541 lines
14 KiB
Go
541 lines
14 KiB
Go
// Copyright 2013 The Go Authors. All rights reserved.
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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.
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package ssh
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import (
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"crypto"
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"crypto/ecdsa"
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"crypto/elliptic"
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"crypto/rand"
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"crypto/subtle"
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"errors"
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"io"
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"math/big"
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"golang.org/x/crypto/curve25519"
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)
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const (
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kexAlgoDH1SHA1 = "diffie-hellman-group1-sha1"
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kexAlgoDH14SHA1 = "diffie-hellman-group14-sha1"
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kexAlgoECDH256 = "ecdh-sha2-nistp256"
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kexAlgoECDH384 = "ecdh-sha2-nistp384"
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kexAlgoECDH521 = "ecdh-sha2-nistp521"
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kexAlgoCurve25519SHA256 = "curve25519-sha256@libssh.org"
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)
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// kexResult captures the outcome of a key exchange.
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type kexResult struct {
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// Session hash. See also RFC 4253, section 8.
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H []byte
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// Shared secret. See also RFC 4253, section 8.
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K []byte
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// Host key as hashed into H.
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HostKey []byte
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// Signature of H.
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Signature []byte
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// A cryptographic hash function that matches the security
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// level of the key exchange algorithm. It is used for
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// calculating H, and for deriving keys from H and K.
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Hash crypto.Hash
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// The session ID, which is the first H computed. This is used
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// to derive key material inside the transport.
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SessionID []byte
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}
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// handshakeMagics contains data that is always included in the
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// session hash.
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type handshakeMagics struct {
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clientVersion, serverVersion []byte
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clientKexInit, serverKexInit []byte
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}
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func (m *handshakeMagics) write(w io.Writer) {
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writeString(w, m.clientVersion)
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writeString(w, m.serverVersion)
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writeString(w, m.clientKexInit)
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writeString(w, m.serverKexInit)
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}
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// kexAlgorithm abstracts different key exchange algorithms.
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type kexAlgorithm interface {
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// Server runs server-side key agreement, signing the result
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// with a hostkey.
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Server(p packetConn, rand io.Reader, magics *handshakeMagics, s Signer) (*kexResult, error)
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// Client runs the client-side key agreement. Caller is
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// responsible for verifying the host key signature.
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Client(p packetConn, rand io.Reader, magics *handshakeMagics) (*kexResult, error)
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}
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// dhGroup is a multiplicative group suitable for implementing Diffie-Hellman key agreement.
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type dhGroup struct {
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g, p, pMinus1 *big.Int
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}
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func (group *dhGroup) diffieHellman(theirPublic, myPrivate *big.Int) (*big.Int, error) {
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if theirPublic.Cmp(bigOne) <= 0 || theirPublic.Cmp(group.pMinus1) >= 0 {
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return nil, errors.New("ssh: DH parameter out of bounds")
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}
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return new(big.Int).Exp(theirPublic, myPrivate, group.p), nil
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}
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func (group *dhGroup) Client(c packetConn, randSource io.Reader, magics *handshakeMagics) (*kexResult, error) {
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hashFunc := crypto.SHA1
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var x *big.Int
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for {
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var err error
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if x, err = rand.Int(randSource, group.pMinus1); err != nil {
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return nil, err
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}
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if x.Sign() > 0 {
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break
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}
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}
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X := new(big.Int).Exp(group.g, x, group.p)
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kexDHInit := kexDHInitMsg{
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X: X,
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}
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if err := c.writePacket(Marshal(&kexDHInit)); err != nil {
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return nil, err
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}
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packet, err := c.readPacket()
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if err != nil {
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return nil, err
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}
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var kexDHReply kexDHReplyMsg
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if err = Unmarshal(packet, &kexDHReply); err != nil {
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return nil, err
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}
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ki, err := group.diffieHellman(kexDHReply.Y, x)
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if err != nil {
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return nil, err
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}
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h := hashFunc.New()
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magics.write(h)
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writeString(h, kexDHReply.HostKey)
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writeInt(h, X)
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writeInt(h, kexDHReply.Y)
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K := make([]byte, intLength(ki))
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marshalInt(K, ki)
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h.Write(K)
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return &kexResult{
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H: h.Sum(nil),
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K: K,
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HostKey: kexDHReply.HostKey,
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Signature: kexDHReply.Signature,
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Hash: crypto.SHA1,
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}, nil
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}
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func (group *dhGroup) Server(c packetConn, randSource io.Reader, magics *handshakeMagics, priv Signer) (result *kexResult, err error) {
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hashFunc := crypto.SHA1
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packet, err := c.readPacket()
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if err != nil {
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return
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}
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var kexDHInit kexDHInitMsg
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if err = Unmarshal(packet, &kexDHInit); err != nil {
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return
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}
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var y *big.Int
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for {
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if y, err = rand.Int(randSource, group.pMinus1); err != nil {
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return
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}
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if y.Sign() > 0 {
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break
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}
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}
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Y := new(big.Int).Exp(group.g, y, group.p)
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ki, err := group.diffieHellman(kexDHInit.X, y)
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if err != nil {
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return nil, err
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}
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hostKeyBytes := priv.PublicKey().Marshal()
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h := hashFunc.New()
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magics.write(h)
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writeString(h, hostKeyBytes)
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writeInt(h, kexDHInit.X)
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writeInt(h, Y)
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K := make([]byte, intLength(ki))
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marshalInt(K, ki)
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h.Write(K)
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H := h.Sum(nil)
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// H is already a hash, but the hostkey signing will apply its
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// own key-specific hash algorithm.
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sig, err := signAndMarshal(priv, randSource, H)
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if err != nil {
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return nil, err
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}
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kexDHReply := kexDHReplyMsg{
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HostKey: hostKeyBytes,
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Y: Y,
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Signature: sig,
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}
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packet = Marshal(&kexDHReply)
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err = c.writePacket(packet)
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return &kexResult{
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H: H,
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K: K,
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HostKey: hostKeyBytes,
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Signature: sig,
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Hash: crypto.SHA1,
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}, nil
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}
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// ecdh performs Elliptic Curve Diffie-Hellman key exchange as
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// described in RFC 5656, section 4.
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type ecdh struct {
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curve elliptic.Curve
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}
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func (kex *ecdh) Client(c packetConn, rand io.Reader, magics *handshakeMagics) (*kexResult, error) {
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ephKey, err := ecdsa.GenerateKey(kex.curve, rand)
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if err != nil {
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return nil, err
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}
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kexInit := kexECDHInitMsg{
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ClientPubKey: elliptic.Marshal(kex.curve, ephKey.PublicKey.X, ephKey.PublicKey.Y),
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}
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serialized := Marshal(&kexInit)
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if err := c.writePacket(serialized); err != nil {
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return nil, err
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}
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packet, err := c.readPacket()
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if err != nil {
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return nil, err
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}
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var reply kexECDHReplyMsg
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if err = Unmarshal(packet, &reply); err != nil {
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return nil, err
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}
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x, y, err := unmarshalECKey(kex.curve, reply.EphemeralPubKey)
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if err != nil {
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return nil, err
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}
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// generate shared secret
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secret, _ := kex.curve.ScalarMult(x, y, ephKey.D.Bytes())
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h := ecHash(kex.curve).New()
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magics.write(h)
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writeString(h, reply.HostKey)
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writeString(h, kexInit.ClientPubKey)
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writeString(h, reply.EphemeralPubKey)
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K := make([]byte, intLength(secret))
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marshalInt(K, secret)
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h.Write(K)
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return &kexResult{
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H: h.Sum(nil),
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K: K,
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HostKey: reply.HostKey,
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Signature: reply.Signature,
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Hash: ecHash(kex.curve),
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}, nil
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}
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// unmarshalECKey parses and checks an EC key.
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func unmarshalECKey(curve elliptic.Curve, pubkey []byte) (x, y *big.Int, err error) {
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x, y = elliptic.Unmarshal(curve, pubkey)
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if x == nil {
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return nil, nil, errors.New("ssh: elliptic.Unmarshal failure")
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}
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if !validateECPublicKey(curve, x, y) {
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return nil, nil, errors.New("ssh: public key not on curve")
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}
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return x, y, nil
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}
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// validateECPublicKey checks that the point is a valid public key for
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// the given curve. See [SEC1], 3.2.2
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func validateECPublicKey(curve elliptic.Curve, x, y *big.Int) bool {
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if x.Sign() == 0 && y.Sign() == 0 {
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return false
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}
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if x.Cmp(curve.Params().P) >= 0 {
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return false
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}
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if y.Cmp(curve.Params().P) >= 0 {
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return false
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}
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if !curve.IsOnCurve(x, y) {
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return false
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}
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// We don't check if N * PubKey == 0, since
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//
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// - the NIST curves have cofactor = 1, so this is implicit.
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// (We don't foresee an implementation that supports non NIST
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// curves)
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//
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// - for ephemeral keys, we don't need to worry about small
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// subgroup attacks.
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return true
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}
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func (kex *ecdh) Server(c packetConn, rand io.Reader, magics *handshakeMagics, priv Signer) (result *kexResult, err error) {
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packet, err := c.readPacket()
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if err != nil {
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return nil, err
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}
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var kexECDHInit kexECDHInitMsg
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if err = Unmarshal(packet, &kexECDHInit); err != nil {
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return nil, err
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}
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clientX, clientY, err := unmarshalECKey(kex.curve, kexECDHInit.ClientPubKey)
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if err != nil {
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return nil, err
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}
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// We could cache this key across multiple users/multiple
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// connection attempts, but the benefit is small. OpenSSH
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// generates a new key for each incoming connection.
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ephKey, err := ecdsa.GenerateKey(kex.curve, rand)
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if err != nil {
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return nil, err
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}
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hostKeyBytes := priv.PublicKey().Marshal()
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serializedEphKey := elliptic.Marshal(kex.curve, ephKey.PublicKey.X, ephKey.PublicKey.Y)
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// generate shared secret
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secret, _ := kex.curve.ScalarMult(clientX, clientY, ephKey.D.Bytes())
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h := ecHash(kex.curve).New()
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magics.write(h)
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writeString(h, hostKeyBytes)
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writeString(h, kexECDHInit.ClientPubKey)
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writeString(h, serializedEphKey)
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K := make([]byte, intLength(secret))
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marshalInt(K, secret)
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h.Write(K)
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H := h.Sum(nil)
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// H is already a hash, but the hostkey signing will apply its
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// own key-specific hash algorithm.
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sig, err := signAndMarshal(priv, rand, H)
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if err != nil {
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return nil, err
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}
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reply := kexECDHReplyMsg{
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EphemeralPubKey: serializedEphKey,
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HostKey: hostKeyBytes,
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Signature: sig,
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}
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serialized := Marshal(&reply)
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if err := c.writePacket(serialized); err != nil {
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return nil, err
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}
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return &kexResult{
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H: H,
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K: K,
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HostKey: reply.HostKey,
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Signature: sig,
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Hash: ecHash(kex.curve),
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}, nil
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}
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var kexAlgoMap = map[string]kexAlgorithm{}
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func init() {
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// This is the group called diffie-hellman-group1-sha1 in RFC
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// 4253 and Oakley Group 2 in RFC 2409.
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p, _ := new(big.Int).SetString("FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE649286651ECE65381FFFFFFFFFFFFFFFF", 16)
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kexAlgoMap[kexAlgoDH1SHA1] = &dhGroup{
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g: new(big.Int).SetInt64(2),
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p: p,
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pMinus1: new(big.Int).Sub(p, bigOne),
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}
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// This is the group called diffie-hellman-group14-sha1 in RFC
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// 4253 and Oakley Group 14 in RFC 3526.
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p, _ = new(big.Int).SetString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kexAlgoMap[kexAlgoDH14SHA1] = &dhGroup{
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g: new(big.Int).SetInt64(2),
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p: p,
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pMinus1: new(big.Int).Sub(p, bigOne),
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}
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kexAlgoMap[kexAlgoECDH521] = &ecdh{elliptic.P521()}
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kexAlgoMap[kexAlgoECDH384] = &ecdh{elliptic.P384()}
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kexAlgoMap[kexAlgoECDH256] = &ecdh{elliptic.P256()}
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kexAlgoMap[kexAlgoCurve25519SHA256] = &curve25519sha256{}
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}
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// curve25519sha256 implements the curve25519-sha256@libssh.org key
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// agreement protocol, as described in
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// https://git.libssh.org/projects/libssh.git/tree/doc/curve25519-sha256@libssh.org.txt
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type curve25519sha256 struct{}
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type curve25519KeyPair struct {
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priv [32]byte
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pub [32]byte
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}
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func (kp *curve25519KeyPair) generate(rand io.Reader) error {
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if _, err := io.ReadFull(rand, kp.priv[:]); err != nil {
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return err
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}
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curve25519.ScalarBaseMult(&kp.pub, &kp.priv)
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return nil
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}
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// curve25519Zeros is just an array of 32 zero bytes so that we have something
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// convenient to compare against in order to reject curve25519 points with the
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// wrong order.
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var curve25519Zeros [32]byte
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func (kex *curve25519sha256) Client(c packetConn, rand io.Reader, magics *handshakeMagics) (*kexResult, error) {
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var kp curve25519KeyPair
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if err := kp.generate(rand); err != nil {
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return nil, err
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}
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if err := c.writePacket(Marshal(&kexECDHInitMsg{kp.pub[:]})); err != nil {
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return nil, err
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}
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packet, err := c.readPacket()
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if err != nil {
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return nil, err
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}
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var reply kexECDHReplyMsg
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if err = Unmarshal(packet, &reply); err != nil {
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return nil, err
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}
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if len(reply.EphemeralPubKey) != 32 {
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return nil, errors.New("ssh: peer's curve25519 public value has wrong length")
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}
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var servPub, secret [32]byte
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copy(servPub[:], reply.EphemeralPubKey)
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curve25519.ScalarMult(&secret, &kp.priv, &servPub)
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if subtle.ConstantTimeCompare(secret[:], curve25519Zeros[:]) == 1 {
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return nil, errors.New("ssh: peer's curve25519 public value has wrong order")
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}
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h := crypto.SHA256.New()
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magics.write(h)
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writeString(h, reply.HostKey)
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writeString(h, kp.pub[:])
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writeString(h, reply.EphemeralPubKey)
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ki := new(big.Int).SetBytes(secret[:])
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K := make([]byte, intLength(ki))
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marshalInt(K, ki)
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h.Write(K)
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return &kexResult{
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H: h.Sum(nil),
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K: K,
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HostKey: reply.HostKey,
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Signature: reply.Signature,
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Hash: crypto.SHA256,
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}, nil
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}
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func (kex *curve25519sha256) Server(c packetConn, rand io.Reader, magics *handshakeMagics, priv Signer) (result *kexResult, err error) {
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packet, err := c.readPacket()
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if err != nil {
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return
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}
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var kexInit kexECDHInitMsg
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if err = Unmarshal(packet, &kexInit); err != nil {
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return
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}
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if len(kexInit.ClientPubKey) != 32 {
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return nil, errors.New("ssh: peer's curve25519 public value has wrong length")
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}
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var kp curve25519KeyPair
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if err := kp.generate(rand); err != nil {
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return nil, err
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}
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var clientPub, secret [32]byte
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copy(clientPub[:], kexInit.ClientPubKey)
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curve25519.ScalarMult(&secret, &kp.priv, &clientPub)
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if subtle.ConstantTimeCompare(secret[:], curve25519Zeros[:]) == 1 {
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return nil, errors.New("ssh: peer's curve25519 public value has wrong order")
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}
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hostKeyBytes := priv.PublicKey().Marshal()
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|
|
h := crypto.SHA256.New()
|
|
magics.write(h)
|
|
writeString(h, hostKeyBytes)
|
|
writeString(h, kexInit.ClientPubKey)
|
|
writeString(h, kp.pub[:])
|
|
|
|
ki := new(big.Int).SetBytes(secret[:])
|
|
K := make([]byte, intLength(ki))
|
|
marshalInt(K, ki)
|
|
h.Write(K)
|
|
|
|
H := h.Sum(nil)
|
|
|
|
sig, err := signAndMarshal(priv, rand, H)
|
|
if err != nil {
|
|
return nil, err
|
|
}
|
|
|
|
reply := kexECDHReplyMsg{
|
|
EphemeralPubKey: kp.pub[:],
|
|
HostKey: hostKeyBytes,
|
|
Signature: sig,
|
|
}
|
|
if err := c.writePacket(Marshal(&reply)); err != nil {
|
|
return nil, err
|
|
}
|
|
return &kexResult{
|
|
H: H,
|
|
K: K,
|
|
HostKey: hostKeyBytes,
|
|
Signature: sig,
|
|
Hash: crypto.SHA256,
|
|
}, nil
|
|
}
|