feat: encrypt private keys saved on disk and in database (#682)

Co-authored-by: Kyle Mendell <kmendell@ofkm.us>

backend/internal/utils/crypto/crypto.go

@@ -0,0 +1,69 @@
+package crypto
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"errors"
+	"fmt"
+	"io"
+)
+
+// ErrDecrypt is returned by Decrypt when the operation failed for any reason
+var ErrDecrypt = errors.New("failed to decrypt data")
+
+// Encrypt a byte slice using AES-GCM and a random nonce
+// Important: do not encrypt more than ~4 billion messages with the same key!
+func Encrypt(key []byte, plaintext []byte, associatedData []byte) (ciphertext []byte, err error) {
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create block cipher: %w", err)
+	}
+	aead, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create AEAD cipher: %w", err)
+	}
+
+	// Generate a random nonce
+	nonce := make([]byte, aead.NonceSize())
+	_, err = io.ReadFull(rand.Reader, nonce)
+	if err != nil {
+		return nil, fmt.Errorf("failed to generate random nonce: %w", err)
+	}
+
+	// Allocate the slice for the result, with additional space for the nonce and overhead
+	ciphertext = make([]byte, 0, len(plaintext)+aead.NonceSize()+aead.Overhead())
+	ciphertext = append(ciphertext, nonce...)
+
+	// Encrypt the plaintext
+	// Tag is automatically added at the end
+	ciphertext = aead.Seal(ciphertext, nonce, plaintext, associatedData)
+
+	return ciphertext, nil
+}
+
+// Decrypt a byte slice using AES-GCM
+func Decrypt(key []byte, ciphertext []byte, associatedData []byte) (plaintext []byte, err error) {
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create block cipher: %w", err)
+	}
+	aead, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create AEAD cipher: %w", err)
+	}
+
+	// Extract the nonce
+	if len(ciphertext) < (aead.NonceSize() + aead.Overhead()) {
+		return nil, ErrDecrypt
+	}
+
+	// Decrypt the data
+	plaintext, err = aead.Open(nil, ciphertext[:aead.NonceSize()], ciphertext[aead.NonceSize():], associatedData)
+	if err != nil {
+		// Note: we do not return the exact error here, to avoid disclosing information
+		return nil, ErrDecrypt
+	}
+
+	return plaintext, nil
+}