Should I Trim the Decrypted String after mcrypt_decrypt?
I have a code that goes something like:
$cipher_alg = MCRYPT_RIJNDAEL_128;
$decrypted_string = mcrypt_decrypt($cipher_alg, $key,
$encrypted_string , MCRYPT_MODE
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Actually both mcrypt_encrypt() and mcrypt_decrypt() as well as the other en-/decryption functons (like mcrypt_generic() or mdecrypt_generic()) do pad the
$dataparameter to a length ofn * <. The padding character is the> NULcharacter (\x0or\0) whereas the<depends on the cipher and the block cipher modes used. You should have a look at Block cipher modes of operation and Padding (cryptography).> The following is the output of mcrypt_get_block_size() for each of the available ciphers and modes on my machine. Obviously the function does not take into account that modes such as CFB, OFB and CTR do not require any special measures to handle messages whose lengths are not multiples of the block size, since they all work by XORing the plaintext with the output of the block cipher (quote from Wikipedia). CBC which is used in your example always requires that the final block is padded before encryption.
cast-128 cbc: 8 bytes cfb: 8 bytes ctr: 8 bytes ecb: 8 bytes ncfb: 8 bytes nofb: 8 bytes ofb: 8 bytes stream: not supported gost cbc: 8 bytes cfb: 8 bytes ctr: 8 bytes ecb: 8 bytes ncfb: 8 bytes nofb: 8 bytes ofb: 8 bytes stream: not supported rijndael-128 cbc: 16 bytes cfb: 16 bytes ctr: 16 bytes ecb: 16 bytes ncfb: 16 bytes nofb: 16 bytes ofb: 16 bytes stream: not supported twofish cbc: 16 bytes cfb: 16 bytes ctr: 16 bytes ecb: 16 bytes ncfb: 16 bytes nofb: 16 bytes ofb: 16 bytes stream: not supported arcfour cbc: not supported cfb: not supported ctr: not supported ecb: not supported ncfb: not supported nofb: not supported ofb: not supported stream: 1 bytes cast-256 cbc: 16 bytes cfb: 16 bytes ctr: 16 bytes ecb: 16 bytes ncfb: 16 bytes nofb: 16 bytes ofb: 16 bytes stream: not supported loki97 cbc: 16 bytes cfb: 16 bytes ctr: 16 bytes ecb: 16 bytes ncfb: 16 bytes nofb: 16 bytes ofb: 16 bytes stream: not supported rijndael-192 cbc: 24 bytes cfb: 24 bytes ctr: 24 bytes ecb: 24 bytes ncfb: 24 bytes nofb: 24 bytes ofb: 24 bytes stream: not supported saferplus cbc: 16 bytes cfb: 16 bytes ctr: 16 bytes ecb: 16 bytes ncfb: 16 bytes nofb: 16 bytes ofb: 16 bytes stream: not supported wake cbc: not supported cfb: not supported ctr: not supported ecb: not supported ncfb: not supported nofb: not supported ofb: not supported stream: 1 bytes blowfish-compat cbc: 8 bytes cfb: 8 bytes ctr: 8 bytes ecb: 8 bytes ncfb: 8 bytes nofb: 8 bytes ofb: 8 bytes stream: not supported des cbc: 8 bytes cfb: 8 bytes ctr: 8 bytes ecb: 8 bytes ncfb: 8 bytes nofb: 8 bytes ofb: 8 bytes stream: not supported rijndael-256 cbc: 32 bytes cfb: 32 bytes ctr: 32 bytes ecb: 32 bytes ncfb: 32 bytes nofb: 32 bytes ofb: 32 bytes stream: not supported serpent cbc: 16 bytes cfb: 16 bytes ctr: 16 bytes ecb: 16 bytes ncfb: 16 bytes nofb: 16 bytes ofb: 16 bytes stream: not supported xtea cbc: 8 bytes cfb: 8 bytes ctr: 8 bytes ecb: 8 bytes ncfb: 8 bytes nofb: 8 bytes ofb: 8 bytes stream: not supported blowfish cbc: 8 bytes cfb: 8 bytes ctr: 8 bytes ecb: 8 bytes ncfb: 8 bytes nofb: 8 bytes ofb: 8 bytes stream: not supported enigma cbc: not supported cfb: not supported ctr: not supported ecb: not supported ncfb: not supported nofb: not supported ofb: not supported stream: 1 bytes rc2 cbc: 8 bytes cfb: 8 bytes ctr: 8 bytes ecb: 8 bytes ncfb: 8 bytes nofb: 8 bytes ofb: 8 bytes stream: not supported tripledes cbc: 8 bytes cfb: 8 bytes ctr: 8 bytes ecb: 8 bytes ncfb: 8 bytes nofb: 8 bytes ofb: 8 bytes stream: not supportedTherefore you have to rtrim() the output of the decryption functions to get the original string if your cipher operates on fixed length blocks:
$output = rtrim($decrypted, "\0");
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