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