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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(key []byte) hash.Hash
   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 SHA-1 based constant time MAC.
   251  func macSHA1(key []byte) hash.Hash {
   252  	return hmac.New(newConstantTimeHash(sha1.New), key)
   253  }
   254  
   255  // macSHA256 returns a SHA-256 based MAC. This is only supported in TLS 1.2 and
   256  // is currently only used in disabled-by-default cipher suites.
   257  func macSHA256(key []byte) hash.Hash {
   258  	return hmac.New(sha256.New, key)
   259  }
   260  
   261  type aead interface {
   262  	cipher.AEAD
   263  
   264  	// explicitNonceLen returns the number of bytes of explicit nonce
   265  	// included in each record. This is eight for older AEADs and
   266  	// zero for modern ones.
   267  	explicitNonceLen() int
   268  }
   269  
   270  const (
   271  	aeadNonceLength   = 12
   272  	noncePrefixLength = 4
   273  )
   274  
   275  // prefixNonceAEAD wraps an AEAD and prefixes a fixed portion of the nonce to
   276  // each call.
   277  type prefixNonceAEAD struct {
   278  	// nonce contains the fixed part of the nonce in the first four bytes.
   279  	nonce [aeadNonceLength]byte
   280  	aead  cipher.AEAD
   281  }
   282  
   283  func (f *prefixNonceAEAD) NonceSize() int        { return aeadNonceLength - noncePrefixLength }
   284  func (f *prefixNonceAEAD) Overhead() int         { return f.aead.Overhead() }
   285  func (f *prefixNonceAEAD) explicitNonceLen() int { return f.NonceSize() }
   286  
   287  func (f *prefixNonceAEAD) Seal(out, nonce, plaintext, additionalData []byte) []byte {
   288  	copy(f.nonce[4:], nonce)
   289  	return f.aead.Seal(out, f.nonce[:], plaintext, additionalData)
   290  }
   291  
   292  func (f *prefixNonceAEAD) Open(out, nonce, ciphertext, additionalData []byte) ([]byte, error) {
   293  	copy(f.nonce[4:], nonce)
   294  	return f.aead.Open(out, f.nonce[:], ciphertext, additionalData)
   295  }
   296  
   297  // xoredNonceAEAD wraps an AEAD by XORing in a fixed pattern to the nonce
   298  // before each call.
   299  type xorNonceAEAD struct {
   300  	nonceMask [aeadNonceLength]byte
   301  	aead      cipher.AEAD
   302  }
   303  
   304  func (f *xorNonceAEAD) NonceSize() int        { return 8 } // 64-bit sequence number
   305  func (f *xorNonceAEAD) Overhead() int         { return f.aead.Overhead() }
   306  func (f *xorNonceAEAD) explicitNonceLen() int { return 0 }
   307  
   308  func (f *xorNonceAEAD) Seal(out, nonce, plaintext, additionalData []byte) []byte {
   309  	for i, b := range nonce {
   310  		f.nonceMask[4+i] ^= b
   311  	}
   312  	result := f.aead.Seal(out, f.nonceMask[:], plaintext, additionalData)
   313  	for i, b := range nonce {
   314  		f.nonceMask[4+i] ^= b
   315  	}
   316  
   317  	return result
   318  }
   319  
   320  func (f *xorNonceAEAD) Open(out, nonce, ciphertext, additionalData []byte) ([]byte, error) {
   321  	for i, b := range nonce {
   322  		f.nonceMask[4+i] ^= b
   323  	}
   324  	result, err := f.aead.Open(out, f.nonceMask[:], ciphertext, additionalData)
   325  	for i, b := range nonce {
   326  		f.nonceMask[4+i] ^= b
   327  	}
   328  
   329  	return result, err
   330  }
   331  
   332  func aeadAESGCM(key, noncePrefix []byte) aead {
   333  	if len(noncePrefix) != noncePrefixLength {
   334  		panic("tls: internal error: wrong nonce length")
   335  	}
   336  	aes, err := aes.NewCipher(key)
   337  	if err != nil {
   338  		panic(err)
   339  	}
   340  	aead, err := cipher.NewGCM(aes)
   341  	if err != nil {
   342  		panic(err)
   343  	}
   344  
   345  	ret := &prefixNonceAEAD{aead: aead}
   346  	copy(ret.nonce[:], noncePrefix)
   347  	return ret
   348  }
   349  
   350  func aeadAESGCMTLS13(key, nonceMask []byte) aead {
   351  	if len(nonceMask) != aeadNonceLength {
   352  		panic("tls: internal error: wrong nonce length")
   353  	}
   354  	aes, err := aes.NewCipher(key)
   355  	if err != nil {
   356  		panic(err)
   357  	}
   358  	aead, err := cipher.NewGCM(aes)
   359  	if err != nil {
   360  		panic(err)
   361  	}
   362  
   363  	ret := &xorNonceAEAD{aead: aead}
   364  	copy(ret.nonceMask[:], nonceMask)
   365  	return ret
   366  }
   367  
   368  func aeadChaCha20Poly1305(key, nonceMask []byte) aead {
   369  	if len(nonceMask) != aeadNonceLength {
   370  		panic("tls: internal error: wrong nonce length")
   371  	}
   372  	aead, err := chacha20poly1305.New(key)
   373  	if err != nil {
   374  		panic(err)
   375  	}
   376  
   377  	ret := &xorNonceAEAD{aead: aead}
   378  	copy(ret.nonceMask[:], nonceMask)
   379  	return ret
   380  }
   381  
   382  type constantTimeHash interface {
   383  	hash.Hash
   384  	ConstantTimeSum(b []byte) []byte
   385  }
   386  
   387  // cthWrapper wraps any hash.Hash that implements ConstantTimeSum, and replaces
   388  // with that all calls to Sum. It's used to obtain a ConstantTimeSum-based HMAC.
   389  type cthWrapper struct {
   390  	h constantTimeHash
   391  }
   392  
   393  func (c *cthWrapper) Size() int                   { return c.h.Size() }
   394  func (c *cthWrapper) BlockSize() int              { return c.h.BlockSize() }
   395  func (c *cthWrapper) Reset()                      { c.h.Reset() }
   396  func (c *cthWrapper) Write(p []byte) (int, error) { return c.h.Write(p) }
   397  func (c *cthWrapper) Sum(b []byte) []byte         { return c.h.ConstantTimeSum(b) }
   398  
   399  func newConstantTimeHash(h func() hash.Hash) func() hash.Hash {
   400  	return func() hash.Hash {
   401  		return &cthWrapper{h().(constantTimeHash)}
   402  	}
   403  }
   404  
   405  // tls10MAC implements the TLS 1.0 MAC function. RFC 2246, Section 6.2.3.
   406  func tls10MAC(h hash.Hash, out, seq, header, data, extra []byte) []byte {
   407  	h.Reset()
   408  	h.Write(seq)
   409  	h.Write(header)
   410  	h.Write(data)
   411  	res := h.Sum(out)
   412  	if extra != nil {
   413  		h.Write(extra)
   414  	}
   415  	return res
   416  }
   417  
   418  func rsaKA(version uint16) keyAgreement {
   419  	return rsaKeyAgreement{}
   420  }
   421  
   422  func ecdheECDSAKA(version uint16) keyAgreement {
   423  	return &ecdheKeyAgreement{
   424  		isRSA:   false,
   425  		version: version,
   426  	}
   427  }
   428  
   429  func ecdheRSAKA(version uint16) keyAgreement {
   430  	return &ecdheKeyAgreement{
   431  		isRSA:   true,
   432  		version: version,
   433  	}
   434  }
   435  
   436  // mutualCipherSuite returns a cipherSuite given a list of supported
   437  // ciphersuites and the id requested by the peer.
   438  func mutualCipherSuite(have []uint16, want uint16) *cipherSuite {
   439  	for _, id := range have {
   440  		if id == want {
   441  			return cipherSuiteByID(id)
   442  		}
   443  	}
   444  	return nil
   445  }
   446  
   447  func cipherSuiteByID(id uint16) *cipherSuite {
   448  	for _, cipherSuite := range cipherSuites {
   449  		if cipherSuite.id == id {
   450  			return cipherSuite
   451  		}
   452  	}
   453  	return nil
   454  }
   455  
   456  func mutualCipherSuiteTLS13(have []uint16, want uint16) *cipherSuiteTLS13 {
   457  	for _, id := range have {
   458  		if id == want {
   459  			return cipherSuiteTLS13ByID(id)
   460  		}
   461  	}
   462  	return nil
   463  }
   464  
   465  func cipherSuiteTLS13ByID(id uint16) *cipherSuiteTLS13 {
   466  	for _, cipherSuite := range cipherSuitesTLS13 {
   467  		if cipherSuite.id == id {
   468  			return cipherSuite
   469  		}
   470  	}
   471  	return nil
   472  }
   473  
   474  // A list of cipher suite IDs that are, or have been, implemented by this
   475  // package.
   476  //
   477  // See https://www.iana.org/assignments/tls-parameters/tls-parameters.xml
   478  const (
   479  	// TLS 1.0 - 1.2 cipher suites.
   480  	TLS_RSA_WITH_RC4_128_SHA                      uint16 = 0x0005
   481  	TLS_RSA_WITH_3DES_EDE_CBC_SHA                 uint16 = 0x000a
   482  	TLS_RSA_WITH_AES_128_CBC_SHA                  uint16 = 0x002f
   483  	TLS_RSA_WITH_AES_256_CBC_SHA                  uint16 = 0x0035
   484  	TLS_RSA_WITH_AES_128_CBC_SHA256               uint16 = 0x003c
   485  	TLS_RSA_WITH_AES_128_GCM_SHA256               uint16 = 0x009c
   486  	TLS_RSA_WITH_AES_256_GCM_SHA384               uint16 = 0x009d
   487  	TLS_ECDHE_ECDSA_WITH_RC4_128_SHA              uint16 = 0xc007
   488  	TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA          uint16 = 0xc009
   489  	TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA          uint16 = 0xc00a
   490  	TLS_ECDHE_RSA_WITH_RC4_128_SHA                uint16 = 0xc011
   491  	TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA           uint16 = 0xc012
   492  	TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA            uint16 = 0xc013
   493  	TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA            uint16 = 0xc014
   494  	TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256       uint16 = 0xc023
   495  	TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256         uint16 = 0xc027
   496  	TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256         uint16 = 0xc02f
   497  	TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256       uint16 = 0xc02b
   498  	TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384         uint16 = 0xc030
   499  	TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384       uint16 = 0xc02c
   500  	TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256   uint16 = 0xcca8
   501  	TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256 uint16 = 0xcca9
   502  
   503  	// TLS 1.3 cipher suites.
   504  	TLS_AES_128_GCM_SHA256       uint16 = 0x1301
   505  	TLS_AES_256_GCM_SHA384       uint16 = 0x1302
   506  	TLS_CHACHA20_POLY1305_SHA256 uint16 = 0x1303
   507  
   508  	// TLS_FALLBACK_SCSV isn't a standard cipher suite but an indicator
   509  	// that the client is doing version fallback. See RFC 7507.
   510  	TLS_FALLBACK_SCSV uint16 = 0x5600
   511  
   512  	// Legacy names for the corresponding cipher suites with the correct _SHA256
   513  	// suffix, retained for backward compatibility.
   514  	TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305   = TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256
   515  	TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305 = TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256
   516  )
   517  

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