AES Decryption with ECB and PKCS5 Padding (Go and Java)

AES Decryption with ECB and PKCS5 Padding (Go and Java)

This article demonstrates AES decryption using Electronic Codebook (ECB) mode and PKCS5 padding, providing code examples in both Go and Java.

Go Implementation

func AesDecryptECBPKCS5(encrypted []byte, key []byte) (decrypted []byte, err error) {
	cipher, err := aes.NewCipher(key)
	if err != nil {
		return
	}
	decrypted = make([]byte, len(encrypted))
	// Decrypt each block
	for bs, be := 0, cipher.BlockSize(); bs < len(encrypted); bs, be = bs+cipher.BlockSize(), be+cipher.BlockSize() {
		cipher.Decrypt(decrypted[bs:be], encrypted[bs:be])
	}

	trim := 0
	if len(decrypted) > 0 {
		trim = len(decrypted) - int(decrypted[len(decrypted)-1])
	}

	return decrypted[:trim], nil
}

Java Implementation

import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;

public class AesDecryptECBPKCS5 {
    public static byte[] aesDecryptECBPKCS5(byte[] encrypted, byte[] key) throws Exception {
        SecretKeySpec secretKeySpec = new SecretKeySpec(key, 'AES');
        Cipher cipher = Cipher.getInstance('AES/ECB/PKCS5Padding');
        cipher.init(Cipher.DECRYPT_MODE, secretKeySpec);

        byte[] decrypted = cipher.doFinal(encrypted);

        int trim = 0;
        if (decrypted.length > 0) {
            trim = decrypted.length - (int) decrypted[decrypted.length - 1];
        }

        byte[] result = new byte[trim];
        System.arraycopy(decrypted, 0, result, 0, trim);
        return result;
    }
}

Explanation:

• AES: The Advanced Encryption Standard (AES) is a symmetric block cipher widely used for secure communication.
• ECB (Electronic Codebook): ECB is a basic encryption mode where each block of plaintext is encrypted independently. While simple, ECB can be vulnerable to attacks if used without proper padding.
• PKCS5 Padding: PKCS5 padding ensures that the plaintext data is a multiple of the block size. This is crucial for ECB mode to prevent attacks that exploit incomplete blocks.

Code Breakdown:

1. Key and Cipher Initialization:

   • The code first creates a cipher object (Go: aes.NewCipher, Java: Cipher.getInstance) using the provided key. This sets up the encryption algorithm and mode.

2. Decryption:

   • The Decrypt function (Go) or doFinal method (Java) is used to decrypt the encrypted data. Each block of data is decrypted individually.

3. PKCS5 Padding Removal:

   • The code removes the PKCS5 padding from the decrypted data. This involves identifying the padding bytes and trimming them off.

Note: ECB is generally not recommended for sensitive data due to its potential vulnerabilities. For most applications, other modes like CBC or GCM are preferred.