PHP Security and Encryption Encrypting and Decrypting Data

PHP Security and Encryption

Encrypting and Decrypting Data

Problem

You want to encrypt and decrypt data using one of a variety of popular algorithms.

Solution

Use PHP’s mcrypt extension:

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

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

       $encrypted_data = mcrypt_encrypt($algorithm, $key, $data, $mode, $iv);
       $plain_text = base64_encode($encrypted_data);
       echo $plain_text . "\n";

       $encrypted_data = base64_decode($plain_text);
       $decoded = mcrypt_decrypt($algorithm, $key, $encrypted_data, $mode, $iv);
       // trim() will remove any trailing NULL bytes that mcrypt_decrypt() may
       // have added to pad the output to be a whole number of 8-byte blocks
       echo trim($decoded) . "\n";

This prints:

Cd4Uzc1c5lDxxWc7rXv+mbsElwj2ENrYg5HAPiaOpe7Wr8UAG5aXD9CoG6NdKoOWLSumg9ffSnE=
The chicken escapes at dawn. Send help with Mr. Blue.

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 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.

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 OK 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 discrete 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 plain text with a given key always produces the same cipher text. Constants to set each mode are listed in Table.

Table 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
MCRYPT_MODE_STREAM  Stream Cipher mode, for algorithms such as RC4 and WAKE
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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:

       $iv = mcrypt_create_iv(mcrypt_get_iv_size($algorithm, $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 pseudodevice /dev/urandom, and MCRYPT_RAND uses an internal random number generator. Not all operating systems support random-generating pseudodevices. Make sure to call srand() before using MCRYPT_RAND in order to get a nonrepeating random number stream.