add string index lookup operation

src/core/config/Categories.json

@@ -324,6 +324,7 @@
             "Filter",
             "Head",
             "Tail",
+            "String index lookup",
             "Count occurrences",
             "Expand alphabet range",
             "Drop bytes",

src/core/operations/StringIndexLookup.mjs

@@ -0,0 +1,89 @@
+/**
+ * @author skyswordw
+ * @copyright Crown Copyright 2026
+ * @license Apache-2.0
+ */
+
+import Operation from "../Operation.mjs";
+import Utils from "../Utils.mjs";
+import { DELIM_OPTIONS } from "../lib/Delim.mjs";
+import OperationError from "../errors/OperationError.mjs";
+
+/**
+ * String index lookup operation
+ */
+class StringIndexLookup extends Operation {
+
+    /**
+     * StringIndexLookup constructor
+     */
+    constructor() {
+        super();
+
+        this.name = "String index lookup";
+        this.module = "Default";
+        this.description = "Looks up characters from a source string using an input list of indexes. This is useful for deobfuscating command lines that build strings by indexing into a lookup string.";
+        this.inputType = "string";
+        this.outputType = "string";
+        this.args = [
+            {
+                "name": "Source string",
+                "type": "binaryString",
+                "value": ""
+            },
+            {
+                "name": "Delimiter",
+                "type": "option",
+                "value": DELIM_OPTIONS
+            },
+            {
+                "name": "Indexing",
+                "type": "option",
+                "value": ["Zero-based", "One-based"]
+            },
+            {
+                "name": "Skip invalid indexes",
+                "type": "boolean",
+                "value": true
+            }
+        ];
+    }
+
+    /**
+     * @param {string} input
+     * @param {Object[]} args
+     * @returns {string}
+     *
+     * @throws {OperationError} if an index is invalid and invalid indexes are not skipped
+     */
+    run(input, args) {
+        const source = [...(args[0] || "")],
+            delim = Utils.charRep(args[1] || "Space"),
+            indexOffset = args[2] === "One-based" ? 1 : 0,
+            skipInvalid = args[3] !== false;
+
+        return input.split(delim).reduce((output, rawIndex) => {
+            const trimmedIndex = rawIndex.trim();
+
+            if (trimmedIndex.length === 0) return output;
+
+            const parsedIndex = Number(trimmedIndex);
+
+            if (!Number.isInteger(parsedIndex)) {
+                if (skipInvalid) return output;
+                throw new OperationError(`Invalid index: ${trimmedIndex}`);
+            }
+
+            const sourceIndex = parsedIndex - indexOffset;
+            if (sourceIndex < 0 || sourceIndex >= source.length) {
+                if (skipInvalid) return output;
+                throw new OperationError(`Index out of range: ${parsedIndex}`);
+            }
+
+            return output + source[sourceIndex];
+        }, "");
+    }
+
+}
+
+export default StringIndexLookup;

tests/operations/index.mjs

@@ -169,6 +169,7 @@ import "./tests/RC6.mjs";
 // import "./tests/SplitColourChannels.mjs"; // Cannot test operations that use the File type yet
 import "./tests/SQLBeautify.mjs";
 import "./tests/StrUtils.mjs";
+import "./tests/StringIndexLookup.mjs";
 import "./tests/StripIPv4Header.mjs";
 import "./tests/StripTCPHeader.mjs";
 import "./tests/StripUDPHeader.mjs";

tests/operations/tests/StringIndexLookup.mjs

@@ -0,0 +1,66 @@
+/**
+ * String index lookup tests.
+ *
+ * @author skyswordw
+ * @copyright Crown Copyright 2026
+ * @licence Apache-2.0
+ */
+import TestRegister from "../../lib/TestRegister.mjs";
+
+TestRegister.addTests([
+    {
+        name: "String index lookup: zero-based semicolon list",
+        input: "0;1;2;3;4;5;6;7;8;9",
+        expectedOutput: "powershell",
+        recipeConfig: [
+            {
+                op: "String index lookup",
+                args: ["powershell", "Semi-colon", "Zero-based", false]
+            }
+        ]
+    },
+    {
+        name: "String index lookup: one-based comma list",
+        input: "1, 2, 3, 4, 5",
+        expectedOutput: "abcde",
+        recipeConfig: [
+            {
+                op: "String index lookup",
+                args: ["abcde", "Comma", "One-based", false]
+            }
+        ]
+    },
+    {
+        name: "String index lookup: skip invalid indexes",
+        input: "0;99;1;-1;2",
+        expectedOutput: "abc",
+        recipeConfig: [
+            {
+                op: "String index lookup",
+                args: ["abc", "Semi-colon", "Zero-based", true]
+            }
+        ]
+    },
+    {
+        name: "String index lookup: default skips invalid tokens",
+        input: "0;foo;1;99;2",
+        expectedOutput: "abc",
+        recipeConfig: [
+            {
+                op: "String index lookup",
+                args: ["abc", "Semi-colon", "Zero-based"]
+            }
+        ]
+    },
+    {
+        name: "String index lookup: strict out of range",
+        input: "3",
+        expectedOutput: "Index out of range: 3",
+        recipeConfig: [
+            {
+                op: "String index lookup",
+                args: ["abc", "Semi-colon", "Zero-based", false]
+            }
+        ]
+    }
+]);