1 files changed, 150 insertions, 0 deletions
diff --git a/backend/internal/keys/keys.go b/backend/internal/keys/keys.go
new file mode 100644
index 0000000..ac73173
--- /dev/null
+++ b/backend/internal/keys/keys.go
@@ -0,0 +1,150 @@
+package keys
+
+import (
+	"context"
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"fmt"
+	"io"
+)
+
+var CSRNG = rand.Reader
+
+//go:generate mockgen -destination mock_keys_test.go -package keys_test -typed . KeyManagementService
+type KeyManagementService interface {
+	io.Closer
+
+	// Encrypt encrypts the given plaintext using the key with the given key name.
+	Encrypt(ctx context.Context, keyName string, plaintext []byte) ([]byte, error)
+
+	// Decrypt decrypts the given ciphertext using the key with the given key name.
+	Decrypt(ctx context.Context, keyName string, ciphertext []byte) ([]byte, error)
+}
+
+// Encrypt encrypts the given plaintext using a hybrid encryption scheme.
+//
+// It first generates a random AES 256-bit key and encrypts the plaintext with it.
+// Then, it encrypts the AES key using the KMS.
+//
+// It returns the ciphertext, the encrypted AES key, and any errors that occurred.
+func Encrypt(
+	ctx context.Context,
+	kms KeyManagementService,
+	keyName string,
+	plaintext []byte,
+) (ciphertext []byte, encryptedKey []byte, err error) {
+	// Generate a random AES key
+	aesKey := make([]byte, 32)
+	if _, err = io.ReadFull(CSRNG, aesKey); err != nil {
+		return nil, nil, fmt.Errorf("unable to generate AES key: %w", err)
+	}
+
+	// Encrypt the plaintext using the AES key
+	ciphertext, err = encrypt(aesKey, plaintext)
+	if err != nil {
+		return nil, nil, fmt.Errorf("unable to encrypt plaintext: %w", err)
+	}
+
+	// Encrypt the AES key using the KMS
+	encryptedKey, err = kms.Encrypt(ctx, keyName, aesKey)
+	if err != nil {
+		return nil, nil, fmt.Errorf("unable to encrypt AES key: %w", err)
+	}
+
+	return ciphertext, encryptedKey, nil
+}
+
+// EncryptWithKey encrypts the given plaintext using a hybrid encryption scheme.
+//
+// This works similarly to Encrypt, but instead of generating a new AES key, it uses a given already encrypted AES key.
+func EncryptWithKey(
+	ctx context.Context,
+	kms KeyManagementService,
+	keyName string,
+	encryptedKey []byte,
+	plaintext []byte,
+) ([]byte, error) {
+	// Decrypt the AES key
+	aesKey, err := kms.Decrypt(ctx, keyName, encryptedKey)
+	if err != nil {
+		return nil, fmt.Errorf("unable to decrypt AES key: %w", err)
+	}
+
+	// Encrypt the plaintext using the AES key
+	ciphertext, err := encrypt(aesKey, plaintext)
+	if err != nil {
+		return nil, fmt.Errorf("unable to encrypt plaintext: %w", err)
+	}
+
+	return ciphertext, nil
+}
+
+func encrypt(aesKey []byte, plaintext []byte) ([]byte, error) {
+	// Create an AES cipher
+	blockCipher, err := aes.NewCipher(aesKey)
+	if err != nil {
+		return nil, fmt.Errorf("unable to create AES cipher: %w", err)
+	}
+
+	gcm, err := cipher.NewGCM(blockCipher)
+	if err != nil {
+		return nil, fmt.Errorf("unable to create GCM: %w", err)
+	}
+
+	// Generate a random nonce
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err = io.ReadFull(CSRNG, nonce); err != nil {
+		return nil, fmt.Errorf("unable to generate nonce: %w", err)
+	}
+
+	// Encrypt the plaintext
+	ciphertext := gcm.Seal(nonce, nonce, plaintext, nil)
+	return ciphertext, nil
+}
+
+// Decrypt decrypts the given ciphertext using a hybrid encryption scheme.
+//
+// It first decrypts the AES key using the KMS.
+// Then, it decrypts the ciphertext using the decrypted AES key.
+//
+// It returns the plaintext and any errors that occurred.
+func Decrypt(
+	ctx context.Context,
+	kms KeyManagementService,
+	keyName string,
+	ciphertext []byte,
+	encryptedKey []byte,
+) ([]byte, error) {
+	// Decrypt the AES key
+	aesKey, err := kms.Decrypt(ctx, keyName, encryptedKey)
+	if err != nil {
+		return nil, fmt.Errorf("unable to decrypt AES key: %w", err)
+	}
+
+	// Create an AES cipher
+	blockCipher, err := aes.NewCipher(aesKey)
+	if err != nil {
+		return nil, fmt.Errorf("unable to create AES cipher: %w", err)
+	}
+
+	gcm, err := cipher.NewGCM(blockCipher)
+	if err != nil {
+		return nil, fmt.Errorf("unable to create GCM: %w", err)
+	}
+
+	// Extract the nonce from the ciphertext
+	nonceSize := gcm.NonceSize()
+	if len(ciphertext) < nonceSize {
+		return nil, fmt.Errorf("ciphertext is too short")
+	}
+	nonce, ciphertext := ciphertext[:nonceSize], ciphertext[nonceSize:]
+
+	// Decrypt the ciphertext
+	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
+	if err != nil {
+		return nil, fmt.Errorf("unable to decrypt ciphertext: %w", err)
+	}
+
+	return plaintext, nil
+}