...
Run Format

Source file src/crypto/cipher/gcm.go

  // Copyright 2013 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 cipher
  
  import (
  	"crypto/subtle"
  	"errors"
  )
  
  // AEAD is a cipher mode providing authenticated encryption with associated
  // data. For a description of the methodology, see
  //	https://en.wikipedia.org/wiki/Authenticated_encryption
  type AEAD interface {
  	// NonceSize returns the size of the nonce that must be passed to Seal
  	// and Open.
  	NonceSize() int
  
  	// Overhead returns the maximum difference between the lengths of a
  	// plaintext and its ciphertext.
  	Overhead() int
  
  	// Seal encrypts and authenticates plaintext, authenticates the
  	// additional data and appends the result to dst, returning the updated
  	// slice. The nonce must be NonceSize() bytes long and unique for all
  	// time, for a given key.
  	//
  	// The plaintext and dst may alias exactly or not at all. To reuse
  	// plaintext's storage for the encrypted output, use plaintext[:0] as dst.
  	Seal(dst, nonce, plaintext, additionalData []byte) []byte
  
  	// Open decrypts and authenticates ciphertext, authenticates the
  	// additional data and, if successful, appends the resulting plaintext
  	// to dst, returning the updated slice. The nonce must be NonceSize()
  	// bytes long and both it and the additional data must match the
  	// value passed to Seal.
  	//
  	// The ciphertext and dst may alias exactly or not at all. To reuse
  	// ciphertext's storage for the decrypted output, use ciphertext[:0] as dst.
  	//
  	// Even if the function fails, the contents of dst, up to its capacity,
  	// may be overwritten.
  	Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error)
  }
  
  // gcmAble is an interface implemented by ciphers that have a specific optimized
  // implementation of GCM, like crypto/aes. NewGCM will check for this interface
  // and return the specific AEAD if found.
  type gcmAble interface {
  	NewGCM(int) (AEAD, error)
  }
  
  // gcmFieldElement represents a value in GF(2¹²⁸). In order to reflect the GCM
  // standard and make getUint64 suitable for marshaling these values, the bits
  // are stored backwards. For example:
  //   the coefficient of x⁰ can be obtained by v.low >> 63.
  //   the coefficient of x⁶³ can be obtained by v.low & 1.
  //   the coefficient of x⁶⁴ can be obtained by v.high >> 63.
  //   the coefficient of x¹²⁷ can be obtained by v.high & 1.
  type gcmFieldElement struct {
  	low, high uint64
  }
  
  // gcm represents a Galois Counter Mode with a specific key. See
  // http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/proposedmodes/gcm/gcm-revised-spec.pdf
  type gcm struct {
  	cipher    Block
  	nonceSize int
  	// productTable contains the first sixteen powers of the key, H.
  	// However, they are in bit reversed order. See NewGCMWithNonceSize.
  	productTable [16]gcmFieldElement
  }
  
  // NewGCM returns the given 128-bit, block cipher wrapped in Galois Counter Mode
  // with the standard nonce length.
  //
  // In general, the GHASH operation performed by this implementation of GCM is not constant-time.
  // An exception is when the underlying Block was created by aes.NewCipher
  // on systems with hardware support for AES. See the crypto/aes package documentation for details.
  func NewGCM(cipher Block) (AEAD, error) {
  	return NewGCMWithNonceSize(cipher, gcmStandardNonceSize)
  }
  
  // NewGCMWithNonceSize returns the given 128-bit, block cipher wrapped in Galois
  // Counter Mode, which accepts nonces of the given length.
  //
  // Only use this function if you require compatibility with an existing
  // cryptosystem that uses non-standard nonce lengths. All other users should use
  // NewGCM, which is faster and more resistant to misuse.
  func NewGCMWithNonceSize(cipher Block, size int) (AEAD, error) {
  	if cipher, ok := cipher.(gcmAble); ok {
  		return cipher.NewGCM(size)
  	}
  
  	if cipher.BlockSize() != gcmBlockSize {
  		return nil, errors.New("cipher: NewGCM requires 128-bit block cipher")
  	}
  
  	var key [gcmBlockSize]byte
  	cipher.Encrypt(key[:], key[:])
  
  	g := &gcm{cipher: cipher, nonceSize: size}
  
  	// We precompute 16 multiples of |key|. However, when we do lookups
  	// into this table we'll be using bits from a field element and
  	// therefore the bits will be in the reverse order. So normally one
  	// would expect, say, 4*key to be in index 4 of the table but due to
  	// this bit ordering it will actually be in index 0010 (base 2) = 2.
  	x := gcmFieldElement{
  		getUint64(key[:8]),
  		getUint64(key[8:]),
  	}
  	g.productTable[reverseBits(1)] = x
  
  	for i := 2; i < 16; i += 2 {
  		g.productTable[reverseBits(i)] = gcmDouble(&g.productTable[reverseBits(i/2)])
  		g.productTable[reverseBits(i+1)] = gcmAdd(&g.productTable[reverseBits(i)], &x)
  	}
  
  	return g, nil
  }
  
  const (
  	gcmBlockSize         = 16
  	gcmTagSize           = 16
  	gcmStandardNonceSize = 12
  )
  
  func (g *gcm) NonceSize() int {
  	return g.nonceSize
  }
  
  func (*gcm) Overhead() int {
  	return gcmTagSize
  }
  
  func (g *gcm) Seal(dst, nonce, plaintext, data []byte) []byte {
  	if len(nonce) != g.nonceSize {
  		panic("cipher: incorrect nonce length given to GCM")
  	}
  	if uint64(len(plaintext)) > ((1<<32)-2)*uint64(g.cipher.BlockSize()) {
  		panic("cipher: message too large for GCM")
  	}
  
  	ret, out := sliceForAppend(dst, len(plaintext)+gcmTagSize)
  
  	var counter, tagMask [gcmBlockSize]byte
  	g.deriveCounter(&counter, nonce)
  
  	g.cipher.Encrypt(tagMask[:], counter[:])
  	gcmInc32(&counter)
  
  	g.counterCrypt(out, plaintext, &counter)
  	g.auth(out[len(plaintext):], out[:len(plaintext)], data, &tagMask)
  
  	return ret
  }
  
  var errOpen = errors.New("cipher: message authentication failed")
  
  func (g *gcm) Open(dst, nonce, ciphertext, data []byte) ([]byte, error) {
  	if len(nonce) != g.nonceSize {
  		panic("cipher: incorrect nonce length given to GCM")
  	}
  
  	if len(ciphertext) < gcmTagSize {
  		return nil, errOpen
  	}
  	if uint64(len(ciphertext)) > ((1<<32)-2)*uint64(g.cipher.BlockSize())+gcmTagSize {
  		return nil, errOpen
  	}
  
  	tag := ciphertext[len(ciphertext)-gcmTagSize:]
  	ciphertext = ciphertext[:len(ciphertext)-gcmTagSize]
  
  	var counter, tagMask [gcmBlockSize]byte
  	g.deriveCounter(&counter, nonce)
  
  	g.cipher.Encrypt(tagMask[:], counter[:])
  	gcmInc32(&counter)
  
  	var expectedTag [gcmTagSize]byte
  	g.auth(expectedTag[:], ciphertext, data, &tagMask)
  
  	ret, out := sliceForAppend(dst, len(ciphertext))
  
  	if subtle.ConstantTimeCompare(expectedTag[:], tag) != 1 {
  		// The AESNI code decrypts and authenticates concurrently, and
  		// so overwrites dst in the event of a tag mismatch. That
  		// behavior is mimicked here in order to be consistent across
  		// platforms.
  		for i := range out {
  			out[i] = 0
  		}
  		return nil, errOpen
  	}
  
  	g.counterCrypt(out, ciphertext, &counter)
  
  	return ret, nil
  }
  
  // reverseBits reverses the order of the bits of 4-bit number in i.
  func reverseBits(i int) int {
  	i = ((i << 2) & 0xc) | ((i >> 2) & 0x3)
  	i = ((i << 1) & 0xa) | ((i >> 1) & 0x5)
  	return i
  }
  
  // gcmAdd adds two elements of GF(2¹²⁸) and returns the sum.
  func gcmAdd(x, y *gcmFieldElement) gcmFieldElement {
  	// Addition in a characteristic 2 field is just XOR.
  	return gcmFieldElement{x.low ^ y.low, x.high ^ y.high}
  }
  
  // gcmDouble returns the result of doubling an element of GF(2¹²⁸).
  func gcmDouble(x *gcmFieldElement) (double gcmFieldElement) {
  	msbSet := x.high&1 == 1
  
  	// Because of the bit-ordering, doubling is actually a right shift.
  	double.high = x.high >> 1
  	double.high |= x.low << 63
  	double.low = x.low >> 1
  
  	// If the most-significant bit was set before shifting then it,
  	// conceptually, becomes a term of x^128. This is greater than the
  	// irreducible polynomial so the result has to be reduced. The
  	// irreducible polynomial is 1+x+x^2+x^7+x^128. We can subtract that to
  	// eliminate the term at x^128 which also means subtracting the other
  	// four terms. In characteristic 2 fields, subtraction == addition ==
  	// XOR.
  	if msbSet {
  		double.low ^= 0xe100000000000000
  	}
  
  	return
  }
  
  var gcmReductionTable = []uint16{
  	0x0000, 0x1c20, 0x3840, 0x2460, 0x7080, 0x6ca0, 0x48c0, 0x54e0,
  	0xe100, 0xfd20, 0xd940, 0xc560, 0x9180, 0x8da0, 0xa9c0, 0xb5e0,
  }
  
  // mul sets y to y*H, where H is the GCM key, fixed during NewGCMWithNonceSize.
  func (g *gcm) mul(y *gcmFieldElement) {
  	var z gcmFieldElement
  
  	for i := 0; i < 2; i++ {
  		word := y.high
  		if i == 1 {
  			word = y.low
  		}
  
  		// Multiplication works by multiplying z by 16 and adding in
  		// one of the precomputed multiples of H.
  		for j := 0; j < 64; j += 4 {
  			msw := z.high & 0xf
  			z.high >>= 4
  			z.high |= z.low << 60
  			z.low >>= 4
  			z.low ^= uint64(gcmReductionTable[msw]) << 48
  
  			// the values in |table| are ordered for
  			// little-endian bit positions. See the comment
  			// in NewGCMWithNonceSize.
  			t := &g.productTable[word&0xf]
  
  			z.low ^= t.low
  			z.high ^= t.high
  			word >>= 4
  		}
  	}
  
  	*y = z
  }
  
  // updateBlocks extends y with more polynomial terms from blocks, based on
  // Horner's rule. There must be a multiple of gcmBlockSize bytes in blocks.
  func (g *gcm) updateBlocks(y *gcmFieldElement, blocks []byte) {
  	for len(blocks) > 0 {
  		y.low ^= getUint64(blocks)
  		y.high ^= getUint64(blocks[8:])
  		g.mul(y)
  		blocks = blocks[gcmBlockSize:]
  	}
  }
  
  // update extends y with more polynomial terms from data. If data is not a
  // multiple of gcmBlockSize bytes long then the remainder is zero padded.
  func (g *gcm) update(y *gcmFieldElement, data []byte) {
  	fullBlocks := (len(data) >> 4) << 4
  	g.updateBlocks(y, data[:fullBlocks])
  
  	if len(data) != fullBlocks {
  		var partialBlock [gcmBlockSize]byte
  		copy(partialBlock[:], data[fullBlocks:])
  		g.updateBlocks(y, partialBlock[:])
  	}
  }
  
  // gcmInc32 treats the final four bytes of counterBlock as a big-endian value
  // and increments it.
  func gcmInc32(counterBlock *[16]byte) {
  	for i := gcmBlockSize - 1; i >= gcmBlockSize-4; i-- {
  		counterBlock[i]++
  		if counterBlock[i] != 0 {
  			break
  		}
  	}
  }
  
  // sliceForAppend takes a slice and a requested number of bytes. It returns a
  // slice with the contents of the given slice followed by that many bytes and a
  // 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)
  		putUint64(counter[:8], y.low)
  		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)
  
  	putUint64(out, y.low)
  	putUint64(out[8:], y.high)
  
  	xorWords(out, out, tagMask[:])
  }
  
  func getUint64(data []byte) uint64 {
  	r := uint64(data[0])<<56 |
  		uint64(data[1])<<48 |
  		uint64(data[2])<<40 |
  		uint64(data[3])<<32 |
  		uint64(data[4])<<24 |
  		uint64(data[5])<<16 |
  		uint64(data[6])<<8 |
  		uint64(data[7])
  	return r
  }
  
  func putUint64(out []byte, v uint64) {
  	out[0] = byte(v >> 56)
  	out[1] = byte(v >> 48)
  	out[2] = byte(v >> 40)
  	out[3] = byte(v >> 32)
  	out[4] = byte(v >> 24)
  	out[5] = byte(v >> 16)
  	out[6] = byte(v >> 8)
  	out[7] = byte(v)
  }
  

View as plain text