46 lines
1007 B
Go
46 lines
1007 B
Go
package rfc6979
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import (
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"crypto/dsa"
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"hash"
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"math/big"
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)
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// SignDSA signs an arbitrary length hash (which should be the result of hashing
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// a larger message) using the private key, priv. It returns the signature as a
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// 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 SignDSA(priv *dsa.PrivateKey, hash []byte, alg func() hash.Hash) (r, s *big.Int, err error) {
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n := priv.Q.BitLen()
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if n&7 != 0 {
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err = dsa.ErrInvalidPublicKey
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return
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}
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n >>= 3
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generateSecret(priv.Q, priv.X, alg, hash, func(k *big.Int) bool {
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inv := new(big.Int).ModInverse(k, priv.Q)
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r = new(big.Int).Exp(priv.G, k, priv.P)
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r.Mod(r, priv.Q)
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if r.Sign() == 0 {
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return false
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}
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z := new(big.Int).SetBytes(hash)
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s = new(big.Int).Mul(priv.X, r)
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s.Add(s, z)
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s.Mod(s, priv.Q)
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s.Mul(s, inv)
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s.Mod(s, priv.Q)
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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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