Solidity RSA Library

Simple RSA verification library for Solidity contracts.

Library Setup

# Using NPM
/home/ricmoo/my-project> npm install rsa-contracts

From your Solidity:

import "rsa-contracts/contract/lib-rsa.sol";

contract MyContract {
    function validateRSA3072(bytes32 hash, bytes32[12] memory pubkey,
      bytes32[12] memory sig) public returns (bool) {
        return RSA3072.recoverHash(pubkey, 65537, sig) == hash;
    }
}

API

The API for each of the supported keypair sizes is the same, only differing in the number of bytes32 words that make up the modulus and signature, which must match the keypair size.

// RSA 1024-bit
RSA1024.recoverHash(bytes32[4] modulus, uint exponent, bytes32[4] sig)
  returns (bytes32)

// RSA 2048-bit
RSA2048.recoverHash(bytes32[8] modulus, uint exponent, bytes32[8] sig)
  returns (bytes32)

// RSA 3072-bit
RSA3072.recoverHash(bytes32[12] modulus, uint exponent, bytes32[12] sig)
  returns (bytes32)

// RSA 4096-bit
RSA4096.recoverHash(bytes32[16] modulus, uint exponent, bytes32[16] sig)
  returns (bytes32)

RSA public keys have two components, the modulus (also called n) and the exponent (also called e).

The coresponding private key can be used to efficiently compute a signature such that:

digest = (signature ** exponent) % modulus

So, during signing the payload is hashed using SHA2-256 into a digest, then encoded with PKCS#1 v1.5 to pad the 256-bit digest up to the necessary key size (for example, 3072-bits). The private key then computes the signature.

To verify the signed content, these contract methods can be used to recover the original hash which can then be checked against the expected value.

require(RSA3072.recoverHash(modulus, 65537, signature) == digest);

The vast majority of RSA usage expect the exponent 65537, so usually it can be omitted from the public key and you need only provide the modulus, hard-coding the exponent where necessary. That depends on the system you are using though.

License

MIT License