Source file src/crypto/cipher/gcm.go

Documentation: crypto/cipher

     1  // Copyright 2013 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 cipher
     6  
     7  import (
     8  	subtleoverlap "crypto/internal/subtle"
     9  	"crypto/subtle"
    10  	"encoding/binary"
    11  	"errors"
    12  )
    13  
    14  // AEAD is a cipher mode providing authenticated encryption with associated
    15  // data. For a description of the methodology, see
    16  //	https://en.wikipedia.org/wiki/Authenticated_encryption
    17  type AEAD interface {
    18  	// NonceSize returns the size of the nonce that must be passed to Seal
    19  	// and Open.
    20  	NonceSize() int
    21  
    22  	// Overhead returns the maximum difference between the lengths of a
    23  	// plaintext and its ciphertext.
    24  	Overhead() int
    25  
    26  	// Seal encrypts and authenticates plaintext, authenticates the
    27  	// additional data and appends the result to dst, returning the updated
    28  	// slice. The nonce must be NonceSize() bytes long and unique for all
    29  	// time, for a given key.
    30  	//
    31  	// To reuse plaintext's storage for the encrypted output, use plaintext[:0]
    32  	// as dst. Otherwise, the remaining capacity of dst must not overlap plaintext.
    33  	Seal(dst, nonce, plaintext, additionalData []byte) []byte
    34  
    35  	// Open decrypts and authenticates ciphertext, authenticates the
    36  	// additional data and, if successful, appends the resulting plaintext
    37  	// to dst, returning the updated slice. The nonce must be NonceSize()
    38  	// bytes long and both it and the additional data must match the
    39  	// value passed to Seal.
    40  	//
    41  	// To reuse ciphertext's storage for the decrypted output, use ciphertext[:0]
    42  	// as dst. Otherwise, the remaining capacity of dst must not overlap plaintext.
    43  	//
    44  	// Even if the function fails, the contents of dst, up to its capacity,
    45  	// may be overwritten.
    46  	Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error)
    47  }
    48  
    49  // gcmAble is an interface implemented by ciphers that have a specific optimized
    50  // implementation of GCM, like crypto/aes. NewGCM will check for this interface
    51  // and return the specific AEAD if found.
    52  type gcmAble interface {
    53  	NewGCM(nonceSize, tagSize int) (AEAD, error)
    54  }
    55  
    56  // gcmFieldElement represents a value in GF(2¹²⁸). In order to reflect the GCM
    57  // standard and make binary.BigEndian suitable for marshaling these values, the
    58  // bits are stored in big endian order. For example:
    59  //   the coefficient of x⁰ can be obtained by v.low >> 63.
    60  //   the coefficient of x⁶³ can be obtained by v.low & 1.
    61  //   the coefficient of x⁶⁴ can be obtained by v.high >> 63.
    62  //   the coefficient of x¹²⁷ can be obtained by v.high & 1.
    63  type gcmFieldElement struct {
    64  	low, high uint64
    65  }
    66  
    67  // gcm represents a Galois Counter Mode with a specific key. See
    68  // https://csrc.nist.gov/groups/ST/toolkit/BCM/documents/proposedmodes/gcm/gcm-revised-spec.pdf
    69  type gcm struct {
    70  	cipher    Block
    71  	nonceSize int
    72  	tagSize   int
    73  	// productTable contains the first sixteen powers of the key, H.
    74  	// However, they are in bit reversed order. See NewGCMWithNonceSize.
    75  	productTable [16]gcmFieldElement
    76  }
    77  
    78  // NewGCM returns the given 128-bit, block cipher wrapped in Galois Counter Mode
    79  // with the standard nonce length.
    80  //
    81  // In general, the GHASH operation performed by this implementation of GCM is not constant-time.
    82  // An exception is when the underlying Block was created by aes.NewCipher
    83  // on systems with hardware support for AES. See the crypto/aes package documentation for details.
    84  func NewGCM(cipher Block) (AEAD, error) {
    85  	return newGCMWithNonceAndTagSize(cipher, gcmStandardNonceSize, gcmTagSize)
    86  }
    87  
    88  // NewGCMWithNonceSize returns the given 128-bit, block cipher wrapped in Galois
    89  // Counter Mode, which accepts nonces of the given length. The length must not
    90  // be zero.
    91  //
    92  // Only use this function if you require compatibility with an existing
    93  // cryptosystem that uses non-standard nonce lengths. All other users should use
    94  // NewGCM, which is faster and more resistant to misuse.
    95  func NewGCMWithNonceSize(cipher Block, size int) (AEAD, error) {
    96  	return newGCMWithNonceAndTagSize(cipher, size, gcmTagSize)
    97  }
    98  
    99  // NewGCMWithTagSize returns the given 128-bit, block cipher wrapped in Galois
   100  // Counter Mode, which generates tags with the given length.
   101  //
   102  // Tag sizes between 12 and 16 bytes are allowed.
   103  //
   104  // Only use this function if you require compatibility with an existing
   105  // cryptosystem that uses non-standard tag lengths. All other users should use
   106  // NewGCM, which is more resistant to misuse.
   107  func NewGCMWithTagSize(cipher Block, tagSize int) (AEAD, error) {
   108  	return newGCMWithNonceAndTagSize(cipher, gcmStandardNonceSize, tagSize)
   109  }
   110  
   111  func newGCMWithNonceAndTagSize(cipher Block, nonceSize, tagSize int) (AEAD, error) {
   112  	if tagSize < gcmMinimumTagSize || tagSize > gcmBlockSize {
   113  		return nil, errors.New("cipher: incorrect tag size given to GCM")
   114  	}
   115  
   116  	if nonceSize <= 0 {
   117  		return nil, errors.New("cipher: the nonce can't have zero length, or the security of the key will be immediately compromised")
   118  	}
   119  
   120  	if cipher, ok := cipher.(gcmAble); ok {
   121  		return cipher.NewGCM(nonceSize, tagSize)
   122  	}
   123  
   124  	if cipher.BlockSize() != gcmBlockSize {
   125  		return nil, errors.New("cipher: NewGCM requires 128-bit block cipher")
   126  	}
   127  
   128  	var key [gcmBlockSize]byte
   129  	cipher.Encrypt(key[:], key[:])
   130  
   131  	g := &gcm{cipher: cipher, nonceSize: nonceSize, tagSize: tagSize}
   132  
   133  	// We precompute 16 multiples of |key|. However, when we do lookups
   134  	// into this table we'll be using bits from a field element and
   135  	// therefore the bits will be in the reverse order. So normally one
   136  	// would expect, say, 4*key to be in index 4 of the table but due to
   137  	// this bit ordering it will actually be in index 0010 (base 2) = 2.
   138  	x := gcmFieldElement{
   139  		binary.BigEndian.Uint64(key[:8]),
   140  		binary.BigEndian.Uint64(key[8:]),
   141  	}
   142  	g.productTable[reverseBits(1)] = x
   143  
   144  	for i := 2; i < 16; i += 2 {
   145  		g.productTable[reverseBits(i)] = gcmDouble(&g.productTable[reverseBits(i/2)])
   146  		g.productTable[reverseBits(i+1)] = gcmAdd(&g.productTable[reverseBits(i)], &x)
   147  	}
   148  
   149  	return g, nil
   150  }
   151  
   152  const (
   153  	gcmBlockSize         = 16
   154  	gcmTagSize           = 16
   155  	gcmMinimumTagSize    = 12 // NIST SP 800-38D recommends tags with 12 or more bytes.
   156  	gcmStandardNonceSize = 12
   157  )
   158  
   159  func (g *gcm) NonceSize() int {
   160  	return g.nonceSize
   161  }
   162  
   163  func (g *gcm) Overhead() int {
   164  	return g.tagSize
   165  }
   166  
   167  func (g *gcm) Seal(dst, nonce, plaintext, data []byte) []byte {
   168  	if len(nonce) != g.nonceSize {
   169  		panic("crypto/cipher: incorrect nonce length given to GCM")
   170  	}
   171  	if uint64(len(plaintext)) > ((1<<32)-2)*uint64(g.cipher.BlockSize()) {
   172  		panic("crypto/cipher: message too large for GCM")
   173  	}
   174  
   175  	ret, out := sliceForAppend(dst, len(plaintext)+g.tagSize)
   176  	if subtleoverlap.InexactOverlap(out, plaintext) {
   177  		panic("crypto/cipher: invalid buffer overlap")
   178  	}
   179  
   180  	var counter, tagMask [gcmBlockSize]byte
   181  	g.deriveCounter(&counter, nonce)
   182  
   183  	g.cipher.Encrypt(tagMask[:], counter[:])
   184  	gcmInc32(&counter)
   185  
   186  	g.counterCrypt(out, plaintext, &counter)
   187  
   188  	var tag [gcmTagSize]byte
   189  	g.auth(tag[:], out[:len(plaintext)], data, &tagMask)
   190  	copy(out[len(plaintext):], tag[:])
   191  
   192  	return ret
   193  }
   194  
   195  var errOpen = errors.New("cipher: message authentication failed")
   196  
   197  func (g *gcm) Open(dst, nonce, ciphertext, data []byte) ([]byte, error) {
   198  	if len(nonce) != g.nonceSize {
   199  		panic("crypto/cipher: incorrect nonce length given to GCM")
   200  	}
   201  	// Sanity check to prevent the authentication from always succeeding if an implementation
   202  	// leaves tagSize uninitialized, for example.
   203  	if g.tagSize < gcmMinimumTagSize {
   204  		panic("crypto/cipher: incorrect GCM tag size")
   205  	}
   206  
   207  	if len(ciphertext) < g.tagSize {
   208  		return nil, errOpen
   209  	}
   210  	if uint64(len(ciphertext)) > ((1<<32)-2)*uint64(g.cipher.BlockSize())+uint64(g.tagSize) {
   211  		return nil, errOpen
   212  	}
   213  
   214  	tag := ciphertext[len(ciphertext)-g.tagSize:]
   215  	ciphertext = ciphertext[:len(ciphertext)-g.tagSize]
   216  
   217  	var counter, tagMask [gcmBlockSize]byte
   218  	g.deriveCounter(&counter, nonce)
   219  
   220  	g.cipher.Encrypt(tagMask[:], counter[:])
   221  	gcmInc32(&counter)
   222  
   223  	var expectedTag [gcmTagSize]byte
   224  	g.auth(expectedTag[:], ciphertext, data, &tagMask)
   225  
   226  	ret, out := sliceForAppend(dst, len(ciphertext))
   227  	if subtleoverlap.InexactOverlap(out, ciphertext) {
   228  		panic("crypto/cipher: invalid buffer overlap")
   229  	}
   230  
   231  	if subtle.ConstantTimeCompare(expectedTag[:g.tagSize], tag) != 1 {
   232  		// The AESNI code decrypts and authenticates concurrently, and
   233  		// so overwrites dst in the event of a tag mismatch. That
   234  		// behavior is mimicked here in order to be consistent across
   235  		// platforms.
   236  		for i := range out {
   237  			out[i] = 0
   238  		}
   239  		return nil, errOpen
   240  	}
   241  
   242  	g.counterCrypt(out, ciphertext, &counter)
   243  
   244  	return ret, nil
   245  }
   246  
   247  // reverseBits reverses the order of the bits of 4-bit number in i.
   248  func reverseBits(i int) int {
   249  	i = ((i << 2) & 0xc) | ((i >> 2) & 0x3)
   250  	i = ((i << 1) & 0xa) | ((i >> 1) & 0x5)
   251  	return i
   252  }
   253  
   254  // gcmAdd adds two elements of GF(2¹²⁸) and returns the sum.
   255  func gcmAdd(x, y *gcmFieldElement) gcmFieldElement {
   256  	// Addition in a characteristic 2 field is just XOR.
   257  	return gcmFieldElement{x.low ^ y.low, x.high ^ y.high}
   258  }
   259  
   260  // gcmDouble returns the result of doubling an element of GF(2¹²⁸).
   261  func gcmDouble(x *gcmFieldElement) (double gcmFieldElement) {
   262  	msbSet := x.high&1 == 1
   263  
   264  	// Because of the bit-ordering, doubling is actually a right shift.
   265  	double.high = x.high >> 1
   266  	double.high |= x.low << 63
   267  	double.low = x.low >> 1
   268  
   269  	// If the most-significant bit was set before shifting then it,
   270  	// conceptually, becomes a term of x^128. This is greater than the
   271  	// irreducible polynomial so the result has to be reduced. The
   272  	// irreducible polynomial is 1+x+x^2+x^7+x^128. We can subtract that to
   273  	// eliminate the term at x^128 which also means subtracting the other
   274  	// four terms. In characteristic 2 fields, subtraction == addition ==
   275  	// XOR.
   276  	if msbSet {
   277  		double.low ^= 0xe100000000000000
   278  	}
   279  
   280  	return
   281  }
   282  
   283  var gcmReductionTable = []uint16{
   284  	0x0000, 0x1c20, 0x3840, 0x2460, 0x7080, 0x6ca0, 0x48c0, 0x54e0,
   285  	0xe100, 0xfd20, 0xd940, 0xc560, 0x9180, 0x8da0, 0xa9c0, 0xb5e0,
   286  }
   287  
   288  // mul sets y to y*H, where H is the GCM key, fixed during NewGCMWithNonceSize.
   289  func (g *gcm) mul(y *gcmFieldElement) {
   290  	var z gcmFieldElement
   291  
   292  	for i := 0; i < 2; i++ {
   293  		word := y.high
   294  		if i == 1 {
   295  			word = y.low
   296  		}
   297  
   298  		// Multiplication works by multiplying z by 16 and adding in
   299  		// one of the precomputed multiples of H.
   300  		for j := 0; j < 64; j += 4 {
   301  			msw := z.high & 0xf
   302  			z.high >>= 4
   303  			z.high |= z.low << 60
   304  			z.low >>= 4
   305  			z.low ^= uint64(gcmReductionTable[msw]) << 48
   306  
   307  			// the values in |table| are ordered for
   308  			// little-endian bit positions. See the comment
   309  			// in NewGCMWithNonceSize.
   310  			t := &g.productTable[word&0xf]
   311  
   312  			z.low ^= t.low
   313  			z.high ^= t.high
   314  			word >>= 4
   315  		}
   316  	}
   317  
   318  	*y = z
   319  }
   320  
   321  // updateBlocks extends y with more polynomial terms from blocks, based on
   322  // Horner's rule. There must be a multiple of gcmBlockSize bytes in blocks.
   323  func (g *gcm) updateBlocks(y *gcmFieldElement, blocks []byte) {
   324  	for len(blocks) > 0 {
   325  		y.low ^= binary.BigEndian.Uint64(blocks)
   326  		y.high ^= binary.BigEndian.Uint64(blocks[8:])
   327  		g.mul(y)
   328  		blocks = blocks[gcmBlockSize:]
   329  	}
   330  }
   331  
   332  // update extends y with more polynomial terms from data. If data is not a
   333  // multiple of gcmBlockSize bytes long then the remainder is zero padded.
   334  func (g *gcm) update(y *gcmFieldElement, data []byte) {
   335  	fullBlocks := (len(data) >> 4) << 4
   336  	g.updateBlocks(y, data[:fullBlocks])
   337  
   338  	if len(data) != fullBlocks {
   339  		var partialBlock [gcmBlockSize]byte
   340  		copy(partialBlock[:], data[fullBlocks:])
   341  		g.updateBlocks(y, partialBlock[:])
   342  	}
   343  }
   344  
   345  // gcmInc32 treats the final four bytes of counterBlock as a big-endian value
   346  // and increments it.
   347  func gcmInc32(counterBlock *[16]byte) {
   348  	ctr := counterBlock[len(counterBlock)-4:]
   349  	binary.BigEndian.PutUint32(ctr, binary.BigEndian.Uint32(ctr)+1)
   350  }
   351  
   352  // sliceForAppend takes a slice and a requested number of bytes. It returns a
   353  // slice with the contents of the given slice followed by that many bytes and a
   354  // second slice that aliases into it and contains only the extra bytes. If the
   355  // original slice has sufficient capacity then no allocation is performed.
   356  func sliceForAppend(in []byte, n int) (head, tail []byte) {
   357  	if total := len(in) + n; cap(in) >= total {
   358  		head = in[:total]
   359  	} else {
   360  		head = make([]byte, total)
   361  		copy(head, in)
   362  	}
   363  	tail = head[len(in):]
   364  	return
   365  }
   366  
   367  // counterCrypt crypts in to out using g.cipher in counter mode.
   368  func (g *gcm) counterCrypt(out, in []byte, counter *[gcmBlockSize]byte) {
   369  	var mask [gcmBlockSize]byte
   370  
   371  	for len(in) >= gcmBlockSize {
   372  		g.cipher.Encrypt(mask[:], counter[:])
   373  		gcmInc32(counter)
   374  
   375  		xorWords(out, in, mask[:])
   376  		out = out[gcmBlockSize:]
   377  		in = in[gcmBlockSize:]
   378  	}
   379  
   380  	if len(in) > 0 {
   381  		g.cipher.Encrypt(mask[:], counter[:])
   382  		gcmInc32(counter)
   383  		xorBytes(out, in, mask[:])
   384  	}
   385  }
   386  
   387  // deriveCounter computes the initial GCM counter state from the given nonce.
   388  // See NIST SP 800-38D, section 7.1. This assumes that counter is filled with
   389  // zeros on entry.
   390  func (g *gcm) deriveCounter(counter *[gcmBlockSize]byte, nonce []byte) {
   391  	// GCM has two modes of operation with respect to the initial counter
   392  	// state: a "fast path" for 96-bit (12-byte) nonces, and a "slow path"
   393  	// for nonces of other lengths. For a 96-bit nonce, the nonce, along
   394  	// with a four-byte big-endian counter starting at one, is used
   395  	// directly as the starting counter. For other nonce sizes, the counter
   396  	// is computed by passing it through the GHASH function.
   397  	if len(nonce) == gcmStandardNonceSize {
   398  		copy(counter[:], nonce)
   399  		counter[gcmBlockSize-1] = 1
   400  	} else {
   401  		var y gcmFieldElement
   402  		g.update(&y, nonce)
   403  		y.high ^= uint64(len(nonce)) * 8
   404  		g.mul(&y)
   405  		binary.BigEndian.PutUint64(counter[:8], y.low)
   406  		binary.BigEndian.PutUint64(counter[8:], y.high)
   407  	}
   408  }
   409  
   410  // auth calculates GHASH(ciphertext, additionalData), masks the result with
   411  // tagMask and writes the result to out.
   412  func (g *gcm) auth(out, ciphertext, additionalData []byte, tagMask *[gcmTagSize]byte) {
   413  	var y gcmFieldElement
   414  	g.update(&y, additionalData)
   415  	g.update(&y, ciphertext)
   416  
   417  	y.low ^= uint64(len(additionalData)) * 8
   418  	y.high ^= uint64(len(ciphertext)) * 8
   419  
   420  	g.mul(&y)
   421  
   422  	binary.BigEndian.PutUint64(out, y.low)
   423  	binary.BigEndian.PutUint64(out[8:], y.high)
   424  
   425  	xorWords(out, out, tagMask[:])
   426  }
   427  

View as plain text