From 8d04b0fde96562a0806299dbfbf6cf7b5f3dbc26 Mon Sep 17 00:00:00 2001
From: Steve Cliff <steve@devcloud.guru>
Date: Sun, 21 Jun 2026 23:32:23 +0100
Subject: [PATCH] feat(crypto): AES-256-GCM field encryption keyed from
 EMCLI_KEY

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 internal/crypto/crypto.go      | 65 ++++++++++++++++++++++++++++++++
 internal/crypto/crypto_test.go | 69 ++++++++++++++++++++++++++++++++++
 2 files changed, 134 insertions(+)
 create mode 100644 internal/crypto/crypto.go
 create mode 100644 internal/crypto/crypto_test.go

diff --git a/internal/crypto/crypto.go b/internal/crypto/crypto.go
new file mode 100644
index 0000000..1d00c10
--- /dev/null
+++ b/internal/crypto/crypto.go
@@ -0,0 +1,65 @@
+// Package crypto provides AES-256-GCM field encryption keyed from EMCLI_KEY.
+package crypto
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"encoding/base64"
+	"errors"
+	"io"
+	"os"
+)
+
+var (
+	ErrNoKey  = errors.New("EMCLI_KEY is not set")
+	ErrBadKey = errors.New("EMCLI_KEY must be base64 of exactly 32 bytes")
+)
+
+// KeyFromEnv reads and validates the AES-256 key from EMCLI_KEY.
+func KeyFromEnv() ([]byte, error) {
+	raw := os.Getenv("EMCLI_KEY")
+	if raw == "" {
+		return nil, ErrNoKey
+	}
+	key, err := base64.StdEncoding.DecodeString(raw)
+	if err != nil || len(key) != 32 {
+		return nil, ErrBadKey
+	}
+	return key, nil
+}
+
+func newGCM(key []byte) (cipher.AEAD, error) {
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+	return cipher.NewGCM(block)
+}
+
+// Seal encrypts plaintext, returning nonce||ciphertext.
+func Seal(key, plaintext []byte) ([]byte, error) {
+	gcm, err := newGCM(key)
+	if err != nil {
+		return nil, err
+	}
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return nil, err
+	}
+	return gcm.Seal(nonce, nonce, plaintext, nil), nil
+}
+
+// Open reverses Seal. A wrong key or tampered blob returns an error.
+func Open(key, blob []byte) ([]byte, error) {
+	gcm, err := newGCM(key)
+	if err != nil {
+		return nil, err
+	}
+	ns := gcm.NonceSize()
+	if len(blob) < ns {
+		return nil, errors.New("ciphertext too short")
+	}
+	nonce, ct := blob[:ns], blob[ns:]
+	return gcm.Open(nil, nonce, ct, nil)
+}
diff --git a/internal/crypto/crypto_test.go b/internal/crypto/crypto_test.go
new file mode 100644
index 0000000..62e86f7
--- /dev/null
+++ b/internal/crypto/crypto_test.go
@@ -0,0 +1,69 @@
+package crypto
+
+import (
+	"bytes"
+	"encoding/base64"
+	"testing"
+)
+
+func testKey() []byte {
+	k := make([]byte, 32)
+	for i := range k {
+		k[i] = byte(i)
+	}
+	return k
+}
+
+func TestSealOpenRoundTrip(t *testing.T) {
+	key := testKey()
+	msg := []byte("hunter2-the-password")
+	blob, err := Seal(key, msg)
+	if err != nil {
+		t.Fatalf("Seal: %v", err)
+	}
+	if bytes.Contains(blob, msg) {
+		t.Fatal("ciphertext must not contain plaintext")
+	}
+	got, err := Open(key, blob)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	if !bytes.Equal(got, msg) {
+		t.Fatalf("round-trip mismatch: %q", got)
+	}
+}
+
+func TestSealUsesRandomNonce(t *testing.T) {
+	key := testKey()
+	a, _ := Seal(key, []byte("x"))
+	b, _ := Seal(key, []byte("x"))
+	if bytes.Equal(a, b) {
+		t.Fatal("two seals of same plaintext must differ (random nonce)")
+	}
+}
+
+func TestOpenWrongKeyFails(t *testing.T) {
+	blob, _ := Seal(testKey(), []byte("secret"))
+	wrong := make([]byte, 32) // all zeros
+	if _, err := Open(wrong, blob); err == nil {
+		t.Fatal("Open with wrong key must fail")
+	}
+}
+
+func TestKeyFromEnv(t *testing.T) {
+	t.Setenv("EMCLI_KEY", base64.StdEncoding.EncodeToString(testKey()))
+	k, err := KeyFromEnv()
+	if err != nil || len(k) != 32 {
+		t.Fatalf("KeyFromEnv: key=%d err=%v", len(k), err)
+	}
+
+	t.Setenv("EMCLI_KEY", "")
+	if _, err := KeyFromEnv(); err != ErrNoKey {
+		t.Fatalf("empty key: want ErrNoKey, got %v", err)
+	}
+
+	t.Setenv("EMCLI_KEY", base64.StdEncoding.EncodeToString([]byte("tooshort")))
+	if _, err := KeyFromEnv(); err != ErrBadKey {
+		t.Fatalf("short key: want ErrBadKey, got %v", err)
+	}
+}