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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;
// (:glance): also loaded by the glance, which shows the favourite code (GlanceCode).
(:glance)
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;
}
}
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