sha3

Overview ¶

Package sha3 implements the SHA-3 fixed-output-length hash functions and the SHAKE variable-output-length hash functions defined by FIPS-202.

Both types of hash function use the "sponge" construction and the Keccak permutation. For a detailed specification see http://keccak.noekeon.org/

Guidance ¶

If you aren't sure what function you need, use SHAKE256 with at least 64 bytes of output. The SHAKE instances are faster than the SHA3 instances; the latter have to allocate memory to conform to the hash.Hash interface.

If you need a secret-key MAC (message authentication code), prepend the secret key to the input, hash with SHAKE256 and read at least 32 bytes of output.

Security strengths ¶

The SHA3-x (x equals 224, 256, 384, or 512) functions have a security strength against preimage attacks of x bits. Since they only produce "x" bits of output, their collision-resistance is only "x/2" bits.

The SHAKE-256 and -128 functions have a generic security strength of 256 and 128 bits against all attacks, provided that at least 2x bits of their output is used. Requesting more than 64 or 32 bytes of output, respectively, does not increase the collision-resistance of the SHAKE functions.

The sponge construction ¶

A sponge builds a pseudo-random function from a public pseudo-random permutation, by applying the permutation to a state of "rate + capacity" bytes, but hiding "capacity" of the bytes.

A sponge starts out with a zero state. To hash an input using a sponge, up to "rate" bytes of the input are XORed into the sponge's state. The sponge is then "full" and the permutation is applied to "empty" it. This process is repeated until all the input has been "absorbed". The input is then padded. The digest is "squeezed" from the sponge in the same way, except that output output is copied out instead of input being XORed in.

A sponge is parameterized by its generic security strength, which is equal to half its capacity; capacity + rate is equal to the permutation's width. Since the KeccakF-1600 permutation is 1600 bits (200 bytes) wide, this means that the security strength of a sponge instance is equal to (1600 - bitrate) / 2.

Recommendations ¶

The SHAKE functions are recommended for most new uses. They can produce output of arbitrary length. SHAKE256, with an output length of at least 64 bytes, provides 256-bit security against all attacks. The Keccak team recommends it for most applications upgrading from SHA2-512. (NIST chose a much stronger, but much slower, sponge instance for SHA3-512.)

The SHA-3 functions are "drop-in" replacements for the SHA-2 functions. They produce output of the same length, with the same security strengths against all attacks. This means, in particular, that SHA3-256 only has 128-bit collision resistance, because its output length is 32 bytes.

k := []byte("this is a secret key; you should generate a strong random key that's at least 32 bytes long")
buf := []byte("and this is some data to authenticate")
// A MAC with 32 bytes of output has 256-bit security strength -- if you use at least a 32-byte-long key.
h := make([]byte, 32)
d := NewShake256()
// Write the key into the hash.
d.Write(k)
// Now write the data.
d.Write(buf)
// Read 32 bytes of output from the hash into h.
d.Read(h)
fmt.Printf("%x\n", h)

Output:

78de2974bd2711d5549ffd32b753ef0f5fa80a0db2556db60f0987eb8a9218ff

buf := []byte("some data to hash")
// A hash needs to be 64 bytes long to have 256-bit collision resistance.
h := make([]byte, 64)
// Compute a 64-byte hash of buf and put it in h.
ShakeSum256(h, buf)
fmt.Printf("%x\n", h)

Output:

0f65fe41fc353e52c55667bb9e2b27bfcc8476f2c413e9437d272ee3194a4e3146d05ec04a25d16b8f577c19b82d16b1424c3e022e783d2b4da98de3658d363d