ceremonyclient/nekryptology/internal/hash.go
2023-07-05 00:32:28 -05:00

94 lines
2.4 KiB
Go

//
// Copyright Coinbase, Inc. All Rights Reserved.
//
// SPDX-License-Identifier: Apache-2.0
//
package internal
import (
"bytes"
"crypto/sha256"
"fmt"
"golang.org/x/crypto/hkdf"
)
// Hash computes the HKDF over many values
// iteratively such that each value is hashed separately
// and based on preceding values
//
// The first value is computed as okm_0 = KDF(f || value) where
// f is a byte slice of 32 0xFF
// salt is zero-filled byte slice with length equal to the hash output length
// info is the protocol name
// okm is the 32 byte output
//
// The each subsequent iteration is computed by as okm_i = KDF(f_i || value || okm_{i-1})
// where f_i = 2^b - 1 - i such that there are 0xFF bytes prior to the value.
// f_1 changes the first byte to 0xFE, f_2 to 0xFD. The previous okm is appended to the value
// to provide cryptographic domain separation.
// See https://signal.org/docs/specifications/x3dh/#cryptographic-notation
// and https://signal.org/docs/specifications/xeddsa/#hash-functions
// for more details.
// This uses the KDF function similar to X3DH for each `value`
// But changes the key just like XEdDSA where the prefix bytes change by a single bit
func Hash(info []byte, values ...[]byte) ([]byte, error) {
// Don't accept any nil arguments
if anyNil(values...) {
return nil, ErrNilArguments
}
salt := make([]byte, 32)
okm := make([]byte, 32)
f := bytes.Repeat([]byte{0xFF}, 32)
for _, b := range values {
ikm := append(f, b...)
ikm = append(ikm, okm...)
kdf := hkdf.New(sha256.New, ikm, salt, info)
n, err := kdf.Read(okm)
if err != nil {
return nil, err
}
if n != len(okm) {
return nil, fmt.Errorf("unable to read expected number of bytes want=%v got=%v", len(okm), n)
}
ByteSub(f)
}
return okm, nil
}
func anyNil(values ...[]byte) bool {
for _, x := range values {
if x == nil {
return true
}
}
return false
}
// ByteSub is a constant time algorithm for subtracting
// 1 from the array as if it were a big number.
// 0 is considered a wrap which resets to 0xFF
func ByteSub(b []byte) {
m := byte(1)
for i := 0; i < len(b); i++ {
b[i] -= m
// If b[i] > 0, s == 0
// If b[i] == 0, s == 1
// Computing IsNonZero(b[i])
s1 := int8(b[i]) >> 7
s2 := -int8(b[i]) >> 7
s := byte((s1 | s2) + 1)
// If s == 0, don't subtract anymore
// s == 1, continue subtracting
m = s & m
// If s == 0 this does nothing
// If s == 1 reset this value to 0xFF
b[i] |= -s
}
}