mcrypt encrypt adding s bunch of '%00' to end of string

Question

Working with OAuth and encrypting the keys with the following function with a string which we'll call 'foo' (actually an OAuth token)

public function encrypt( $text )
{
    // add end of text delimiter
    $data = mcrypt_encrypt( MCRYPT_RIJNDAEL_128, $this->key, $text, MCRYPT_MODE_ECB, $this->iv );
    return base64_encode( $data );
}

When I decrypt it using the inverse function, I end up with:

Function:

    public function decrypt( $text )
    {
        $text = base64_decode( $text );
        return mcrypt_decrypt( MCRYPT_RIJNDAEL_128, $this->key, $text, MCRYPT_MODE_ECB, $this->iv );
    }

Result:

foo%00%00%00%00%00%00%00%00%00%00%00%00%00%00

Edit:

Looking at it a little more, I realized that it is actually URL encoding to %00, which means that my strings are somehow being padded by null characters? So I am currently using trim() to get rid of them, but I would like to understand why this is happening.

Answer 1

Rijndael is a block cypher, which means that it operates on chunks of data of a particular length (128 bits in this case). This means that if the length of the input text is not a multiple of the block size, it must be padded out to fit. In this case, the padding is zeros; there are a number of other possible padding schemes that could be used, but if you want them with PHP's mcrypt you'll have to apply them manually.