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

@@ -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)
+}