54 lines
1.6 KiB
Go
54 lines
1.6 KiB
Go
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// Copyright 2019 Google LLC
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//
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file or at
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// https://developers.google.com/open-source/licenses/bsd
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//
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// Original code from https://github.com/FiloSottile/age/blob/bbab440e198a4d67ba78591176c7853e62d29e04/internal/age/ssh.go
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package convert
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import (
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"crypto/ed25519"
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"crypto/sha512"
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"math/big"
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"golang.org/x/crypto/curve25519"
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)
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var curve25519P, _ = new(big.Int).SetString("57896044618658097711785492504343953926634992332820282019728792003956564819949", 10)
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func Ed25519PrivateKeyToCurve25519(pk ed25519.PrivateKey) []byte {
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h := sha512.New()
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_, _ = h.Write(pk.Seed())
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out := h.Sum(nil)
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return out[:curve25519.ScalarSize]
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}
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func Ed25519PublicKeyToCurve25519(pk ed25519.PublicKey) []byte {
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// ed25519.PublicKey is a little endian representation of the y-coordinate,
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// with the most significant bit set based on the sign of the x-coordinate.
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bigEndianY := make([]byte, ed25519.PublicKeySize)
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for i, b := range pk {
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bigEndianY[ed25519.PublicKeySize-i-1] = b
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}
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bigEndianY[0] &= 0b0111_1111
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// The Montgomery u-coordinate is derived through the bilinear map
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// u = (1 + y) / (1 - y)
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// See https://blog.filippo.io/using-ed25519-keys-for-encryption.
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y := new(big.Int).SetBytes(bigEndianY)
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denom := big.NewInt(1)
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denom.ModInverse(denom.Sub(denom, y), curve25519P) // 1 / (1 - y)
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u := y.Mul(y.Add(y, big.NewInt(1)), denom)
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u.Mod(u, curve25519P)
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out := make([]byte, curve25519.PointSize)
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uBytes := u.Bytes()
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for i, b := range uBytes {
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out[len(uBytes)-i-1] = b
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}
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return out
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}
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