERC721AssetData {
-    assetProxyId: AssetProxyId;
+    assetProxyId: string;
     tokenAddress: string;
     tokenId: BigNumber;
     receiverData: string;