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14.8. Encrypting and Decrypting Data

14.8.3. Discussion

The mcrypt extension is an interface with mcrypt, a library that implements many different encryption algorithms. The data is encrypted and decrypted by mcrypt_encrypt( ) and mcrypt_decrypt( ), respectively. They each take five arguments. The first is the algorithm to use. To find which algorithms mcrypt supports on your system, call mcrypt_list_algorithms( ). The full list of mcrypt algorithms is shown in Table 14-1. The second argument is the encryption key; the third argument is the data to encrypt or decrypt. The fourth argument is the mode for the encryption or decryption (a list of supported modes is returned by mcrypt_list_modes( )). The fifth argument is an initialization vector (IV), used by some modes as part of the encryption or decryption process.

Table 14-1 lists all the possible mcrypt algorithms, including the constant value used to indicate the algorithm, the key and block sizes in bits, and whether the algorithm is supported by libmcrypt 2.2.x and 2.4.x.

Table 14-1. mcrypt algorithm constants

Algorithm constant

Description

Key size

Block size

2.2.x

2.4.x

MCRYPT_3DES

Triple DES

168 (112 effective)

64

Yes

Yes

MCRYPT_TRIPLEDES

Triple DES

168 (112 effective)

64

No

Yes

MCRYPT_3WAY

3way (Joan Daemen)

96

96

Yes

No

MCRYPT_THREEWAY

3way

96

96

Yes

Yes

MCRYPT_BLOWFISH

Blowfish (Bruce Schneier)

Up to 448

64

No

Yes

MCRYPT_BLOWFISH_COMPAT

Blowfish with compatibility to other implementations

Up to 448

64

No

Yes

MCRYPT_BLOWFISH_128

Blowfish

128

64

Yes

No

MCRYPT_BLOWFISH_192

Blowfish

192

64

Yes

 

MCRYPT_BLOWFISH_256

Blowfish

256

64

Yes

No

MCRYPT_BLOWFISH_448

Blowfish

448

64

Yes

No

MCRYPT_CAST_128

CAST (Carlisle Adams and Stafford Tavares)

128

64

Yes

Yes

MCRYPT_CAST_256

CAST

256

128

Yes

Yes

MCRYPT_CRYPT

One-rotor Unix crypt

104

8

 

Yes

MCRYPT_ENIGNA

One-rotor Unix crypt

104

8

No

Yes

MCRYPT_DES

U.S. Data Encryption Standard

56

64

Yes

Yes

MCRYPT_GOST

Soviet Gosudarstvennyi Standard ("Government Standard")

256

64

Yes

Yes

MCRYPT_IDEA

International Data Encryption Algorithm

128

64

Yes

Yes

MCRYPT_LOKI97

LOKI97 (Lawrie Brown, Josef Pieprzyk)

128, 192, or 256

64

Yes

Yes

MCRYPT_MARS

MARS (IBM)

128-448

128

No

Yes

MCRYPT_PANAMA

PANAMA (Joan Daemen, Craig Clapp)

-

Stream

No

Yes

MCRYPT_RC2

Rivest Cipher 2

8-1024

64

No

Yes

MCRYPT_RC2_1024

Rivest Cipher 2

1024

64

Yes

No

MCRYPT_RC2_128

Rivest Cipher 2

128

64

Yes

No

MCRYPT_RC2_256

Rivest Cipher 2

256

64

Yes

No

MCRYPT_RC4

Rivest Cipher 4

Up to 2048

Stream

Yes

No

MCRYPT_ARCFOUR

Non-trademarked RC4 compatible

Up to 2048

Stream

No

Yes

MCRYPT_ARCFOUR_IV

Arcfour with Initialization Vector

Up to 2048

Stream

No

Yes

MCRYPT_RC6

Rivest Cipher 6

128, 192, or 256

128

No

Yes

MCRYPT_RC6_128

Rivest Cipher 6

128

128

Yes

No

MCRYPT_RC6_192

Rivest Cipher 6

192

128

Yes

No

MCRYPT_RC6_256

Rivest Cipher 6

256

128

Yes

No

MCRYPT_RIJNDAEL_128

Rijndael (Joan Daemen, Vincent Rijmen)

128

128

Yes

Yes

MCRYPT_RIJNDAEL_192

Rijndael

192

192

Yes

Yes

MCRYPT_RIJNDAEL_256

Rijndael

256

256

Yes

Yes

MCRYPT_SAFERPLUS

SAFER+ (based on SAFER)

128, 192, or 256

128

Yes

Yes

MCRYPT_SAFER_128

Secure And Fast Encryption Routine with strengthened key schedule

128

64

Yes

Yes

MCRYPT_SAFER_64

Secure And Fast Encryption Routine with strengthened key

64

64

Yes

Yes

MCRYPT_SERPENT

Serpent (Ross Anderson, Eli Biham, Lars Knudsen)

128, 192, or 256

128

No

Yes

MCRYPT_SERPENT_128

Serpent

128

128

Yes

No

MCRYPT_SERPENT_192

Serpent

192

128

Yes

No

MCRYPT_SERPENT_256

Serpent

256

128

Yes

No

MCRYPT_SKIPJACK

U.S. NSA Clipper Escrowed Encryption Standard

80

64

No

Yes

MCRYPT_TWOFISH

Twofish (Counterpane Systems)

128, 192, or 256

128

No

Yes

MCRYPT_TWOFISH_128

Twofish

128

128

Yes

No

MCRYPT_TWOFISH_192

Twofish

192

128

Yes

No

MCRYPT_TWOFISH_256

Twofish

256

128

Yes

No

MCRYPT_WAKE

Word Auto Key Encryption (David Wheeler)

256

32

No

Yes

MCRYPT_XTEA

Extended Tiny Encryption Algorithm (David Wheeler, Roger Needham)

128

64

Yes

Yes

Except for the data to encrypt or decrypt, all the other arguments must be the same when encrypting and decrypting. If you're using a mode that requires an initialization vector, it's okay to pass the initialization vector in the clear with the encrypted text.

The different modes are appropriate in different circumstances. Cipher Block Chaining (CBC) mode encrypts the data in blocks, and uses the encrypted value of each block (as well as the key) to compute the encrypted value of the next block. The initialization vector affects the encrypted value of the first block. Cipher Feedback (CFB) and Output Feedback (OFB) also use an initialization vector, but they encrypt data in units smaller than the block size. Note that OFB mode has security problems if you encrypt data in smaller units than its block size. Electronic Code Book (ECB) mode encrypts data in discreet blocks that don't depend on each other. ECB mode doesn't use an initialization vector. It is also less secure than other modes for repeated use, because the same plaintext with a given key always produces the same ciphertext. Constants to set each mode are listed in Table 14-2.

Table 14-2. mcrypt mode constants

Mode constant

Description

MCRYPT_MODE_ECB

Electronic Code Book mode

MCRYPT_MODE_CBC

Cipher Block Chaining mode

MCRYPT_MODE_CFB

Cipher Feedback mode

MCRYPT_MODE_OFB

Output Feedback mode with 8 bits of feedback

MCRYPT_MODE_NOFB

Output Feedback mode with n bits of feedback, where n is the block size of the algorithm used (libmcrypt 2.4 and higher only)

MCRYPT_MODE_STREAM

Stream Cipher mode, for algorithms such as RC4 and WAKE (libmcrypt 2.4 and higher only)

Different algorithms have different block sizes. You can retrieve the block size for a particular algorithm with mcrypt_get_block_size( ) . Similarly, the initialization vector size is determined by the algorithm and the mode. mcrypt_create_iv( ) and mcrypt_get_iv_size( ) make it easy to create an appropriate random initialization vector:

$iv = mcrypt_create_iv(mcrypt_get_iv_size($alg,$mode),MCRYPT_DEV_URANDOM);

The first argument to mcrypt_create_iv( ) is the size of the vector, and the second is a source of randomness. You have three choices for the source of randomness. MCRYPT_DEV_RANDOM reads from the pseudodevice /dev/random, MCRYPT_DEV_URANDOM reads from the pseudo-device /dev/urandom, and MCRYPT_RAND uses an internal random number generator. Not all operating systems support random-generating pseudo-devices. Make sure to call srand( ) before using MCRYPT_RAND in order to get a nonrepeating random number stream.

The code and examples in this recipe are compatible with mcrypt 2.4. PHP's mcrypt interface supports both mcrypt 2.2 and mcrypt 2.4, but there are differences between the two. With mcrypt 2.2, PHP supports only the following mcrypt functions: mcrypt_ecb( ), mcrypt_cbc( ), mcrypt_cfb( ), mcrypt_ofb( ), mcrypt_get_key_size( ), mcrypt_get_block_size( ), mcrypt_get_cipher_name( ), and mcrypt_create_iv( ). To encrypt or decrypt data with mcrypt 2.2, call the appropriate mcrypt_MODE( ) function, based on what mode you want to use, and pass it an argument that instructs it to encrypt or decrypt. The following code is the mcrypt 2.2-compatible version of the code in the Solution:

$key  = 'That golden key that opes the palace of eternity.';
$data = 'The chicken escapes at dawn. Send help with Mr. Blue.';
$alg = MCRYPT_BLOWFISH;

$iv = mcrypt_create_iv(mcrypt_get_block_size($alg),MCRYPT_DEV_URANDOM);
$encrypted_data = mcrypt_cbc($alg,$key,$data,MCRYPT_ENCRYPT);
$plain_text = base64_encode($encrypted_data);

print $plain_text."\n";

$decoded = mcrypt_cbc($alg,$key,base64_decode($plain_text),MCRYPT_DECRYPT);

print $decoded."\n";

14.8.4. See Also

Documentation on the mcrypt extension at http://www.php.net/mcrypt; the mcrypt library is available at http://mcrypt.hellug.gr/; choosing an appropriate algorithm and using it securely requires care and planning: for more information about mcrypt and the cipher algorithms it uses, see the online PHP manual section on mcrypt, the mcrypt home page, and the manpages for /dev/random and /dev/urandom; good books about cryptography include Applied Cryptography, by Bruce Schneier (Wiley) and Cryptography: Theory and Practice, by Douglas R. Stinson (Chapman & Hall).



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