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Asked: 2026-05-26T15:52:21+00:00

I’m trying to use ARCFOUR algorithm in my PHP code: $td = mcrypt_module_open(MCRYPT_ARCFOUR, ”,

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I’m trying to use ARCFOUR algorithm in my PHP code:

$td = mcrypt_module_open(MCRYPT_ARCFOUR, '', MCRYPT_MODE_CBC, '');
$iv = mcrypt_create_iv (mcrypt_enc_get_iv_size($td), MCRYPT_RAND);
mcrypt_generic_init($td, $key, $iv);
$output = mcrypt_generic($td, $input);
mcrypt_generic_deinit($td);
mcrypt_module_close($td);

and the problem is that the first line returns warning:

mcrypt_module_open(): Could not open encryption module

My settings:

  • From php_info() output:

    command configure: … “–with-mcrypt=static” …

    If I’m correct it means I do not need a DLL for mcrypt extension.

    Supported ciphers: cast-128 gost rijndael-128 twofish arcfour cast-256 loki97 rijndael-192 saferplus wake blowfish-compat des rijndael-256 serpent xtea blowfish enigma rc2 tripledes

  • PHP version 5.3.8

  • Wamp 2.2a (32 bits)

Thank you for help!

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1 Answer

  1. Editorial Team

    As of PHP 5.3.6 and 5.4.0 RC6, the arcfour, wake and enigma mcrypt ciphers require the use of stream mode. They will not initialize any of the other modes, nor can any of the other ciphers use stream mode.

    This may be true of earlier PHP versions as well.

    Demo code, with the @ present to silence the “can’t open module” warning:

    foreach(mcrypt_list_algorithms() as $alg) {
    printf("\n%20s:", $alg);
    foreach(mcrypt_list_modes() as $mode) {
        $mc = @mcrypt_module_open($alg, null, $mode, null);
        if(is_resource($mc)) echo "\t$mode";
        else echo "\t!!FAIL!!:$mode";
    }
}
echo "\n";

    Demo output:

            cast-128:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
            gost:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
    rijndael-128:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
         twofish:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
         arcfour:   !!FAIL!!:cbc    !!FAIL!!:cfb    !!FAIL!!:ctr    !!FAIL!!:ecb    !!FAIL!!:ncfb   !!FAIL!!:nofb   !!FAIL!!:ofb    stream
        cast-256:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
          loki97:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
    rijndael-192:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
       saferplus:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
            wake:   !!FAIL!!:cbc    !!FAIL!!:cfb    !!FAIL!!:ctr    !!FAIL!!:ecb    !!FAIL!!:ncfb   !!FAIL!!:nofb   !!FAIL!!:ofb    stream
 blowfish-compat:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
             des:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
    rijndael-256:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
         serpent:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
            xtea:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
        blowfish:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
          enigma:   !!FAIL!!:cbc    !!FAIL!!:cfb    !!FAIL!!:ctr    !!FAIL!!:ecb    !!FAIL!!:ncfb   !!FAIL!!:nofb   !!FAIL!!:ofb    stream
             rc2:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream
       tripledes:   cbc cfb ctr ecb ncfb    nofb    ofb !!FAIL!!:stream

    It looks like you (and I) are seeing PHP bug 49311, which was closed after no feedback in 2009. RC4, WAKE and Enigma are broken. Code to demonstrate the problem:

    foreach(mcrypt_list_algorithms() as $algo) {
    echo $algo; 
    $td = mcrypt_module_open($algo, '', MCRYPT_MODE_CBC, '');
    echo "\n";
}

    Output on my system, from the PHP interactive prompt:

    cast-128
gost
rijndael-128
twofish
arcfourPHP Warning:  mcrypt_module_open(): Could not open encryption module in php shell code on line 1
PHP Stack trace:
PHP   1. {main}() php shell code:0
PHP   2. mcrypt_module_open() php shell code:1

cast-256
loki97
rijndael-192
saferplus
wakePHP Warning:  mcrypt_module_open(): Could not open encryption module in php shell code on line 1
PHP Stack trace:
PHP   1. {main}() php shell code:0
PHP   2. mcrypt_module_open() php shell code:1

blowfish-compat
des
rijndael-256
serpent
xtea
blowfish
enigmaPHP Warning:  mcrypt_module_open(): Could not open encryption module in php shell code on line 1
PHP Stack trace:
PHP   1. {main}() php shell code:0
PHP   2. mcrypt_module_open() php shell code:1

rc2
tripledes

    Until (unless) they fix this bug or a workaround is found, you’ll want to choose a different encryption algorithm.

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