feat: add payment encryption helpers

Add AES-GCM encryption utilities for payment credentials with key length and ciphertext validation.

payment/crypto.go

@@ -0,0 +1,70 @@
+package payment
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"encoding/base64"
+	"fmt"
+	"io"
+)
+
+var encryptionKey []byte
+
+func InitEncryptionKey(key string) {
+	encryptionKey = []byte(key)
+}
+
+func Encrypt(plaintext string) (string, error) {
+	gcm, err := newGCM()
+	if err != nil {
+		return "", err
+	}
+
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return "", err
+	}
+
+	ciphertext := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
+	return base64.StdEncoding.EncodeToString(ciphertext), nil
+}
+
+func Decrypt(encoded string) (string, error) {
+	gcm, err := newGCM()
+	if err != nil {
+		return "", err
+	}
+
+	ciphertext, err := base64.StdEncoding.DecodeString(encoded)
+	if err != nil {
+		return "", err
+	}
+
+	nonceSize := gcm.NonceSize()
+	if len(ciphertext) < nonceSize+gcm.Overhead() {
+		return "", fmt.Errorf("ciphertext too short")
+	}
+
+	nonce := ciphertext[:nonceSize]
+	encrypted := ciphertext[nonceSize:]
+	plaintext, err := gcm.Open(nil, nonce, encrypted, nil)
+	if err != nil {
+		return "", err
+	}
+
+	return string(plaintext), nil
+}
+
+func newGCM() (cipher.AEAD, error) {
+	if len(encryptionKey) != 32 {
+		return nil, fmt.Errorf("invalid encryption key length: got %d, want 32", len(encryptionKey))
+	}
+
+	block, err := aes.NewCipher(encryptionKey)
+	if err != nil {
+		return nil, err
+	}
+
+	return cipher.NewGCM(block)
+}

payment/crypto_test.go

@@ -0,0 +1,64 @@
+package payment
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+const testEncryptionKey = "12345678901234567890123456789012"
+
+func setTestEncryptionKey(t *testing.T, key string) {
+	t.Helper()
+	previousKey := encryptionKey
+	InitEncryptionKey(key)
+	t.Cleanup(func() {
+		encryptionKey = previousKey
+	})
+}
+
+func TestEncryptDecryptRoundTrip(t *testing.T) {
+	setTestEncryptionKey(t, testEncryptionKey)
+
+	encrypted, err := Encrypt("hello payment")
+	require.NoError(t, err)
+	require.NotEmpty(t, encrypted)
+
+	decrypted, err := Decrypt(encrypted)
+	require.NoError(t, err)
+	require.Equal(t, "hello payment", decrypted)
+}
+
+func TestEncryptRejectsInvalidKeyLength(t *testing.T) {
+	setTestEncryptionKey(t, "short")
+
+	_, err := Encrypt("hello payment")
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "invalid encryption key length")
+}
+
+func TestDecryptRejectsInvalidBase64(t *testing.T) {
+	setTestEncryptionKey(t, testEncryptionKey)
+
+	_, err := Decrypt("not-base64")
+	require.Error(t, err)
+}
+
+func TestDecryptRejectsTooShortCiphertext(t *testing.T) {
+	setTestEncryptionKey(t, testEncryptionKey)
+
+	_, err := Decrypt("c2hvcnQ=")
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "ciphertext too short")
+}
+
+func TestEncryptUsesRandomNonce(t *testing.T) {
+	setTestEncryptionKey(t, testEncryptionKey)
+
+	first, err := Encrypt("same plaintext")
+	require.NoError(t, err)
+	second, err := Encrypt("same plaintext")
+	require.NoError(t, err)
+
+	require.NotEqual(t, first, second)
+}