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// Copyright 2024 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package ssh
import (
"crypto"
"crypto/mlkem"
"crypto/sha256"
"errors"
"fmt"
"io"
"golang.org/x/crypto/curve25519"
)
// mlkem768WithCurve25519sha256 implements the hybrid ML-KEM768 with
// curve25519-sha256 key exchange method, as described by
// draft-kampanakis-curdle-ssh-pq-ke-05 section 2.3.3.
type mlkem768WithCurve25519sha256 struct{}
func (kex *mlkem768WithCurve25519sha256) Client(c packetConn, rand io.Reader, magics *handshakeMagics) (*kexResult, error) {
var c25519kp curve25519KeyPair
if err := c25519kp.generate(rand); err != nil {
return nil, err
}
seed := make([]byte, mlkem.SeedSize)
if _, err := io.ReadFull(rand, seed); err != nil {
return nil, err
}
mlkemDk, err := mlkem.NewDecapsulationKey768(seed)
if err != nil {
return nil, err
}
hybridKey := append(mlkemDk.EncapsulationKey().Bytes(), c25519kp.pub[:]...)
if err := c.writePacket(Marshal(&kexECDHInitMsg{hybridKey})); err != nil {
return nil, err
}
packet, err := c.readPacket()
if err != nil {
return nil, err
}
var reply kexECDHReplyMsg
if err = Unmarshal(packet, &reply); err != nil {
return nil, err
}
if len(reply.EphemeralPubKey) != mlkem.CiphertextSize768+32 {
return nil, errors.New("ssh: peer's mlkem768x25519 public value has wrong length")
}
// Perform KEM decapsulate operation to obtain shared key from ML-KEM.
mlkem768Secret, err := mlkemDk.Decapsulate(reply.EphemeralPubKey[:mlkem.CiphertextSize768])
if err != nil {
return nil, err
}
// Complete Curve25519 ECDH to obtain its shared key.
c25519Secret, err := curve25519.X25519(c25519kp.priv[:], reply.EphemeralPubKey[mlkem.CiphertextSize768:])
if err != nil {
return nil, fmt.Errorf("ssh: peer's mlkem768x25519 public value is not valid: %w", err)
}
// Compute actual shared key.
h := sha256.New()
h.Write(mlkem768Secret)
h.Write(c25519Secret)
secret := h.Sum(nil)
h.Reset()
magics.write(h)
writeString(h, reply.HostKey)
writeString(h, hybridKey)
writeString(h, reply.EphemeralPubKey)
K := make([]byte, stringLength(len(secret)))
marshalString(K, secret)
h.Write(K)
return &kexResult{
H: h.Sum(nil),
K: K,
HostKey: reply.HostKey,
Signature: reply.Signature,
Hash: crypto.SHA256,
}, nil
}
func (kex *mlkem768WithCurve25519sha256) Server(c packetConn, rand io.Reader, magics *handshakeMagics, priv AlgorithmSigner, algo string) (*kexResult, error) {
packet, err := c.readPacket()
if err != nil {
return nil, err
}
var kexInit kexECDHInitMsg
if err = Unmarshal(packet, &kexInit); err != nil {
return nil, err
}
if len(kexInit.ClientPubKey) != mlkem.EncapsulationKeySize768+32 {
return nil, errors.New("ssh: peer's ML-KEM768/curve25519 public value has wrong length")
}
encapsulationKey, err := mlkem.NewEncapsulationKey768(kexInit.ClientPubKey[:mlkem.EncapsulationKeySize768])
if err != nil {
return nil, fmt.Errorf("ssh: peer's ML-KEM768 encapsulation key is not valid: %w", err)
}
// Perform KEM encapsulate operation to obtain ciphertext and shared key.
mlkem768Secret, mlkem768Ciphertext := encapsulationKey.Encapsulate()
// Perform server side of Curve25519 ECDH to obtain server public value and
// shared key.
var c25519kp curve25519KeyPair
if err := c25519kp.generate(rand); err != nil {
return nil, err
}
c25519Secret, err := curve25519.X25519(c25519kp.priv[:], kexInit.ClientPubKey[mlkem.EncapsulationKeySize768:])
if err != nil {
return nil, fmt.Errorf("ssh: peer's ML-KEM768/curve25519 public value is not valid: %w", err)
}
hybridKey := append(mlkem768Ciphertext, c25519kp.pub[:]...)
// Compute actual shared key.
h := sha256.New()
h.Write(mlkem768Secret)
h.Write(c25519Secret)
secret := h.Sum(nil)
hostKeyBytes := priv.PublicKey().Marshal()
h.Reset()
magics.write(h)
writeString(h, hostKeyBytes)
writeString(h, kexInit.ClientPubKey)
writeString(h, hybridKey)
K := make([]byte, stringLength(len(secret)))
marshalString(K, secret)
h.Write(K)
H := h.Sum(nil)
sig, err := signAndMarshal(priv, rand, H, algo)
if err != nil {
return nil, err
}
reply := kexECDHReplyMsg{
EphemeralPubKey: hybridKey,
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
}
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