// 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 file. package tls import ( "crypto" "crypto/md5" "crypto/rsa" "crypto/sha1" "crypto/x509" "errors" "fmt" "io" "crypto/ecdh" ) // a keyAgreement implements the client and server side of a TLS 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") 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 the given hash function (for >= TLS 1.2) or using a default based on // the sigType (for earlier TLS versions). For Ed25519 signatures, which don't // do pre-hashing, it returns the concatenation of the slices. func hashForServerKeyExchange(sigType uint8, hashFunc crypto.Hash, version uint16, slices ...[]byte) []byte { if sigType == signatureEd25519 { var signed []byte for _, slice := range slices { signed = append(signed, slice...) } return signed } if version >= VersionTLS12 { h := hashFunc.New() for _, slice := range slices { h.Write(slice) } digest := h.Sum(nil) return digest } 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 key *ecdh.PrivateKey // ckx and preMasterSecret are generated in processServerKeyExchange // and returned in generateClientKeyExchange. ckx *clientKeyExchangeMsg preMasterSecret []byte } func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) { var curveID CurveID for _, c := range clientHello.supportedCurves { if config.supportsCurve(c) { curveID = c break } } if curveID == 0 { return nil, errors.New("tls: no supported elliptic curves offered") } if _, ok := curveForCurveID(curveID); !ok { return nil, errors.New("tls: CurvePreferences includes unsupported curve") } key, err := generateECDHEKey(config.rand(), 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(curveID >> 8) serverECDHEParams[2] = byte(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 signatureAlgorithm SignatureScheme var sigType uint8 var sigHash crypto.Hash if ka.version >= VersionTLS12 { signatureAlgorithm, err = selectSignatureScheme(ka.version, cert, clientHello.supportedSignatureAlgorithms) if err != nil { return nil, err } sigType, sigHash, err = typeAndHashFromSignatureScheme(signatureAlgorithm) if err != nil { return nil, err } } else { sigType, sigHash, err = legacyTypeAndHashFromPublicKey(priv.Public()) if err != nil { return nil, err } } if (sigType == signaturePKCS1v15 || sigType == signatureRSAPSS) != ka.isRSA { return nil, errors.New("tls: certificate cannot be used with the selected cipher suite") } signed := hashForServerKeyExchange(sigType, sigHash, ka.version, clientHello.random, hello.random, serverECDHEParams) signOpts := crypto.SignerOpts(sigHash) if sigType == signatureRSAPSS { signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: sigHash} } sig, err := priv.Sign(config.rand(), signed, signOpts) 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(signatureAlgorithm >> 8) k[1] = byte(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") } 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 _, ok := curveForCurveID(curveID); !ok { return errors.New("tls: server selected unsupported curve") } key, err := generateECDHEKey(config.rand(), 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 { signatureAlgorithm := SignatureScheme(sig[0])<<8 | SignatureScheme(sig[1]) sig = sig[2:] if len(sig) < 2 { return errServerKeyExchange } if !isSupportedSignatureAlgorithm(signatureAlgorithm, clientHello.supportedSignatureAlgorithms) { return errors.New("tls: certificate used with invalid signature algorithm") } sigType, sigHash, err = typeAndHashFromSignatureScheme(signatureAlgorithm) if err != nil { return err } } else { sigType, sigHash, err = legacyTypeAndHashFromPublicKey(cert.PublicKey) if err != nil { return err } } if (sigType == signaturePKCS1v15 || sigType == signatureRSAPSS) != ka.isRSA { return errServerKeyExchange } sigLen := int(sig[0])<<8 | int(sig[1]) if sigLen+2 != len(sig) { return errServerKeyExchange } sig = sig[2:] signed := hashForServerKeyExchange(sigType, sigHash, ka.version, 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()) } 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 }