IPFS

MultiHash

IPFS uses MultiHash to describe file paths

Currently, they are using sha-256, 32 bytes for their multihash, from which we can get the actual hash using

export function getBytes32FromMultiash(multihash) {
  const decoded = bs58.decode(multihash);

  return {
    digest: `0x${decoded.slice(2).toString('hex')}`,
    hashFunction: decoded[0],
    size: decoded[1],
  };
}

where the digest part is the hash.

To convert bytes32 to multihash, apply

export function getMultihashFromBytes32(digest) {
  const hashFunction = 18;
  const size = 32;

  // cut off leading "0x"
  const hashBytes = Buffer.from(digest.slice(2), 'hex');

  // prepend hashFunction and digest size
  const multihashBytes = new (hashBytes.constructor)(2 + hashBytes.length);
  multihashBytes[0] = hashFunction;
  multihashBytes[1] = size;
  multihashBytes.set(hashBytes, 2);

  return bs58.encode(multihashBytes);
}

See this file for details.

Store IPFS Multihash in Solidity

Since the hash function and the size stay unchanged, we only need to store the digest part in a bytes32.