mcrypt

mcrypt-go

A Go library for Rijndael-256 encryption and decryption, designed to provide full compatibility with PHP's legacy mcrypt extension (MCRYPT_RIJNDAEL_256).

Why does this library exist?

Go's standard crypto/aes package implements the FIPS 197 AES standard. Although AES is based on Rijndael, AES strictly limits the block size to 128 bits (16 bytes), regardless of key length.

However, PHP's legacy mcrypt extension allowed using Rijndael with a 256-bit block size (32 bytes) via the MCRYPT_RIJNDAEL_256 constant.

Standard Go does not support 256-bit blocks. Attempting to decrypt data encrypted with MCRYPT_RIJNDAEL_256 using crypto/aes will fail or produce corrupted output.

This library implements the Rijndael-256 block cipher natively in Go, conforming to the standard cipher.Block interface, with ECB and CBC modes that exactly match PHP's behavior (including NUL-byte padding).

Features

• 100% PHP mcrypt compatibility: Encrypts and decrypts data identically to mcrypt_decrypt / mcrypt_encrypt with MCRYPT_RIJNDAEL_256.
• All standard block cipher modes: ECB, CBC, CFB, OFB, and CTR with a 32-byte (256-bit) block size.
• NUL-byte padding: Matches PHP mcrypt's default padding behavior.
• Standard Go interface: Implements cipher.Block for interoperability with Go's crypto tooling.
• URL-safe Base64: Handles URL-safe Base64 payloads (swapping -/_ for +//).
• Cryptographically secure IV generation: GenerateIV() using crypto/rand.
• Stream cipher wrapper: NewStreamWriter / NewStreamReader for chunk-based encryption of large data.
• Sensitive memory clearing: ClearMem for timing-safe zeroing of sensitive data.
• Specific error types: ErrInvalidIV, ErrInvalidKey, ErrInvalidNonce, ErrInvalidCiphertext, ErrEmptyPassword.

Installation

go get codeberg.org/simplicer/mcrypt-go

Usage

PHP mcrypt interoperability

If you're migrating a PHP API that encrypts credentials like this:

function decrypt($psw, $data){
    $data = str_replace(array('-','_'), array('+','/'), $data);
    $mod4 = strlen($data) % 4;
    if ($mod4) $data .= substr('====', $mod4);
    $decoded = base64_decode($data);
    return mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $psw, $decoded, MCRYPT_MODE_ECB);
}

The exact equivalent in Go:

package main

import (
	"fmt"
	"log"

	"codeberg.org/simplicer/mcrypt-go"
)

func main() {
	partnerPassword := "h3SdkVj2#!dkj3-D$5P"
	payload := "TQ3qLcs6FmgOBel-AAOhO97uE4SCRvj-uVbABOM_uNVF6k8JjwbLF1i7OFTfvedWGh9ZnMnW1PoKNmCKiYWT57kPmOVJC8Qi6K0wtKQxfbtenWCBv8ZricPjXXdJnvuPAvIRdDKO9wodxM_6OiSQp_8Dzku-3UMCRYo1cBCKv1Y"

	decrypted, err := mcrypt.DecryptPHP(partnerPassword, payload)
	if err != nil {
		log.Fatalf("decrypt error: %v", err)
	}

	fmt.Printf("Decrypted: %s\n", decrypted)
	// Output: {"password":"r3fZNZmHL2ra3h!","auth":"p79itgjfhbvou4df","admin":"ARB588SF-ITNIC","tech":"ARB5TECH-ITNIC"}

	encrypted, err := mcrypt.EncryptPHP(partnerPassword, decrypted)
	if err != nil {
		log.Fatalf("encrypt error: %v", err)
	}
	fmt.Printf("Encrypted: %s\n", encrypted)
}

CBC mode with IV

key := mcrypt.PadKey([]byte("my-secret-key"))
iv := make([]byte, mcrypt.BlockSize) // 32 bytes for Rijndael-256

ciphertext, err := mcrypt.EncryptCBC(key, plaintext, iv)
decrypted, err := mcrypt.DecryptCBC(key, ciphertext, iv)
result := mcrypt.UnpadNull(decrypted)

CFB mode

key := mcrypt.PadKey([]byte("my-secret-key"))
iv := make([]byte, mcrypt.BlockSize)
// Use GenerateIV() for cryptographically random IV
ciphertext, err := mcrypt.EncryptCFB(key, plaintext, iv)
decrypted, err := mcrypt.DecryptCFB(key, ciphertext, iv)

OFB mode

ciphertext, err := mcrypt.EncryptOFB(key, plaintext, iv)
decrypted, err := mcrypt.DecryptOFB(key, ciphertext, iv) // symmetric

CTR mode

nonce := make([]byte, mcrypt.BlockSize) // or use mcrypt.GenerateIV()
ciphertext, err := mcrypt.EncryptCTR(key, plaintext, nonce)
decrypted, err := mcrypt.DecryptCTR(key, ciphertext, nonce) // symmetric

Stream cipher (large data)

w := mcrypt.NewStreamWriter(key, mcrypt.CTR, nonce, os.Stdout)
defer w.Close()
io.Copy(w, largeFile)

r := mcrypt.NewStreamReader(key, mcrypt.CTR, nonce, encryptedFile)
io.Copy(os.Stdout, r)

Secure IV generation

iv, err := mcrypt.GenerateIV() // 32 bytes from crypto/rand
if err != nil {
    log.Fatal(err)
}
// Use iv with EncryptCBC/EncryptCFB/EncryptOFB — never reuse an IV

Memory clearing

mcrypt.ClearMem(sensitiveData) // zero memory timing-safely

Raw block cipher access

For advanced use cases where you need the raw Rijndael-256 block cipher:

block, err := mcrypt.NewCipher(key) // key must be 16, 24, or 32 bytes
if err != nil {
    log.Fatal(err)
}

// Use with any crypto/cipher mode that accepts cipher.Block.
// Remember the block size is always 32 bytes:
// cipher.NewCFBEncrypter(block, iv32bytes)

API reference

Error types

Testing

go test -v ./...

Versioning

This project follows Semantic Versioning with Conventional Commits. See CHANGELOG.md for details.

License

GNU General Public License v3.0 (GPL-3.0) — a strong copyleft license. Any application or library that uses or incorporates this package must be published and distributed under the same open-source terms. See LICENSE for details.

Author

Antonio Villamarin — antonio [at] simplicer [dot] com