tzhash/tz/hash.go

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// Package tz contains Tillich-Zemor checksum implementations
// using different backends.
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//
// Copyright 2022 (c) NSPCC
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package tz
import (
"errors"
)
// Concat performs combining of hashes based on homomorphic property.
func Concat(hs [][]byte) ([]byte, error) {
var b, c sl2
b = id
for i := range hs {
if err := c.UnmarshalBinary(hs[i]); err != nil {
return nil, err
}
b.Mul(&b, &c)
}
return b.MarshalBinary()
}
// Validate checks if hashes in hs combined are equal to h.
func Validate(h []byte, hs [][]byte) (bool, error) {
var (
b []byte
got, expected [Size]byte
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err error
)
if len(h) != Size {
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return false, errors.New("invalid hash")
} else if len(hs) == 0 {
return false, errors.New("empty slice")
}
copy(expected[:], h)
b, err = Concat(hs)
if err != nil {
return false, errors.New("cant concatenate hashes")
}
copy(got[:], b)
return expected == got, nil
}
// SubtractR returns hash a, such that Concat(a, b) == c
// This is possible, because Tillich-Zemor hash is actually a matrix
// which can be inversed.
func SubtractR(c, b []byte) (a []byte, err error) {
var p1, p2, r sl2
if err = r.UnmarshalBinary(c); err != nil {
return nil, err
}
if err = p2.UnmarshalBinary(b); err != nil {
return nil, err
}
p1 = *Inv(&p2)
p1.Mul(&r, &p1)
return p1.MarshalBinary()
}
// SubtractL returns hash b, such that Concat(a, b) == c
// This is possible, because Tillich-Zemor hash is actually a matrix
// which can be inversed.
func SubtractL(c, a []byte) (b []byte, err error) {
var p1, p2, r sl2
if err = r.UnmarshalBinary(c); err != nil {
return nil, err
}
if err = p1.UnmarshalBinary(a); err != nil {
return nil, err
}
p2 = *Inv(&p1)
p2.Mul(&p2, &r)
return p2.MarshalBinary()
}