forked from TrueCloudLab/rfc6979
56 lines
1.3 KiB
Go
56 lines
1.3 KiB
Go
package rfc6979
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import (
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"crypto/ecdsa"
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"crypto/elliptic"
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"hash"
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"math/big"
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)
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// SignECDSA signs an arbitrary length hash (which should be the result of
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// hashing a larger message) using the private key, priv. It returns the
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// signature as a pair of integers.
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//
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// Note that FIPS 186-3 section 4.6 specifies that the hash should be truncated
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// to the byte-length of the subgroup. This function does not perform that
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// truncation itself.
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func SignECDSA(priv *ecdsa.PrivateKey, hash []byte, alg func() hash.Hash) (r, s *big.Int, err error) {
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c := priv.PublicKey.Curve
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N := c.Params().N
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generateSecret(N, priv.D, alg, hash, func(k *big.Int) bool {
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inv := new(big.Int).ModInverse(k, N)
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r, _ = priv.Curve.ScalarBaseMult(k.Bytes())
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r.Mod(r, N)
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if r.Sign() == 0 {
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return false
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}
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e := hashToInt(hash, c)
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s = new(big.Int).Mul(priv.D, r)
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s.Add(s, e)
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s.Mul(s, inv)
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s.Mod(s, N)
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return s.Sign() != 0
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})
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return
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}
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// copied from crypto/ecdsa
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func hashToInt(hash []byte, c elliptic.Curve) *big.Int {
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orderBits := c.Params().N.BitLen()
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orderBytes := (orderBits + 7) / 8
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if len(hash) > orderBytes {
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hash = hash[:orderBytes]
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}
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ret := new(big.Int).SetBytes(hash)
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excess := len(hash)*8 - orderBits
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if excess > 0 {
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ret.Rsh(ret, uint(excess))
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}
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return ret
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}
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