all: fix problematic comments

[gostls13.git] / src / crypto / tls / cipher_suites.go

// 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/aes"
        "crypto/cipher"
        "crypto/des"
        "crypto/hmac"
        "crypto/internal/boring"
        "crypto/rc4"
        "crypto/sha1"
        "crypto/sha256"
        "fmt"
        "hash"
        "internal/cpu"
        "runtime"

        "golang.org/x/crypto/chacha20poly1305"
)

// CipherSuite is a TLS cipher suite. Note that most functions in this package
// accept and expose cipher suite IDs instead of this type.
type CipherSuite struct {
        ID   uint16
        Name string

        // Supported versions is the list of TLS protocol versions that can
        // negotiate this cipher suite.
        SupportedVersions []uint16

        // Insecure is true if the cipher suite has known security issues
        // due to its primitives, design, or implementation.
        Insecure bool
}

var (
        supportedUpToTLS12 = []uint16{VersionTLS10, VersionTLS11, VersionTLS12}
        supportedOnlyTLS12 = []uint16{VersionTLS12}
        supportedOnlyTLS13 = []uint16{VersionTLS13}
)

// CipherSuites returns a list of cipher suites currently implemented by this
// package, excluding those with security issues, which are returned by
// InsecureCipherSuites.
//
// The list is sorted by ID. Note that the default cipher suites selected by
// this package might depend on logic that can't be captured by a static list,
// and might not match those returned by this function.
func CipherSuites() []*CipherSuite {
        return []*CipherSuite{
                {TLS_RSA_WITH_AES_128_CBC_SHA, "TLS_RSA_WITH_AES_128_CBC_SHA", supportedUpToTLS12, false},
                {TLS_RSA_WITH_AES_256_CBC_SHA, "TLS_RSA_WITH_AES_256_CBC_SHA", supportedUpToTLS12, false},
                {TLS_RSA_WITH_AES_128_GCM_SHA256, "TLS_RSA_WITH_AES_128_GCM_SHA256", supportedOnlyTLS12, false},
                {TLS_RSA_WITH_AES_256_GCM_SHA384, "TLS_RSA_WITH_AES_256_GCM_SHA384", supportedOnlyTLS12, false},

                {TLS_AES_128_GCM_SHA256, "TLS_AES_128_GCM_SHA256", supportedOnlyTLS13, false},
                {TLS_AES_256_GCM_SHA384, "TLS_AES_256_GCM_SHA384", supportedOnlyTLS13, false},
                {TLS_CHACHA20_POLY1305_SHA256, "TLS_CHACHA20_POLY1305_SHA256", supportedOnlyTLS13, false},

                {TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA, "TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA", supportedUpToTLS12, false},
                {TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA, "TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA", supportedUpToTLS12, false},
                {TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA, "TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA", supportedUpToTLS12, false},
                {TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA, "TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA", supportedUpToTLS12, false},
                {TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, "TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256", supportedOnlyTLS12, false},
                {TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, "TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384", supportedOnlyTLS12, false},
                {TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256", supportedOnlyTLS12, false},
                {TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384, "TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384", supportedOnlyTLS12, false},
                {TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256, "TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256", supportedOnlyTLS12, false},
                {TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256, "TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256", supportedOnlyTLS12, false},
        }
}

// InsecureCipherSuites returns a list of cipher suites currently implemented by
// this package and which have security issues.
//
// Most applications should not use the cipher suites in this list, and should
// only use those returned by CipherSuites.
func InsecureCipherSuites() []*CipherSuite {
        // This list includes RC4, CBC_SHA256, and 3DES cipher suites. See
        // cipherSuitesPreferenceOrder for details.
        return []*CipherSuite{
                {TLS_RSA_WITH_RC4_128_SHA, "TLS_RSA_WITH_RC4_128_SHA", supportedUpToTLS12, true},
                {TLS_RSA_WITH_3DES_EDE_CBC_SHA, "TLS_RSA_WITH_3DES_EDE_CBC_SHA", supportedUpToTLS12, true},
                {TLS_RSA_WITH_AES_128_CBC_SHA256, "TLS_RSA_WITH_AES_128_CBC_SHA256", supportedOnlyTLS12, true},
                {TLS_ECDHE_ECDSA_WITH_RC4_128_SHA, "TLS_ECDHE_ECDSA_WITH_RC4_128_SHA", supportedUpToTLS12, true},
                {TLS_ECDHE_RSA_WITH_RC4_128_SHA, "TLS_ECDHE_RSA_WITH_RC4_128_SHA", supportedUpToTLS12, true},
                {TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA, "TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA", supportedUpToTLS12, true},
                {TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256, "TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256", supportedOnlyTLS12, true},
                {TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256, "TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256", supportedOnlyTLS12, true},
        }
}

// CipherSuiteName returns the standard name for the passed cipher suite ID
// (e.g. "TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256"), or a fallback representation
// of the ID value if the cipher suite is not implemented by this package.
func CipherSuiteName(id uint16) string {
        for _, c := range CipherSuites() {
                if c.ID == id {
                        return c.Name
                }
        }
        for _, c := range InsecureCipherSuites() {
                if c.ID == id {
                        return c.Name
                }
        }
        return fmt.Sprintf("0x%04X", id)
}

const (
        // suiteECDHE indicates that the cipher suite involves elliptic curve
        // Diffie-Hellman. This means that it should only be selected when the
        // client indicates that it supports ECC with a curve and point format
        // that we're happy with.
        suiteECDHE = 1 << iota
        // suiteECSign indicates that the cipher suite involves an ECDSA or
        // EdDSA signature and therefore may only be selected when the server's
        // certificate is ECDSA or EdDSA. If this is not set then the cipher suite
        // is RSA based.
        suiteECSign
        // suiteTLS12 indicates that the cipher suite should only be advertised
        // and accepted when using TLS 1.2.
        suiteTLS12
        // suiteSHA384 indicates that the cipher suite uses SHA384 as the
        // handshake hash.
        suiteSHA384
)

// A cipherSuite is a TLS 1.0–1.2 cipher suite, and defines the key exchange
// mechanism, as well as the cipher+MAC pair or the AEAD.
type cipherSuite struct {
        id uint16
        // the lengths, in bytes, of the key material needed for each component.
        keyLen int
        macLen int
        ivLen  int
        ka     func(version uint16) keyAgreement
        // flags is a bitmask of the suite* values, above.
        flags  int
        cipher func(key, iv []byte, isRead bool) any
        mac    func(key []byte) hash.Hash
        aead   func(key, fixedNonce []byte) aead
}

var cipherSuites = []*cipherSuite{ // TODO: replace with a map, since the order doesn't matter.
        {TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305, 32, 0, 12, ecdheRSAKA, suiteECDHE | suiteTLS12, nil, nil, aeadChaCha20Poly1305},
        {TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305, 32, 0, 12, ecdheECDSAKA, suiteECDHE | suiteECSign | suiteTLS12, nil, nil, aeadChaCha20Poly1305},
        {TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, 16, 0, 4, ecdheRSAKA, suiteECDHE | suiteTLS12, nil, nil, aeadAESGCM},
        {TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, 16, 0, 4, ecdheECDSAKA, suiteECDHE | suiteECSign | suiteTLS12, nil, nil, aeadAESGCM},
        {TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384, 32, 0, 4, ecdheRSAKA, suiteECDHE | suiteTLS12 | suiteSHA384, nil, nil, aeadAESGCM},
        {TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, 32, 0, 4, ecdheECDSAKA, suiteECDHE | suiteECSign | suiteTLS12 | suiteSHA384, nil, nil, aeadAESGCM},
        {TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, ecdheRSAKA, suiteECDHE | suiteTLS12, cipherAES, macSHA256, nil},
        {TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA, 16, 20, 16, ecdheRSAKA, suiteECDHE, cipherAES, macSHA1, nil},
        {TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, ecdheECDSAKA, suiteECDHE | suiteECSign | suiteTLS12, cipherAES, macSHA256, nil},
        {TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA, 16, 20, 16, ecdheECDSAKA, suiteECDHE | suiteECSign, cipherAES, macSHA1, nil},
        {TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA, 32, 20, 16, ecdheRSAKA, suiteECDHE, cipherAES, macSHA1, nil},
        {TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA, 32, 20, 16, ecdheECDSAKA, suiteECDHE | suiteECSign, cipherAES, macSHA1, nil},
        {TLS_RSA_WITH_AES_128_GCM_SHA256, 16, 0, 4, rsaKA, suiteTLS12, nil, nil, aeadAESGCM},
        {TLS_RSA_WITH_AES_256_GCM_SHA384, 32, 0, 4, rsaKA, suiteTLS12 | suiteSHA384, nil, nil, aeadAESGCM},
        {TLS_RSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, rsaKA, suiteTLS12, cipherAES, macSHA256, nil},
        {TLS_RSA_WITH_AES_128_CBC_SHA, 16, 20, 16, rsaKA, 0, cipherAES, macSHA1, nil},
        {TLS_RSA_WITH_AES_256_CBC_SHA, 32, 20, 16, rsaKA, 0, cipherAES, macSHA1, nil},
        {TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA, 24, 20, 8, ecdheRSAKA, suiteECDHE, cipher3DES, macSHA1, nil},
        {TLS_RSA_WITH_3DES_EDE_CBC_SHA, 24, 20, 8, rsaKA, 0, cipher3DES, macSHA1, nil},
        {TLS_RSA_WITH_RC4_128_SHA, 16, 20, 0, rsaKA, 0, cipherRC4, macSHA1, nil},
        {TLS_ECDHE_RSA_WITH_RC4_128_SHA, 16, 20, 0, ecdheRSAKA, suiteECDHE, cipherRC4, macSHA1, nil},
        {TLS_ECDHE_ECDSA_WITH_RC4_128_SHA, 16, 20, 0, ecdheECDSAKA, suiteECDHE | suiteECSign, cipherRC4, macSHA1, nil},
}

// selectCipherSuite returns the first TLS 1.0–1.2 cipher suite from ids which
// is also in supportedIDs and passes the ok filter.
func selectCipherSuite(ids, supportedIDs []uint16, ok func(*cipherSuite) bool) *cipherSuite {
        for _, id := range ids {
                candidate := cipherSuiteByID(id)
                if candidate == nil || !ok(candidate) {
                        continue
                }

                for _, suppID := range supportedIDs {
                        if id == suppID {
                                return candidate
                        }
                }
        }
        return nil
}

// A cipherSuiteTLS13 defines only the pair of the AEAD algorithm and hash
// algorithm to be used with HKDF. See RFC 8446, Appendix B.4.
type cipherSuiteTLS13 struct {
        id     uint16
        keyLen int
        aead   func(key, fixedNonce []byte) aead
        hash   crypto.Hash
}

var cipherSuitesTLS13 = []*cipherSuiteTLS13{ // TODO: replace with a map.
        {TLS_AES_128_GCM_SHA256, 16, aeadAESGCMTLS13, crypto.SHA256},
        {TLS_CHACHA20_POLY1305_SHA256, 32, aeadChaCha20Poly1305, crypto.SHA256},
        {TLS_AES_256_GCM_SHA384, 32, aeadAESGCMTLS13, crypto.SHA384},
}

// cipherSuitesPreferenceOrder is the order in which we'll select (on the
// server) or advertise (on the client) TLS 1.0–1.2 cipher suites.
//
// Cipher suites are filtered but not reordered based on the application and
// peer's preferences, meaning we'll never select a suite lower in this list if
// any higher one is available. This makes it more defensible to keep weaker
// cipher suites enabled, especially on the server side where we get the last
// word, since there are no known downgrade attacks on cipher suites selection.
//
// The list is sorted by applying the following priority rules, stopping at the
// first (most important) applicable one:
//
//   - Anything else comes before RC4
//
//     RC4 has practically exploitable biases. See https://www.rc4nomore.com.
//
//   - Anything else comes before CBC_SHA256
//
//     SHA-256 variants of the CBC ciphersuites don't implement any Lucky13
//     countermeasures. See http://www.isg.rhul.ac.uk/tls/Lucky13.html and
//     https://www.imperialviolet.org/2013/02/04/luckythirteen.html.
//
//   - Anything else comes before 3DES
//
//     3DES has 64-bit blocks, which makes it fundamentally susceptible to
//     birthday attacks. See https://sweet32.info.
//
//   - ECDHE comes before anything else
//
//     Once we got the broken stuff out of the way, the most important
//     property a cipher suite can have is forward secrecy. We don't
//     implement FFDHE, so that means ECDHE.
//
//   - AEADs come before CBC ciphers
//
//     Even with Lucky13 countermeasures, MAC-then-Encrypt CBC cipher suites
//     are fundamentally fragile, and suffered from an endless sequence of
//     padding oracle attacks. See https://eprint.iacr.org/2015/1129,
//     https://www.imperialviolet.org/2014/12/08/poodleagain.html, and
//     https://blog.cloudflare.com/yet-another-padding-oracle-in-openssl-cbc-ciphersuites/.
//
//   - AES comes before ChaCha20
//
//     When AES hardware is available, AES-128-GCM and AES-256-GCM are faster
//     than ChaCha20Poly1305.
//
//     When AES hardware is not available, AES-128-GCM is one or more of: much
//     slower, way more complex, and less safe (because not constant time)
//     than ChaCha20Poly1305.
//
//     We use this list if we think both peers have AES hardware, and
//     cipherSuitesPreferenceOrderNoAES otherwise.
//
//   - AES-128 comes before AES-256
//
//     The only potential advantages of AES-256 are better multi-target
//     margins, and hypothetical post-quantum properties. Neither apply to
//     TLS, and AES-256 is slower due to its four extra rounds (which don't
//     contribute to the advantages above).
//
//   - ECDSA comes before RSA
//
//     The relative order of ECDSA and RSA cipher suites doesn't matter,
//     as they depend on the certificate. Pick one to get a stable order.
var cipherSuitesPreferenceOrder = []uint16{
        // AEADs w/ ECDHE
        TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
        TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
        TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305, TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305,

        // CBC w/ ECDHE
        TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA, TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA,
        TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA, TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA,

        // AEADs w/o ECDHE
        TLS_RSA_WITH_AES_128_GCM_SHA256,
        TLS_RSA_WITH_AES_256_GCM_SHA384,

        // CBC w/o ECDHE
        TLS_RSA_WITH_AES_128_CBC_SHA,
        TLS_RSA_WITH_AES_256_CBC_SHA,

        // 3DES
        TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA,
        TLS_RSA_WITH_3DES_EDE_CBC_SHA,

        // CBC_SHA256
        TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256, TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256,
        TLS_RSA_WITH_AES_128_CBC_SHA256,

        // RC4
        TLS_ECDHE_ECDSA_WITH_RC4_128_SHA, TLS_ECDHE_RSA_WITH_RC4_128_SHA,
        TLS_RSA_WITH_RC4_128_SHA,
}

var cipherSuitesPreferenceOrderNoAES = []uint16{
        // ChaCha20Poly1305
        TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305, TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305,

        // AES-GCM w/ ECDHE
        TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
        TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,

        // The rest of cipherSuitesPreferenceOrder.
        TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA, TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA,
        TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA, TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA,
        TLS_RSA_WITH_AES_128_GCM_SHA256,
        TLS_RSA_WITH_AES_256_GCM_SHA384,
        TLS_RSA_WITH_AES_128_CBC_SHA,
        TLS_RSA_WITH_AES_256_CBC_SHA,
        TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA,
        TLS_RSA_WITH_3DES_EDE_CBC_SHA,
        TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256, TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256,
        TLS_RSA_WITH_AES_128_CBC_SHA256,
        TLS_ECDHE_ECDSA_WITH_RC4_128_SHA, TLS_ECDHE_RSA_WITH_RC4_128_SHA,
        TLS_RSA_WITH_RC4_128_SHA,
}

// disabledCipherSuites are not used unless explicitly listed in
// Config.CipherSuites. They MUST be at the end of cipherSuitesPreferenceOrder.
var disabledCipherSuites = []uint16{
        // CBC_SHA256
        TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256, TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256,
        TLS_RSA_WITH_AES_128_CBC_SHA256,

        // RC4
        TLS_ECDHE_ECDSA_WITH_RC4_128_SHA, TLS_ECDHE_RSA_WITH_RC4_128_SHA,
        TLS_RSA_WITH_RC4_128_SHA,
}

var (
        defaultCipherSuitesLen = len(cipherSuitesPreferenceOrder) - len(disabledCipherSuites)
        defaultCipherSuites    = cipherSuitesPreferenceOrder[:defaultCipherSuitesLen]
)

// defaultCipherSuitesTLS13 is also the preference order, since there are no
// disabled by default TLS 1.3 cipher suites. The same AES vs ChaCha20 logic as
// cipherSuitesPreferenceOrder applies.
var defaultCipherSuitesTLS13 = []uint16{
        TLS_AES_128_GCM_SHA256,
        TLS_AES_256_GCM_SHA384,
        TLS_CHACHA20_POLY1305_SHA256,
}

var defaultCipherSuitesTLS13NoAES = []uint16{
        TLS_CHACHA20_POLY1305_SHA256,
        TLS_AES_128_GCM_SHA256,
        TLS_AES_256_GCM_SHA384,
}

var (
        hasGCMAsmAMD64 = cpu.X86.HasAES && cpu.X86.HasPCLMULQDQ
        hasGCMAsmARM64 = cpu.ARM64.HasAES && cpu.ARM64.HasPMULL
        // Keep in sync with crypto/aes/cipher_s390x.go.
        hasGCMAsmS390X = cpu.S390X.HasAES && cpu.S390X.HasAESCBC && cpu.S390X.HasAESCTR &&
                (cpu.S390X.HasGHASH || cpu.S390X.HasAESGCM)

        hasAESGCMHardwareSupport = runtime.GOARCH == "amd64" && hasGCMAsmAMD64 ||
                runtime.GOARCH == "arm64" && hasGCMAsmARM64 ||
                runtime.GOARCH == "s390x" && hasGCMAsmS390X
)

var aesgcmCiphers = map[uint16]bool{
        // TLS 1.2
        TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256:   true,
        TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384:   true,
        TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256: true,
        TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384: true,
        // TLS 1.3
        TLS_AES_128_GCM_SHA256: true,
        TLS_AES_256_GCM_SHA384: true,
}

var nonAESGCMAEADCiphers = map[uint16]bool{
        // TLS 1.2
        TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305:   true,
        TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305: true,
        // TLS 1.3
        TLS_CHACHA20_POLY1305_SHA256: true,
}

// aesgcmPreferred returns whether the first known cipher in the preference list
// is an AES-GCM cipher, implying the peer has hardware support for it.
func aesgcmPreferred(ciphers []uint16) bool {
        for _, cID := range ciphers {
                if c := cipherSuiteByID(cID); c != nil {
                        return aesgcmCiphers[cID]
                }
                if c := cipherSuiteTLS13ByID(cID); c != nil {
                        return aesgcmCiphers[cID]
                }
        }
        return false
}

func cipherRC4(key, iv []byte, isRead bool) any {
        cipher, _ := rc4.NewCipher(key)
        return cipher
}

func cipher3DES(key, iv []byte, isRead bool) any {
        block, _ := des.NewTripleDESCipher(key)
        if isRead {
                return cipher.NewCBCDecrypter(block, iv)
        }
        return cipher.NewCBCEncrypter(block, iv)
}

func cipherAES(key, iv []byte, isRead bool) any {
        block, _ := aes.NewCipher(key)
        if isRead {
                return cipher.NewCBCDecrypter(block, iv)
        }
        return cipher.NewCBCEncrypter(block, iv)
}

// macSHA1 returns a SHA-1 based constant time MAC.
func macSHA1(key []byte) hash.Hash {
        h := sha1.New
        // The BoringCrypto SHA1 does not have a constant-time
        // checksum function, so don't try to use it.
        if !boring.Enabled {
                h = newConstantTimeHash(h)
        }
        return hmac.New(h, key)
}

// macSHA256 returns a SHA-256 based MAC. This is only supported in TLS 1.2 and
// is currently only used in disabled-by-default cipher suites.
func macSHA256(key []byte) hash.Hash {
        return hmac.New(sha256.New, key)
}

type aead interface {
        cipher.AEAD

        // explicitNonceLen returns the number of bytes of explicit nonce
        // included in each record. This is eight for older AEADs and
        // zero for modern ones.
        explicitNonceLen() int
}

const (
        aeadNonceLength   = 12
        noncePrefixLength = 4
)

// prefixNonceAEAD wraps an AEAD and prefixes a fixed portion of the nonce to
// each call.
type prefixNonceAEAD struct {
        // nonce contains the fixed part of the nonce in the first four bytes.
        nonce [aeadNonceLength]byte
        aead  cipher.AEAD
}

func (f *prefixNonceAEAD) NonceSize() int        { return aeadNonceLength - noncePrefixLength }
func (f *prefixNonceAEAD) Overhead() int         { return f.aead.Overhead() }
func (f *prefixNonceAEAD) explicitNonceLen() int { return f.NonceSize() }

func (f *prefixNonceAEAD) Seal(out, nonce, plaintext, additionalData []byte) []byte {
        copy(f.nonce[4:], nonce)
        return f.aead.Seal(out, f.nonce[:], plaintext, additionalData)
}

func (f *prefixNonceAEAD) Open(out, nonce, ciphertext, additionalData []byte) ([]byte, error) {
        copy(f.nonce[4:], nonce)
        return f.aead.Open(out, f.nonce[:], ciphertext, additionalData)
}

// xorNonceAEAD wraps an AEAD by XORing in a fixed pattern to the nonce
// before each call.
type xorNonceAEAD struct {
        nonceMask [aeadNonceLength]byte
        aead      cipher.AEAD
}

func (f *xorNonceAEAD) NonceSize() int        { return 8 } // 64-bit sequence number
func (f *xorNonceAEAD) Overhead() int         { return f.aead.Overhead() }
func (f *xorNonceAEAD) explicitNonceLen() int { return 0 }

func (f *xorNonceAEAD) Seal(out, nonce, plaintext, additionalData []byte) []byte {
        for i, b := range nonce {
                f.nonceMask[4+i] ^= b
        }
        result := f.aead.Seal(out, f.nonceMask[:], plaintext, additionalData)
        for i, b := range nonce {
                f.nonceMask[4+i] ^= b
        }

        return result
}

func (f *xorNonceAEAD) Open(out, nonce, ciphertext, additionalData []byte) ([]byte, error) {
        for i, b := range nonce {
                f.nonceMask[4+i] ^= b
        }
        result, err := f.aead.Open(out, f.nonceMask[:], ciphertext, additionalData)
        for i, b := range nonce {
                f.nonceMask[4+i] ^= b
        }

        return result, err
}

func aeadAESGCM(key, noncePrefix []byte) aead {
        if len(noncePrefix) != noncePrefixLength {
                panic("tls: internal error: wrong nonce length")
        }
        aes, err := aes.NewCipher(key)
        if err != nil {
                panic(err)
        }
        var aead cipher.AEAD
        if boring.Enabled {
                aead, err = boring.NewGCMTLS(aes)
        } else {
                boring.Unreachable()
                aead, err = cipher.NewGCM(aes)
        }
        if err != nil {
                panic(err)
        }

        ret := &prefixNonceAEAD{aead: aead}
        copy(ret.nonce[:], noncePrefix)
        return ret
}

func aeadAESGCMTLS13(key, nonceMask []byte) aead {
        if len(nonceMask) != aeadNonceLength {
                panic("tls: internal error: wrong nonce length")
        }
        aes, err := aes.NewCipher(key)
        if err != nil {
                panic(err)
        }
        aead, err := cipher.NewGCM(aes)
        if err != nil {
                panic(err)
        }

        ret := &xorNonceAEAD{aead: aead}
        copy(ret.nonceMask[:], nonceMask)
        return ret
}

func aeadChaCha20Poly1305(key, nonceMask []byte) aead {
        if len(nonceMask) != aeadNonceLength {
                panic("tls: internal error: wrong nonce length")
        }
        aead, err := chacha20poly1305.New(key)
        if err != nil {
                panic(err)
        }

        ret := &xorNonceAEAD{aead: aead}
        copy(ret.nonceMask[:], nonceMask)
        return ret
}

type constantTimeHash interface {
        hash.Hash
        ConstantTimeSum(b []byte) []byte
}

// cthWrapper wraps any hash.Hash that implements ConstantTimeSum, and replaces
// with that all calls to Sum. It's used to obtain a ConstantTimeSum-based HMAC.
type cthWrapper struct {
        h constantTimeHash
}

func (c *cthWrapper) Size() int                   { return c.h.Size() }
func (c *cthWrapper) BlockSize() int              { return c.h.BlockSize() }
func (c *cthWrapper) Reset()                      { c.h.Reset() }
func (c *cthWrapper) Write(p []byte) (int, error) { return c.h.Write(p) }
func (c *cthWrapper) Sum(b []byte) []byte         { return c.h.ConstantTimeSum(b) }

func newConstantTimeHash(h func() hash.Hash) func() hash.Hash {
        boring.Unreachable()
        return func() hash.Hash {
                return &cthWrapper{h().(constantTimeHash)}
        }
}

// tls10MAC implements the TLS 1.0 MAC function. RFC 2246, Section 6.2.3.
func tls10MAC(h hash.Hash, out, seq, header, data, extra []byte) []byte {
        h.Reset()
        h.Write(seq)
        h.Write(header)
        h.Write(data)
        res := h.Sum(out)
        if extra != nil {
                h.Write(extra)
        }
        return res
}

func rsaKA(version uint16) keyAgreement {
        return rsaKeyAgreement{}
}

func ecdheECDSAKA(version uint16) keyAgreement {
        return &ecdheKeyAgreement{
                isRSA:   false,
                version: version,
        }
}

func ecdheRSAKA(version uint16) keyAgreement {
        return &ecdheKeyAgreement{
                isRSA:   true,
                version: version,
        }
}

// mutualCipherSuite returns a cipherSuite given a list of supported
// ciphersuites and the id requested by the peer.
func mutualCipherSuite(have []uint16, want uint16) *cipherSuite {
        for _, id := range have {
                if id == want {
                        return cipherSuiteByID(id)
                }
        }
        return nil
}

func cipherSuiteByID(id uint16) *cipherSuite {
        for _, cipherSuite := range cipherSuites {
                if cipherSuite.id == id {
                        return cipherSuite
                }
        }
        return nil
}

func mutualCipherSuiteTLS13(have []uint16, want uint16) *cipherSuiteTLS13 {
        for _, id := range have {
                if id == want {
                        return cipherSuiteTLS13ByID(id)
                }
        }
        return nil
}

func cipherSuiteTLS13ByID(id uint16) *cipherSuiteTLS13 {
        for _, cipherSuite := range cipherSuitesTLS13 {
                if cipherSuite.id == id {
                        return cipherSuite
                }
        }
        return nil
}

// A list of cipher suite IDs that are, or have been, implemented by this
// package.
//
// See https://www.iana.org/assignments/tls-parameters/tls-parameters.xml
const (
        // TLS 1.0 - 1.2 cipher suites.
        TLS_RSA_WITH_RC4_128_SHA                      uint16 = 0x0005
        TLS_RSA_WITH_3DES_EDE_CBC_SHA                 uint16 = 0x000a
        TLS_RSA_WITH_AES_128_CBC_SHA                  uint16 = 0x002f
        TLS_RSA_WITH_AES_256_CBC_SHA                  uint16 = 0x0035
        TLS_RSA_WITH_AES_128_CBC_SHA256               uint16 = 0x003c
        TLS_RSA_WITH_AES_128_GCM_SHA256               uint16 = 0x009c
        TLS_RSA_WITH_AES_256_GCM_SHA384               uint16 = 0x009d
        TLS_ECDHE_ECDSA_WITH_RC4_128_SHA              uint16 = 0xc007
        TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA          uint16 = 0xc009
        TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA          uint16 = 0xc00a
        TLS_ECDHE_RSA_WITH_RC4_128_SHA                uint16 = 0xc011
        TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA           uint16 = 0xc012
        TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA            uint16 = 0xc013
        TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA            uint16 = 0xc014
        TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256       uint16 = 0xc023
        TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256         uint16 = 0xc027
        TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256         uint16 = 0xc02f
        TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256       uint16 = 0xc02b
        TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384         uint16 = 0xc030
        TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384       uint16 = 0xc02c
        TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256   uint16 = 0xcca8
        TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256 uint16 = 0xcca9

        // TLS 1.3 cipher suites.
        TLS_AES_128_GCM_SHA256       uint16 = 0x1301
        TLS_AES_256_GCM_SHA384       uint16 = 0x1302
        TLS_CHACHA20_POLY1305_SHA256 uint16 = 0x1303

        // TLS_FALLBACK_SCSV isn't a standard cipher suite but an indicator
        // that the client is doing version fallback. See RFC 7507.
        TLS_FALLBACK_SCSV uint16 = 0x5600

        // Legacy names for the corresponding cipher suites with the correct _SHA256
        // suffix, retained for backward compatibility.
        TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305   = TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256
        TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305 = TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256
)