// 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/elliptic" "crypto/md5" "crypto/rsa" "crypto/sha1" "errors" "io" "math/big" "golang_org/x/crypto/curve25519" ) 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, sk crypto.PrivateKey, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) { return nil, nil } func (ka rsaKeyAgreement) processClientKeyExchange(config *Config, sk crypto.PrivateKey, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) { if len(ckx.ciphertext) < 2 { return nil, errClientKeyExchange } ciphertext := ckx.ciphertext if version != VersionSSL30 { 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 := sk.(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, pk crypto.PublicKey, skx *serverKeyExchangeMsg) error { return errors.New("tls: unexpected ServerKeyExchange") } func (ka rsaKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, pk crypto.PublicKey) ([]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 } encrypted, err := rsa.EncryptPKCS1v15(config.rand(), pk.(*rsa.PublicKey), 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. func hashForServerKeyExchange(sigType uint8, hashFunc crypto.Hash, version uint16, slices ...[]byte) ([]byte, error) { if version >= VersionTLS12 { h := hashFunc.New() for _, slice := range slices { h.Write(slice) } digest := h.Sum(nil) return digest, nil } if sigType == signatureECDSA { return sha1Hash(slices), nil } return md5SHA1Hash(slices), nil } func curveForCurveID(id CurveID) (elliptic.Curve, bool) { switch id { case CurveP256: return elliptic.P256(), true case CurveP384: return elliptic.P384(), true case CurveP521: return elliptic.P521(), true default: return nil, false } } // 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 // either be ECDSA or RSA. type ecdheKeyAgreement struct { version uint16 isRSA bool privateKey []byte curveid CurveID // publicKey is used to store the peer's public value when X25519 is // being used. publicKey []byte // x and y are used to store the peer's public value when one of the // NIST curves is being used. x, y *big.Int } func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, sk crypto.PrivateKey, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) { preferredCurves := config.curvePreferences() NextCandidate: for _, candidate := range preferredCurves { for _, c := range clientHello.supportedCurves { if candidate == c { ka.curveid = c break NextCandidate } } } if ka.curveid == 0 { return nil, errors.New("tls: no supported elliptic curves offered") } var ecdhePublic []byte if ka.curveid == X25519 { var scalar, public [32]byte if _, err := io.ReadFull(config.rand(), scalar[:]); err != nil { return nil, err } curve25519.ScalarBaseMult(&public, &scalar) ka.privateKey = scalar[:] ecdhePublic = public[:] } else { curve, ok := curveForCurveID(ka.curveid) if !ok { return nil, errors.New("tls: preferredCurves includes unsupported curve") } var x, y *big.Int var err error ka.privateKey, x, y, err = elliptic.GenerateKey(curve, config.rand()) if err != nil { return nil, err } ecdhePublic = elliptic.Marshal(curve, x, y) } // http://tools.ietf.org/html/rfc4492#section-5.4 serverECDHParams := make([]byte, 1+2+1+len(ecdhePublic)) serverECDHParams[0] = 3 // named curve serverECDHParams[1] = byte(ka.curveid >> 8) serverECDHParams[2] = byte(ka.curveid) serverECDHParams[3] = byte(len(ecdhePublic)) copy(serverECDHParams[4:], ecdhePublic) priv, ok := sk.(crypto.Signer) if !ok { return nil, errors.New("tls: certificate private key does not implement crypto.Signer") } signatureAlgorithm, sigType, hashFunc, err := pickSignatureAlgorithm(priv.Public(), clientHello.supportedSignatureAlgorithms, supportedSignatureAlgorithms, ka.version) 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") } digest, err := hashForServerKeyExchange(sigType, hashFunc, ka.version, clientHello.random, hello.random, serverECDHParams) if err != nil { return nil, err } var sig []byte signOpts := crypto.SignerOpts(hashFunc) if sigType == signatureRSAPSS { signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: hashFunc} } sig, err = priv.Sign(config.rand(), digest, 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(serverECDHParams)+sigAndHashLen+2+len(sig)) copy(skx.key, serverECDHParams) k := skx.key[len(serverECDHParams):] 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, sk crypto.PrivateKey, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) { if len(ckx.ciphertext) == 0 || int(ckx.ciphertext[0]) != len(ckx.ciphertext)-1 { return nil, errClientKeyExchange } if ka.curveid == X25519 { if len(ckx.ciphertext) != 1+32 { return nil, errClientKeyExchange } var theirPublic, sharedKey, scalar [32]byte copy(theirPublic[:], ckx.ciphertext[1:]) copy(scalar[:], ka.privateKey) curve25519.ScalarMult(&sharedKey, &scalar, &theirPublic) return sharedKey[:], nil } curve, ok := curveForCurveID(ka.curveid) if !ok { panic("internal error") } x, y := elliptic.Unmarshal(curve, ckx.ciphertext[1:]) // Unmarshal also checks whether the given point is on the curve if x == nil { return nil, errClientKeyExchange } x, _ = curve.ScalarMult(x, y, ka.privateKey) curveSize := (curve.Params().BitSize + 7) >> 3 xBytes := x.Bytes() if len(xBytes) == curveSize { return xBytes, nil } preMasterSecret := make([]byte, curveSize) copy(preMasterSecret[len(preMasterSecret)-len(xBytes):], xBytes) return preMasterSecret, nil } func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHello *clientHelloMsg, serverHello *serverHelloMsg, pk crypto.PublicKey, 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 } serverECDHParams := skx.key[:4+publicLen] publicKey := serverECDHParams[4:] sig := skx.key[4+publicLen:] if len(sig) < 2 { return errServerKeyExchange } if ka.curveid == X25519 { if len(publicKey) != 32 { return errors.New("tls: bad X25519 public value") } ka.publicKey = publicKey } else { curve, ok := curveForCurveID(ka.curveid) if !ok { return errors.New("tls: server selected unsupported curve") } ka.x, ka.y = elliptic.Unmarshal(curve, publicKey) // Unmarshal also checks whether the given point is on the curve if ka.x == nil { return errServerKeyExchange } } var signatureAlgorithm SignatureScheme if ka.version >= VersionTLS12 { // handle SignatureAndHashAlgorithm signatureAlgorithm = SignatureScheme(sig[0])<<8 | SignatureScheme(sig[1]) sig = sig[2:] if len(sig) < 2 { return errServerKeyExchange } } _, sigType, hashFunc, err := pickSignatureAlgorithm(pk, []SignatureScheme{signatureAlgorithm}, clientHello.supportedSignatureAlgorithms, ka.version) 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:] digest, err := hashForServerKeyExchange(sigType, hashFunc, ka.version, clientHello.random, serverHello.random, serverECDHParams) if err != nil { return err } return verifyHandshakeSignature(sigType, pk, hashFunc, digest, sig) } func (ka *ecdheKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, pk crypto.PublicKey) ([]byte, *clientKeyExchangeMsg, error) { if ka.curveid == 0 { return nil, nil, errors.New("tls: missing ServerKeyExchange message") } var serialized, preMasterSecret []byte if ka.curveid == X25519 { var ourPublic, theirPublic, sharedKey, scalar [32]byte if _, err := io.ReadFull(config.rand(), scalar[:]); err != nil { return nil, nil, err } copy(theirPublic[:], ka.publicKey) curve25519.ScalarBaseMult(&ourPublic, &scalar) curve25519.ScalarMult(&sharedKey, &scalar, &theirPublic) serialized = ourPublic[:] preMasterSecret = sharedKey[:] } else { curve, ok := curveForCurveID(ka.curveid) if !ok { panic("internal error") } priv, mx, my, err := elliptic.GenerateKey(curve, config.rand()) if err != nil { return nil, nil, err } x, _ := curve.ScalarMult(ka.x, ka.y, priv) preMasterSecret = make([]byte, (curve.Params().BitSize+7)>>3) xBytes := x.Bytes() copy(preMasterSecret[len(preMasterSecret)-len(xBytes):], xBytes) serialized = elliptic.Marshal(curve, mx, my) } ckx := new(clientKeyExchangeMsg) ckx.ciphertext = make([]byte, 1+len(serialized)) ckx.ciphertext[0] = byte(len(serialized)) copy(ckx.ciphertext[1:], serialized) return preMasterSecret, ckx, nil }