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/*
* Copyright (c) 2014, Yawning Angel <yawning at torproject dot org>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* * Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
*
* * Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
// Package uniformdh implements the Tor Project's UniformDH key exchange
// mechanism as defined in the obfs3 protocol specification. This
// implementation is suitable for obfuscation but MUST NOT BE USED when strong
// security is required as it is not constant time.
package uniformdh
import (
"fmt"
"io"
"math/big"
)
const (
// Size is the size of a UniformDH key or shared secret in bytes.
Size = 1536 / 8
// modpStr is the RFC3526 1536-bit MODP Group (Group 5).
modpStr = "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD1" +
"29024E088A67CC74020BBEA63B139B22514A08798E3404DD" +
"EF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245" +
"E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7ED" +
"EE386BFB5A899FA5AE9F24117C4B1FE649286651ECE45B3D" +
"C2007CB8A163BF0598DA48361C55D39A69163FA8FD24CF5F" +
"83655D23DCA3AD961C62F356208552BB9ED529077096966D" +
"670C354E4ABC9804F1746C08CA237327FFFFFFFFFFFFFFFF"
g = 2
)
var modpGroup *big.Int
var gen *big.Int
// A PrivateKey represents a UniformDH private key.
type PrivateKey struct {
PublicKey
privateKey *big.Int
}
// A PublicKey represents a UniformDH public key.
type PublicKey struct {
bytes []byte
publicKey *big.Int
}
// Bytes returns the byte representation of a PublicKey.
func (pub *PublicKey) Bytes() (pubBytes []byte, err error) {
if len(pub.bytes) != Size || pub.bytes == nil {
return nil, fmt.Errorf("public key is not initialized")
}
pubBytes = make([]byte, Size)
copy(pubBytes, pub.bytes)
return
}
// SetBytes sets the PublicKey from a byte slice.
func (pub *PublicKey) SetBytes(pubBytes []byte) error {
if len(pubBytes) != Size {
return fmt.Errorf("public key length %d is not %d", len(pubBytes), Size)
}
pub.bytes = make([]byte, Size)
copy(pub.bytes, pubBytes)
pub.publicKey = new(big.Int).SetBytes(pub.bytes)
return nil
}
// GenerateKey generates a UniformDH keypair using the random source random.
func GenerateKey(random io.Reader) (priv *PrivateKey, err error) {
privBytes := make([]byte, Size)
if _, err = io.ReadFull(random, privBytes); err != nil {
return
}
priv, err = generateKey(privBytes)
return
}
func generateKey(privBytes []byte) (priv *PrivateKey, err error) {
// This function does all of the actual heavy lifting of creating a public
// key from a raw 192 byte private key. It is split so that the KAT tests
// can be written easily, and not exposed since non-ephemeral keys are a
// terrible idea.
if len(privBytes) != Size {
return nil, fmt.Errorf("invalid private key size %d", len(privBytes))
}
// To pick a private UniformDH key, we pick a random 1536-bit number,
// and make it even by setting its low bit to 0
privBn := new(big.Int).SetBytes(privBytes)
wasEven := privBn.Bit(0) == 0
privBn = privBn.SetBit(privBn, 0, 0)
// Let x be that private key, and X = g^x (mod p).
pubBn := new(big.Int).Exp(gen, privBn, modpGroup)
pubAlt := new(big.Int).Sub(modpGroup, pubBn)
// When someone sends her public key to the other party, she randomly
// decides whether to send X or p-X. Use the lowest most bit of the
// private key here as the random coin flip since it is masked out and not
// used.
//
// Note: The spec doesn't explicitly specify it, but here we prepend zeros
// to the key so that it is always exactly Size bytes.
pubBytes := make([]byte, Size)
if wasEven {
err = prependZeroBytes(pubBytes, pubBn.Bytes())
} else {
err = prependZeroBytes(pubBytes, pubAlt.Bytes())
}
if err != nil {
return
}
priv = new(PrivateKey)
priv.PublicKey.bytes = pubBytes
priv.PublicKey.publicKey = pubBn
priv.privateKey = privBn
return
}
// Handshake generates a shared secret given a PrivateKey and PublicKey.
func Handshake(privateKey *PrivateKey, publicKey *PublicKey) (sharedSecret []byte, err error) {
// When a party wants to calculate the shared secret, she raises the
// foreign public key to her private key.
secretBn := new(big.Int).Exp(publicKey.publicKey, privateKey.privateKey, modpGroup)
sharedSecret = make([]byte, Size)
err = prependZeroBytes(sharedSecret, secretBn.Bytes())
return
}
func prependZeroBytes(dst, src []byte) error {
zeros := len(dst) - len(src)
if zeros < 0 {
return fmt.Errorf("src length is greater than destination: %d", zeros)
}
for i := 0; i < zeros; i++ {
dst[i] = 0
}
copy(dst[zeros:], src)
return nil
}
func init() {
// Load the MODP group and the generator.
var ok bool
modpGroup, ok = new(big.Int).SetString(modpStr, 16)
if !ok {
panic("Failed to load the RFC3526 MODP Group")
}
gen = big.NewInt(g)
}
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