swisspost-evoting-go-poc/pkg/symmetric/aesgcm.go

package symmetric

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"fmt"
)

// Encrypt encrypts plaintext using AES-256-GCM.
// Returns (ciphertext, nonce).
func Encrypt(key, plaintext, associatedData []byte) (ciphertext, nonce []byte, err error) {
	if len(key) != 32 {
		return nil, nil, fmt.Errorf("key must be 32 bytes (AES-256), got %d", len(key))
	}

	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, nil, fmt.Errorf("creating AES cipher: %w", err)
	}

	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, nil, fmt.Errorf("creating GCM: %w", err)
	}

	nonce = make([]byte, gcm.NonceSize())
	if _, err := rand.Read(nonce); err != nil {
		return nil, nil, fmt.Errorf("generating nonce: %w", err)
	}

	ciphertext = gcm.Seal(nil, nonce, plaintext, associatedData)
	return ciphertext, nonce, nil
}

// Decrypt decrypts ciphertext using AES-256-GCM.
func Decrypt(key, ciphertext, nonce, associatedData []byte) ([]byte, error) {
	if len(key) != 32 {
		return nil, fmt.Errorf("key must be 32 bytes (AES-256), got %d", len(key))
	}

	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, fmt.Errorf("creating AES cipher: %w", err)
	}

	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("creating GCM: %w", err)
	}

	plaintext, err := gcm.Open(nil, nonce, ciphertext, associatedData)
	if err != nil {
		return nil, fmt.Errorf("decrypting: %w", err)
	}

	return plaintext, nil
}

// EncryptWithNonce encrypts plaintext using AES-256-GCM with a specified nonce.
func EncryptWithNonce(key, plaintext, nonce, associatedData []byte) ([]byte, error) {
	if len(key) != 32 {
		return nil, fmt.Errorf("key must be 32 bytes (AES-256), got %d", len(key))
	}

	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, fmt.Errorf("creating AES cipher: %w", err)
	}

	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("creating GCM: %w", err)
	}

	return gcm.Seal(nil, nonce, plaintext, associatedData), nil
}