BlockCipher

Block ciphers

The block cipher is one of the two major categories of symmetric ciphers. It operates on fixed-length groups of bits, called blocks. It means that if you want to encrypt 64 bytes of data, and the block size is 128 bits (16 bytes), data will be splitted in 4 blocks of 16 bytes and each block will be encrypted separately. A block cipher is usually used with a cipher mode of operation.

A cipher is said "symmetric" when it uses only one key : the same key is used to encrypt and decrypt data.

Usage

Block cipher classes implements the Cryptopp\BlockCipherInterface interface.

$o = new Cryptopp\BlockCipherAes();

// returns the name of the block cipher
$o->getName();

// returns the block size
$o->getBlockSize();

// indicates if a given key length is valid
$o->isValidKeyLength(10);

Encrypt/decrypt data

A block cipher should not be used as is. Instead, use a cipher mode of operation.

Using the cipher without a mode

To encrypt/decrypt data, use the encrypt() and decrypt() methods. Data length have to be a multiple of the cipher block size.

$o = new Cryptopp\BlockCipherAes();
$o->setKey("your secret key");

$ciphertext = $o->encrypt("this string length has to be a multiple of the cipher block size");

$plaintext = $o->decrypt($ciphertext);

To encrypt/decrypt a single block, use the encryptBlock() and decryptBlock() methods :

$o = new Cryptopp\BlockCipherAes();
$o->setKey("your secret key");

$ciphertext = $o->encrypt("this string length has to match exactly the cipher block size");

$plaintext = $o->decrypt($ciphertext);

Extending

Extending a block cipher class is possible to add some methods, but existing methods are not overwritable. Also, you are required to call the parent constructor.

class MyAes extends Cryptopp\BlockCipherAes
{
    public function __construct()
    {
        parent::__construct();
    }

    public function myMethod()
    {
        // ...
    }
}

$o = new MyAes();

Create a block cipher class

You can create your own block cipher class by implementing the Cryptopp\BlockCipherInterface interface. The following methods are required :

Method signature	Descritption
getName()	Returns the cipher name
getBlockSize()	Returns the block size (in bytes)
encrypt($data)	Encrypts data
decrypt($data)	Decrypts data
encryptBlock($block)	Encrypts a single data block
decryptBlock($block)	Decrypts a single data block
setKey($key)	Sets the key
getKey()	Returns the key
isValidKeyLength($length)	Indicates if a given key length is valid for this algorithm

Block cipher classes

AES

The Advanced Encryption Standard (AES) is based on the Rijndael cipher developed by two Belgian cryptographers, Joan Daemen and Vincent Rijmen. This is the recommended block cipher.

• Class : Cryptopp\BlockCipherAes
• Block size : 128 bits (16 bytes)
• Key size : 128 bits (16 bytes), 192 bits (24 bytes) or 256 bits (32 bytes)