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diff --git a/source/crypto/Aes.mc b/source/crypto/Aes.mc
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+// Author:  cirakg & claude ai
+// Summary: AES-CBC encryption with PKCS#7 padding on top of the native Connect IQ cipher.
+//
+// The native Cipher offers AES-128 and AES-256 in ECB and CBC mode, without
+// padding (CBC input must be a multiple of 16 bytes). This module adds PKCS#7
+// padding. AES-256 is used for records (decided in M2: it works in the
+// simulator on all four test devices and costs nothing extra; the PIN, not
+// the cipher, limits the security of locked entries).
+//
+// Security note: CBC alone does not detect tampering. Records are always
+// sealed with an HMAC as well (SealedBox.mc), and the MAC is checked before
+// anything is decrypted, so padding errors never reach an attacker.
+// Verified with NIST SP 800-38A F.2.1 and F.2.5 (test/crypto/AesTest.mc).
+
+import Toybox.Cryptography;
+import Toybox.Lang;
+
+module Aes {
+
+    const BLOCK_BYTES = 16;
+
+    // A CBC cipher for a 16- or 32-byte key and a 16-byte IV.
+    function cipher(key as ByteArray, iv as ByteArray) as Cryptography.Cipher {
+        if ((key.size() != 16 && key.size() != 32) || iv.size() != BLOCK_BYTES) {
+            throw new Lang.InvalidValueException("AES: key must be 16 or 32 bytes, IV 16 bytes");
+        }
+        return new Cryptography.Cipher({
+            :algorithm => key.size() == 32 ? Cryptography.CIPHER_AES256 : Cryptography.CIPHER_AES128,
+            :mode => Cryptography.MODE_CBC,
+            :key => key,
+            :iv => iv
+        });
+    }
+
+    // CBC without padding; data must be a multiple of 16 bytes.
+    function cbcRaw(key as ByteArray, iv as ByteArray, data as ByteArray, encrypt as Boolean) as ByteArray {
+        var c = cipher(key, iv);
+        return encrypt ? c.encrypt(data) : c.decrypt(data);
+    }
+
+    // Encrypt with PKCS#7 padding: 1 to 16 bytes, each holding the pad length.
+    function cbcEncrypt(key as ByteArray, iv as ByteArray, plaintext as ByteArray) as ByteArray {
+        var padLength = BLOCK_BYTES - (plaintext.size() % BLOCK_BYTES);
+        var padded = Bytes.copy(plaintext);
+        for (var i = 0; i < padLength; i++) {
+            padded.add(padLength);
+        }
+        var out = cbcRaw(key, iv, padded, true);
+        Bytes.wipe(padded);
+        return out;
+    }
+
+    // Decrypt and remove PKCS#7 padding; null when the length or the padding is
+    // wrong.
+    function cbcDecrypt(key as ByteArray, iv as ByteArray, ciphertext as ByteArray) as ByteArray? {
+        var size = ciphertext.size();
+        if (size == 0 || size % BLOCK_BYTES != 0) {
+            return null;
+        }
+        var padded = cbcRaw(key, iv, ciphertext, false);
+        var padLength = padded[size - 1];
+        var valid = padLength >= 1 && padLength <= BLOCK_BYTES;
+        if (valid) {
+            for (var i = size - padLength; i < size; i++) {
+                valid = valid && padded[i] == padLength;
+            }
+        }
+        var plaintext = valid ? padded.slice(0, size - padLength) : null;
+        Bytes.wipe(padded);
+        return plaintext;
+    }
+}