cbc_hmac.go

Documentation: gopkg.in/square/go-jose.v2/cipher

     1  /*-
     2   * Copyright 2014 Square Inc.
     3   *
     4   * Licensed under the Apache License, Version 2.0 (the "License");
     5   * you may not use this file except in compliance with the License.
     6   * You may obtain a copy of the License at
     7   *
     8   *     http://www.apache.org/licenses/LICENSE-2.0
     9   *
    10   * Unless required by applicable law or agreed to in writing, software
    11   * distributed under the License is distributed on an "AS IS" BASIS,
    12   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13   * See the License for the specific language governing permissions and
    14   * limitations under the License.
    15   */
    16  
    17  package josecipher
    18  
    19  import (
    20  	"bytes"
    21  	"crypto/cipher"
    22  	"crypto/hmac"
    23  	"crypto/sha256"
    24  	"crypto/sha512"
    25  	"crypto/subtle"
    26  	"encoding/binary"
    27  	"errors"
    28  	"hash"
    29  )
    30  
    31  const (
    32  	nonceBytes = 16
    33  )
    34  
    35  // NewCBCHMAC instantiates a new AEAD based on CBC+HMAC.
    36  func NewCBCHMAC(key []byte, newBlockCipher func([]byte) (cipher.Block, error)) (cipher.AEAD, error) {
    37  	keySize := len(key) / 2
    38  	integrityKey := key[:keySize]
    39  	encryptionKey := key[keySize:]
    40  
    41  	blockCipher, err := newBlockCipher(encryptionKey)
    42  	if err != nil {
    43  		return nil, err
    44  	}
    45  
    46  	var hash func() hash.Hash
    47  	switch keySize {
    48  	case 16:
    49  		hash = sha256.New
    50  	case 24:
    51  		hash = sha512.New384
    52  	case 32:
    53  		hash = sha512.New
    54  	}
    55  
    56  	return &cbcAEAD{
    57  		hash:         hash,
    58  		blockCipher:  blockCipher,
    59  		authtagBytes: keySize,
    60  		integrityKey: integrityKey,
    61  	}, nil
    62  }
    63  
    64  // An AEAD based on CBC+HMAC
    65  type cbcAEAD struct {
    66  	hash         func() hash.Hash
    67  	authtagBytes int
    68  	integrityKey []byte
    69  	blockCipher  cipher.Block
    70  }
    71  
    72  func (ctx *cbcAEAD) NonceSize() int {
    73  	return nonceBytes
    74  }
    75  
    76  func (ctx *cbcAEAD) Overhead() int {
    77  	// Maximum overhead is block size (for padding) plus auth tag length, where
    78  	// the length of the auth tag is equivalent to the key size.
    79  	return ctx.blockCipher.BlockSize() + ctx.authtagBytes
    80  }
    81  
    82  // Seal encrypts and authenticates the plaintext.
    83  func (ctx *cbcAEAD) Seal(dst, nonce, plaintext, data []byte) []byte {
    84  	// Output buffer -- must take care not to mangle plaintext input.
    85  	ciphertext := make([]byte, uint64(len(plaintext))+uint64(ctx.Overhead()))[:len(plaintext)]
    86  	copy(ciphertext, plaintext)
    87  	ciphertext = padBuffer(ciphertext, ctx.blockCipher.BlockSize())
    88  
    89  	cbc := cipher.NewCBCEncrypter(ctx.blockCipher, nonce)
    90  
    91  	cbc.CryptBlocks(ciphertext, ciphertext)
    92  	authtag := ctx.computeAuthTag(data, nonce, ciphertext)
    93  
    94  	ret, out := resize(dst, uint64(len(dst))+uint64(len(ciphertext))+uint64(len(authtag)))
    95  	copy(out, ciphertext)
    96  	copy(out[len(ciphertext):], authtag)
    97  
    98  	return ret
    99  }
   100  
   101  // Open decrypts and authenticates the ciphertext.
   102  func (ctx *cbcAEAD) Open(dst, nonce, ciphertext, data []byte) ([]byte, error) {
   103  	if len(ciphertext) < ctx.authtagBytes {
   104  		return nil, errors.New("square/go-jose: invalid ciphertext (too short)")
   105  	}
   106  
   107  	offset := len(ciphertext) - ctx.authtagBytes
   108  	expectedTag := ctx.computeAuthTag(data, nonce, ciphertext[:offset])
   109  	match := subtle.ConstantTimeCompare(expectedTag, ciphertext[offset:])
   110  	if match != 1 {
   111  		return nil, errors.New("square/go-jose: invalid ciphertext (auth tag mismatch)")
   112  	}
   113  
   114  	cbc := cipher.NewCBCDecrypter(ctx.blockCipher, nonce)
   115  
   116  	// Make copy of ciphertext buffer, don't want to modify in place
   117  	buffer := append([]byte{}, []byte(ciphertext[:offset])...)
   118  
   119  	if len(buffer)%ctx.blockCipher.BlockSize() > 0 {
   120  		return nil, errors.New("square/go-jose: invalid ciphertext (invalid length)")
   121  	}
   122  
   123  	cbc.CryptBlocks(buffer, buffer)
   124  
   125  	// Remove padding
   126  	plaintext, err := unpadBuffer(buffer, ctx.blockCipher.BlockSize())
   127  	if err != nil {
   128  		return nil, err
   129  	}
   130  
   131  	ret, out := resize(dst, uint64(len(dst))+uint64(len(plaintext)))
   132  	copy(out, plaintext)
   133  
   134  	return ret, nil
   135  }
   136  
   137  // Compute an authentication tag
   138  func (ctx *cbcAEAD) computeAuthTag(aad, nonce, ciphertext []byte) []byte {
   139  	buffer := make([]byte, uint64(len(aad))+uint64(len(nonce))+uint64(len(ciphertext))+8)
   140  	n := 0
   141  	n += copy(buffer, aad)
   142  	n += copy(buffer[n:], nonce)
   143  	n += copy(buffer[n:], ciphertext)
   144  	binary.BigEndian.PutUint64(buffer[n:], uint64(len(aad))*8)
   145  
   146  	// According to documentation, Write() on hash.Hash never fails.
   147  	hmac := hmac.New(ctx.hash, ctx.integrityKey)
   148  	_, _ = hmac.Write(buffer)
   149  
   150  	return hmac.Sum(nil)[:ctx.authtagBytes]
   151  }
   152  
   153  // resize ensures that the given slice has a capacity of at least n bytes.
   154  // If the capacity of the slice is less than n, a new slice is allocated
   155  // and the existing data will be copied.
   156  func resize(in []byte, n uint64) (head, tail []byte) {
   157  	if uint64(cap(in)) >= n {
   158  		head = in[:n]
   159  	} else {
   160  		head = make([]byte, n)
   161  		copy(head, in)
   162  	}
   163  
   164  	tail = head[len(in):]
   165  	return
   166  }
   167  
   168  // Apply padding
   169  func padBuffer(buffer []byte, blockSize int) []byte {
   170  	missing := blockSize - (len(buffer) % blockSize)
   171  	ret, out := resize(buffer, uint64(len(buffer))+uint64(missing))
   172  	padding := bytes.Repeat([]byte{byte(missing)}, missing)
   173  	copy(out, padding)
   174  	return ret
   175  }
   176  
   177  // Remove padding
   178  func unpadBuffer(buffer []byte, blockSize int) ([]byte, error) {
   179  	if len(buffer)%blockSize != 0 {
   180  		return nil, errors.New("square/go-jose: invalid padding")
   181  	}
   182  
   183  	last := buffer[len(buffer)-1]
   184  	count := int(last)
   185  
   186  	if count == 0 || count > blockSize || count > len(buffer) {
   187  		return nil, errors.New("square/go-jose: invalid padding")
   188  	}
   189  
   190  	padding := bytes.Repeat([]byte{last}, count)
   191  	if !bytes.HasSuffix(buffer, padding) {
   192  		return nil, errors.New("square/go-jose: invalid padding")
   193  	}
   194  
   195  	return buffer[:len(buffer)-count], nil
   196  }
   197
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