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# On-Chain Random Oracle
Most blockchains do not have a random source since they are not able to
generate an on-chain random number that can not be predicted beforehand.
Tangerine solves this problem by using threshold signature with BLS scheme.
Since this final threshold signature is only known after they are being
gathered together, one can not predict the signature before sending the
transaction that would be included in the given block.
If you look at the block result using the RPC:
```
{
"jsonrpc": "2.0",
"id": 1,
"result": {
"dexconMeta": "...",
"extraData": "0x",
"gasLimit": "0x7a1200",
"gasUsed": "0x238f8",
"hash": "0x78ba157ae54a4784e1f0edbdac2b14e1f38f519d34d3d3164d75016ac90e8227",
"logsBloom": "...",
"miner": "0xb6a2e270b54f19b4002d6535a71cfdd1ca1f5fc1",
"mixHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
"nonce": "0x0000000000000000",
"number": "0x100",
"parentHash": "0x8da90bea16f9a6edf022030766950636caad7fbd0ef296e7d34da052d0fa4ca7",
"randomness": "0x895736dfafa3ce4e0f051fac7cc218cb092b67e88b7c25f4cf90e496e91c7d10e55a251e0ff6e655fdc8d866b85fe30c",
"receiptsRoot": "0xc627d0acef6f0592ffd775e05b4f6b36dce8065a217bf4c9fec0c538b8b46b7d",
"sha3Uncles": "0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347",
"size": "0x67b",
"stateRoot": "0xb728e453a70d2c6d5f9a00857aa697740c16770a696dd8a6594876c2b6f85a84",
"timestamp": "0x5bcf0935",
"totalDifficulty": "0x20100",
"transactions": [
...
],
"transactionsRoot": "0x1b1e6b2a70de752b563fbeedb0461dca4af0c56e8dd3147b7a7ea3b2a701f6e6",
"uncles": []
}
}
```
You can see there is a `randomness` field which standard Ethereum RPC does not
have. This `randomness` field is actually the threshold signature of the block
signed by the DKG set.
Tangerine Network provides on-chain randomness through a precompiled contract.
Using a precompiled contract instead of introducing a new OP code ensures that
we are compatible with EVM, and eliminates the need to maintain a separate
toolchain.
The precompiled contract is located at
`0xc327ff1025c5b3d2deb5e3f0f161b3f7e557579a`, which is the first 20 bytes of
`Keccak256Hash("Tangerine Network Random")`. The gas cost for each invocation is 64.
To use the random library, simply include the library file then call `Random.rand()` to get the random number.
## Example
```
import "github.com/tangerine-network/random-lib/RandomLib.sol";
contract UseRandom {
event Win();
event Lose();
function readRand() public view returns (uint256) {
return Random.rand();
}
function bet() public {
if (Random.rand() % 100 >= 50) {
emit Win();
} else {
emit Lose();
}
}
}
```
The random variable is generated with the following formula:
```
rand = Keccak( Randomness . Caller . Nonce . RandCallIndex )
```
* `Randomness` is the block randomness signed by DKG set, unique for each block.
* Note: `Randomness` field is not available in the initial round.
* `Caller` is the address of the contract caller.
* `Nonce` is the account nonce of the original caller.
* `RandCallIndex` number of times OP_RAND is called in the current transaction.
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