96 lines
3.5 KiB
Go
96 lines
3.5 KiB
Go
package utils
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import (
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"crypto/sha512"
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"golang.org/x/crypto/curve25519"
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"golang.org/x/crypto/ed25519"
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"math/big"
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)
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// EDH implements diffie hellman using curve25519 keys derived from ed25519 keys
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func EDH(privateKey ed25519.PrivateKey, remotePublicKey ed25519.PublicKey) ([]byte, error) {
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var privKeyBytes [64]byte
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var remotePubKeyBytes [32]byte
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copy(privKeyBytes[:], privateKey[:])
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copy(remotePubKeyBytes[:], remotePublicKey[:])
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var curve25519priv [32]byte
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PrivateKeyToCurve25519(&curve25519priv, &privKeyBytes)
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curve25519pub := ed25519PublicKeyToCurve25519(remotePublicKey)
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secret, err := curve25519.X25519(curve25519priv[:], curve25519pub[:])
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return secret, err
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}
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// https://github.com/FiloSottile/age/blob/master/internal/age/ssh.go#L174
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// Copyright 2019 Google LLC
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//
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//Redistribution and use in source and binary forms, with or without
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//modification, are permitted provided that the following conditions are
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//met:
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//
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// * Redistributions of source code must retain the above copyright
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//notice, this list of conditions and the following disclaimer.
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// * Redistributions in binary form must reproduce the above
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//copyright notice, this list of conditions and the following disclaimer
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//in the documentation and/or other materials provided with the
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//distribution.
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// * Neither the name of Google LLC nor the names of its
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//contributors may be used to endorse or promote products derived from
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//this software without specific prior written permission.
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//
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//THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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//"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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//LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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//A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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//OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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//SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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//LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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//DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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//THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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//(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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//OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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var curve25519P, _ = new(big.Int).SetString("57896044618658097711785492504343953926634992332820282019728792003956564819949", 10)
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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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//
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// u = (1 + y) / (1 - y)
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//
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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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// PrivateKeyToCurve25519 converts an ed25519 private key into a corresponding
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// curve25519 private key
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func PrivateKeyToCurve25519(curve25519Private *[32]byte, privateKey *[64]byte) {
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h := sha512.New()
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h.Write(privateKey[:32])
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digest := h.Sum(nil)
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digest[0] &= 248
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digest[31] &= 127
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digest[31] |= 64
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copy(curve25519Private[:], digest)
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}
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