Decrypting using Go a string encrypted in php using MCRYPT_RIJNDAEL_256

In PHP, mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CBC); returns a value of 32, thus apparently saying that AES-256 wants an Initialization Vector of 32 bytes. But this is deceiving, as said in the comments for mcrypt_encrypt:

>Also, MCRYPT_RIJNDAEL_256 is not AES-256, it's a different variant of the Rijndael block cipher. If you want AES-256 in mcrypt, you have to use MCRYPT_RIJNDAEL_128 with a 32-byte key. OpenSSL makes it more obvious which mode you are using (i.e. 'aes-128-cbc' vs 'aes-256-ctr').

So of course, with an IV of 32 bytes, the following example does not work in Go (it causes a panic).

score := decodePost(c.PostForm("score"))
iv := decodePost(c.PostForm("iv"))

aesKey := getAESKey()
baseAES, err := aes.NewCipher([]byte(aesKey))
if err != nil {
	c.AbortWithError(500, err)
	return
}
block := cipher.NewCBCDecrypter(baseAES, []byte(iv))
block.CryptBlocks(score, score)

Quoting from the docs of crypto/cipher:

>The length of iv must be the same as the Block's block size and must match the iv used to encrypt the data.

(And of course, the AES block size in Go is 16 bytes).

So, finally, how can I decrypt such string in Go?

Since in Golang you have only built-in standard AES crypto, you need to implement the Rijndael crypto yourself.

I found one here: https://github.com/celso-wo/rijndael256/blob/master/rijndael256.go

Several years later, I've been told by a colleague of mine which stumbled upon this issue again that this gist is a good implementation of the algorithm, while alas the one posted by itscaro didn't seem to work:

https://gist.github.com/slifer2015/0d182d99280758a154e3fe12e48664d4

This is essentially the code from the Go crypto/aes library, adjusted to use the 32 byte IV and Rijnadael256 algorithm.

As I've not yet had the time to verify this solution myself I will not yet accept this solution, though if I have time I'll write some tests to make sure that the code does manage to encrypt/decrypt precisely as the PHP implementation.

Just as a warning: always read carefully crypto code you find on the internet, and please only use this algorithm if you do find yourself as well in need of supporting it to decode data outside of your control. For normal symmetric and asymmetric encryption, the go crypto packages provide safe and battle-tested well-supported cryptographic solutions such as RSA, Ed25519 and AES.