path: root/crypto/key_store_passphrase.go

/*
    This file is part of go-ethereum

    go-ethereum is free software: you can redistribute it and/or modify
    it under the terms of the GNU Lesser General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    go-ethereum is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU Lesser General Public License
    along with go-ethereum.  If not, see <http://www.gnu.org/licenses/>.
*/
/**
 * @authors
 *  Gustav Simonsson <gustav.simonsson@gmail.com>
 * @date 2015
 *
 */

/*

This key store behaves as KeyStorePlain with the difference that
the private key is encrypted and on disk uses another JSON encoding.

Cryptography:

1. Encryption key is first 16 bytes of SHA3-256 of first 16 bytes of
   scrypt derived key from user passphrase. Scrypt parameters
   (work factors) [1][2] are defined as constants below.
2. Scrypt salt is 32 random bytes from CSPRNG.
   It's stored in plain next to ciphertext in key file.
3. MAC is SHA3-256 of concatenation of ciphertext and last 16 bytes of scrypt derived key.
4. Plaintext is the EC private key bytes.
5. Encryption algo is AES 128 CBC [3][4]
6. CBC IV is 16 random bytes from CSPRNG.
   It's stored in plain next to ciphertext in key file.
7. Plaintext padding is PKCS #7 [5][6]

Encoding:

1. On disk, the ciphertext, MAC, salt and IV are encoded in a nested JSON object.
   cat a key file to see the structure.
2. byte arrays are base64 JSON strings.
3. The EC private key bytes are in uncompressed form [7].
   They are a big-endian byte slice of the absolute value of D [8][9].

References:

1. http://www.tarsnap.com/scrypt/scrypt-slides.pdf
2. http://stackoverflow.com/questions/11126315/what-are-optimal-scrypt-work-factors
3. http://en.wikipedia.org/wiki/Advanced_Encryption_Standard
4. http://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Cipher-block_chaining_.28CBC.29
5. https://leanpub.com/gocrypto/read#leanpub-auto-block-cipher-modes
6. http://tools.ietf.org/html/rfc2315
7. http://bitcoin.stackexchange.com/questions/3059/what-is-a-compressed-bitcoin-key
8. http://golang.org/pkg/crypto/ecdsa/#PrivateKey
9. https://golang.org/pkg/math/big/#Int.Bytes

*/

package crypto

import (
    "bytes"
    "crypto/aes"
    "crypto/cipher"
    "encoding/json"
    "errors"
    "io"
    "os"
    "path/filepath"

    "code.google.com/p/go-uuid/uuid"
    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/crypto/randentropy"
    "golang.org/x/crypto/scrypt"
)

const (
    keyHeaderVersion = "1"
    keyHeaderKDF     = "scrypt"
    // 2^18 / 8 / 1 uses 256MB memory and approx 1s CPU time on a modern CPU.
    scryptN     = 1 << 18
    scryptr     = 8
    scryptp     = 1
    scryptdkLen = 32
)

type keyStorePassphrase struct {
    keysDirPath string
}

func NewKeyStorePassphrase(path string) KeyStore2 {
    return &keyStorePassphrase{path}
}

func (ks keyStorePassphrase) GenerateNewKey(rand io.Reader, auth string) (key *Key, err error) {
    return GenerateNewKeyDefault(ks, rand, auth)
}

func (ks keyStorePassphrase) GetKey(keyAddr common.Address, auth string) (key *Key, err error) {
    keyBytes, keyId, err := DecryptKey(ks, keyAddr, auth)
    if err != nil {
        return nil, err
    }
    key = &Key{
        Id:         uuid.UUID(keyId),
        Address:    keyAddr,
        PrivateKey: ToECDSA(keyBytes),
    }
    return key, err
}

func (ks keyStorePassphrase) GetKeyAddresses() (addresses []common.Address, err error) {
    return GetKeyAddresses(ks.keysDirPath)
}

func (ks keyStorePassphrase) StoreKey(key *Key, auth string) (err error) {
    authArray := []byte(auth)
    salt := randentropy.GetEntropyCSPRNG(32)
    derivedKey, err := scrypt.Key(authArray, salt, scryptN, scryptr, scryptp, scryptdkLen)
    if err != nil {
        return err
    }

    encryptKey := Sha3(derivedKey[:16])[:16]

    keyBytes := FromECDSA(key.PrivateKey)
    toEncrypt := PKCS7Pad(keyBytes)

    AES128Block, err := aes.NewCipher(encryptKey)
    if err != nil {
        return err
    }

    iv := randentropy.GetEntropyCSPRNG(aes.BlockSize) // 16
    AES128CBCEncrypter := cipher.NewCBCEncrypter(AES128Block, iv)
    cipherText := make([]byte, len(toEncrypt))
    AES128CBCEncrypter.CryptBlocks(cipherText, toEncrypt)

    paramsJSON := scryptParamsJSON{
        N:       scryptN,
        R:       scryptr,
        P:       scryptp,
        DkLen:   scryptdkLen,
        SaltLen: 32,
    }

    keyHeaderJSON := keyHeaderJSON{
        Version:   keyHeaderVersion,
        Kdf:       keyHeaderKDF,
        KdfParams: paramsJSON,
    }

    keyHeaderJSONStr, err := json.Marshal(keyHeaderJSON)
    if err != nil {
        return err
    }

    mac := Sha3(keyHeaderJSONStr, derivedKey[16:32], cipherText)

    cipherStruct := cipherJSON{
        mac,
        salt,
        iv,
        keyHeaderJSON,
        cipherText,
    }
    keyStruct := encryptedKeyJSON{
        key.Id,
        key.Address.Bytes(),
        cipherStruct,
    }
    keyJSON, err := json.Marshal(keyStruct)
    if err != nil {
        return err
    }

    return WriteKeyFile(key.Address, ks.keysDirPath, keyJSON)
}

func (ks keyStorePassphrase) DeleteKey(keyAddr common.Address, auth string) (err error) {
    // only delete if correct passphrase is given
    _, _, err = DecryptKey(ks, keyAddr, auth)
    if err != nil {
        return err
    }

    keyDirPath := filepath.Join(ks.keysDirPath, keyAddr.Hex())
    return os.RemoveAll(keyDirPath)
}

func DecryptKey(ks keyStorePassphrase, keyAddr common.Address, auth string) (keyBytes []byte, keyId []byte, err error) {
    fileContent, err := GetKeyFile(ks.keysDirPath, keyAddr)
    if err != nil {
        return nil, nil, err
    }

    keyProtected := new(encryptedKeyJSON)
    err = json.Unmarshal(fileContent, keyProtected)

    keyId = keyProtected.Id
    mac := keyProtected.Crypto.MAC
    salt := keyProtected.Crypto.Salt
    iv := keyProtected.Crypto.IV
    keyHeader := keyProtected.Crypto.KeyHeader
    cipherText := keyProtected.Crypto.CipherText

    // used in MAC
    keyHeaderJSONStr, err := json.Marshal(keyHeader)
    if err != nil {
        return nil, nil, err
    }

    // TODO: make this more generic when we support different KDF params / key versions
    n := keyHeader.KdfParams.N
    r := keyHeader.KdfParams.R
    p := keyHeader.KdfParams.P
    dkLen := keyHeader.KdfParams.DkLen

    authArray := []byte(auth)
    derivedKey, err := scrypt.Key(authArray, salt, n, r, p, dkLen)
    if err != nil {
        return nil, nil, err
    }

    calculatedMAC := Sha3(keyHeaderJSONStr, derivedKey[16:32], cipherText)
    if !bytes.Equal(calculatedMAC, mac) {
        err = errors.New("Decryption failed: MAC mismatch")
        return nil, nil, err
    }

    plainText, err := aesCBCDecrypt(Sha3(derivedKey[:16])[:16], cipherText, iv)
    if err != nil {
        return nil, nil, err
    }
    return plainText, keyId, err
}