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98 lines
2.7 KiB
Go
98 lines
2.7 KiB
Go
package ecc
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// Package curve25519sign implements a signature scheme based on Curve25519 keys.
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// See https://moderncrypto.org/mail-archive/curves/2014/000205.html for details.
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import (
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"crypto/ed25519"
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"crypto/sha512"
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"filippo.io/edwards25519"
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"filippo.io/edwards25519/field"
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)
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// sign signs the message with privateKey and returns a signature as a byte slice.
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func sign(privateKey *[32]byte, message []byte, random [64]byte) *[64]byte {
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// Calculate Ed25519 public key from Curve25519 private key
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var A edwards25519.Point
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privateKeyScalar, _ := edwards25519.NewScalar().SetBytesWithClamping(privateKey[:])
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A.ScalarBaseMult(privateKeyScalar)
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publicKey := *(*[32]byte)(A.Bytes())
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// Calculate r
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diversifier := [32]byte{
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0xFE, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
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0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
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0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
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0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF}
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var r [64]byte
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hash := sha512.New()
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hash.Write(diversifier[:])
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hash.Write(privateKey[:])
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hash.Write(message)
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hash.Write(random[:])
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hash.Sum(r[:0])
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// Calculate R
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var rReduced *edwards25519.Scalar
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rReduced, _ = edwards25519.NewScalar().SetUniformBytes(r[:])
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var R edwards25519.Point
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R.ScalarBaseMult(rReduced)
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var encodedR [32]byte
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encodedR = *(*[32]byte)(R.Bytes())
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// Calculate S = r + SHA2-512(R || A_ed || msg) * a (mod L)
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var hramDigest [64]byte
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hash.Reset()
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hash.Write(encodedR[:])
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hash.Write(publicKey[:])
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hash.Write(message)
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hash.Sum(hramDigest[:0])
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hramDigestReduced, _ := edwards25519.NewScalar().SetUniformBytes(hramDigest[:])
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sScalar := edwards25519.NewScalar().MultiplyAdd(hramDigestReduced, privateKeyScalar, rReduced)
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s := *(*[32]byte)(sScalar.Bytes())
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signature := new([64]byte)
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copy(signature[:], encodedR[:])
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copy(signature[32:], s[:])
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signature[63] |= publicKey[31] & 0x80
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return signature
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}
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// verify checks whether the message has a valid signature.
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func verify(publicKey [32]byte, message []byte, signature *[64]byte) bool {
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publicKey[31] &= 0x7F
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/* Convert the Curve25519 public key into an Ed25519 public key. In
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particular, convert Curve25519's "montgomery" x-coordinate into an
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Ed25519 "edwards" y-coordinate:
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ed_y = (mont_x - 1) / (mont_x + 1)
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NOTE: mont_x=-1 is converted to ed_y=0 since fe_invert is mod-exp
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Then move the sign bit into the pubkey from the signature.
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*/
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var edY, one, montX, montXMinusOne, montXPlusOne field.Element
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_, _ = montX.SetBytes(publicKey[:])
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_ = one.One()
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montXMinusOne.Subtract(&montX, &one)
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montXPlusOne.Add(&montX, &one)
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montXPlusOne.Invert(&montXPlusOne)
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edY.Multiply(&montXMinusOne, &montXPlusOne)
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A_ed := *(*[32]byte)(edY.Bytes())
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A_ed[31] |= signature[63] & 0x80
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signature[63] &= 0x7F
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return ed25519.Verify(A_ed[:], message, signature[:])
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}
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