427 lines
14 KiB
Go
427 lines
14 KiB
Go
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// Copyright 2013 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package cipher
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import (
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subtleoverlap "crypto/internal/subtle"
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"crypto/subtle"
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"encoding/binary"
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"errors"
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)
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// AEAD is a cipher mode providing authenticated encryption with associated
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// data. For a description of the methodology, see
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// https://en.wikipedia.org/wiki/Authenticated_encryption
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type AEAD interface {
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// NonceSize returns the size of the nonce that must be passed to Seal
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// and Open.
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NonceSize() int
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// Overhead returns the maximum difference between the lengths of a
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// plaintext and its ciphertext.
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Overhead() int
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// Seal encrypts and authenticates plaintext, authenticates the
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// additional data and appends the result to dst, returning the updated
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// slice. The nonce must be NonceSize() bytes long and unique for all
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// time, for a given key.
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//
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// To reuse plaintext's storage for the encrypted output, use plaintext[:0]
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// as dst. Otherwise, the remaining capacity of dst must not overlap plaintext.
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Seal(dst, nonce, plaintext, additionalData []byte) []byte
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// Open decrypts and authenticates ciphertext, authenticates the
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// additional data and, if successful, appends the resulting plaintext
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// to dst, returning the updated slice. The nonce must be NonceSize()
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// bytes long and both it and the additional data must match the
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// value passed to Seal.
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//
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// To reuse ciphertext's storage for the decrypted output, use ciphertext[:0]
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// as dst. Otherwise, the remaining capacity of dst must not overlap plaintext.
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//
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// Even if the function fails, the contents of dst, up to its capacity,
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// may be overwritten.
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Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error)
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}
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// gcmAble is an interface implemented by ciphers that have a specific optimized
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// implementation of GCM, like crypto/aes. NewGCM will check for this interface
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// and return the specific AEAD if found.
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type gcmAble interface {
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NewGCM(nonceSize, tagSize int) (AEAD, error)
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}
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// gcmFieldElement represents a value in GF(2¹²⁸). In order to reflect the GCM
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// standard and make binary.BigEndian suitable for marshaling these values, the
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// bits are stored in big endian order. For example:
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// the coefficient of x⁰ can be obtained by v.low >> 63.
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// the coefficient of x⁶³ can be obtained by v.low & 1.
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// the coefficient of x⁶⁴ can be obtained by v.high >> 63.
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// the coefficient of x¹²⁷ can be obtained by v.high & 1.
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type gcmFieldElement struct {
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low, high uint64
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}
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// gcm represents a Galois Counter Mode with a specific key. See
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// https://csrc.nist.gov/groups/ST/toolkit/BCM/documents/proposedmodes/gcm/gcm-revised-spec.pdf
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type gcm struct {
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cipher Block
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nonceSize int
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tagSize int
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// productTable contains the first sixteen powers of the key, H.
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// However, they are in bit reversed order. See NewGCMWithNonceSize.
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productTable [16]gcmFieldElement
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}
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// NewGCM returns the given 128-bit, block cipher wrapped in Galois Counter Mode
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// with the standard nonce length.
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//
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// In general, the GHASH operation performed by this implementation of GCM is not constant-time.
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// An exception is when the underlying Block was created by aes.NewCipher
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// on systems with hardware support for AES. See the crypto/aes package documentation for details.
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func NewGCM(cipher Block) (AEAD, error) {
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return newGCMWithNonceAndTagSize(cipher, gcmStandardNonceSize, gcmTagSize)
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}
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// NewGCMWithNonceSize returns the given 128-bit, block cipher wrapped in Galois
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// Counter Mode, which accepts nonces of the given length. The length must not
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// be zero.
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//
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// Only use this function if you require compatibility with an existing
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// cryptosystem that uses non-standard nonce lengths. All other users should use
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// NewGCM, which is faster and more resistant to misuse.
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func NewGCMWithNonceSize(cipher Block, size int) (AEAD, error) {
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return newGCMWithNonceAndTagSize(cipher, size, gcmTagSize)
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}
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// NewGCMWithTagSize returns the given 128-bit, block cipher wrapped in Galois
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// Counter Mode, which generates tags with the given length.
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//
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// Tag sizes between 12 and 16 bytes are allowed.
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//
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// Only use this function if you require compatibility with an existing
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// cryptosystem that uses non-standard tag lengths. All other users should use
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// NewGCM, which is more resistant to misuse.
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func NewGCMWithTagSize(cipher Block, tagSize int) (AEAD, error) {
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return newGCMWithNonceAndTagSize(cipher, gcmStandardNonceSize, tagSize)
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}
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func newGCMWithNonceAndTagSize(cipher Block, nonceSize, tagSize int) (AEAD, error) {
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if tagSize < gcmMinimumTagSize || tagSize > gcmBlockSize {
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return nil, errors.New("cipher: incorrect tag size given to GCM")
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}
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if nonceSize <= 0 {
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return nil, errors.New("cipher: the nonce can't have zero length, or the security of the key will be immediately compromised")
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}
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if cipher, ok := cipher.(gcmAble); ok {
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return cipher.NewGCM(nonceSize, tagSize)
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}
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if cipher.BlockSize() != gcmBlockSize {
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return nil, errors.New("cipher: NewGCM requires 128-bit block cipher")
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}
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var key [gcmBlockSize]byte
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cipher.Encrypt(key[:], key[:])
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g := &gcm{cipher: cipher, nonceSize: nonceSize, tagSize: tagSize}
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// We precompute 16 multiples of |key|. However, when we do lookups
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// into this table we'll be using bits from a field element and
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// therefore the bits will be in the reverse order. So normally one
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// would expect, say, 4*key to be in index 4 of the table but due to
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// this bit ordering it will actually be in index 0010 (base 2) = 2.
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x := gcmFieldElement{
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binary.BigEndian.Uint64(key[:8]),
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binary.BigEndian.Uint64(key[8:]),
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}
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g.productTable[reverseBits(1)] = x
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for i := 2; i < 16; i += 2 {
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g.productTable[reverseBits(i)] = gcmDouble(&g.productTable[reverseBits(i/2)])
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g.productTable[reverseBits(i+1)] = gcmAdd(&g.productTable[reverseBits(i)], &x)
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}
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return g, nil
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}
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const (
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gcmBlockSize = 16
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gcmTagSize = 16
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gcmMinimumTagSize = 12 // NIST SP 800-38D recommends tags with 12 or more bytes.
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gcmStandardNonceSize = 12
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)
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func (g *gcm) NonceSize() int {
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return g.nonceSize
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}
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func (g *gcm) Overhead() int {
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return g.tagSize
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}
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func (g *gcm) Seal(dst, nonce, plaintext, data []byte) []byte {
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if len(nonce) != g.nonceSize {
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panic("crypto/cipher: incorrect nonce length given to GCM")
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}
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if uint64(len(plaintext)) > ((1<<32)-2)*uint64(g.cipher.BlockSize()) {
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panic("crypto/cipher: message too large for GCM")
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}
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ret, out := sliceForAppend(dst, len(plaintext)+g.tagSize)
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if subtleoverlap.InexactOverlap(out, plaintext) {
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panic("crypto/cipher: invalid buffer overlap")
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}
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var counter, tagMask [gcmBlockSize]byte
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g.deriveCounter(&counter, nonce)
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g.cipher.Encrypt(tagMask[:], counter[:])
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gcmInc32(&counter)
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g.counterCrypt(out, plaintext, &counter)
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var tag [gcmTagSize]byte
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g.auth(tag[:], out[:len(plaintext)], data, &tagMask)
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copy(out[len(plaintext):], tag[:])
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return ret
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}
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var errOpen = errors.New("cipher: message authentication failed")
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func (g *gcm) Open(dst, nonce, ciphertext, data []byte) ([]byte, error) {
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if len(nonce) != g.nonceSize {
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panic("crypto/cipher: incorrect nonce length given to GCM")
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}
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// Sanity check to prevent the authentication from always succeeding if an implementation
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// leaves tagSize uninitialized, for example.
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if g.tagSize < gcmMinimumTagSize {
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panic("crypto/cipher: incorrect GCM tag size")
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}
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if len(ciphertext) < g.tagSize {
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return nil, errOpen
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}
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if uint64(len(ciphertext)) > ((1<<32)-2)*uint64(g.cipher.BlockSize())+uint64(g.tagSize) {
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return nil, errOpen
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}
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tag := ciphertext[len(ciphertext)-g.tagSize:]
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ciphertext = ciphertext[:len(ciphertext)-g.tagSize]
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var counter, tagMask [gcmBlockSize]byte
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g.deriveCounter(&counter, nonce)
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g.cipher.Encrypt(tagMask[:], counter[:])
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gcmInc32(&counter)
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var expectedTag [gcmTagSize]byte
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g.auth(expectedTag[:], ciphertext, data, &tagMask)
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ret, out := sliceForAppend(dst, len(ciphertext))
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if subtleoverlap.InexactOverlap(out, ciphertext) {
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panic("crypto/cipher: invalid buffer overlap")
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}
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if subtle.ConstantTimeCompare(expectedTag[:g.tagSize], tag) != 1 {
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// The AESNI code decrypts and authenticates concurrently, and
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// so overwrites dst in the event of a tag mismatch. That
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// behavior is mimicked here in order to be consistent across
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// platforms.
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for i := range out {
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out[i] = 0
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}
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return nil, errOpen
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}
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g.counterCrypt(out, ciphertext, &counter)
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return ret, nil
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}
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// reverseBits reverses the order of the bits of 4-bit number in i.
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func reverseBits(i int) int {
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i = ((i << 2) & 0xc) | ((i >> 2) & 0x3)
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i = ((i << 1) & 0xa) | ((i >> 1) & 0x5)
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return i
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}
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// gcmAdd adds two elements of GF(2¹²⁸) and returns the sum.
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func gcmAdd(x, y *gcmFieldElement) gcmFieldElement {
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// Addition in a characteristic 2 field is just XOR.
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return gcmFieldElement{x.low ^ y.low, x.high ^ y.high}
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}
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// gcmDouble returns the result of doubling an element of GF(2¹²⁸).
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func gcmDouble(x *gcmFieldElement) (double gcmFieldElement) {
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msbSet := x.high&1 == 1
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// Because of the bit-ordering, doubling is actually a right shift.
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double.high = x.high >> 1
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double.high |= x.low << 63
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double.low = x.low >> 1
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// If the most-significant bit was set before shifting then it,
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// conceptually, becomes a term of x^128. This is greater than the
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// irreducible polynomial so the result has to be reduced. The
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// irreducible polynomial is 1+x+x^2+x^7+x^128. We can subtract that to
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// eliminate the term at x^128 which also means subtracting the other
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// four terms. In characteristic 2 fields, subtraction == addition ==
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// XOR.
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if msbSet {
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double.low ^= 0xe100000000000000
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}
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return
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}
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var gcmReductionTable = []uint16{
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0x0000, 0x1c20, 0x3840, 0x2460, 0x7080, 0x6ca0, 0x48c0, 0x54e0,
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0xe100, 0xfd20, 0xd940, 0xc560, 0x9180, 0x8da0, 0xa9c0, 0xb5e0,
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}
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// mul sets y to y*H, where H is the GCM key, fixed during NewGCMWithNonceSize.
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func (g *gcm) mul(y *gcmFieldElement) {
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var z gcmFieldElement
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for i := 0; i < 2; i++ {
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word := y.high
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if i == 1 {
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word = y.low
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}
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// Multiplication works by multiplying z by 16 and adding in
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// one of the precomputed multiples of H.
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for j := 0; j < 64; j += 4 {
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msw := z.high & 0xf
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z.high >>= 4
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z.high |= z.low << 60
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z.low >>= 4
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z.low ^= uint64(gcmReductionTable[msw]) << 48
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// the values in |table| are ordered for
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// little-endian bit positions. See the comment
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// in NewGCMWithNonceSize.
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t := &g.productTable[word&0xf]
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z.low ^= t.low
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z.high ^= t.high
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word >>= 4
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}
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}
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*y = z
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}
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// updateBlocks extends y with more polynomial terms from blocks, based on
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// Horner's rule. There must be a multiple of gcmBlockSize bytes in blocks.
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func (g *gcm) updateBlocks(y *gcmFieldElement, blocks []byte) {
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for len(blocks) > 0 {
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y.low ^= binary.BigEndian.Uint64(blocks)
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y.high ^= binary.BigEndian.Uint64(blocks[8:])
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g.mul(y)
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blocks = blocks[gcmBlockSize:]
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}
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}
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// update extends y with more polynomial terms from data. If data is not a
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// multiple of gcmBlockSize bytes long then the remainder is zero padded.
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func (g *gcm) update(y *gcmFieldElement, data []byte) {
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fullBlocks := (len(data) >> 4) << 4
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g.updateBlocks(y, data[:fullBlocks])
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if len(data) != fullBlocks {
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var partialBlock [gcmBlockSize]byte
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copy(partialBlock[:], data[fullBlocks:])
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g.updateBlocks(y, partialBlock[:])
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}
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}
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// gcmInc32 treats the final four bytes of counterBlock as a big-endian value
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// and increments it.
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func gcmInc32(counterBlock *[16]byte) {
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ctr := counterBlock[len(counterBlock)-4:]
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binary.BigEndian.PutUint32(ctr, binary.BigEndian.Uint32(ctr)+1)
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}
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// sliceForAppend takes a slice and a requested number of bytes. It returns a
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// slice with the contents of the given slice followed by that many bytes and a
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|
// second slice that aliases into it and contains only the extra bytes. If the
|
||
|
|
// original slice has sufficient capacity then no allocation is performed.
|
||
|
|
func sliceForAppend(in []byte, n int) (head, tail []byte) {
|
||
|
|
if total := len(in) + n; cap(in) >= total {
|
||
|
|
head = in[:total]
|
||
|
|
} else {
|
||
|
|
head = make([]byte, total)
|
||
|
|
copy(head, in)
|
||
|
|
}
|
||
|
|
tail = head[len(in):]
|
||
|
|
return
|
||
|
|
}
|
||
|
|
|
||
|
|
// counterCrypt crypts in to out using g.cipher in counter mode.
|
||
|
|
func (g *gcm) counterCrypt(out, in []byte, counter *[gcmBlockSize]byte) {
|
||
|
|
var mask [gcmBlockSize]byte
|
||
|
|
|
||
|
|
for len(in) >= gcmBlockSize {
|
||
|
|
g.cipher.Encrypt(mask[:], counter[:])
|
||
|
|
gcmInc32(counter)
|
||
|
|
|
||
|
|
xorWords(out, in, mask[:])
|
||
|
|
out = out[gcmBlockSize:]
|
||
|
|
in = in[gcmBlockSize:]
|
||
|
|
}
|
||
|
|
|
||
|
|
if len(in) > 0 {
|
||
|
|
g.cipher.Encrypt(mask[:], counter[:])
|
||
|
|
gcmInc32(counter)
|
||
|
|
xorBytes(out, in, mask[:])
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
// deriveCounter computes the initial GCM counter state from the given nonce.
|
||
|
|
// See NIST SP 800-38D, section 7.1. This assumes that counter is filled with
|
||
|
|
// zeros on entry.
|
||
|
|
func (g *gcm) deriveCounter(counter *[gcmBlockSize]byte, nonce []byte) {
|
||
|
|
// GCM has two modes of operation with respect to the initial counter
|
||
|
|
// state: a "fast path" for 96-bit (12-byte) nonces, and a "slow path"
|
||
|
|
// for nonces of other lengths. For a 96-bit nonce, the nonce, along
|
||
|
|
// with a four-byte big-endian counter starting at one, is used
|
||
|
|
// directly as the starting counter. For other nonce sizes, the counter
|
||
|
|
// is computed by passing it through the GHASH function.
|
||
|
|
if len(nonce) == gcmStandardNonceSize {
|
||
|
|
copy(counter[:], nonce)
|
||
|
|
counter[gcmBlockSize-1] = 1
|
||
|
|
} else {
|
||
|
|
var y gcmFieldElement
|
||
|
|
g.update(&y, nonce)
|
||
|
|
y.high ^= uint64(len(nonce)) * 8
|
||
|
|
g.mul(&y)
|
||
|
|
binary.BigEndian.PutUint64(counter[:8], y.low)
|
||
|
|
binary.BigEndian.PutUint64(counter[8:], y.high)
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
// auth calculates GHASH(ciphertext, additionalData), masks the result with
|
||
|
|
// tagMask and writes the result to out.
|
||
|
|
func (g *gcm) auth(out, ciphertext, additionalData []byte, tagMask *[gcmTagSize]byte) {
|
||
|
|
var y gcmFieldElement
|
||
|
|
g.update(&y, additionalData)
|
||
|
|
g.update(&y, ciphertext)
|
||
|
|
|
||
|
|
y.low ^= uint64(len(additionalData)) * 8
|
||
|
|
y.high ^= uint64(len(ciphertext)) * 8
|
||
|
|
|
||
|
|
g.mul(&y)
|
||
|
|
|
||
|
|
binary.BigEndian.PutUint64(out, y.low)
|
||
|
|
binary.BigEndian.PutUint64(out[8:], y.high)
|
||
|
|
|
||
|
|
xorWords(out, out, tagMask[:])
|
||
|
|
}
|