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diff --git a/source/crypto/Hmac.mc b/source/crypto/Hmac.mc
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+// Author:  cirakg & claude ai
+// Summary: HMAC (RFC 2104) over SHA-1, SHA-256 and SHA-512; native HMAC-SHA256 where Connect IQ provides it.
+//
+// Connect IQ's native HMAC accepts only SHA-256 (HMAC-SHA1 is rejected with
+// "Invalid :algorithm option value", verified with SDK 9.2.0). Most TOTP
+// secrets use SHA-1, so compute() builds HMAC from the hash functions as
+// RFC 2104 defines it:
+//     HMAC(K, m) = H((K' xor opad) || H((K' xor ipad) || m))
+// where K' is the key padded to the hash's block size (hashed first if it is
+// longer than a block), ipad = 0x36 and opad = 0x5c repeated.
+//
+// Security notes: the padded key copies are wiped after use; the caller's key
+// array is never modified. Verified with RFC 2202 (SHA-1) and RFC 4231
+// (SHA-256, SHA-512), test/crypto/HmacTest.mc.
+
+import Toybox.Cryptography;
+import Toybox.Lang;
+
+module Hmac {
+
+    const IPAD = 0x36;
+    const OPAD = 0x5c;
+
+    // HMAC of message under key. SHA-256 uses the native implementation; SHA-1
+    // and SHA-512 use generic().
+    function compute(alg as Hashes.Algorithm, key as ByteArray, message as ByteArray) as ByteArray {
+        if (alg == Hashes.SHA256) {
+            return nativeSha256(key, message);
+        }
+        return generic(alg, key, message);
+    }
+
+    // RFC 2104 HMAC built on Hashes.digest, for any supported algorithm.
+    function generic(alg as Hashes.Algorithm, key as ByteArray, message as ByteArray) as ByteArray {
+        var block = Hashes.blockSize(alg);
+        var hashedKey = key.size() > block ? Hashes.digest(alg, key) : null;
+        var k = hashedKey != null ? hashedKey : key;
+
+        var inner = new [block]b;
+        var outer = new [block]b;
+        for (var i = 0; i < block; i++) {
+            var keyByte = i < k.size() ? k[i] : 0;
+            inner[i] = keyByte ^ IPAD;
+            outer[i] = keyByte ^ OPAD;
+        }
+        inner.addAll(message);
+        var innerHash = Hashes.digest(alg, inner);
+        outer.addAll(innerHash);
+        var mac = Hashes.digest(alg, outer);
+
+        Bytes.wipe(inner);
+        Bytes.wipe(outer);
+        Bytes.wipe(innerHash);
+        Bytes.wipe(hashedKey);
+        return mac;
+    }
+
+    // Native HMAC-SHA256 (Toybox.Cryptography).
+    function nativeSha256(key as ByteArray, message as ByteArray) as ByteArray {
+        var mac = new Cryptography.HashBasedMessageAuthenticationCode({
+            :algorithm => Cryptography.HASH_SHA256,
+            :key => key
+        });
+        if (message.size() > 0) {
+            mac.update(message);
+        }
+        return mac.digest();
+    }
+}