feat(crypto): AES-256-GCM password encryption

internal/crypto/crypto.go

@@ -0,0 +1,47 @@
+package crypto
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"encoding/base64"
+	"errors"
+	"io"
+)
+
+func Encrypt(key, plaintext []byte) (string, error) {
+	gcm, err := newGCM(key)
+	if err != nil {
+		return "", err
+	}
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return "", err
+	}
+	ct := gcm.Seal(nonce, nonce, plaintext, nil)
+	return base64.StdEncoding.EncodeToString(ct), nil
+}
+
+func Decrypt(key []byte, enc string) ([]byte, error) {
+	gcm, err := newGCM(key)
+	if err != nil {
+		return nil, err
+	}
+	raw, err := base64.StdEncoding.DecodeString(enc)
+	if err != nil {
+		return nil, err
+	}
+	ns := gcm.NonceSize()
+	if len(raw) < ns {
+		return nil, errors.New("ciphertext too short")
+	}
+	return gcm.Open(nil, raw[:ns], raw[ns:], nil)
+}
+
+func newGCM(key []byte) (cipher.AEAD, error) {
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+	return cipher.NewGCM(block)
+}

internal/crypto/crypto_test.go

@@ -0,0 +1,33 @@
+package crypto
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestEncryptDecryptRoundTrip(t *testing.T) {
+	key := make([]byte, 32)
+	enc, err := Encrypt(key, []byte("hunter2"))
+	if err != nil {
+		t.Fatalf("encrypt: %v", err)
+	}
+	if enc == "hunter2" {
+		t.Fatal("ciphertext must not equal plaintext")
+	}
+	got, err := Decrypt(key, enc)
+	if err != nil {
+		t.Fatalf("decrypt: %v", err)
+	}
+	if !bytes.Equal(got, []byte("hunter2")) {
+		t.Fatalf("got %q, want hunter2", got)
+	}
+}
+
+func TestEncryptNonDeterministic(t *testing.T) {
+	key := make([]byte, 32)
+	a, _ := Encrypt(key, []byte("x"))
+	b, _ := Encrypt(key, []byte("x"))
+	if a == b {
+		t.Fatal("two encryptions must differ (random nonce)")
+	}
+}