key_agreement.go

     1  // Copyright 2010 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package tls
     6  
     7  import (
     8  	"crypto"
     9  	"crypto/ecdh"
    10  	"crypto/md5"
    11  	"crypto/rsa"
    12  	"crypto/sha1"
    13  	"crypto/x509"
    14  	"errors"
    15  	"fmt"
    16  	"io"
    17  	"slices"
    18  )
    19  
    20  // A keyAgreement implements the client and server side of a TLS 1.0–1.2 key
    21  // agreement protocol by generating and processing key exchange messages.
    22  type keyAgreement interface {
    23  	// On the server side, the first two methods are called in order.
    24  
    25  	// In the case that the key agreement protocol doesn't use a
    26  	// ServerKeyExchange message, generateServerKeyExchange can return nil,
    27  	// nil.
    28  	generateServerKeyExchange(*Config, *Certificate, *clientHelloMsg, *serverHelloMsg) (*serverKeyExchangeMsg, error)
    29  	processClientKeyExchange(*Config, *Certificate, *clientKeyExchangeMsg, uint16) ([]byte, error)
    30  
    31  	// On the client side, the next two methods are called in order.
    32  
    33  	// This method may not be called if the server doesn't send a
    34  	// ServerKeyExchange message.
    35  	processServerKeyExchange(*Config, *clientHelloMsg, *serverHelloMsg, *x509.Certificate, *serverKeyExchangeMsg) error
    36  	generateClientKeyExchange(*Config, *clientHelloMsg, *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error)
    37  }
    38  
    39  var errClientKeyExchange = errors.New("tls: invalid ClientKeyExchange message")
    40  var errServerKeyExchange = errors.New("tls: invalid ServerKeyExchange message")
    41  
    42  // rsaKeyAgreement implements the standard TLS key agreement where the client
    43  // encrypts the pre-master secret to the server's public key.
    44  type rsaKeyAgreement struct{}
    45  
    46  func (ka rsaKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
    47  	return nil, nil
    48  }
    49  
    50  func (ka rsaKeyAgreement) processClientKeyExchange(config *Config, cert *Certificate, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) {
    51  	if len(ckx.ciphertext) < 2 {
    52  		return nil, errClientKeyExchange
    53  	}
    54  	ciphertextLen := int(ckx.ciphertext[0])<<8 | int(ckx.ciphertext[1])
    55  	if ciphertextLen != len(ckx.ciphertext)-2 {
    56  		return nil, errClientKeyExchange
    57  	}
    58  	ciphertext := ckx.ciphertext[2:]
    59  
    60  	priv, ok := cert.PrivateKey.(crypto.Decrypter)
    61  	if !ok {
    62  		return nil, errors.New("tls: certificate private key does not implement crypto.Decrypter")
    63  	}
    64  	// Perform constant time RSA PKCS #1 v1.5 decryption
    65  	preMasterSecret, err := priv.Decrypt(config.rand(), ciphertext, &rsa.PKCS1v15DecryptOptions{SessionKeyLen: 48})
    66  	if err != nil {
    67  		return nil, err
    68  	}
    69  	// We don't check the version number in the premaster secret. For one,
    70  	// by checking it, we would leak information about the validity of the
    71  	// encrypted pre-master secret. Secondly, it provides only a small
    72  	// benefit against a downgrade attack and some implementations send the
    73  	// wrong version anyway. See the discussion at the end of section
    74  	// 7.4.7.1 of RFC 4346.
    75  	return preMasterSecret, nil
    76  }
    77  
    78  func (ka rsaKeyAgreement) processServerKeyExchange(config *Config, clientHello *clientHelloMsg, serverHello *serverHelloMsg, cert *x509.Certificate, skx *serverKeyExchangeMsg) error {
    79  	return errors.New("tls: unexpected ServerKeyExchange")
    80  }
    81  
    82  func (ka rsaKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, cert *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error) {
    83  	preMasterSecret := make([]byte, 48)
    84  	preMasterSecret[0] = byte(clientHello.vers >> 8)
    85  	preMasterSecret[1] = byte(clientHello.vers)
    86  	_, err := io.ReadFull(config.rand(), preMasterSecret[2:])
    87  	if err != nil {
    88  		return nil, nil, err
    89  	}
    90  
    91  	rsaKey, ok := cert.PublicKey.(*rsa.PublicKey)
    92  	if !ok {
    93  		return nil, nil, errors.New("tls: server certificate contains incorrect key type for selected ciphersuite")
    94  	}
    95  	encrypted, err := rsa.EncryptPKCS1v15(config.rand(), rsaKey, preMasterSecret)
    96  	if err != nil {
    97  		return nil, nil, err
    98  	}
    99  	ckx := new(clientKeyExchangeMsg)
   100  	ckx.ciphertext = make([]byte, len(encrypted)+2)
   101  	ckx.ciphertext[0] = byte(len(encrypted) >> 8)
   102  	ckx.ciphertext[1] = byte(len(encrypted))
   103  	copy(ckx.ciphertext[2:], encrypted)
   104  	return preMasterSecret, ckx, nil
   105  }
   106  
   107  // sha1Hash calculates a SHA1 hash over the given byte slices.
   108  func sha1Hash(slices [][]byte) []byte {
   109  	hsha1 := sha1.New()
   110  	for _, slice := range slices {
   111  		hsha1.Write(slice)
   112  	}
   113  	return hsha1.Sum(nil)
   114  }
   115  
   116  // md5SHA1Hash implements TLS 1.0's hybrid hash function which consists of the
   117  // concatenation of an MD5 and SHA1 hash.
   118  func md5SHA1Hash(slices [][]byte) []byte {
   119  	md5sha1 := make([]byte, md5.Size+sha1.Size)
   120  	hmd5 := md5.New()
   121  	for _, slice := range slices {
   122  		hmd5.Write(slice)
   123  	}
   124  	copy(md5sha1, hmd5.Sum(nil))
   125  	copy(md5sha1[md5.Size:], sha1Hash(slices))
   126  	return md5sha1
   127  }
   128  
   129  // hashForServerKeyExchange hashes the given slices and returns their digest
   130  // using the given hash function (for TLS 1.2) or using a default based on
   131  // the sigType (for earlier TLS versions). For Ed25519 signatures, which don't
   132  // do pre-hashing, it returns the concatenation of the slices.
   133  func hashForServerKeyExchange(sigType uint8, hashFunc crypto.Hash, version uint16, slices ...[]byte) []byte {
   134  	if sigType == signatureEd25519 {
   135  		var signed []byte
   136  		for _, slice := range slices {
   137  			signed = append(signed, slice...)
   138  		}
   139  		return signed
   140  	}
   141  	if version >= VersionTLS12 {
   142  		h := hashFunc.New()
   143  		for _, slice := range slices {
   144  			h.Write(slice)
   145  		}
   146  		digest := h.Sum(nil)
   147  		return digest
   148  	}
   149  	if sigType == signatureECDSA {
   150  		return sha1Hash(slices)
   151  	}
   152  	return md5SHA1Hash(slices)
   153  }
   154  
   155  // ecdheKeyAgreement implements a TLS key agreement where the server
   156  // generates an ephemeral EC public/private key pair and signs it. The
   157  // pre-master secret is then calculated using ECDH. The signature may
   158  // be ECDSA, Ed25519 or RSA.
   159  type ecdheKeyAgreement struct {
   160  	version uint16
   161  	isRSA   bool
   162  	key     *ecdh.PrivateKey
   163  
   164  	// ckx and preMasterSecret are generated in processServerKeyExchange
   165  	// and returned in generateClientKeyExchange.
   166  	ckx             *clientKeyExchangeMsg
   167  	preMasterSecret []byte
   168  
   169  	// curveID and signatureAlgorithm are set by processServerKeyExchange and
   170  	// generateServerKeyExchange.
   171  	curveID            CurveID
   172  	signatureAlgorithm SignatureScheme
   173  }
   174  
   175  func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
   176  	for _, c := range clientHello.supportedCurves {
   177  		if config.supportsCurve(ka.version, c) {
   178  			ka.curveID = c
   179  			break
   180  		}
   181  	}
   182  
   183  	if ka.curveID == 0 {
   184  		return nil, errors.New("tls: no supported elliptic curves offered")
   185  	}
   186  	if _, ok := curveForCurveID(ka.curveID); !ok {
   187  		return nil, errors.New("tls: CurvePreferences includes unsupported curve")
   188  	}
   189  
   190  	key, err := generateECDHEKey(config.rand(), ka.curveID)
   191  	if err != nil {
   192  		return nil, err
   193  	}
   194  	ka.key = key
   195  
   196  	// See RFC 4492, Section 5.4.
   197  	ecdhePublic := key.PublicKey().Bytes()
   198  	serverECDHEParams := make([]byte, 1+2+1+len(ecdhePublic))
   199  	serverECDHEParams[0] = 3 // named curve
   200  	serverECDHEParams[1] = byte(ka.curveID >> 8)
   201  	serverECDHEParams[2] = byte(ka.curveID)
   202  	serverECDHEParams[3] = byte(len(ecdhePublic))
   203  	copy(serverECDHEParams[4:], ecdhePublic)
   204  
   205  	priv, ok := cert.PrivateKey.(crypto.Signer)
   206  	if !ok {
   207  		return nil, fmt.Errorf("tls: certificate private key of type %T does not implement crypto.Signer", cert.PrivateKey)
   208  	}
   209  
   210  	var sigType uint8
   211  	var sigHash crypto.Hash
   212  	if ka.version >= VersionTLS12 {
   213  		ka.signatureAlgorithm, err = selectSignatureScheme(ka.version, cert, clientHello.supportedSignatureAlgorithms)
   214  		if err != nil {
   215  			return nil, err
   216  		}
   217  		sigType, sigHash, err = typeAndHashFromSignatureScheme(ka.signatureAlgorithm)
   218  		if err != nil {
   219  			return nil, err
   220  		}
   221  		if sigHash == crypto.SHA1 {
   222  			tlssha1.Value() // ensure godebug is initialized
   223  			tlssha1.IncNonDefault()
   224  		}
   225  	} else {
   226  		sigType, sigHash, err = legacyTypeAndHashFromPublicKey(priv.Public())
   227  		if err != nil {
   228  			return nil, err
   229  		}
   230  	}
   231  	if (sigType == signaturePKCS1v15 || sigType == signatureRSAPSS) != ka.isRSA {
   232  		return nil, errors.New("tls: certificate cannot be used with the selected cipher suite")
   233  	}
   234  
   235  	signed := hashForServerKeyExchange(sigType, sigHash, ka.version, clientHello.random, hello.random, serverECDHEParams)
   236  
   237  	signOpts := crypto.SignerOpts(sigHash)
   238  	if sigType == signatureRSAPSS {
   239  		signOpts = &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: sigHash}
   240  	}
   241  	sig, err := priv.Sign(config.rand(), signed, signOpts)
   242  	if err != nil {
   243  		return nil, errors.New("tls: failed to sign ECDHE parameters: " + err.Error())
   244  	}
   245  
   246  	skx := new(serverKeyExchangeMsg)
   247  	sigAndHashLen := 0
   248  	if ka.version >= VersionTLS12 {
   249  		sigAndHashLen = 2
   250  	}
   251  	skx.key = make([]byte, len(serverECDHEParams)+sigAndHashLen+2+len(sig))
   252  	copy(skx.key, serverECDHEParams)
   253  	k := skx.key[len(serverECDHEParams):]
   254  	if ka.version >= VersionTLS12 {
   255  		k[0] = byte(ka.signatureAlgorithm >> 8)
   256  		k[1] = byte(ka.signatureAlgorithm)
   257  		k = k[2:]
   258  	}
   259  	k[0] = byte(len(sig) >> 8)
   260  	k[1] = byte(len(sig))
   261  	copy(k[2:], sig)
   262  
   263  	return skx, nil
   264  }
   265  
   266  func (ka *ecdheKeyAgreement) processClientKeyExchange(config *Config, cert *Certificate, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) {
   267  	if len(ckx.ciphertext) == 0 || int(ckx.ciphertext[0]) != len(ckx.ciphertext)-1 {
   268  		return nil, errClientKeyExchange
   269  	}
   270  
   271  	peerKey, err := ka.key.Curve().NewPublicKey(ckx.ciphertext[1:])
   272  	if err != nil {
   273  		return nil, errClientKeyExchange
   274  	}
   275  	preMasterSecret, err := ka.key.ECDH(peerKey)
   276  	if err != nil {
   277  		return nil, errClientKeyExchange
   278  	}
   279  
   280  	return preMasterSecret, nil
   281  }
   282  
   283  func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHello *clientHelloMsg, serverHello *serverHelloMsg, cert *x509.Certificate, skx *serverKeyExchangeMsg) error {
   284  	if len(skx.key) < 4 {
   285  		return errServerKeyExchange
   286  	}
   287  	if skx.key[0] != 3 { // named curve
   288  		return errors.New("tls: server selected unsupported curve")
   289  	}
   290  	ka.curveID = CurveID(skx.key[1])<<8 | CurveID(skx.key[2])
   291  
   292  	publicLen := int(skx.key[3])
   293  	if publicLen+4 > len(skx.key) {
   294  		return errServerKeyExchange
   295  	}
   296  	serverECDHEParams := skx.key[:4+publicLen]
   297  	publicKey := serverECDHEParams[4:]
   298  
   299  	sig := skx.key[4+publicLen:]
   300  	if len(sig) < 2 {
   301  		return errServerKeyExchange
   302  	}
   303  
   304  	if !slices.Contains(clientHello.supportedCurves, ka.curveID) {
   305  		return errors.New("tls: server selected unoffered curve")
   306  	}
   307  
   308  	if _, ok := curveForCurveID(ka.curveID); !ok {
   309  		return errors.New("tls: server selected unsupported curve")
   310  	}
   311  
   312  	key, err := generateECDHEKey(config.rand(), ka.curveID)
   313  	if err != nil {
   314  		return err
   315  	}
   316  	ka.key = key
   317  
   318  	peerKey, err := key.Curve().NewPublicKey(publicKey)
   319  	if err != nil {
   320  		return errServerKeyExchange
   321  	}
   322  	ka.preMasterSecret, err = key.ECDH(peerKey)
   323  	if err != nil {
   324  		return errServerKeyExchange
   325  	}
   326  
   327  	ourPublicKey := key.PublicKey().Bytes()
   328  	ka.ckx = new(clientKeyExchangeMsg)
   329  	ka.ckx.ciphertext = make([]byte, 1+len(ourPublicKey))
   330  	ka.ckx.ciphertext[0] = byte(len(ourPublicKey))
   331  	copy(ka.ckx.ciphertext[1:], ourPublicKey)
   332  
   333  	var sigType uint8
   334  	var sigHash crypto.Hash
   335  	if ka.version >= VersionTLS12 {
   336  		ka.signatureAlgorithm = SignatureScheme(sig[0])<<8 | SignatureScheme(sig[1])
   337  		sig = sig[2:]
   338  		if len(sig) < 2 {
   339  			return errServerKeyExchange
   340  		}
   341  
   342  		if !isSupportedSignatureAlgorithm(ka.signatureAlgorithm, clientHello.supportedSignatureAlgorithms) {
   343  			return errors.New("tls: certificate used with invalid signature algorithm")
   344  		}
   345  		sigType, sigHash, err = typeAndHashFromSignatureScheme(ka.signatureAlgorithm)
   346  		if err != nil {
   347  			return err
   348  		}
   349  		if sigHash == crypto.SHA1 {
   350  			tlssha1.Value() // ensure godebug is initialized
   351  			tlssha1.IncNonDefault()
   352  		}
   353  	} else {
   354  		sigType, sigHash, err = legacyTypeAndHashFromPublicKey(cert.PublicKey)
   355  		if err != nil {
   356  			return err
   357  		}
   358  	}
   359  	if (sigType == signaturePKCS1v15 || sigType == signatureRSAPSS) != ka.isRSA {
   360  		return errServerKeyExchange
   361  	}
   362  
   363  	sigLen := int(sig[0])<<8 | int(sig[1])
   364  	if sigLen+2 != len(sig) {
   365  		return errServerKeyExchange
   366  	}
   367  	sig = sig[2:]
   368  
   369  	signed := hashForServerKeyExchange(sigType, sigHash, ka.version, clientHello.random, serverHello.random, serverECDHEParams)
   370  	if err := verifyHandshakeSignature(sigType, cert.PublicKey, sigHash, signed, sig); err != nil {
   371  		return errors.New("tls: invalid signature by the server certificate: " + err.Error())
   372  	}
   373  	return nil
   374  }
   375  
   376  func (ka *ecdheKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, cert *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error) {
   377  	if ka.ckx == nil {
   378  		return nil, nil, errors.New("tls: missing ServerKeyExchange message")
   379  	}
   380  
   381  	return ka.preMasterSecret, ka.ckx, nil
   382  }
   383
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