Source file src/pkg/crypto/tls/cipher_suites.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/aes"
    10		"crypto/cipher"
    11		"crypto/des"
    12		"crypto/hmac"
    13		"crypto/rc4"
    14		"crypto/sha1"
    15		"crypto/sha256"
    16		"crypto/x509"
    17		"golang.org/x/crypto/chacha20poly1305"
    18		"hash"
    19	)
    20	
    21	// a keyAgreement implements the client and server side of a TLS key agreement
    22	// protocol by generating and processing key exchange messages.
    23	type keyAgreement interface {
    24		// On the server side, the first two methods are called in order.
    25	
    26		// In the case that the key agreement protocol doesn't use a
    27		// ServerKeyExchange message, generateServerKeyExchange can return nil,
    28		// nil.
    29		generateServerKeyExchange(*Config, *Certificate, *clientHelloMsg, *serverHelloMsg) (*serverKeyExchangeMsg, error)
    30		processClientKeyExchange(*Config, *Certificate, *clientKeyExchangeMsg, uint16) ([]byte, error)
    31	
    32		// On the client side, the next two methods are called in order.
    33	
    34		// This method may not be called if the server doesn't send a
    35		// ServerKeyExchange message.
    36		processServerKeyExchange(*Config, *clientHelloMsg, *serverHelloMsg, *x509.Certificate, *serverKeyExchangeMsg) error
    37		generateClientKeyExchange(*Config, *clientHelloMsg, *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error)
    38	}
    39	
    40	const (
    41		// suiteECDH indicates that the cipher suite involves elliptic curve
    42		// Diffie-Hellman. This means that it should only be selected when the
    43		// client indicates that it supports ECC with a curve and point format
    44		// that we're happy with.
    45		suiteECDHE = 1 << iota
    46		// suiteECSign indicates that the cipher suite involves an ECDSA or
    47		// EdDSA signature and therefore may only be selected when the server's
    48		// certificate is ECDSA or EdDSA. If this is not set then the cipher suite
    49		// is RSA based.
    50		suiteECSign
    51		// suiteTLS12 indicates that the cipher suite should only be advertised
    52		// and accepted when using TLS 1.2.
    53		suiteTLS12
    54		// suiteSHA384 indicates that the cipher suite uses SHA384 as the
    55		// handshake hash.
    56		suiteSHA384
    57		// suiteDefaultOff indicates that this cipher suite is not included by
    58		// default.
    59		suiteDefaultOff
    60	)
    61	
    62	// A cipherSuite is a specific combination of key agreement, cipher and MAC function.
    63	type cipherSuite struct {
    64		id uint16
    65		// the lengths, in bytes, of the key material needed for each component.
    66		keyLen int
    67		macLen int
    68		ivLen  int
    69		ka     func(version uint16) keyAgreement
    70		// flags is a bitmask of the suite* values, above.
    71		flags  int
    72		cipher func(key, iv []byte, isRead bool) interface{}
    73		mac    func(version uint16, macKey []byte) macFunction
    74		aead   func(key, fixedNonce []byte) aead
    75	}
    76	
    77	var cipherSuites = []*cipherSuite{
    78		// Ciphersuite order is chosen so that ECDHE comes before plain RSA and
    79		// AEADs are the top preference.
    80		{TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305, 32, 0, 12, ecdheRSAKA, suiteECDHE | suiteTLS12, nil, nil, aeadChaCha20Poly1305},
    81		{TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305, 32, 0, 12, ecdheECDSAKA, suiteECDHE | suiteECSign | suiteTLS12, nil, nil, aeadChaCha20Poly1305},
    82		{TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, 16, 0, 4, ecdheRSAKA, suiteECDHE | suiteTLS12, nil, nil, aeadAESGCM},
    83		{TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, 16, 0, 4, ecdheECDSAKA, suiteECDHE | suiteECSign | suiteTLS12, nil, nil, aeadAESGCM},
    84		{TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384, 32, 0, 4, ecdheRSAKA, suiteECDHE | suiteTLS12 | suiteSHA384, nil, nil, aeadAESGCM},
    85		{TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, 32, 0, 4, ecdheECDSAKA, suiteECDHE | suiteECSign | suiteTLS12 | suiteSHA384, nil, nil, aeadAESGCM},
    86		{TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, ecdheRSAKA, suiteECDHE | suiteTLS12 | suiteDefaultOff, cipherAES, macSHA256, nil},
    87		{TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA, 16, 20, 16, ecdheRSAKA, suiteECDHE, cipherAES, macSHA1, nil},
    88		{TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, ecdheECDSAKA, suiteECDHE | suiteECSign | suiteTLS12 | suiteDefaultOff, cipherAES, macSHA256, nil},
    89		{TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA, 16, 20, 16, ecdheECDSAKA, suiteECDHE | suiteECSign, cipherAES, macSHA1, nil},
    90		{TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA, 32, 20, 16, ecdheRSAKA, suiteECDHE, cipherAES, macSHA1, nil},
    91		{TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA, 32, 20, 16, ecdheECDSAKA, suiteECDHE | suiteECSign, cipherAES, macSHA1, nil},
    92		{TLS_RSA_WITH_AES_128_GCM_SHA256, 16, 0, 4, rsaKA, suiteTLS12, nil, nil, aeadAESGCM},
    93		{TLS_RSA_WITH_AES_256_GCM_SHA384, 32, 0, 4, rsaKA, suiteTLS12 | suiteSHA384, nil, nil, aeadAESGCM},
    94		{TLS_RSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, rsaKA, suiteTLS12 | suiteDefaultOff, cipherAES, macSHA256, nil},
    95		{TLS_RSA_WITH_AES_128_CBC_SHA, 16, 20, 16, rsaKA, 0, cipherAES, macSHA1, nil},
    96		{TLS_RSA_WITH_AES_256_CBC_SHA, 32, 20, 16, rsaKA, 0, cipherAES, macSHA1, nil},
    97		{TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA, 24, 20, 8, ecdheRSAKA, suiteECDHE, cipher3DES, macSHA1, nil},
    98		{TLS_RSA_WITH_3DES_EDE_CBC_SHA, 24, 20, 8, rsaKA, 0, cipher3DES, macSHA1, nil},
    99	
   100		// RC4-based cipher suites are disabled by default.
   101		{TLS_RSA_WITH_RC4_128_SHA, 16, 20, 0, rsaKA, suiteDefaultOff, cipherRC4, macSHA1, nil},
   102		{TLS_ECDHE_RSA_WITH_RC4_128_SHA, 16, 20, 0, ecdheRSAKA, suiteECDHE | suiteDefaultOff, cipherRC4, macSHA1, nil},
   103		{TLS_ECDHE_ECDSA_WITH_RC4_128_SHA, 16, 20, 0, ecdheECDSAKA, suiteECDHE | suiteECSign | suiteDefaultOff, cipherRC4, macSHA1, nil},
   104	}
   105	
   106	// A cipherSuiteTLS13 defines only the pair of the AEAD algorithm and hash
   107	// algorithm to be used with HKDF. See RFC 8446, Appendix B.4.
   108	type cipherSuiteTLS13 struct {
   109		id     uint16
   110		keyLen int
   111		aead   func(key, fixedNonce []byte) aead
   112		hash   crypto.Hash
   113	}
   114	
   115	var cipherSuitesTLS13 = []*cipherSuiteTLS13{
   116		{TLS_AES_128_GCM_SHA256, 16, aeadAESGCMTLS13, crypto.SHA256},
   117		{TLS_CHACHA20_POLY1305_SHA256, 32, aeadChaCha20Poly1305, crypto.SHA256},
   118		{TLS_AES_256_GCM_SHA384, 32, aeadAESGCMTLS13, crypto.SHA384},
   119	}
   120	
   121	func cipherRC4(key, iv []byte, isRead bool) interface{} {
   122		cipher, _ := rc4.NewCipher(key)
   123		return cipher
   124	}
   125	
   126	func cipher3DES(key, iv []byte, isRead bool) interface{} {
   127		block, _ := des.NewTripleDESCipher(key)
   128		if isRead {
   129			return cipher.NewCBCDecrypter(block, iv)
   130		}
   131		return cipher.NewCBCEncrypter(block, iv)
   132	}
   133	
   134	func cipherAES(key, iv []byte, isRead bool) interface{} {
   135		block, _ := aes.NewCipher(key)
   136		if isRead {
   137			return cipher.NewCBCDecrypter(block, iv)
   138		}
   139		return cipher.NewCBCEncrypter(block, iv)
   140	}
   141	
   142	// macSHA1 returns a macFunction for the given protocol version.
   143	func macSHA1(version uint16, key []byte) macFunction {
   144		if version == VersionSSL30 {
   145			mac := ssl30MAC{
   146				h:   sha1.New(),
   147				key: make([]byte, len(key)),
   148			}
   149			copy(mac.key, key)
   150			return mac
   151		}
   152		return tls10MAC{h: hmac.New(newConstantTimeHash(sha1.New), key)}
   153	}
   154	
   155	// macSHA256 returns a SHA-256 based MAC. These are only supported in TLS 1.2
   156	// so the given version is ignored.
   157	func macSHA256(version uint16, key []byte) macFunction {
   158		return tls10MAC{h: hmac.New(sha256.New, key)}
   159	}
   160	
   161	type macFunction interface {
   162		// Size returns the length of the MAC.
   163		Size() int
   164		// MAC appends the MAC of (seq, header, data) to out. The extra data is fed
   165		// into the MAC after obtaining the result to normalize timing. The result
   166		// is only valid until the next invocation of MAC as the buffer is reused.
   167		MAC(seq, header, data, extra []byte) []byte
   168	}
   169	
   170	type aead interface {
   171		cipher.AEAD
   172	
   173		// explicitNonceLen returns the number of bytes of explicit nonce
   174		// included in each record. This is eight for older AEADs and
   175		// zero for modern ones.
   176		explicitNonceLen() int
   177	}
   178	
   179	const (
   180		aeadNonceLength   = 12
   181		noncePrefixLength = 4
   182	)
   183	
   184	// prefixNonceAEAD wraps an AEAD and prefixes a fixed portion of the nonce to
   185	// each call.
   186	type prefixNonceAEAD struct {
   187		// nonce contains the fixed part of the nonce in the first four bytes.
   188		nonce [aeadNonceLength]byte
   189		aead  cipher.AEAD
   190	}
   191	
   192	func (f *prefixNonceAEAD) NonceSize() int        { return aeadNonceLength - noncePrefixLength }
   193	func (f *prefixNonceAEAD) Overhead() int         { return f.aead.Overhead() }
   194	func (f *prefixNonceAEAD) explicitNonceLen() int { return f.NonceSize() }
   195	
   196	func (f *prefixNonceAEAD) Seal(out, nonce, plaintext, additionalData []byte) []byte {
   197		copy(f.nonce[4:], nonce)
   198		return f.aead.Seal(out, f.nonce[:], plaintext, additionalData)
   199	}
   200	
   201	func (f *prefixNonceAEAD) Open(out, nonce, ciphertext, additionalData []byte) ([]byte, error) {
   202		copy(f.nonce[4:], nonce)
   203		return f.aead.Open(out, f.nonce[:], ciphertext, additionalData)
   204	}
   205	
   206	// xoredNonceAEAD wraps an AEAD by XORing in a fixed pattern to the nonce
   207	// before each call.
   208	type xorNonceAEAD struct {
   209		nonceMask [aeadNonceLength]byte
   210		aead      cipher.AEAD
   211	}
   212	
   213	func (f *xorNonceAEAD) NonceSize() int        { return 8 } // 64-bit sequence number
   214	func (f *xorNonceAEAD) Overhead() int         { return f.aead.Overhead() }
   215	func (f *xorNonceAEAD) explicitNonceLen() int { return 0 }
   216	
   217	func (f *xorNonceAEAD) Seal(out, nonce, plaintext, additionalData []byte) []byte {
   218		for i, b := range nonce {
   219			f.nonceMask[4+i] ^= b
   220		}
   221		result := f.aead.Seal(out, f.nonceMask[:], plaintext, additionalData)
   222		for i, b := range nonce {
   223			f.nonceMask[4+i] ^= b
   224		}
   225	
   226		return result
   227	}
   228	
   229	func (f *xorNonceAEAD) Open(out, nonce, ciphertext, additionalData []byte) ([]byte, error) {
   230		for i, b := range nonce {
   231			f.nonceMask[4+i] ^= b
   232		}
   233		result, err := f.aead.Open(out, f.nonceMask[:], ciphertext, additionalData)
   234		for i, b := range nonce {
   235			f.nonceMask[4+i] ^= b
   236		}
   237	
   238		return result, err
   239	}
   240	
   241	func aeadAESGCM(key, noncePrefix []byte) aead {
   242		if len(noncePrefix) != noncePrefixLength {
   243			panic("tls: internal error: wrong nonce length")
   244		}
   245		aes, err := aes.NewCipher(key)
   246		if err != nil {
   247			panic(err)
   248		}
   249		aead, err := cipher.NewGCM(aes)
   250		if err != nil {
   251			panic(err)
   252		}
   253	
   254		ret := &prefixNonceAEAD{aead: aead}
   255		copy(ret.nonce[:], noncePrefix)
   256		return ret
   257	}
   258	
   259	func aeadAESGCMTLS13(key, nonceMask []byte) aead {
   260		if len(nonceMask) != aeadNonceLength {
   261			panic("tls: internal error: wrong nonce length")
   262		}
   263		aes, err := aes.NewCipher(key)
   264		if err != nil {
   265			panic(err)
   266		}
   267		aead, err := cipher.NewGCM(aes)
   268		if err != nil {
   269			panic(err)
   270		}
   271	
   272		ret := &xorNonceAEAD{aead: aead}
   273		copy(ret.nonceMask[:], nonceMask)
   274		return ret
   275	}
   276	
   277	func aeadChaCha20Poly1305(key, nonceMask []byte) aead {
   278		if len(nonceMask) != aeadNonceLength {
   279			panic("tls: internal error: wrong nonce length")
   280		}
   281		aead, err := chacha20poly1305.New(key)
   282		if err != nil {
   283			panic(err)
   284		}
   285	
   286		ret := &xorNonceAEAD{aead: aead}
   287		copy(ret.nonceMask[:], nonceMask)
   288		return ret
   289	}
   290	
   291	// ssl30MAC implements the SSLv3 MAC function, as defined in
   292	// www.mozilla.org/projects/security/pki/nss/ssl/draft302.txt section 5.2.3.1
   293	type ssl30MAC struct {
   294		h   hash.Hash
   295		key []byte
   296		buf []byte
   297	}
   298	
   299	func (s ssl30MAC) Size() int {
   300		return s.h.Size()
   301	}
   302	
   303	var ssl30Pad1 = [48]byte{0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36}
   304	
   305	var ssl30Pad2 = [48]byte{0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c}
   306	
   307	// MAC does not offer constant timing guarantees for SSL v3.0, since it's deemed
   308	// useless considering the similar, protocol-level POODLE vulnerability.
   309	func (s ssl30MAC) MAC(seq, header, data, extra []byte) []byte {
   310		padLength := 48
   311		if s.h.Size() == 20 {
   312			padLength = 40
   313		}
   314	
   315		s.h.Reset()
   316		s.h.Write(s.key)
   317		s.h.Write(ssl30Pad1[:padLength])
   318		s.h.Write(seq)
   319		s.h.Write(header[:1])
   320		s.h.Write(header[3:5])
   321		s.h.Write(data)
   322		s.buf = s.h.Sum(s.buf[:0])
   323	
   324		s.h.Reset()
   325		s.h.Write(s.key)
   326		s.h.Write(ssl30Pad2[:padLength])
   327		s.h.Write(s.buf)
   328		return s.h.Sum(s.buf[:0])
   329	}
   330	
   331	type constantTimeHash interface {
   332		hash.Hash
   333		ConstantTimeSum(b []byte) []byte
   334	}
   335	
   336	// cthWrapper wraps any hash.Hash that implements ConstantTimeSum, and replaces
   337	// with that all calls to Sum. It's used to obtain a ConstantTimeSum-based HMAC.
   338	type cthWrapper struct {
   339		h constantTimeHash
   340	}
   341	
   342	func (c *cthWrapper) Size() int                   { return c.h.Size() }
   343	func (c *cthWrapper) BlockSize() int              { return c.h.BlockSize() }
   344	func (c *cthWrapper) Reset()                      { c.h.Reset() }
   345	func (c *cthWrapper) Write(p []byte) (int, error) { return c.h.Write(p) }
   346	func (c *cthWrapper) Sum(b []byte) []byte         { return c.h.ConstantTimeSum(b) }
   347	
   348	func newConstantTimeHash(h func() hash.Hash) func() hash.Hash {
   349		return func() hash.Hash {
   350			return &cthWrapper{h().(constantTimeHash)}
   351		}
   352	}
   353	
   354	// tls10MAC implements the TLS 1.0 MAC function. RFC 2246, Section 6.2.3.
   355	type tls10MAC struct {
   356		h   hash.Hash
   357		buf []byte
   358	}
   359	
   360	func (s tls10MAC) Size() int {
   361		return s.h.Size()
   362	}
   363	
   364	// MAC is guaranteed to take constant time, as long as
   365	// len(seq)+len(header)+len(data)+len(extra) is constant. extra is not fed into
   366	// the MAC, but is only provided to make the timing profile constant.
   367	func (s tls10MAC) MAC(seq, header, data, extra []byte) []byte {
   368		s.h.Reset()
   369		s.h.Write(seq)
   370		s.h.Write(header)
   371		s.h.Write(data)
   372		res := s.h.Sum(s.buf[:0])
   373		if extra != nil {
   374			s.h.Write(extra)
   375		}
   376		return res
   377	}
   378	
   379	func rsaKA(version uint16) keyAgreement {
   380		return rsaKeyAgreement{}
   381	}
   382	
   383	func ecdheECDSAKA(version uint16) keyAgreement {
   384		return &ecdheKeyAgreement{
   385			isRSA:   false,
   386			version: version,
   387		}
   388	}
   389	
   390	func ecdheRSAKA(version uint16) keyAgreement {
   391		return &ecdheKeyAgreement{
   392			isRSA:   true,
   393			version: version,
   394		}
   395	}
   396	
   397	// mutualCipherSuite returns a cipherSuite given a list of supported
   398	// ciphersuites and the id requested by the peer.
   399	func mutualCipherSuite(have []uint16, want uint16) *cipherSuite {
   400		for _, id := range have {
   401			if id == want {
   402				return cipherSuiteByID(id)
   403			}
   404		}
   405		return nil
   406	}
   407	
   408	func cipherSuiteByID(id uint16) *cipherSuite {
   409		for _, cipherSuite := range cipherSuites {
   410			if cipherSuite.id == id {
   411				return cipherSuite
   412			}
   413		}
   414		return nil
   415	}
   416	
   417	func mutualCipherSuiteTLS13(have []uint16, want uint16) *cipherSuiteTLS13 {
   418		for _, id := range have {
   419			if id == want {
   420				return cipherSuiteTLS13ByID(id)
   421			}
   422		}
   423		return nil
   424	}
   425	
   426	func cipherSuiteTLS13ByID(id uint16) *cipherSuiteTLS13 {
   427		for _, cipherSuite := range cipherSuitesTLS13 {
   428			if cipherSuite.id == id {
   429				return cipherSuite
   430			}
   431		}
   432		return nil
   433	}
   434	
   435	// A list of cipher suite IDs that are, or have been, implemented by this
   436	// package.
   437	//
   438	// Taken from https://www.iana.org/assignments/tls-parameters/tls-parameters.xml
   439	const (
   440		// TLS 1.0 - 1.2 cipher suites.
   441		TLS_RSA_WITH_RC4_128_SHA                uint16 = 0x0005
   442		TLS_RSA_WITH_3DES_EDE_CBC_SHA           uint16 = 0x000a
   443		TLS_RSA_WITH_AES_128_CBC_SHA            uint16 = 0x002f
   444		TLS_RSA_WITH_AES_256_CBC_SHA            uint16 = 0x0035
   445		TLS_RSA_WITH_AES_128_CBC_SHA256         uint16 = 0x003c
   446		TLS_RSA_WITH_AES_128_GCM_SHA256         uint16 = 0x009c
   447		TLS_RSA_WITH_AES_256_GCM_SHA384         uint16 = 0x009d
   448		TLS_ECDHE_ECDSA_WITH_RC4_128_SHA        uint16 = 0xc007
   449		TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA    uint16 = 0xc009
   450		TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA    uint16 = 0xc00a
   451		TLS_ECDHE_RSA_WITH_RC4_128_SHA          uint16 = 0xc011
   452		TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA     uint16 = 0xc012
   453		TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA      uint16 = 0xc013
   454		TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA      uint16 = 0xc014
   455		TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 uint16 = 0xc023
   456		TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256   uint16 = 0xc027
   457		TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256   uint16 = 0xc02f
   458		TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 uint16 = 0xc02b
   459		TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384   uint16 = 0xc030
   460		TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 uint16 = 0xc02c
   461		TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305    uint16 = 0xcca8
   462		TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305  uint16 = 0xcca9
   463	
   464		// TLS 1.3 cipher suites.
   465		TLS_AES_128_GCM_SHA256       uint16 = 0x1301
   466		TLS_AES_256_GCM_SHA384       uint16 = 0x1302
   467		TLS_CHACHA20_POLY1305_SHA256 uint16 = 0x1303
   468	
   469		// TLS_FALLBACK_SCSV isn't a standard cipher suite but an indicator
   470		// that the client is doing version fallback. See RFC 7507.
   471		TLS_FALLBACK_SCSV uint16 = 0x5600
   472	)
   473	

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