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diff --git a/source/crypto/Hkdf.mc b/source/crypto/Hkdf.mc
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+// Author:  cirakg & claude ai
+// Summary: HKDF-SHA256 (RFC 5869) on the native HMAC, small enough for the glance.
+//
+// The key derivation SealedBox and KeyManager use for every sub-key. It
+// lives apart from Kdf (PBKDF2, which needs the job system) so the glance
+// can open the sealed favourite code (GlanceCode) with only the crypto it
+// needs. Verified with RFC 5869 test cases 1 to 3 (test/crypto/KdfAesTest.mc)
+// and in the start-up self-test.
+
+import Toybox.Lang;
+
+(:glance)
+module Hkdf {
+
+    const HASH_BYTES = 32;
+
+    // Extract with salt (empty means 32 zero bytes), then expand to
+    // `length` bytes with info. length <= 255 * 32.
+    function derive(ikm as ByteArray, salt as ByteArray, info as ByteArray, length as Number) as ByteArray {
+        if (length < 1 || length > 255 * HASH_BYTES) {
+            throw new Lang.InvalidValueException("HKDF: bad key length");
+        }
+        var prk = Hmac.nativeSha256(salt.size() > 0 ? salt : new [HASH_BYTES]b, ikm);
+        var okm = new [0]b;
+        var block = new [0]b;
+        for (var counter = 1; okm.size() < length; counter++) {
+            var input = Bytes.concat(block, info);
+            input.add(counter);
+            Bytes.wipe(block);
+            block = Hmac.nativeSha256(prk, input);
+            okm.addAll(block);
+        }
+        Bytes.wipe(prk);
+        Bytes.wipe(block);
+        var key = okm.slice(0, length);
+        Bytes.wipe(okm);
+        return key;
+    }
+}