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Source file src/crypto/tls/cipher_suites.go

Documentation: crypto/tls

     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/aes"
    10  	"crypto/cipher"
    11  	"crypto/des"
    12  	"crypto/hmac"
    13  	"crypto/rc4"
    14  	"crypto/sha1"
    15  	"crypto/sha256"
    16  	"crypto/x509"
    17  	"fmt"
    18  	"hash"
    19  
    20  	"golang.org/x/crypto/chacha20poly1305"
    21  )
    22  
    23  // CipherSuite is a TLS cipher suite. Note that most functions in this package
    24  // accept and expose cipher suite IDs instead of this type.
    25  type CipherSuite struct {
    26  	ID   uint16
    27  	Name string
    28  
    29  	// Supported versions is the list of TLS protocol versions that can
    30  	// negotiate this cipher suite.
    31  	SupportedVersions []uint16
    32  
    33  	// Insecure is true if the cipher suite has known security issues
    34  	// due to its primitives, design, or implementation.
    35  	Insecure bool
    36  }
    37  
    38  var (
    39  	supportedUpToTLS12 = []uint16{VersionTLS10, VersionTLS11, VersionTLS12}
    40  	supportedOnlyTLS12 = []uint16{VersionTLS12}
    41  	supportedOnlyTLS13 = []uint16{VersionTLS13}
    42  )
    43  
    44  // CipherSuites returns a list of cipher suites currently implemented by this
    45  // package, excluding those with security issues, which are returned by
    46  // InsecureCipherSuites.
    47  //
    48  // The list is sorted by ID. Note that the default cipher suites selected by
    49  // this package might depend on logic that can't be captured by a static list.
    50  func CipherSuites() []*CipherSuite {
    51  	return []*CipherSuite{
    52  		{TLS_RSA_WITH_3DES_EDE_CBC_SHA, "TLS_RSA_WITH_3DES_EDE_CBC_SHA", supportedUpToTLS12, false},
    53  		{TLS_RSA_WITH_AES_128_CBC_SHA, "TLS_RSA_WITH_AES_128_CBC_SHA", supportedUpToTLS12, false},
    54  		{TLS_RSA_WITH_AES_256_CBC_SHA, "TLS_RSA_WITH_AES_256_CBC_SHA", supportedUpToTLS12, false},
    55  		{TLS_RSA_WITH_AES_128_GCM_SHA256, "TLS_RSA_WITH_AES_128_GCM_SHA256", supportedOnlyTLS12, false},
    56  		{TLS_RSA_WITH_AES_256_GCM_SHA384, "TLS_RSA_WITH_AES_256_GCM_SHA384", supportedOnlyTLS12, false},
    57  
    58  		{TLS_AES_128_GCM_SHA256, "TLS_AES_128_GCM_SHA256", supportedOnlyTLS13, false},
    59  		{TLS_AES_256_GCM_SHA384, "TLS_AES_256_GCM_SHA384", supportedOnlyTLS13, false},
    60  		{TLS_CHACHA20_POLY1305_SHA256, "TLS_CHACHA20_POLY1305_SHA256", supportedOnlyTLS13, false},
    61  
    62  		{TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA, "TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA", supportedUpToTLS12, false},
    63  		{TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA, "TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA", supportedUpToTLS12, false},
    64  		{TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA, "TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA", supportedUpToTLS12, false},
    65  		{TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA, "TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA", supportedUpToTLS12, false},
    66  		{TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA, "TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA", supportedUpToTLS12, false},
    67  		{TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, "TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256", supportedOnlyTLS12, false},
    68  		{TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, "TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384", supportedOnlyTLS12, false},
    69  		{TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256", supportedOnlyTLS12, false},
    70  		{TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384, "TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384", supportedOnlyTLS12, false},
    71  		{TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256, "TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256", supportedOnlyTLS12, false},
    72  		{TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256, "TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256", supportedOnlyTLS12, false},
    73  	}
    74  }
    75  
    76  // InsecureCipherSuites returns a list of cipher suites currently implemented by
    77  // this package and which have security issues.
    78  //
    79  // Most applications should not use the cipher suites in this list, and should
    80  // only use those returned by CipherSuites.
    81  func InsecureCipherSuites() []*CipherSuite {
    82  	// RC4 suites are broken because RC4 is.
    83  	// CBC-SHA256 suites have no Lucky13 countermeasures.
    84  	return []*CipherSuite{
    85  		{TLS_RSA_WITH_RC4_128_SHA, "TLS_RSA_WITH_RC4_128_SHA", supportedUpToTLS12, true},
    86  		{TLS_RSA_WITH_AES_128_CBC_SHA256, "TLS_RSA_WITH_AES_128_CBC_SHA256", supportedOnlyTLS12, true},
    87  		{TLS_ECDHE_ECDSA_WITH_RC4_128_SHA, "TLS_ECDHE_ECDSA_WITH_RC4_128_SHA", supportedUpToTLS12, true},
    88  		{TLS_ECDHE_RSA_WITH_RC4_128_SHA, "TLS_ECDHE_RSA_WITH_RC4_128_SHA", supportedUpToTLS12, true},
    89  		{TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256, "TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256", supportedOnlyTLS12, true},
    90  		{TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256, "TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256", supportedOnlyTLS12, true},
    91  	}
    92  }
    93  
    94  // CipherSuiteName returns the standard name for the passed cipher suite ID
    95  // (e.g. "TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256"), or a fallback representation
    96  // of the ID value if the cipher suite is not implemented by this package.
    97  func CipherSuiteName(id uint16) string {
    98  	for _, c := range CipherSuites() {
    99  		if c.ID == id {
   100  			return c.Name
   101  		}
   102  	}
   103  	for _, c := range InsecureCipherSuites() {
   104  		if c.ID == id {
   105  			return c.Name
   106  		}
   107  	}
   108  	return fmt.Sprintf("0x%04X", id)
   109  }
   110  
   111  // a keyAgreement implements the client and server side of a TLS key agreement
   112  // protocol by generating and processing key exchange messages.
   113  type keyAgreement interface {
   114  	// On the server side, the first two methods are called in order.
   115  
   116  	// In the case that the key agreement protocol doesn't use a
   117  	// ServerKeyExchange message, generateServerKeyExchange can return nil,
   118  	// nil.
   119  	generateServerKeyExchange(*Config, *Certificate, *clientHelloMsg, *serverHelloMsg) (*serverKeyExchangeMsg, error)
   120  	processClientKeyExchange(*Config, *Certificate, *clientKeyExchangeMsg, uint16) ([]byte, error)
   121  
   122  	// On the client side, the next two methods are called in order.
   123  
   124  	// This method may not be called if the server doesn't send a
   125  	// ServerKeyExchange message.
   126  	processServerKeyExchange(*Config, *clientHelloMsg, *serverHelloMsg, *x509.Certificate, *serverKeyExchangeMsg) error
   127  	generateClientKeyExchange(*Config, *clientHelloMsg, *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error)
   128  }
   129  
   130  const (
   131  	// suiteECDHE indicates that the cipher suite involves elliptic curve
   132  	// Diffie-Hellman. This means that it should only be selected when the
   133  	// client indicates that it supports ECC with a curve and point format
   134  	// that we're happy with.
   135  	suiteECDHE = 1 << iota
   136  	// suiteECSign indicates that the cipher suite involves an ECDSA or
   137  	// EdDSA signature and therefore may only be selected when the server's
   138  	// certificate is ECDSA or EdDSA. If this is not set then the cipher suite
   139  	// is RSA based.
   140  	suiteECSign
   141  	// suiteTLS12 indicates that the cipher suite should only be advertised
   142  	// and accepted when using TLS 1.2.
   143  	suiteTLS12
   144  	// suiteSHA384 indicates that the cipher suite uses SHA384 as the
   145  	// handshake hash.
   146  	suiteSHA384
   147  	// suiteDefaultOff indicates that this cipher suite is not included by
   148  	// default.
   149  	suiteDefaultOff
   150  )
   151  
   152  // A cipherSuite is a specific combination of key agreement, cipher and MAC function.
   153  type cipherSuite struct {
   154  	id uint16
   155  	// the lengths, in bytes, of the key material needed for each component.
   156  	keyLen int
   157  	macLen int
   158  	ivLen  int
   159  	ka     func(version uint16) keyAgreement
   160  	// flags is a bitmask of the suite* values, above.
   161  	flags  int
   162  	cipher func(key, iv []byte, isRead bool) interface{}
   163  	mac    func(version uint16, macKey []byte) macFunction
   164  	aead   func(key, fixedNonce []byte) aead
   165  }
   166  
   167  var cipherSuites = []*cipherSuite{
   168  	// Ciphersuite order is chosen so that ECDHE comes before plain RSA and
   169  	// AEADs are the top preference.
   170  	{TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305, 32, 0, 12, ecdheRSAKA, suiteECDHE | suiteTLS12, nil, nil, aeadChaCha20Poly1305},
   171  	{TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305, 32, 0, 12, ecdheECDSAKA, suiteECDHE | suiteECSign | suiteTLS12, nil, nil, aeadChaCha20Poly1305},
   172  	{TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, 16, 0, 4, ecdheRSAKA, suiteECDHE | suiteTLS12, nil, nil, aeadAESGCM},
   173  	{TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, 16, 0, 4, ecdheECDSAKA, suiteECDHE | suiteECSign | suiteTLS12, nil, nil, aeadAESGCM},
   174  	{TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384, 32, 0, 4, ecdheRSAKA, suiteECDHE | suiteTLS12 | suiteSHA384, nil, nil, aeadAESGCM},
   175  	{TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, 32, 0, 4, ecdheECDSAKA, suiteECDHE | suiteECSign | suiteTLS12 | suiteSHA384, nil, nil, aeadAESGCM},
   176  	{TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, ecdheRSAKA, suiteECDHE | suiteTLS12 | suiteDefaultOff, cipherAES, macSHA256, nil},
   177  	{TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA, 16, 20, 16, ecdheRSAKA, suiteECDHE, cipherAES, macSHA1, nil},
   178  	{TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, ecdheECDSAKA, suiteECDHE | suiteECSign | suiteTLS12 | suiteDefaultOff, cipherAES, macSHA256, nil},
   179  	{TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA, 16, 20, 16, ecdheECDSAKA, suiteECDHE | suiteECSign, cipherAES, macSHA1, nil},
   180  	{TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA, 32, 20, 16, ecdheRSAKA, suiteECDHE, cipherAES, macSHA1, nil},
   181  	{TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA, 32, 20, 16, ecdheECDSAKA, suiteECDHE | suiteECSign, cipherAES, macSHA1, nil},
   182  	{TLS_RSA_WITH_AES_128_GCM_SHA256, 16, 0, 4, rsaKA, suiteTLS12, nil, nil, aeadAESGCM},
   183  	{TLS_RSA_WITH_AES_256_GCM_SHA384, 32, 0, 4, rsaKA, suiteTLS12 | suiteSHA384, nil, nil, aeadAESGCM},
   184  	{TLS_RSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, rsaKA, suiteTLS12 | suiteDefaultOff, cipherAES, macSHA256, nil},
   185  	{TLS_RSA_WITH_AES_128_CBC_SHA, 16, 20, 16, rsaKA, 0, cipherAES, macSHA1, nil},
   186  	{TLS_RSA_WITH_AES_256_CBC_SHA, 32, 20, 16, rsaKA, 0, cipherAES, macSHA1, nil},
   187  	{TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA, 24, 20, 8, ecdheRSAKA, suiteECDHE, cipher3DES, macSHA1, nil},
   188  	{TLS_RSA_WITH_3DES_EDE_CBC_SHA, 24, 20, 8, rsaKA, 0, cipher3DES, macSHA1, nil},
   189  
   190  	// RC4-based cipher suites are disabled by default.
   191  	{TLS_RSA_WITH_RC4_128_SHA, 16, 20, 0, rsaKA, suiteDefaultOff, cipherRC4, macSHA1, nil},
   192  	{TLS_ECDHE_RSA_WITH_RC4_128_SHA, 16, 20, 0, ecdheRSAKA, suiteECDHE | suiteDefaultOff, cipherRC4, macSHA1, nil},
   193  	{TLS_ECDHE_ECDSA_WITH_RC4_128_SHA, 16, 20, 0, ecdheECDSAKA, suiteECDHE | suiteECSign | suiteDefaultOff, cipherRC4, macSHA1, nil},
   194  }
   195  
   196  // selectCipherSuite returns the first cipher suite from ids which is also in
   197  // supportedIDs and passes the ok filter.
   198  func selectCipherSuite(ids, supportedIDs []uint16, ok func(*cipherSuite) bool) *cipherSuite {
   199  	for _, id := range ids {
   200  		candidate := cipherSuiteByID(id)
   201  		if candidate == nil || !ok(candidate) {
   202  			continue
   203  		}
   204  
   205  		for _, suppID := range supportedIDs {
   206  			if id == suppID {
   207  				return candidate
   208  			}
   209  		}
   210  	}
   211  	return nil
   212  }
   213  
   214  // A cipherSuiteTLS13 defines only the pair of the AEAD algorithm and hash
   215  // algorithm to be used with HKDF. See RFC 8446, Appendix B.4.
   216  type cipherSuiteTLS13 struct {
   217  	id     uint16
   218  	keyLen int
   219  	aead   func(key, fixedNonce []byte) aead
   220  	hash   crypto.Hash
   221  }
   222  
   223  var cipherSuitesTLS13 = []*cipherSuiteTLS13{
   224  	{TLS_AES_128_GCM_SHA256, 16, aeadAESGCMTLS13, crypto.SHA256},
   225  	{TLS_CHACHA20_POLY1305_SHA256, 32, aeadChaCha20Poly1305, crypto.SHA256},
   226  	{TLS_AES_256_GCM_SHA384, 32, aeadAESGCMTLS13, crypto.SHA384},
   227  }
   228  
   229  func cipherRC4(key, iv []byte, isRead bool) interface{} {
   230  	cipher, _ := rc4.NewCipher(key)
   231  	return cipher
   232  }
   233  
   234  func cipher3DES(key, iv []byte, isRead bool) interface{} {
   235  	block, _ := des.NewTripleDESCipher(key)
   236  	if isRead {
   237  		return cipher.NewCBCDecrypter(block, iv)
   238  	}
   239  	return cipher.NewCBCEncrypter(block, iv)
   240  }
   241  
   242  func cipherAES(key, iv []byte, isRead bool) interface{} {
   243  	block, _ := aes.NewCipher(key)
   244  	if isRead {
   245  		return cipher.NewCBCDecrypter(block, iv)
   246  	}
   247  	return cipher.NewCBCEncrypter(block, iv)
   248  }
   249  
   250  // macSHA1 returns a macFunction for the given protocol version.
   251  func macSHA1(version uint16, key []byte) macFunction {
   252  	return tls10MAC{h: hmac.New(newConstantTimeHash(sha1.New), key)}
   253  }
   254  
   255  // macSHA256 returns a SHA-256 based MAC. These are only supported in TLS 1.2
   256  // so the given version is ignored.
   257  func macSHA256(version uint16, key []byte) macFunction {
   258  	return tls10MAC{h: hmac.New(sha256.New, key)}
   259  }
   260  
   261  type macFunction interface {
   262  	// Size returns the length of the MAC.
   263  	Size() int
   264  	// MAC appends the MAC of (seq, header, data) to out. The extra data is fed
   265  	// into the MAC after obtaining the result to normalize timing. The result
   266  	// is only valid until the next invocation of MAC as the buffer is reused.
   267  	MAC(seq, header, data, extra []byte) []byte
   268  }
   269  
   270  type aead interface {
   271  	cipher.AEAD
   272  
   273  	// explicitNonceLen returns the number of bytes of explicit nonce
   274  	// included in each record. This is eight for older AEADs and
   275  	// zero for modern ones.
   276  	explicitNonceLen() int
   277  }
   278  
   279  const (
   280  	aeadNonceLength   = 12
   281  	noncePrefixLength = 4
   282  )
   283  
   284  // prefixNonceAEAD wraps an AEAD and prefixes a fixed portion of the nonce to
   285  // each call.
   286  type prefixNonceAEAD struct {
   287  	// nonce contains the fixed part of the nonce in the first four bytes.
   288  	nonce [aeadNonceLength]byte
   289  	aead  cipher.AEAD
   290  }
   291  
   292  func (f *prefixNonceAEAD) NonceSize() int        { return aeadNonceLength - noncePrefixLength }
   293  func (f *prefixNonceAEAD) Overhead() int         { return f.aead.Overhead() }
   294  func (f *prefixNonceAEAD) explicitNonceLen() int { return f.NonceSize() }
   295  
   296  func (f *prefixNonceAEAD) Seal(out, nonce, plaintext, additionalData []byte) []byte {
   297  	copy(f.nonce[4:], nonce)
   298  	return f.aead.Seal(out, f.nonce[:], plaintext, additionalData)
   299  }
   300  
   301  func (f *prefixNonceAEAD) Open(out, nonce, ciphertext, additionalData []byte) ([]byte, error) {
   302  	copy(f.nonce[4:], nonce)
   303  	return f.aead.Open(out, f.nonce[:], ciphertext, additionalData)
   304  }
   305  
   306  // xoredNonceAEAD wraps an AEAD by XORing in a fixed pattern to the nonce
   307  // before each call.
   308  type xorNonceAEAD struct {
   309  	nonceMask [aeadNonceLength]byte
   310  	aead      cipher.AEAD
   311  }
   312  
   313  func (f *xorNonceAEAD) NonceSize() int        { return 8 } // 64-bit sequence number
   314  func (f *xorNonceAEAD) Overhead() int         { return f.aead.Overhead() }
   315  func (f *xorNonceAEAD) explicitNonceLen() int { return 0 }
   316  
   317  func (f *xorNonceAEAD) Seal(out, nonce, plaintext, additionalData []byte) []byte {
   318  	for i, b := range nonce {
   319  		f.nonceMask[4+i] ^= b
   320  	}
   321  	result := f.aead.Seal(out, f.nonceMask[:], plaintext, additionalData)
   322  	for i, b := range nonce {
   323  		f.nonceMask[4+i] ^= b
   324  	}
   325  
   326  	return result
   327  }
   328  
   329  func (f *xorNonceAEAD) Open(out, nonce, ciphertext, additionalData []byte) ([]byte, error) {
   330  	for i, b := range nonce {
   331  		f.nonceMask[4+i] ^= b
   332  	}
   333  	result, err := f.aead.Open(out, f.nonceMask[:], ciphertext, additionalData)
   334  	for i, b := range nonce {
   335  		f.nonceMask[4+i] ^= b
   336  	}
   337  
   338  	return result, err
   339  }
   340  
   341  func aeadAESGCM(key, noncePrefix []byte) aead {
   342  	if len(noncePrefix) != noncePrefixLength {
   343  		panic("tls: internal error: wrong nonce length")
   344  	}
   345  	aes, err := aes.NewCipher(key)
   346  	if err != nil {
   347  		panic(err)
   348  	}
   349  	aead, err := cipher.NewGCM(aes)
   350  	if err != nil {
   351  		panic(err)
   352  	}
   353  
   354  	ret := &prefixNonceAEAD{aead: aead}
   355  	copy(ret.nonce[:], noncePrefix)
   356  	return ret
   357  }
   358  
   359  func aeadAESGCMTLS13(key, nonceMask []byte) aead {
   360  	if len(nonceMask) != aeadNonceLength {
   361  		panic("tls: internal error: wrong nonce length")
   362  	}
   363  	aes, err := aes.NewCipher(key)
   364  	if err != nil {
   365  		panic(err)
   366  	}
   367  	aead, err := cipher.NewGCM(aes)
   368  	if err != nil {
   369  		panic(err)
   370  	}
   371  
   372  	ret := &xorNonceAEAD{aead: aead}
   373  	copy(ret.nonceMask[:], nonceMask)
   374  	return ret
   375  }
   376  
   377  func aeadChaCha20Poly1305(key, nonceMask []byte) aead {
   378  	if len(nonceMask) != aeadNonceLength {
   379  		panic("tls: internal error: wrong nonce length")
   380  	}
   381  	aead, err := chacha20poly1305.New(key)
   382  	if err != nil {
   383  		panic(err)
   384  	}
   385  
   386  	ret := &xorNonceAEAD{aead: aead}
   387  	copy(ret.nonceMask[:], nonceMask)
   388  	return ret
   389  }
   390  
   391  type constantTimeHash interface {
   392  	hash.Hash
   393  	ConstantTimeSum(b []byte) []byte
   394  }
   395  
   396  // cthWrapper wraps any hash.Hash that implements ConstantTimeSum, and replaces
   397  // with that all calls to Sum. It's used to obtain a ConstantTimeSum-based HMAC.
   398  type cthWrapper struct {
   399  	h constantTimeHash
   400  }
   401  
   402  func (c *cthWrapper) Size() int                   { return c.h.Size() }
   403  func (c *cthWrapper) BlockSize() int              { return c.h.BlockSize() }
   404  func (c *cthWrapper) Reset()                      { c.h.Reset() }
   405  func (c *cthWrapper) Write(p []byte) (int, error) { return c.h.Write(p) }
   406  func (c *cthWrapper) Sum(b []byte) []byte         { return c.h.ConstantTimeSum(b) }
   407  
   408  func newConstantTimeHash(h func() hash.Hash) func() hash.Hash {
   409  	return func() hash.Hash {
   410  		return &cthWrapper{h().(constantTimeHash)}
   411  	}
   412  }
   413  
   414  // tls10MAC implements the TLS 1.0 MAC function. RFC 2246, Section 6.2.3.
   415  type tls10MAC struct {
   416  	h   hash.Hash
   417  	buf []byte
   418  }
   419  
   420  func (s tls10MAC) Size() int {
   421  	return s.h.Size()
   422  }
   423  
   424  // MAC is guaranteed to take constant time, as long as
   425  // len(seq)+len(header)+len(data)+len(extra) is constant. extra is not fed into
   426  // the MAC, but is only provided to make the timing profile constant.
   427  func (s tls10MAC) MAC(seq, header, data, extra []byte) []byte {
   428  	s.h.Reset()
   429  	s.h.Write(seq)
   430  	s.h.Write(header)
   431  	s.h.Write(data)
   432  	res := s.h.Sum(s.buf[:0])
   433  	if extra != nil {
   434  		s.h.Write(extra)
   435  	}
   436  	return res
   437  }
   438  
   439  func rsaKA(version uint16) keyAgreement {
   440  	return rsaKeyAgreement{}
   441  }
   442  
   443  func ecdheECDSAKA(version uint16) keyAgreement {
   444  	return &ecdheKeyAgreement{
   445  		isRSA:   false,
   446  		version: version,
   447  	}
   448  }
   449  
   450  func ecdheRSAKA(version uint16) keyAgreement {
   451  	return &ecdheKeyAgreement{
   452  		isRSA:   true,
   453  		version: version,
   454  	}
   455  }
   456  
   457  // mutualCipherSuite returns a cipherSuite given a list of supported
   458  // ciphersuites and the id requested by the peer.
   459  func mutualCipherSuite(have []uint16, want uint16) *cipherSuite {
   460  	for _, id := range have {
   461  		if id == want {
   462  			return cipherSuiteByID(id)
   463  		}
   464  	}
   465  	return nil
   466  }
   467  
   468  func cipherSuiteByID(id uint16) *cipherSuite {
   469  	for _, cipherSuite := range cipherSuites {
   470  		if cipherSuite.id == id {
   471  			return cipherSuite
   472  		}
   473  	}
   474  	return nil
   475  }
   476  
   477  func mutualCipherSuiteTLS13(have []uint16, want uint16) *cipherSuiteTLS13 {
   478  	for _, id := range have {
   479  		if id == want {
   480  			return cipherSuiteTLS13ByID(id)
   481  		}
   482  	}
   483  	return nil
   484  }
   485  
   486  func cipherSuiteTLS13ByID(id uint16) *cipherSuiteTLS13 {
   487  	for _, cipherSuite := range cipherSuitesTLS13 {
   488  		if cipherSuite.id == id {
   489  			return cipherSuite
   490  		}
   491  	}
   492  	return nil
   493  }
   494  
   495  // A list of cipher suite IDs that are, or have been, implemented by this
   496  // package.
   497  //
   498  // See https://www.iana.org/assignments/tls-parameters/tls-parameters.xml
   499  const (
   500  	// TLS 1.0 - 1.2 cipher suites.
   501  	TLS_RSA_WITH_RC4_128_SHA                      uint16 = 0x0005
   502  	TLS_RSA_WITH_3DES_EDE_CBC_SHA                 uint16 = 0x000a
   503  	TLS_RSA_WITH_AES_128_CBC_SHA                  uint16 = 0x002f
   504  	TLS_RSA_WITH_AES_256_CBC_SHA                  uint16 = 0x0035
   505  	TLS_RSA_WITH_AES_128_CBC_SHA256               uint16 = 0x003c
   506  	TLS_RSA_WITH_AES_128_GCM_SHA256               uint16 = 0x009c
   507  	TLS_RSA_WITH_AES_256_GCM_SHA384               uint16 = 0x009d
   508  	TLS_ECDHE_ECDSA_WITH_RC4_128_SHA              uint16 = 0xc007
   509  	TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA          uint16 = 0xc009
   510  	TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA          uint16 = 0xc00a
   511  	TLS_ECDHE_RSA_WITH_RC4_128_SHA                uint16 = 0xc011
   512  	TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA           uint16 = 0xc012
   513  	TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA            uint16 = 0xc013
   514  	TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA            uint16 = 0xc014
   515  	TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256       uint16 = 0xc023
   516  	TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256         uint16 = 0xc027
   517  	TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256         uint16 = 0xc02f
   518  	TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256       uint16 = 0xc02b
   519  	TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384         uint16 = 0xc030
   520  	TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384       uint16 = 0xc02c
   521  	TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256   uint16 = 0xcca8
   522  	TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256 uint16 = 0xcca9
   523  
   524  	// TLS 1.3 cipher suites.
   525  	TLS_AES_128_GCM_SHA256       uint16 = 0x1301
   526  	TLS_AES_256_GCM_SHA384       uint16 = 0x1302
   527  	TLS_CHACHA20_POLY1305_SHA256 uint16 = 0x1303
   528  
   529  	// TLS_FALLBACK_SCSV isn't a standard cipher suite but an indicator
   530  	// that the client is doing version fallback. See RFC 7507.
   531  	TLS_FALLBACK_SCSV uint16 = 0x5600
   532  
   533  	// Legacy names for the corresponding cipher suites with the correct _SHA256
   534  	// suffix, retained for backward compatibility.
   535  	TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305   = TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256
   536  	TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305 = TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256
   537  )
   538  

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