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+// Author:  cirakg & claude ai
+// Summary: Byte-array helpers: hex and UTF-8 conversion, concatenation, wiping and constant-time comparison.
+//
+// Security notes:
+//   * wipe() overwrites a buffer in place. Call it on every buffer that held a
+//     key or a decrypted secret as soon as it is no longer needed. Monkey C
+//     can't guarantee no other copy exists (README 7.8), so this is best effort.
+//   * equal() always looks at every byte, so comparing a MAC doesn't reveal
+//     through its running time how many leading bytes matched.
+
+import Toybox.Lang;
+import Toybox.StringUtil;
+
+module Bytes {
+
+    // Hex text (any case, even length) to bytes. Empty text gives an empty array.
+    function fromHex(hex as String) as ByteArray {
+        if (hex.length() == 0) {
+            return []b;
+        }
+        return StringUtil.convertEncodedString(hex, {
+            :fromRepresentation => StringUtil.REPRESENTATION_STRING_HEX,
+            :toRepresentation => StringUtil.REPRESENTATION_BYTE_ARRAY
+        }) as ByteArray;
+    }
+
+    // Bytes to lowercase hex text.
+    function toHex(data as ByteArray) as String {
+        if (data.size() == 0) {
+            return "";
+        }
+        return (StringUtil.convertEncodedString(data, {
+            :fromRepresentation => StringUtil.REPRESENTATION_BYTE_ARRAY,
+            :toRepresentation => StringUtil.REPRESENTATION_STRING_HEX
+        }) as String).toLower();
+    }
+
+    // The UTF-8 bytes of a string.
+    function fromUtf8(text as String) as ByteArray {
+        if (text.length() == 0) {
+            return []b;
+        }
+        return StringUtil.convertEncodedString(text, {
+            :fromRepresentation => StringUtil.REPRESENTATION_STRING_PLAIN_TEXT,
+            :toRepresentation => StringUtil.REPRESENTATION_BYTE_ARRAY,
+            :encoding => StringUtil.CHAR_ENCODING_UTF8
+        }) as ByteArray;
+    }
+
+    // A new array holding a followed by b; neither input is changed.
+    function concat(a as ByteArray, b as ByteArray) as ByteArray {
+        var out = new [0]b;
+        out.addAll(a);
+        out.addAll(b);
+        return out;
+    }
+
+    // A new array with a copy of data.
+    function copy(data as ByteArray) as ByteArray {
+        var out = new [0]b;
+        out.addAll(data);
+        return out;
+    }
+
+    // Overwrite every byte with zero. Accepts null so callers can wipe fields
+    // that may not have been set.
+    function wipe(data as ByteArray?) as Void {
+        if (data == null) {
+            return;
+        }
+        for (var i = 0; i < data.size(); i++) {
+            data[i] = 0;
+        }
+    }
+
+    // True when both arrays have the same length and content. The running time
+    // depends only on the length, not on where the first difference is.
+    function equal(a as ByteArray, b as ByteArray) as Boolean {
+        if (a.size() != b.size()) {
+            return false;
+        }
+        var diff = 0;
+        for (var i = 0; i < a.size(); i++) {
+            diff = diff | (a[i] ^ b[i]);
+        }
+        return diff == 0;
+    }
+
+    // XOR source into target, byte by byte (same length required).
+    function xorInto(target as ByteArray, source as ByteArray) as Void {
+        for (var i = 0; i < target.size(); i++) {
+            target[i] = target[i] ^ source[i];
+        }
+    }
+
+    // An unsigned 64-bit value as 8 bytes, most significant first (the HOTP
+    // counter encoding of RFC 4226).
+    function uint64BigEndian(value as Long) as ByteArray {
+        var out = new [8]b;
+        var v = value;
+        for (var i = 7; i >= 0; i--) {
+            out[i] = (v & 0xFFl).toNumber();
+            v = v >> 8;
+        }
+        return out;
+    }
+
+    // An unsigned 32-bit value as 4 bytes, most significant first.
+    function uint32BigEndian(value as Number) as ByteArray {
+        var out = new [4]b;
+        out.encodeNumber(value, Lang.NUMBER_FORMAT_UINT32, {:offset => 0, :endianness => Lang.ENDIAN_BIG});
+        return out;
+    }
+}