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I have been trying to figure out how to decrypt strings using javascript that were encoded using codeiginter's encryption library.

So far I found this as a guide php to js-mcrypt

But I could not figure out how to supply the iv variable. Because codeiginter randomly generates it upon encryption.

My sample code is 

//PHP Side
    $this->encrypt->encode('apple','1234567');
    //The result is : 2lek4Q1mz4CJtTy2ot/uJWlfeGKuGiUKuKkR5Utkwc1nSWjf3JqG8gOhNmS13mt25QVbgP/2QOuffpn7rhIOmQ==


//JS Side
var encrypted = '2lek4Q1mz4CJtTy2ot/uJWlfeGKuGiUKuKkR5Utkwc1nSWjf3JqG8gOhNmS13mt25QVbgP/2QOuffpn7rhIOmQ==';



var key = 'fcea920f7412b5da7be0cf42b8c93759';//md5 version of "1234567"

var iv = 'some 32 length string';// I don't know how to get the IV because it constantly change in PHP

var decrypted = mcrypt.Decrypt(atob(encrypted), iv, key, 'rijndael-256', 'cbc');

                console.log(decrypted);
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1 Answers1

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A random iv is generally pre-pended to the encrypted data.

Simple encryption of 5 bytes ('apple') with padding using 'rijndael-256' would produce 32 bytes of output. In this case the encrypted output is 88-bytes so the iv is probably there along with something else.

Also mcrypt is somewhat brain-dead in that it does not support the standard PKCS#7 (AKA PKCS#5) padding so that is also an interoperability problem.

Note: 'rijndael-256' means a block size of 256-bits, not a key size and AES is essentially Rijndael with a block size of 128-bits, it is best to use a block size of 128-bits and be compatible with AES.

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