71 lines
1.7 KiB
Go
71 lines
1.7 KiB
Go
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package bls12381
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import (
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"crypto/sha256"
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"errors"
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)
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func hashToFpXMDSHA256(msg []byte, domain []byte, count int) ([]*fe, error) {
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randBytes, err := expandMsgSHA256XMD(msg, domain, count*64)
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if err != nil {
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return nil, err
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}
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els := make([]*fe, count)
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for i := 0; i < count; i++ {
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els[i], err = from64Bytes(randBytes[i*64 : (i+1)*64])
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if err != nil {
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return nil, err
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}
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}
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return els, nil
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}
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func expandMsgSHA256XMD(msg []byte, domain []byte, outLen int) ([]byte, error) {
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h := sha256.New()
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domainLen := uint8(len(domain))
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if domainLen > 255 {
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return nil, errors.New("invalid domain length")
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}
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// DST_prime = DST || I2OSP(len(DST), 1)
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// b_0 = H(Z_pad || msg || l_i_b_str || I2OSP(0, 1) || DST_prime)
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_, _ = h.Write(make([]byte, h.BlockSize()))
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_, _ = h.Write(msg)
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_, _ = h.Write([]byte{uint8(outLen >> 8), uint8(outLen)})
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_, _ = h.Write([]byte{0})
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_, _ = h.Write(domain)
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_, _ = h.Write([]byte{domainLen})
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b0 := h.Sum(nil)
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// b_1 = H(b_0 || I2OSP(1, 1) || DST_prime)
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h.Reset()
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_, _ = h.Write(b0)
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_, _ = h.Write([]byte{1})
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_, _ = h.Write(domain)
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_, _ = h.Write([]byte{domainLen})
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b1 := h.Sum(nil)
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// b_i = H(strxor(b_0, b_(i - 1)) || I2OSP(i, 1) || DST_prime)
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ell := (outLen + h.Size() - 1) / h.Size()
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bi := b1
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out := make([]byte, outLen)
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for i := 1; i < ell; i++ {
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h.Reset()
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// b_i = H(strxor(b_0, b_(i - 1)) || I2OSP(i, 1) || DST_prime)
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tmp := make([]byte, h.Size())
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for j := 0; j < h.Size(); j++ {
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tmp[j] = b0[j] ^ bi[j]
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}
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_, _ = h.Write(tmp)
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_, _ = h.Write([]byte{1 + uint8(i)})
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_, _ = h.Write(domain)
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_, _ = h.Write([]byte{domainLen})
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// b_1 || ... || b_(ell - 1)
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copy(out[(i-1)*h.Size():i*h.Size()], bi[:])
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bi = h.Sum(nil)
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}
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// b_ell
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copy(out[(ell-1)*h.Size():], bi[:])
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return out[:outLen], nil
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}
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