Golang subtle.ConstantTimeSelect函數代碼示例

func decryptPKCS1v15(rand io.Reader, priv *PrivateKey, ciphertext []byte) (valid int, msg []byte, err error) {
	k := (priv.N.BitLen() + 7) / 8
	if k < 11 {
		err = DecryptionError{}
		return
	}

	c := new(big.Int).SetBytes(ciphertext)
	m, err := decrypt(rand, priv, c)
	if err != nil {
		return
	}

	em := leftPad(m.Bytes(), k)
	firstByteIsZero := subtle.ConstantTimeByteEq(em[0], 0)
	secondByteIsTwo := subtle.ConstantTimeByteEq(em[1], 2)

	// The remainder of the plaintext must be a string of non-zero random
	// octets, followed by a 0, followed by the message.
	//   lookingForIndex: 1 iff we are still looking for the zero.
	//   index: the offset of the first zero byte.
	var lookingForIndex, index int
	lookingForIndex = 1

	for i := 2; i < len(em); i++ {
		equals0 := subtle.ConstantTimeByteEq(em[i], 0)
		index = subtle.ConstantTimeSelect(lookingForIndex&equals0, i, index)
		lookingForIndex = subtle.ConstantTimeSelect(equals0, 0, lookingForIndex)
	}

	valid = firstByteIsZero & secondByteIsTwo & (^lookingForIndex & 1)
	msg = em[index+1:]
	return
}

// Decrypt takes two integers, resulting from an ElGamal encryption, and
// returns the plaintext of the message. An error can result only if the
// ciphertext is invalid. Users should keep in mind that this is a padding
// oracle and thus, if exposed to an adaptive chosen ciphertext attack, can
// be used to break the cryptosystem.  See ``Chosen Ciphertext Attacks
// Against Protocols Based on the RSA Encryption Standard PKCS #1'', Daniel
// Bleichenbacher, Advances in Cryptology (Crypto '98),
func Decrypt(priv *PrivateKey, c1, c2 *big.Int) (msg []byte, err error) {
	s := new(big.Int).Exp(c1, priv.X, priv.P)
	s.ModInverse(s, priv.P)
	s.Mul(s, c2)
	s.Mod(s, priv.P)
	em := s.Bytes()

	firstByteIsTwo := subtle.ConstantTimeByteEq(em[0], 2)

	// The remainder of the plaintext must be a string of non-zero random
	// octets, followed by a 0, followed by the message.
	//   lookingForIndex: 1 iff we are still looking for the zero.
	//   index: the offset of the first zero byte.
	var lookingForIndex, index int
	lookingForIndex = 1

	for i := 1; i < len(em); i++ {
		equals0 := subtle.ConstantTimeByteEq(em[i], 0)
		index = subtle.ConstantTimeSelect(lookingForIndex&equals0, i, index)
		lookingForIndex = subtle.ConstantTimeSelect(equals0, 0, lookingForIndex)
	}

	if firstByteIsTwo != 1 || lookingForIndex != 0 || index < 9 {
		return nil, errors.New("elgamal: decryption error")
	}
	return em[index+1:], nil
}

func main() {
	log.Printf("%d", subtle.ConstantTimeByteEq(43, 65))
	log.Printf("%d", subtle.ConstantTimeCompare([]byte("batman"), []byte("robin ")))

	bytes := make([]byte, 6)
	subtle.ConstantTimeCopy(1, bytes, []byte("batman"))
	log.Printf("%s", bytes)

	log.Printf("%d", subtle.ConstantTimeEq(256, 255))
	log.Printf("%d", subtle.ConstantTimeSelect(1, 2, 3))
	log.Printf("%d", subtle.ConstantTimeSelect(0, 2, 3))
}

// Decrypt implements the crypto.Decrypter operation for the given key.
func (key *PrivateKey) Decrypt(rand io.Reader, msg []byte, opts crypto.DecrypterOpts) ([]byte, error) {
	switch opts := opts.(type) {
	case *rsa.PKCS1v15DecryptOptions:
		ptxt, decyptErr := key.execute(gokeyless.OpRSADecrypt, msg)

		// If opts.SessionKeyLen is set, we must perform a variation of
		// rsa.DecryptPKCS1v15SessionKey to ensure the entire operation
		// is performed in constant time regardless of padding errors.
		if l := opts.SessionKeyLen; l > 0 {
			plaintext := make([]byte, l)
			if _, err := io.ReadFull(rand, plaintext); err != nil {
				return nil, err
			}
			valid := subtle.ConstantTimeEq(int32(len(ptxt)), int32(l))
			v2 := subtle.ConstantTimeLessOrEq(l, len(ptxt))
			l2 := subtle.ConstantTimeSelect(v2, l, len(ptxt))
			subtle.ConstantTimeCopy(valid, plaintext[:l2], ptxt[:l2])
			return plaintext, nil
		}
		// Otherwise, we can just return the error like rsa.DecryptPKCS1v15.
		return ptxt, decyptErr
	default:
		return nil, errors.New("invalid options for Decrypt")
	}
}

// Decrypt implements the crypto.Decrypter operation for the given key.
func (key *PrivateKey) Decrypt(rand io.Reader, msg []byte, opts crypto.DecrypterOpts) ([]byte, error) {
	opts1v15, ok := opts.(*rsa.PKCS1v15DecryptOptions)
	if opts != nil && !ok {
		return nil, errors.New("invalid options for Decrypt")
	}

	ptxt, err := key.execute(gokeyless.OpRSADecrypt, msg)
	if err != nil {
		return nil, err
	}

	if ok {
		// If opts.SessionKeyLen is set, we must perform a variation of
		// rsa.DecryptPKCS1v15SessionKey to ensure the entire operation
		// is performed in constant time regardless of padding errors.
		if l := opts1v15.SessionKeyLen; l > 0 {
			plaintext := make([]byte, l)
			if _, err := io.ReadFull(rand, plaintext); err != nil {
				return nil, err
			}
			valid := subtle.ConstantTimeEq(int32(len(ptxt)), int32(l))
			v2 := subtle.ConstantTimeLessOrEq(l, len(ptxt))
			l2 := subtle.ConstantTimeSelect(v2, l, len(ptxt))
			subtle.ConstantTimeCopy(valid, plaintext[:l2], ptxt[:l2])
			return plaintext, nil
		}
	}
	return ptxt, nil
}

// New returns a hash.Hash computing the CMac checksum.
// If the block cipher is not supported by CMac
// (see package doc), a non-nil error is returned.
func New(c cipher.Block) (hash.Hash, error) {
	if c == nil {
		return nil, errors.New("the cipher.Block must not be nil")
	}
	bs := c.BlockSize()

	var p int
	switch bs {
	default:
		return nil, errors.New("cipher block size not supported")
	case 8:
		p = p64
	case 16:
		p = p128
	case 32:
		p = p256
	case 64:
		p = p512
	case 128:
		p = p1024
	}

	m := &macFunc{
		cipher: c,
		k0:     make([]byte, bs),
		k1:     make([]byte, bs),
		buf:    make([]byte, bs),
	}
	c.Encrypt(m.k0, m.k0)

	v := shift(m.k0, m.k0)
	m.k0[bs-1] ^= byte(subtle.ConstantTimeSelect(v, p, 0))

	v = shift(m.k1, m.k0)
	m.k1[bs-1] ^= byte(subtle.ConstantTimeSelect(v, p, 0))

	return m, nil
}

// decryptPKCS1v15 decrypts ciphertext using priv and blinds the operation if
// rand is not nil. It returns one or zero in valid that indicates whether the
// plaintext was correctly structured. In either case, the plaintext is
// returned in em so that it may be read independently of whether it was valid
// in order to maintain constant memory access patterns. If the plaintext was
// valid then index contains the index of the original message in em.
func decryptPKCS1v15(rand io.Reader, priv *PrivateKey, ciphertext []byte) (valid int, em []byte, index int, err error) {
	k := (priv.N.BitLen() + 7) / 8
	if k < 11 {
		err = ErrDecryption
		return
	}

	c := new(big.Int).SetBytes(ciphertext)
	m, err := decrypt(rand, priv, c)
	if err != nil {
		return
	}

	em = leftPad(m.Bytes(), k)
	firstByteIsZero := subtle.ConstantTimeByteEq(em[0], 0)
	secondByteIsTwo := subtle.ConstantTimeByteEq(em[1], 2)

	// The remainder of the plaintext must be a string of non-zero random
	// octets, followed by a 0, followed by the message.
	//   lookingForIndex: 1 iff we are still looking for the zero.
	//   index: the offset of the first zero byte.
	lookingForIndex := 1

	for i := 2; i < len(em); i++ {
		equals0 := subtle.ConstantTimeByteEq(em[i], 0)
		index = subtle.ConstantTimeSelect(lookingForIndex&equals0, i, index)
		lookingForIndex = subtle.ConstantTimeSelect(equals0, 0, lookingForIndex)
	}

	// The PS padding must be at least 8 bytes long, and it starts two
	// bytes into em.
	validPS := subtle.ConstantTimeLessOrEq(2+8, index)

	valid = firstByteIsZero & secondByteIsTwo & (^lookingForIndex & 1) & validPS
	index = subtle.ConstantTimeSelect(valid, index+1, 0)
	return valid, em, index, nil
}

// pkcsUnpad implements PKCS#7 un-padding.  If the padding is valid a valid value of 1 is returned.
// If the padding is invalid, valid is returned as 0.  Any unpaddeddata value should not be used
// if pcksUnpad determines the padding is invalid.  A logic error returns an error.  If you
// have not already authenticated the ciphertext, reporting a padding error, even through side channels
// (like timing), leaves you open to padding oracle attacks, so beware.
//
// I am implementing pkcsPad with constant time operations to forestall timing attacks that might
// be used to create a padding oracle. Since this package always authenticates first,
// timing and padding oracle attacks are ineffective because ciphertexts cannot be
// forged or manipulated with more than insignificant probability of success.
// In such a case constant time operation is unimportant, but constant timing may be important if this
// code is reused elsewhere.
func pkcsUnpad(data []byte, blocklen int) (valid int, unpaddeddata []byte, err error) {

	if blocklen > math.MaxUint8 {
		err = fmt.Errorf("Unpadding error: Blocklen %d exceeds maximum one byte unsigned integer",
			blocklen, math.MaxUint8)
		return
	}

	origlen := len(data)

	if origlen < blocklen {
		err = fmt.Errorf("Unpadding error: Data length %d is less than blocklen %d",
			origlen, blocklen)
		return
	}

	if origlen%blocklen != 0 {
		err = fmt.Errorf("Unpadding error: Data length %d is not a multiple of blocklen %d",
			origlen, blocklen)
		return
	}

	padchar := data[origlen-1]
	padcharlen := int(padchar)

	datalen := origlen - padcharlen

	valid = subtle.ConstantTimeLessOrEq(padcharlen, blocklen)

	for i := 1; i <= blocklen; i++ {
		// valid = (i > padcharlen || data[origlen-i] == padchar) && valid
		iLePadcharlen := subtle.ConstantTimeLessOrEq(i, padcharlen)
		isPadChar := subtle.ConstantTimeByteEq(data[origlen-i], padchar)
		stillvalid := subtle.ConstantTimeSelect(iLePadcharlen, isPadChar, 1)
		valid &= stillvalid
	}

	unpaddeddata = data[:datalen] // This data should not be used if invalid.
	// Returning it in any case simplifies constant timing
	return
}

// DecryptOAEP decrypts ciphertext using RSA-OAEP.
// If random != nil, DecryptOAEP uses RSA blinding to avoid timing side-channel attacks.
func DecryptOAEP(hash hash.Hash, random io.Reader, priv *PrivateKey, ciphertext []byte, label []byte) (msg []byte, err error) {
	if err := checkPub(&priv.PublicKey); err != nil {
		return nil, err
	}
	k := (priv.N.BitLen() + 7) / 8
	if len(ciphertext) > k ||
		k < hash.Size()*2+2 {
		err = ErrDecryption
		return
	}

	c := new(big.Int).SetBytes(ciphertext)

	m, err := decrypt(random, priv, c)
	if err != nil {
		return
	}

	hash.Write(label)
	lHash := hash.Sum(nil)
	hash.Reset()

	// Converting the plaintext number to bytes will strip any
	// leading zeros so we may have to left pad. We do this unconditionally
	// to avoid leaking timing information. (Although we still probably
	// leak the number of leading zeros. It's not clear that we can do
	// anything about this.)
	em := leftPad(m.Bytes(), k)

	firstByteIsZero := subtle.ConstantTimeByteEq(em[0], 0)

	seed := em[1 : hash.Size()+1]
	db := em[hash.Size()+1:]

	mgf1XOR(seed, hash, db)
	mgf1XOR(db, hash, seed)

	lHash2 := db[0:hash.Size()]

	// We have to validate the plaintext in constant time in order to avoid
	// attacks like: J. Manger. A Chosen Ciphertext Attack on RSA Optimal
	// Asymmetric Encryption Padding (OAEP) as Standardized in PKCS #1
	// v2.0. In J. Kilian, editor, Advances in Cryptology.
	lHash2Good := subtle.ConstantTimeCompare(lHash, lHash2)

	// The remainder of the plaintext must be zero or more 0x00, followed
	// by 0x01, followed by the message.
	//   lookingForIndex: 1 iff we are still looking for the 0x01
	//   index: the offset of the first 0x01 byte
	//   invalid: 1 iff we saw a non-zero byte before the 0x01.
	var lookingForIndex, index, invalid int
	lookingForIndex = 1
	rest := db[hash.Size():]

	for i := 0; i < len(rest); i++ {
		equals0 := subtle.ConstantTimeByteEq(rest[i], 0)
		equals1 := subtle.ConstantTimeByteEq(rest[i], 1)
		index = subtle.ConstantTimeSelect(lookingForIndex&equals1, i, index)
		lookingForIndex = subtle.ConstantTimeSelect(equals1, 0, lookingForIndex)
		invalid = subtle.ConstantTimeSelect(lookingForIndex&^equals0, 1, invalid)
	}

	if firstByteIsZero&lHash2Good&^invalid&^lookingForIndex != 1 {
		err = ErrDecryption
		return
	}

	msg = rest[index+1:]
	return
}

// decrypt checks and strips the mac and decrypts the data in b. Returns a
// success boolean, the number of bytes to skip from the start of the record in
// order to get the application payload, and an optional alert value.
func (hc *halfConn) decrypt(b *block) (ok bool, prefixLen int, alertValue alert) {
	// pull out payload
	payload := b.data[recordHeaderLen:]

	macSize := 0
	if hc.mac != nil {
		macSize = hc.mac.Size()
	}

	paddingGood := byte(255)
	paddingLen := 0
	explicitIVLen := 0

	// decrypt
	if hc.cipher != nil {
		switch c := hc.cipher.(type) {
		case cipher.Stream:
			c.XORKeyStream(payload, payload)
		case cipher.AEAD:
			explicitIVLen = 8
			if len(payload) < explicitIVLen {
				return false, 0, alertBadRecordMAC
			}
			nonce := payload[:8]
			payload = payload[8:]

			copy(hc.additionalData[:], hc.seq[:])
			copy(hc.additionalData[8:], b.data[:3])
			n := len(payload) - c.Overhead()
			hc.additionalData[11] = byte(n >> 8)
			hc.additionalData[12] = byte(n)
			var err error
			payload, err = c.Open(payload[:0], nonce, payload, hc.additionalData[:])
			if err != nil {
				return false, 0, alertBadRecordMAC
			}
			b.resize(recordHeaderLen + explicitIVLen + len(payload))
		case cbcMode:
			blockSize := c.BlockSize()
			if hc.version >= VersionTLS11 {
				explicitIVLen = blockSize
			}

			if len(payload)%blockSize != 0 || len(payload) < roundUp(explicitIVLen+macSize+1, blockSize) {
				return false, 0, alertBadRecordMAC
			}

			if explicitIVLen > 0 {
				c.SetIV(payload[:explicitIVLen])
				payload = payload[explicitIVLen:]
			}
			c.CryptBlocks(payload, payload)
			if hc.version == VersionSSL30 {
				paddingLen, paddingGood = extractPaddingSSL30(payload)
			} else {
				paddingLen, paddingGood = extractPadding(payload)

				// To protect against CBC padding oracles like Lucky13, the data
				// past paddingLen (which is secret) is passed to the MAC
				// function as extra data, to be fed into the HMAC after
				// computing the digest. This makes the MAC constant time as
				// long as the digest computation is constant time and does not
				// affect the subsequent write.
			}
		default:
			panic("unknown cipher type")
		}
	}

	// check, strip mac
	if hc.mac != nil {
		if len(payload) < macSize {
			return false, 0, alertBadRecordMAC
		}

		// strip mac off payload, b.data
		n := len(payload) - macSize - paddingLen
		n = subtle.ConstantTimeSelect(int(uint32(n)>>31), 0, n) // if n < 0 { n = 0 }
		b.data[3] = byte(n >> 8)
		b.data[4] = byte(n)
		remoteMAC := payload[n : n+macSize]
		localMAC := hc.mac.MAC(hc.inDigestBuf, hc.seq[0:], b.data[:recordHeaderLen], payload[:n], payload[n+macSize:])

		if subtle.ConstantTimeCompare(localMAC, remoteMAC) != 1 || paddingGood != 255 {
			return false, 0, alertBadRecordMAC
		}
		hc.inDigestBuf = localMAC

		b.resize(recordHeaderLen + explicitIVLen + n)
	}
	hc.incSeq()

	return true, recordHeaderLen + explicitIVLen, 0
}