In this blog post, we will explore the Go code for AES-256 encryption and decryption. AES (Advanced Encryption Standard) is a symmetric encryption algorithm widely used for securing sensitive data. AES-256 refers to AES with a key length of 256 bits, providing a high level of security.

EncryptAES256 Function 

func EncryptAES256(data []byte, passphrase string) (string, error) {
	if len(passphrase) < 32 {
		return "", errors.New("Passphrase must be 32 bytes")
	} else if len(passphrase) > 32 {
		// Use the first 32 bytes.
		passphrase = passphrase[:32]
	}

	block, err := aes.NewCipher([]byte(passphrase))
	if err != nil {
		return "", err
	}

	ciphertext := make([]byte, aes.BlockSize+len(data))
	iv := ciphertext[:aes.BlockSize]
	if _, err := io.ReadFull(rand.Reader, iv); err != nil {
		return "", err
	}

	stream := cipher.NewCFBEncrypter(block, iv)
	stream.XORKeyStream(ciphertext[aes.BlockSize:], data)

	return base64.StdEncoding.EncodeToString(ciphertext), nil
}

1. The function takes a slice of data and a passphrase as input parameters. 
2. It checks the length of the passphrase. AES-256 requires a 32-byte passphrase, so if the provided passphrase is shorter than 32 bytes, an error is returned.
3. If the passphrase is longer than 32 bytes, it is truncated to the first 32 bytes.
4. The function creates a new AES cipher block using the passphrase. 
5. An initialization vector (IV) is generated with a length equal to the AES block size (16 bytes) using io.ReadFull(rand.Reader, iv).
6. A cipher feedback (CFB) encrypter is created using the AES cipher block and IV.
7. The encryption process is performed by XORing the encrypted data with the original data using the XORKeyStream method of the encrypter.
8. The ciphertext, consisting of the IV followed by the encrypted data, is encoded using base64 and returned as a string.

DecryptAES256 Function

func DecryptAES256(ciphertext string, passphrase string) ([]byte, error) {
	if len(passphrase) < 32 {
		return nil, errors.New("Passphrase must be 32 bytes")
	} else if len(passphrase) > 32 {
		// Use the first 32 bytes.
		passphrase = passphrase[:32]
	}

	key := []byte(passphrase)
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, err
	}

	ciphertextBytes, err := base64.StdEncoding.DecodeString(ciphertext)
	if err != nil {
		return nil, err
	}

	if len(ciphertextBytes) < aes.BlockSize {
		return nil, errors.New("ciphertext too short")
	}

	iv := ciphertextBytes[:aes.BlockSize]
	ciphertextBytes = ciphertextBytes[aes.BlockSize:]
    
    stream := cipher.NewCFBDecrypter(block, iv)

	stream.XORKeyStream(ciphertextBytes, ciphertextBytes)

	return ciphertextBytes, nil
}