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crypto/ecdsa: truncate hash values.

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There is some disagreement about how to deal with hash values larger
than the curve order size. We choose to follow OpenSSL's lead here.

R=bradfitzgo, r
CC=golang-dev
https://golang.org/cl/4273059
This commit is contained in:
Adam Langley 2011-03-16 10:42:41 -04:00
parent e44c057369
commit 7b094182e4
1 changed files with 23 additions and 2 deletions
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25
src/pkg/crypto/ecdsa/ecdsa.go
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@ -9,6 +9,8 @@ package ecdsa
// References:
// [NSA]: Suite B implementor's guide to FIPS 186-3,
// http://www.nsa.gov/ia/_files/ecdsa.pdf
// [SECG]: SECG, SEC1
// http://www.secg.org/download/aid-780/sec1-v2.pdf
import (
"big"
@ -61,6 +63,25 @@ func GenerateKey(c *elliptic.Curve, rand io.Reader) (priv *PrivateKey, err os.Er
return
}
// hashToInt converts a hash value to an integer. There is some disagreement
// about how this is done. [NSA] suggests that this is done in the obvious
// manner, but [SECG] truncates the hash to the bit-length of the curve order
// first. We follow [SECG] because that's what OpenSSL does.
func hashToInt(hash []byte, c *elliptic.Curve) *big.Int {
orderBits := c.N.BitLen()
orderBytes := (orderBits + 7) / 8
if len(hash) > orderBytes {
hash = hash[:orderBytes]
}
ret := new(big.Int).SetBytes(hash)
excess := orderBytes*8 - orderBits
if excess > 0 {
ret.Rsh(ret, uint(excess))
}
return ret
}
// Sign signs an arbitrary length hash (which should be the result of hashing a
// larger message) using the private key, priv. It returns the signature as a
// pair of integers. The security of the private key depends on the entropy of
@ -86,7 +107,7 @@ func Sign(rand io.Reader, priv *PrivateKey, hash []byte) (r, s *big.Int, err os.
}
}
e := new(big.Int).SetBytes(hash)
e := hashToInt(hash, c)
s = new(big.Int).Mul(priv.D, r)
s.Add(s, e)
s.Mul(s, kInv)
@ -111,7 +132,7 @@ func Verify(pub *PublicKey, hash []byte, r, s *big.Int) bool {
if r.Cmp(c.N) >= 0 || s.Cmp(c.N) >= 0 {
return false
}
e := new(big.Int).SetBytes(hash)
e := hashToInt(hash, c)
w := new(big.Int).ModInverse(s, c.N)
u1 := e.Mul(e, w)