gchq · voo7ieX9 · Dec 15, 2025
diff --git a/src/core/config/Categories.json b/src/core/config/Categories.json
@@ -31,6 +31,8 @@
             "To Base64",
             "From Base64",
             "Show Base64 offsets",
+            "To Base65536",
+            "From Base65536",
             "To Base92",
             "From Base92",
             "To Base85",

diff --git a/src/core/lib/Base65536.mjs b/src/core/lib/Base65536.mjs
@@ -0,0 +1,172 @@
+/**
+ * Base65536 resources.
+ *
+ * @author voo7ieX9
+ * @copyright Crown Copyright 2025
+ * @license Apache-2.0
+ */
+
+/**
+ * BLOCK_START lookup table for encoding
+ * Maps the second byte (b2) to the starting Unicode code point
+ */
+const BLOCK_START = {
+    0: 13312, 1: 13568, 2: 13824, 3: 14080, 4: 14336, 5: 14592, 6: 14848, 7:
+    15104, 8: 15360, 9: 15616, 10: 15872, 11: 16128, 12: 16384, 13: 16640, 14:
+    16896, 15: 17152, 16: 17408, 17: 17664, 18: 17920, 19: 18176, 20: 18432,
+    21: 18688, 22: 18944, 23: 19200, 24: 19456, 25: 19968, 26: 20224, 27:
+    20480, 28: 20736, 29: 20992, 30: 21248, 31: 21504, 32: 21760, 33: 22016,
+    34: 22272, 35: 22528, 36: 22784, 37: 23040, 38: 23296, 39: 23552, 40:
+    23808, 41: 24064, 42: 24320, 43: 24576, 44: 24832, 45: 25088, 46: 25344,
+    47: 25600, 48: 25856, 49: 26112, 50: 26368, 51: 26624, 52: 26880, 53:
+    27136, 54: 27392, 55: 27648, 56: 27904, 57: 28160, 58: 28416, 59: 28672,
+    60: 28928, 61: 29184, 62: 29440, 63: 29696, 64: 29952, 65: 30208, 66:
+    30464, 67: 30720, 68: 30976, 69: 31232, 70: 31488, 71: 31744, 72: 32000,
+    73: 32256, 74: 32512, 75: 32768, 76: 33024, 77: 33280, 78: 33536, 79:
+    33792, 80: 34048, 81: 34304, 82: 34560, 83: 34816, 84: 35072, 85: 35328,
+    86: 35584, 87: 35840, 88: 36096, 89: 36352, 90: 36608, 91: 36864, 92:
+    37120, 93: 37376, 94: 37632, 95: 37888, 96: 38144, 97: 38400, 98: 38656,
+    99: 38912, 100: 39168, 101: 39424, 102: 39680, 103: 39936, 104: 40192, 105:
+    40448, 106: 41216, 107: 41472, 108: 41728, 109: 42240, 110: 67072, 111:
+    73728, 112: 73984, 113: 74240, 114: 77824, 115: 78080, 116: 78336, 117:
+    78592, 118: 82944, 119: 83200, 120: 92160, 121: 92416, 122: 131072, 123:
+    131328, 124: 131584, 125: 131840, 126: 132096, 127: 132352, 128: 132608,
+    129: 132864, 130: 133120, 131: 133376, 132: 133632, 133: 133888, 134:
+    134144, 135: 134400, 136: 134656, 137: 134912, 138: 135168, 139: 135424,
+    140: 135680, 141: 135936, 142: 136192, 143: 136448, 144: 136704, 145:
+    136960, 146: 137216, 147: 137472, 148: 137728, 149: 137984, 150: 138240,
+    151: 138496, 152: 138752, 153: 139008, 154: 139264, 155: 139520, 156:
+    139776, 157: 140032, 158: 140288, 159: 140544, 160: 140800, 161: 141056,
+    162: 141312, 163: 141568, 164: 141824, 165: 142080, 166: 142336, 167:
+    142592, 168: 142848, 169: 143104, 170: 143360, 171: 143616, 172: 143872,
+    173: 144128, 174: 144384, 175: 144640, 176: 144896, 177: 145152, 178:
+    145408, 179: 145664, 180: 145920, 181: 146176, 182: 146432, 183: 146688,
+    184: 146944, 185: 147200, 186: 147456, 187: 147712, 188: 147968, 189:
+    148224, 190: 148480, 191: 148736, 192: 148992, 193: 149248, 194: 149504,
+    195: 149760, 196: 150016, 197: 150272, 198: 150528, 199: 150784, 200:
+    151040, 201: 151296, 202: 151552, 203: 151808, 204: 152064, 205: 152320,
+    206: 152576, 207: 152832, 208: 153088, 209: 153344, 210: 153600, 211:
+    153856, 212: 154112, 213: 154368, 214: 154624, 215: 154880, 216: 155136,
+    217: 155392, 218: 155648, 219: 155904, 220: 156160, 221: 156416, 222:
+    156672, 223: 156928, 224: 157184, 225: 157440, 226: 157696, 227: 157952,
+    228: 158208, 229: 158464, 230: 158720, 231: 158976, 232: 159232, 233:
+    159488, 234: 159744, 235: 160000, 236: 160256, 237: 160512, 238: 160768,
+    239: 161024, 240: 161280, 241: 161536, 242: 161792, 243: 162048, 244:
+    162304, 245: 162560, 246: 162816, 247: 163072, 248: 163328, 249: 163584,
+    250: 163840, 251: 164096, 252: 164352, 253: 164608, 254: 164864, 255:
+    165120, "-1": 5376
+};
+
+/**
+ * B2 lookup table for decoding
+ * Maps Unicode code point (minus the first byte) to the second byte (b2)
+ */
+const B2 = {
+    5376: -1, 13312: 0, 13568: 1, 13824: 2, 14080: 3, 14336: 4, 14592: 5,
+    14848: 6, 15104: 7, 15360: 8, 15616: 9, 15872: 10, 16128: 11, 16384: 12,
+    16640: 13, 16896: 14, 17152: 15, 17408: 16, 17664: 17, 17920: 18, 18176:
+    19, 18432: 20, 18688: 21, 18944: 22, 19200: 23, 19456: 24, 19968: 25,
+    20224: 26, 20480: 27, 20736: 28, 20992: 29, 21248: 30, 21504: 31, 21760:
+    32, 22016: 33, 22272: 34, 22528: 35, 22784: 36, 23040: 37, 23296: 38,
+    23552: 39, 23808: 40, 24064: 41, 24320: 42, 24576: 43, 24832: 44, 25088:
+    45, 25344: 46, 25600: 47, 25856: 48, 26112: 49, 26368: 50, 26624: 51,
+    26880: 52, 27136: 53, 27392: 54, 27648: 55, 27904: 56, 28160: 57, 28416:
+    58, 28672: 59, 28928: 60, 29184: 61, 29440: 62, 29696: 63, 29952: 64,
+    30208: 65, 30464: 66, 30720: 67, 30976: 68, 31232: 69, 31488: 70, 31744:
+    71, 32000: 72, 32256: 73, 32512: 74, 32768: 75, 33024: 76, 33280: 77,
+    33536: 78, 33792: 79, 34048: 80, 34304: 81, 34560: 82, 34816: 83, 35072:
+    84, 35328: 85, 35584: 86, 35840: 87, 36096: 88, 36352: 89, 36608: 90,
+    36864: 91, 37120: 92, 37376: 93, 37632: 94, 37888: 95, 38144: 96, 38400:
+    97, 38656: 98, 38912: 99, 39168: 100, 39424: 101, 39680: 102, 39936: 103,
+    40192: 104, 40448: 105, 41216: 106, 41472: 107, 41728: 108, 42240: 109,
+    67072: 110, 73728: 111, 73984: 112, 74240: 113, 77824: 114, 78080: 115,
+    78336: 116, 78592: 117, 82944: 118, 83200: 119, 92160: 120, 92416: 121,
+    131072: 122, 131328: 123, 131584: 124, 131840: 125, 132096: 126, 132352:
+    127, 132608: 128, 132864: 129, 133120: 130, 133376: 131, 133632: 132,
+    133888: 133, 134144: 134, 134400: 135, 134656: 136, 134912: 137, 135168:
+    138, 135424: 139, 135680: 140, 135936: 141, 136192: 142, 136448: 143,
+    136704: 144, 136960: 145, 137216: 146, 137472: 147, 137728: 148, 137984:
+    149, 138240: 150, 138496: 151, 138752: 152, 139008: 153, 139264: 154,
+    139520: 155, 139776: 156, 140032: 157, 140288: 158, 140544: 159, 140800:
+    160, 141056: 161, 141312: 162, 141568: 163, 141824: 164, 142080: 165,
+    142336: 166, 142592: 167, 142848: 168, 143104: 169, 143360: 170, 143616:
+    171, 143872: 172, 144128: 173, 144384: 174, 144640: 175, 144896: 176,
+    145152: 177, 145408: 178, 145664: 179, 145920: 180, 146176: 181, 146432:
+    182, 146688: 183, 146944: 184, 147200: 185, 147456: 186, 147712: 187,
+    147968: 188, 148224: 189, 148480: 190, 148736: 191, 148992: 192, 149248:
+    193, 149504: 194, 149760: 195, 150016: 196, 150272: 197, 150528: 198,
+    150784: 199, 151040: 200, 151296: 201, 151552: 202, 151808: 203, 152064:
+    204, 152320: 205, 152576: 206, 152832: 207, 153088: 208, 153344: 209,
+    153600: 210, 153856: 211, 154112: 212, 154368: 213, 154624: 214, 154880:
+    215, 155136: 216, 155392: 217, 155648: 218, 155904: 219, 156160: 220,
+    156416: 221, 156672: 222, 156928: 223, 157184: 224, 157440: 225, 157696:
+    226, 157952: 227, 158208: 228, 158464: 229, 158720: 230, 158976: 231,
+    159232: 232, 159488: 233, 159744: 234, 160000: 235, 160256: 236, 160512:
+    237, 160768: 238, 161024: 239, 161280: 240, 161536: 241, 161792: 242,
+    162048: 243, 162304: 244, 162560: 245, 162816: 246, 163072: 247, 163328:
+    248, 163584: 249, 163840: 250, 164096: 251, 164352: 252, 164608: 253,
+    164864: 254, 165120: 255
+};
+
+/**
+ * Encode bytes to Base65536 string.
+ *
+ * @param {Uint8Array} data
+ * @returns {string}
+ */
+export function encode(data) {
+    let result = "";
+    const length = data.length;
+
+    for (let i = 0; i < length; i += 2) {
+        const b1 = data[i];
+        const b2 = (i + 1 < length) ? data[i + 1] : -1;
+        const codePoint = BLOCK_START[b2] + b1;
+        result += String.fromCodePoint(codePoint);
+    }
+
+    return result;
+}
+
+/**
+ * Decode Base65536 string to bytes.
+ *
+ * @param {string} str
+ * @returns {Uint8Array}
+ */
+export function decode(str) {
+    const result = [];
+    let done = false;
+
+    for (let i = 0; i < str.length; i++) {
+        const codePoint = str.codePointAt(i);
+
+        // Handle surrogate pairs (code points > 0xFFFF)
+        if (codePoint > 0xFFFF) {
+            i++; // Skip the next code unit as it's part of the surrogate pair
+        }
+
+        const b1 = codePoint & 0xFF;
+        const blockStart = codePoint - b1;
+
+        if (!Object.prototype.hasOwnProperty.call(B2, blockStart)) {
+            throw new Error(`Invalid base65536 code point: ${codePoint}`);
+        }
+
+        const b2 = B2[blockStart];
+
+        if (b2 === -1) {
+            // Final byte
+            if (done) {
+                throw new Error("base65536 sequence continued after final byte");
+            }
+            result.push(b1);
+            done = true;
+        } else {
+            result.push(b1);
+            result.push(b2);
+        }
+    }
+
+    return new Uint8Array(result);
+}
diff --git a/src/core/operations/FromBase65536.mjs b/src/core/operations/FromBase65536.mjs
@@ -0,0 +1,42 @@
+/**
+ * @author voo7ieX9
+ * @copyright Crown Copyright 2025
+ * @license Apache-2.0
+ */
+
+import Operation from "../Operation.mjs";
+import {decode} from "../lib/Base65536.mjs";
+
+/**
+ * From Base65536 operation
+ */
+class FromBase65536 extends Operation {
+
+    /**
+     * FromBase65536 constructor
+     */
+    constructor() {
+        super();
+
+        this.name = "From Base65536";
+        this.module = "Default";
+        this.description = "Base65536 is a binary encoding that uses Unicode code points to represent data. This operation decodes a Base65536 string back into its original form.<br><br>e.g. <code>驨ꍬ啯𒁷ꍲᕤ</code> becomes <code>hello world</code>";
+        this.infoURL = "https://github.com/qntm/base65536";
+        this.inputType = "string";
+        this.outputType = "byteArray";
+        this.args = [];
+    }
+
+    /**
+     * @param {string} input
+     * @param {Object[]} args
+     * @returns {byteArray}
+     */
+    run(input, args) {
+        const result = decode(input);
+        return Array.from(result);
+    }
+
+}
+
+export default FromBase65536;
diff --git a/src/core/operations/ToBase65536.mjs b/src/core/operations/ToBase65536.mjs
@@ -0,0 +1,42 @@
+/**
+ * @author voo7ieX9
+ * @copyright Crown Copyright 2025
+ * @license Apache-2.0
+ */
+
+import Operation from "../Operation.mjs";
+import {encode} from "../lib/Base65536.mjs";
+
+/**
+ * To Base65536 operation
+ */
+class ToBase65536 extends Operation {
+
+    /**
+     * ToBase65536 constructor
+     */
+    constructor() {
+        super();
+
+        this.name = "To Base65536";
+        this.module = "Default";
+        this.description = "Base65536 is a binary encoding that uses Unicode code points to represent data. This operation encodes data into a Unicode string.<br><br>e.g. <code>hello world</code> becomes <code>驨ꍬ啯𒁷ꍲᕤ</code>";
+        this.infoURL = "https://github.com/qntm/base65536";
+        this.inputType = "ArrayBuffer";
+        this.outputType = "string";
+        this.args = [];
+    }
+
+    /**
+     * @param {ArrayBuffer} input
+     * @param {Object[]} args
+     * @returns {string}
+     */
+    run(input, args) {
+        const data = new Uint8Array(input);
+        return encode(data);
+    }
+
+}
+
+export default ToBase65536;