PHP mcrypt_decrypt. Same result for two differents strings

Question

I am using mcrypt_encrypt/decrypt to create a coupon whose code is passed through the URL.

public function getReservationIdFromHash($hash) {
    $base64Hash = pack('H*',$hash);
    $encrypted = base64_decode($base64Hash);
    $key = pack("H*", '0123456789abcdef0123456789abcdef');
    $iv =  pack("H*", "abcdef9876543210abcdef9876543210");
    $str = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, $encrypted, MCRYPT_MODE_CBC, $iv);
    return $str;    
}

public function encodeCode($code) {

    $key = pack("H*", '0123456789abcdef0123456789abcdef');
    $iv =  pack("H*", "abcdef9876543210abcdef9876543210");
    $cypherText = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $code, MCRYPT_MODE_CBC, $iv);
    $base64 = base64_encode($cypherText);
    $arr = unpack('H*',$base64);
    $dev = array_pop($arr);
    return $dev;       
}

But I am experiencing a weird problem because this encrypted code (which is the result of my function):

785738496771754c66595869566d73446970667444413d3d

and this:

785738496771754c66595869566d73446970667444413d3d44

always are decode to: "XXXXXXXXXXX" (the string I am using to testing), but my common sense says me that they shouldn't be decoded to the exact equal string.

I think I am doing something wrong but I can't imagine what is.

Thanks

zaph · Accepted Answer · 2016-03-04 17:27:26Z

1

Display $encrypted, you will probably see the same thing.

In short: if you supply the same (and valid) input to encryption/decryption function you will get the same output. In this case the two inputs must not be the same.

Both look like a hexadecimal display of Base64 encoded data.

The first output is 24 ASCII characters
xW8IgquLfYXiVmsDipftDA==,
the second output is 25 ASCII characters
xW8IgquLfYXiVmsDipftDA==D.

The first ends in ASCII == which can only occur at the end of a Base64 encoding. The second ends in ASCII ==D. My guess is that the Base64 decoding is stopping at the == and ignoring additional characters since that can only occur at the end of a Base64 encoded value.

Also the length of the second is not a valid Base64 length, Base64 encoding are always a multiple of 4-bytes with end padding with 0, 1 or 2 = characters.

edited Mar 4, 2016 at 17:27

answered Mar 4, 2016 at 17:22

zaph

113k21 gold badges196 silver badges234 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

PHP mcrypt_decrypt. Same result for two differents strings

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related