MultiHash

All hashes in IPFS are encoded as a MultiHash, a self-describing hash format. The actual hash function used depends on security requirements; "sha2-256" is todays default.

A multihash is used to identify a peer, key and content. For background information, see hash concept.

var hello = Encoding.UTF8.GetBytes("Hello world");
var mh = MultiHash.ComputeHash(hello, "sha2-256");

Format

The binary representation consists of the hash code, the digest's length and value. The code and length are encoded as varints.

The textual representation is usually the Base58 encoding of the binary format. Base32 encoding is used when case insensity is required.

From the above example, the following is produced

Name	Value
hash code	0x12
digest length	0x20
digest value	64ec88ca00b268e5ba1a35678a1b5316d212f4f366b2477232534a8aeca37f3c
binary	12 20 64ec88ca00b268e5ba1a35678a1b5316d212f4f366b2477232534a8aeca37f3c
base 58	QmV8cfu6n4NT5xRr2AHdKxFMTZEJrA44qgrBCr739BN9Wb
base 32	ciqgj3eiziale2hfxindkz4kdnjrnuqs6tzwnmshoizfgsuk5srx6pa

Algorithms

IPFS assigns a unique Name and Code to a hashing algorithm. See hashtable.csv for the currently defined hashing algorithms.

These algorithms are implemented:

• blake2b-160, blake2b-256 blake2b-384 and blake2b-512
• blake2s-128, blake2s-160, blake2s-224 a nd blake2s-256
• keccak-224, keccak-256, keccak-384 and keccak-512
• md4 and md5
• sha1
• sha2-256, sha2-512 and dbl-sha2-256
• sha3-224, sha3-256, sha3-384 and sha3-512
• shake-128 and shake-256

The identity hash is also implemented; which just returns the input bytes. This is used to inline a small amount of data into a CID.

Registry

The hashing registry contains the metadata on hashing algorithms. You can use Register to add a new hashing algorithm.

Example

Using an hashing algorithm. Note that ComputeHash can take a byte array or a Stream.

public void GetHasher()
{
    using (var hasher = HashingAlgorithm.GetAlgorithm("sha3-256"))
    {
        Assert.IsNotNull(hasher);
        var input = new byte[] { 0xe9 };
        var expected = "f0d04dd1e6cfc29a4460d521796852f25d9ef8d28b44ee91ff5b759d72c1e6d6".ToHexBuffer();

        var actual = hasher.ComputeHash(input);
        CollectionAssert.AreEqual(expected, actual);
    }
}