1 files changed, 89 insertions, 0 deletions
diff --git a/source/crypto/SealedBox.mc b/source/crypto/SealedBox.mc
new file mode 100644
index 0000000..0804b96
--- /dev/null
+++ b/source/crypto/SealedBox.mc
@@ -0,0 +1,89 @@
+// Author:  cirakg & claude ai
+// Summary: Authenticated encryption of records: AES-256-CBC plus HMAC-SHA256, encrypt-then-MAC (README 7.3).
+//
+// Format of a sealed box (one ByteArray):
+//     version (1 byte, = 1) | IV (16 bytes) | AES-256-CBC ciphertext | tag (32 bytes)
+//     tag = HMAC-SHA256(macKey, "OTPW-REC-1" || len(aad) as 4 bytes || aad || version || IV || ciphertext)
+//
+// * Two keys come from one master key with HKDF (labels "OTPW enc v1" and
+//   "OTPW mac v1"), so encryption and authentication never share a key.
+// * aad ("additional authenticated data") is authenticated but not encrypted;
+//   storage puts the record ID and header there (M3), so a record can't be
+//   swapped for another one or have its header changed unnoticed. Its length
+//   is part of the MAC input, so aad and ciphertext can't be shifted against
+//   each other.
+// * open() checks the tag in constant time BEFORE decrypting, and returns
+//   null for any problem (wrong key, tampering, truncation, unknown version)
+//   without saying which.
+// * Every seal uses a fresh random IV from Cryptography.randomBytes.
+
+import Toybox.Cryptography;
+import Toybox.Lang;
+
+module SealedBox {
+
+    const VERSION = 1;
+    const IV_BYTES = 16;
+    const TAG_BYTES = 32;
+    const KEY_BYTES = 32;
+    const MIN_SIZE = 1 + IV_BYTES + 16 + TAG_BYTES; // ciphertext is at least one block
+
+    // The encryption and MAC keys derived from one master key.
+    class Keys {
+        var encKey as ByteArray;
+        var macKey as ByteArray;
+
+        function initialize(masterKey as ByteArray) {
+            encKey = Kdf.hkdf(masterKey, []b, Bytes.fromUtf8("OTPW enc v1"), KEY_BYTES);
+            macKey = Kdf.hkdf(masterKey, []b, Bytes.fromUtf8("OTPW mac v1"), KEY_BYTES);
+        }
+
+        // Wipe both keys (when the session locks).
+        function wipe() as Void {
+            Bytes.wipe(encKey);
+            Bytes.wipe(macKey);
+        }
+    }
+
+    // Seal plaintext with a fresh random IV.
+    function seal(keys as Keys, aad as ByteArray, plaintext as ByteArray) as ByteArray {
+        return sealWithIv(keys, aad, plaintext, Cryptography.randomBytes(IV_BYTES));
+    }
+
+    // Seal with a given IV. Only tests call this directly (fixed IVs make the
+    // output reproducible); reusing an IV in real use would be a mistake.
+    function sealWithIv(keys as Keys, aad as ByteArray, plaintext as ByteArray, iv as ByteArray) as ByteArray {
+        var ciphertext = Aes.cbcEncrypt(keys.encKey, iv, plaintext);
+        var box = [VERSION]b;
+        box.addAll(iv);
+        box.addAll(ciphertext);
+        box.addAll(tag(keys, aad, box));
+        return box;
+    }
+
+    // The plaintext, or null if the box is malformed, was sealed with another
+    // key or aad, or was changed in any way.
+    function open(keys as Keys, aad as ByteArray, box as ByteArray) as ByteArray? {
+        var size = box.size();
+        if (size < MIN_SIZE || (size - 1 - IV_BYTES - TAG_BYTES) % Aes.BLOCK_BYTES != 0 || box[0] != VERSION) {
+            return null;
+        }
+        var body = box.slice(0, size - TAG_BYTES);
+        var expected = tag(keys, aad, body);
+        var valid = Bytes.equal(expected, box.slice(size - TAG_BYTES, size));
+        Bytes.wipe(expected);
+        if (!valid) {
+            return null;
+        }
+        return Aes.cbcDecrypt(keys.encKey, box.slice(1, 1 + IV_BYTES), box.slice(1 + IV_BYTES, size - TAG_BYTES));
+    }
+
+    // HMAC over the label, the aad (with its length) and the box body.
+    function tag(keys as Keys, aad as ByteArray, body as ByteArray) as ByteArray {
+        var input = Bytes.fromUtf8("OTPW-REC-1");
+        input.addAll(Bytes.uint32BigEndian(aad.size()));
+        input.addAll(aad);
+        input.addAll(body);
+        return Hmac.nativeSha256(keys.macKey, input);
+    }
+}