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Source file src/crypto/aes/aes_gcm.go

Documentation: crypto/aes

     1  // Copyright 2015 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  // +build amd64 arm64
     6  
     7  package aes
     8  
     9  import (
    10  	"crypto/cipher"
    11  	subtleoverlap "crypto/internal/subtle"
    12  	"crypto/subtle"
    13  	"errors"
    14  )
    15  
    16  // The following functions are defined in gcm_*.s.
    17  
    18  //go:noescape
    19  func gcmAesInit(productTable *[256]byte, ks []uint32)
    20  
    21  //go:noescape
    22  func gcmAesData(productTable *[256]byte, data []byte, T *[16]byte)
    23  
    24  //go:noescape
    25  func gcmAesEnc(productTable *[256]byte, dst, src []byte, ctr, T *[16]byte, ks []uint32)
    26  
    27  //go:noescape
    28  func gcmAesDec(productTable *[256]byte, dst, src []byte, ctr, T *[16]byte, ks []uint32)
    29  
    30  //go:noescape
    31  func gcmAesFinish(productTable *[256]byte, tagMask, T *[16]byte, pLen, dLen uint64)
    32  
    33  const (
    34  	gcmBlockSize         = 16
    35  	gcmTagSize           = 16
    36  	gcmMinimumTagSize    = 12 // NIST SP 800-38D recommends tags with 12 or more bytes.
    37  	gcmStandardNonceSize = 12
    38  )
    39  
    40  var errOpen = errors.New("cipher: message authentication failed")
    41  
    42  // aesCipherGCM implements crypto/cipher.gcmAble so that crypto/cipher.NewGCM
    43  // will use the optimised implementation in this file when possible. Instances
    44  // of this type only exist when hasGCMAsm returns true.
    45  type aesCipherGCM struct {
    46  	aesCipherAsm
    47  }
    48  
    49  // Assert that aesCipherGCM implements the gcmAble interface.
    50  var _ gcmAble = (*aesCipherGCM)(nil)
    51  
    52  // NewGCM returns the AES cipher wrapped in Galois Counter Mode. This is only
    53  // called by crypto/cipher.NewGCM via the gcmAble interface.
    54  func (c *aesCipherGCM) NewGCM(nonceSize, tagSize int) (cipher.AEAD, error) {
    55  	g := &gcmAsm{ks: c.enc, nonceSize: nonceSize, tagSize: tagSize}
    56  	gcmAesInit(&g.productTable, g.ks)
    57  	return g, nil
    58  }
    59  
    60  type gcmAsm struct {
    61  	// ks is the key schedule, the length of which depends on the size of
    62  	// the AES key.
    63  	ks []uint32
    64  	// productTable contains pre-computed multiples of the binary-field
    65  	// element used in GHASH.
    66  	productTable [256]byte
    67  	// nonceSize contains the expected size of the nonce, in bytes.
    68  	nonceSize int
    69  	// tagSize contains the size of the tag, in bytes.
    70  	tagSize int
    71  }
    72  
    73  func (g *gcmAsm) NonceSize() int {
    74  	return g.nonceSize
    75  }
    76  
    77  func (g *gcmAsm) Overhead() int {
    78  	return g.tagSize
    79  }
    80  
    81  // sliceForAppend takes a slice and a requested number of bytes. It returns a
    82  // slice with the contents of the given slice followed by that many bytes and a
    83  // second slice that aliases into it and contains only the extra bytes. If the
    84  // original slice has sufficient capacity then no allocation is performed.
    85  func sliceForAppend(in []byte, n int) (head, tail []byte) {
    86  	if total := len(in) + n; cap(in) >= total {
    87  		head = in[:total]
    88  	} else {
    89  		head = make([]byte, total)
    90  		copy(head, in)
    91  	}
    92  	tail = head[len(in):]
    93  	return
    94  }
    95  
    96  // Seal encrypts and authenticates plaintext. See the cipher.AEAD interface for
    97  // details.
    98  func (g *gcmAsm) Seal(dst, nonce, plaintext, data []byte) []byte {
    99  	if len(nonce) != g.nonceSize {
   100  		panic("crypto/cipher: incorrect nonce length given to GCM")
   101  	}
   102  	if uint64(len(plaintext)) > ((1<<32)-2)*BlockSize {
   103  		panic("crypto/cipher: message too large for GCM")
   104  	}
   105  
   106  	var counter, tagMask [gcmBlockSize]byte
   107  
   108  	if len(nonce) == gcmStandardNonceSize {
   109  		// Init counter to nonce||1
   110  		copy(counter[:], nonce)
   111  		counter[gcmBlockSize-1] = 1
   112  	} else {
   113  		// Otherwise counter = GHASH(nonce)
   114  		gcmAesData(&g.productTable, nonce, &counter)
   115  		gcmAesFinish(&g.productTable, &tagMask, &counter, uint64(len(nonce)), uint64(0))
   116  	}
   117  
   118  	encryptBlockAsm(len(g.ks)/4-1, &g.ks[0], &tagMask[0], &counter[0])
   119  
   120  	var tagOut [gcmTagSize]byte
   121  	gcmAesData(&g.productTable, data, &tagOut)
   122  
   123  	ret, out := sliceForAppend(dst, len(plaintext)+g.tagSize)
   124  	if subtleoverlap.InexactOverlap(out[:len(plaintext)], plaintext) {
   125  		panic("crypto/cipher: invalid buffer overlap")
   126  	}
   127  	if len(plaintext) > 0 {
   128  		gcmAesEnc(&g.productTable, out, plaintext, &counter, &tagOut, g.ks)
   129  	}
   130  	gcmAesFinish(&g.productTable, &tagMask, &tagOut, uint64(len(plaintext)), uint64(len(data)))
   131  	copy(out[len(plaintext):], tagOut[:])
   132  
   133  	return ret
   134  }
   135  
   136  // Open authenticates and decrypts ciphertext. See the cipher.AEAD interface
   137  // for details.
   138  func (g *gcmAsm) Open(dst, nonce, ciphertext, data []byte) ([]byte, error) {
   139  	if len(nonce) != g.nonceSize {
   140  		panic("crypto/cipher: incorrect nonce length given to GCM")
   141  	}
   142  	// Sanity check to prevent the authentication from always succeeding if an implementation
   143  	// leaves tagSize uninitialized, for example.
   144  	if g.tagSize < gcmMinimumTagSize {
   145  		panic("crypto/cipher: incorrect GCM tag size")
   146  	}
   147  
   148  	if len(ciphertext) < g.tagSize {
   149  		return nil, errOpen
   150  	}
   151  	if uint64(len(ciphertext)) > ((1<<32)-2)*uint64(BlockSize)+uint64(g.tagSize) {
   152  		return nil, errOpen
   153  	}
   154  
   155  	tag := ciphertext[len(ciphertext)-g.tagSize:]
   156  	ciphertext = ciphertext[:len(ciphertext)-g.tagSize]
   157  
   158  	// See GCM spec, section 7.1.
   159  	var counter, tagMask [gcmBlockSize]byte
   160  
   161  	if len(nonce) == gcmStandardNonceSize {
   162  		// Init counter to nonce||1
   163  		copy(counter[:], nonce)
   164  		counter[gcmBlockSize-1] = 1
   165  	} else {
   166  		// Otherwise counter = GHASH(nonce)
   167  		gcmAesData(&g.productTable, nonce, &counter)
   168  		gcmAesFinish(&g.productTable, &tagMask, &counter, uint64(len(nonce)), uint64(0))
   169  	}
   170  
   171  	encryptBlockAsm(len(g.ks)/4-1, &g.ks[0], &tagMask[0], &counter[0])
   172  
   173  	var expectedTag [gcmTagSize]byte
   174  	gcmAesData(&g.productTable, data, &expectedTag)
   175  
   176  	ret, out := sliceForAppend(dst, len(ciphertext))
   177  	if subtleoverlap.InexactOverlap(out, ciphertext) {
   178  		panic("crypto/cipher: invalid buffer overlap")
   179  	}
   180  	if len(ciphertext) > 0 {
   181  		gcmAesDec(&g.productTable, out, ciphertext, &counter, &expectedTag, g.ks)
   182  	}
   183  	gcmAesFinish(&g.productTable, &tagMask, &expectedTag, uint64(len(ciphertext)), uint64(len(data)))
   184  
   185  	if subtle.ConstantTimeCompare(expectedTag[:g.tagSize], tag) != 1 {
   186  		for i := range out {
   187  			out[i] = 0
   188  		}
   189  		return nil, errOpen
   190  	}
   191  
   192  	return ret, nil
   193  }
   194  

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