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// Copyright 2010 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-Go file.
package reality
import (
"crypto"
"crypto/ecdh"
"crypto/md5"
"crypto/rsa"
"crypto/sha1"
"crypto/x509"
"errors"
"fmt"
"io"
"slices"
)
// A keyAgreement implements the client and server side of a TLS 1.0–1.2 key
// agreement protocol by generating and processing key exchange messages.
type keyAgreement interface {
// On the server side, the first two methods are called in order.
// In the case that the key agreement protocol doesn't use a
// ServerKeyExchange message, generateServerKeyExchange can return nil,
// nil.
generateServerKeyExchange(*Config, *Certificate, *clientHelloMsg, *serverHelloMsg) (*serverKeyExchangeMsg, error)
processClientKeyExchange(*Config, *Certificate, *clientKeyExchangeMsg, uint16) ([]byte, error)
// On the client side, the next two methods are called in order.
// This method may not be called if the server doesn't send a
// ServerKeyExchange message.
processServerKeyExchange(*Config, *clientHelloMsg, *serverHelloMsg, *x509.Certificate, *serverKeyExchangeMsg) error
generateClientKeyExchange(*Config, *clientHelloMsg, *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error)
}
var errClientKeyExchange = errors.New("tls: invalid ClientKeyExchange message")
var errServerKeyExchange = errors.New("tls: invalid ServerKeyExchange message")
// rsaKeyAgreement implements the standard TLS key agreement where the client
// encrypts the pre-master secret to the server's public key.
type rsaKeyAgreement struct{}
func (ka rsaKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
return nil, nil
}
func (ka rsaKeyAgreement) processClientKeyExchange(config *Config, cert *Certificate, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) {
if len(ckx.ciphertext) < 2 {
return nil, errClientKeyExchange
}
ciphertextLen := int(ckx.ciphertext[0])<<8 | int(ckx.ciphertext[1])
if ciphertextLen != len(ckx.ciphertext)-2 {
return nil, errClientKeyExchange
}
ciphertext := ckx.ciphertext[2:]
priv, ok := cert.PrivateKey.(crypto.Decrypter)
if !ok {
return nil, errors.New("tls: certificate private key does not implement crypto.Decrypter")
}
// Perform constant time RSA PKCS #1 v1.5 decryption
preMasterSecret, err := priv.Decrypt(config.rand(), ciphertext, &rsa.PKCS1v15DecryptOptions{SessionKeyLen: 48})
if err != nil {
return nil, err
}
// We don't check the version number in the premaster secret. For one,
// by checking it, we would leak information about the validity of the
// encrypted pre-master secret. Secondly, it provides only a small
// benefit against a downgrade attack and some implementations send the
// wrong version anyway. See the discussion at the end of section
// 7.4.7.1 of RFC 4346.
return preMasterSecret, nil
}
func (ka rsaKeyAgreement) processServerKeyExchange(config *Config, clientHello *clientHelloMsg, serverHello *serverHelloMsg, cert *x509.Certificate, skx *serverKeyExchangeMsg) error {
return errors.New("tls: unexpected ServerKeyExchange")
}
func (ka rsaKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, cert *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error) {
preMasterSecret := make([]byte, 48)
preMasterSecret[0] = byte(clientHello.vers >> 8)
preMasterSecret[1] = byte(clientHello.vers)
_, err := io.ReadFull(config.rand(), preMasterSecret[2:])
if err != nil {
return nil, nil, err
}
rsaKey, ok := cert.PublicKey.(*rsa.PublicKey)
if !ok {
return nil, nil, errors.New("tls: server certificate contains incorrect key type for selected ciphersuite")
}
encrypted, err := rsa.EncryptPKCS1v15(config.rand(), rsaKey, preMasterSecret)
if err != nil {
return nil, nil, err
}
ckx := new(clientKeyExchangeMsg)
ckx.ciphertext = make([]byte, len(encrypted)+2)
ckx.ciphertext[0] = byte(len(encrypted) >> 8)
ckx.ciphertext[1] = byte(len(encrypted))
copy(ckx.ciphertext[2:], encrypted)
return preMasterSecret, ckx, nil
}
// sha1Hash calculates a SHA1 hash over the given byte slices.
func sha1Hash(slices [][]byte) []byte {
hsha1 := sha1.New()
for _, slice := range slices {
hsha1.Write(slice)
}
return hsha1.Sum(nil)
}
// md5SHA1Hash implements TLS 1.0's hybrid hash function which consists of the
// concatenation of an MD5 and SHA1 hash.
func md5SHA1Hash(slices [][]byte) []byte {
md5sha1 := make([]byte, md5.Size+sha1.Size)
hmd5 := md5.New()
for _, slice := range slices {
hmd5.Write(slice)
}
copy(md5sha1, hmd5.Sum(nil))
copy(md5sha1[md5.Size:], sha1Hash(slices))
return md5sha1
}
// hashForServerKeyExchange hashes the given slices and returns their digest
// using a hash based on the sigType. It can only be used for TLS 1.0 and 1.1.
func hashForServerKeyExchange(sigType uint8, slices ...[]byte) []byte {
if sigType == signatureECDSA {
return sha1Hash(slices)
}
return md5SHA1Hash(slices)
}
// ecdheKeyAgreement implements a TLS key agreement where the server
// generates an ephemeral EC public/private key pair and signs it. The
// pre-master secret is then calculated using ECDH. The signature may
// be ECDSA, Ed25519 or RSA.
type ecdheKeyAgreement struct {
version uint16
isRSA bool
// ckx and preMasterSecret are generated in processServerKeyExchange
// and returned in generateClientKeyExchange.
ckx *clientKeyExchangeMsg
preMasterSecret []byte
// curveID, signatureAlgorithm, and key are set by processServerKeyExchange
// and generateServerKeyExchange.
curveID CurveID
signatureAlgorithm SignatureScheme
key *ecdh.PrivateKey
}
func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
for _, c := range clientHello.supportedCurves {
if config.supportsCurve(ka.version, c) {
ka.curveID = c
break
}
}
if ka.curveID == 0 {
return nil, errors.New("tls: no supported elliptic curves offered")
}
if _, ok := curveForCurveID(ka.curveID); !ok {
return nil, errors.New("tls: CurvePreferences includes unsupported curve")
}
key, err := generateECDHEKey(config.rand(), ka.curveID)
if err != nil {
return nil, err
}
ka.key = key
// See RFC 4492, Section 5.4.
ecdhePublic := key.PublicKey().Bytes()
serverECDHEParams := make([]byte, 1+2+1+len(ecdhePublic))
serverECDHEParams[0] = 3 // named curve
serverECDHEParams[1] = byte(ka.curveID >> 8)
serverECDHEParams[2] = byte(ka.curveID)
serverECDHEParams[3] = byte(len(ecdhePublic))
copy(serverECDHEParams[4:], ecdhePublic)
priv, ok := cert.PrivateKey.(crypto.Signer)
if !ok {
return nil, fmt.Errorf("tls: certificate private key of type %T does not implement crypto.Signer", cert.PrivateKey)
}
var sig []byte
if ka.version >= VersionTLS12 {
ka.signatureAlgorithm, err = selectSignatureScheme(ka.version, cert, clientHello.supportedSignatureAlgorithms)
if err != nil {
return nil, err
}
sigType, sigHash, err := typeAndHashFromSignatureScheme(ka.signatureAlgorithm)
if err != nil {
return nil, err
}
signed := slices.Concat(clientHello.random, hello.random, serverECDHEParams)
if (sigType == signaturePKCS1v15 || sigType == signatureRSAPSS) != ka.isRSA {
return nil, errors.New("tls: certificate cannot be used with the selected cipher suite")
}
signOpts := crypto.SignerOpts(sigHash)
if sigType == signatureRSAPSS {
signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: sigHash}
}
sig, err = crypto.SignMessage(priv, config.rand(), signed, signOpts)
if err != nil {
return nil, errors.New("tls: failed to sign ECDHE parameters: " + err.Error())
}
} else {
sigType, sigHash, err := legacyTypeAndHashFromPublicKey(priv.Public())
if err != nil {
return nil, err
}
signed := hashForServerKeyExchange(sigType, clientHello.random, hello.random, serverECDHEParams)
if (sigType == signaturePKCS1v15) != ka.isRSA {
return nil, errors.New("tls: certificate cannot be used with the selected cipher suite")
}
sig, err = priv.Sign(config.rand(), signed, sigHash)
if err != nil {
return nil, errors.New("tls: failed to sign ECDHE parameters: " + err.Error())
}
}
skx := new(serverKeyExchangeMsg)
sigAndHashLen := 0
if ka.version >= VersionTLS12 {
sigAndHashLen = 2
}
skx.key = make([]byte, len(serverECDHEParams)+sigAndHashLen+2+len(sig))
copy(skx.key, serverECDHEParams)
k := skx.key[len(serverECDHEParams):]
if ka.version >= VersionTLS12 {
k[0] = byte(ka.signatureAlgorithm >> 8)
k[1] = byte(ka.signatureAlgorithm)
k = k[2:]
}
k[0] = byte(len(sig) >> 8)
k[1] = byte(len(sig))
copy(k[2:], sig)
return skx, nil
}
func (ka *ecdheKeyAgreement) processClientKeyExchange(config *Config, cert *Certificate, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) {
if len(ckx.ciphertext) == 0 || int(ckx.ciphertext[0]) != len(ckx.ciphertext)-1 {
return nil, errClientKeyExchange
}
peerKey, err := ka.key.Curve().NewPublicKey(ckx.ciphertext[1:])
if err != nil {
return nil, errClientKeyExchange
}
preMasterSecret, err := ka.key.ECDH(peerKey)
if err != nil {
return nil, errClientKeyExchange
}
return preMasterSecret, nil
}
func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHello *clientHelloMsg, serverHello *serverHelloMsg, cert *x509.Certificate, skx *serverKeyExchangeMsg) error {
if len(skx.key) < 4 {
return errServerKeyExchange
}
if skx.key[0] != 3 { // named curve
return errors.New("tls: server selected unsupported curve")
}
ka.curveID = CurveID(skx.key[1])<<8 | CurveID(skx.key[2])
publicLen := int(skx.key[3])
if publicLen+4 > len(skx.key) {
return errServerKeyExchange
}
serverECDHEParams := skx.key[:4+publicLen]
publicKey := serverECDHEParams[4:]
sig := skx.key[4+publicLen:]
if len(sig) < 2 {
return errServerKeyExchange
}
if ka.version >= VersionTLS12 {
ka.signatureAlgorithm = SignatureScheme(sig[0])<<8 | SignatureScheme(sig[1])
sig = sig[2:]
if len(sig) < 2 {
return errServerKeyExchange
}
}
sigLen := int(sig[0])<<8 | int(sig[1])
if sigLen+2 != len(sig) {
return errServerKeyExchange
}
sig = sig[2:]
if !slices.Contains(clientHello.supportedCurves, ka.curveID) {
return errors.New("tls: server selected unoffered curve")
}
if _, ok := curveForCurveID(ka.curveID); !ok {
return errors.New("tls: server selected unsupported curve")
}
key, err := generateECDHEKey(config.rand(), ka.curveID)
if err != nil {
return err
}
ka.key = key
peerKey, err := key.Curve().NewPublicKey(publicKey)
if err != nil {
return errServerKeyExchange
}
ka.preMasterSecret, err = key.ECDH(peerKey)
if err != nil {
return errServerKeyExchange
}
ourPublicKey := key.PublicKey().Bytes()
ka.ckx = new(clientKeyExchangeMsg)
ka.ckx.ciphertext = make([]byte, 1+len(ourPublicKey))
ka.ckx.ciphertext[0] = byte(len(ourPublicKey))
copy(ka.ckx.ciphertext[1:], ourPublicKey)
var sigType uint8
var sigHash crypto.Hash
if ka.version >= VersionTLS12 {
if !isSupportedSignatureAlgorithm(ka.signatureAlgorithm, clientHello.supportedSignatureAlgorithms) {
return errors.New("tls: certificate used with invalid signature algorithm")
}
sigType, sigHash, err = typeAndHashFromSignatureScheme(ka.signatureAlgorithm)
if err != nil {
return err
}
if (sigType == signaturePKCS1v15 || sigType == signatureRSAPSS) != ka.isRSA {
return errServerKeyExchange
}
signed := slices.Concat(clientHello.random, serverHello.random, serverECDHEParams)
if err := verifyHandshakeSignature(sigType, cert.PublicKey, sigHash, signed, sig); err != nil {
return errors.New("tls: invalid signature by the server certificate: " + err.Error())
}
} else {
sigType, sigHash, err = legacyTypeAndHashFromPublicKey(cert.PublicKey)
if err != nil {
return err
}
if (sigType == signaturePKCS1v15) != ka.isRSA {
return errServerKeyExchange
}
signed := hashForServerKeyExchange(sigType, clientHello.random, serverHello.random, serverECDHEParams)
if err := verifyLegacyHandshakeSignature(sigType, cert.PublicKey, sigHash, signed, sig); err != nil {
return errors.New("tls: invalid signature by the server certificate: " + err.Error())
}
}
return nil
}
func (ka *ecdheKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, cert *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error) {
if ka.ckx == nil {
return nil, nil, errors.New("tls: missing ServerKeyExchange message")
}
return ka.preMasterSecret, ka.ckx, nil
}
// generateECDHEKey returns a PrivateKey that implements Diffie-Hellman
// according to RFC 8446, Section 4.2.8.2.
func generateECDHEKey(rand io.Reader, curveID CurveID) (*ecdh.PrivateKey, error) {
curve, ok := curveForCurveID(curveID)
if !ok {
return nil, errors.New("tls: internal error: unsupported curve")
}
return curve.GenerateKey(rand)
}
func curveForCurveID(id CurveID) (ecdh.Curve, bool) {
switch id {
case X25519:
return ecdh.X25519(), true
case CurveP256:
return ecdh.P256(), true
case CurveP384:
return ecdh.P384(), true
case CurveP521:
return ecdh.P521(), true
default:
return nil, false
}
}
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