init: initial commit

internal/crypto/crypto.go

@@ -0,0 +1,65 @@
+package crypto
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"fmt"
+	"io"
+)
+
+const appSecret = "ai-expert-sidebar-v1-local-©2026"
+
+// deriveKey produces a 32-byte AES key from the application secret.
+func deriveKey() []byte {
+	sum := sha256.Sum256([]byte(appSecret))
+	return sum[:]
+}
+
+// EncryptAPIKey encrypts a plaintext API key. Returns "" for empty input.
+func EncryptAPIKey(plaintext string) (string, error) {
+	if plaintext == "" {
+		return "", nil
+	}
+	block, err := aes.NewCipher(deriveKey())
+	if err != nil {
+		return "", err
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", err
+	}
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err = io.ReadFull(rand.Reader, nonce); err != nil {
+		return "", err
+	}
+	sealed := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
+	return base64.StdEncoding.EncodeToString(sealed), nil
+}
+
+// DecryptAPIKey decrypts a base64-encoded AES-256-GCM ciphertext. Returns "" for empty input.
+func DecryptAPIKey(ciphertext64 string) (string, error) {
+	if ciphertext64 == "" {
+		return "", nil
+	}
+	data, err := base64.StdEncoding.DecodeString(ciphertext64)
+	if err != nil {
+		return "", fmt.Errorf("base64 decode: %w", err)
+	}
+	block, err := aes.NewCipher(deriveKey())
+	if err != nil {
+		return "", err
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", err
+	}
+	ns := gcm.NonceSize()
+	if len(data) < ns {
+		return "", fmt.Errorf("ciphertext too short")
+	}
+	plain, err := gcm.Open(nil, data[:ns], data[ns:], nil)
+	return string(plain), err
+}