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

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