diff --git a/src/core/config/Categories.json b/src/core/config/Categories.json
index 434c8bb6..e627a1f0 100644
--- a/src/core/config/Categories.json
+++ b/src/core/config/Categories.json
@@ -220,6 +220,7 @@
             "Subtract",
             "Multiply",
             "Divide",
+            "Modular Exponentiation",
             "Mean",
             "Median",
             "Standard Deviation",
diff --git a/src/core/operations/ModularExponentiation.mjs b/src/core/operations/ModularExponentiation.mjs
new file mode 100644
index 00000000..43898d86
--- /dev/null
+++ b/src/core/operations/ModularExponentiation.mjs
@@ -0,0 +1,143 @@
+/**
+ * @author p-leriche [philip.leriche@cantab.net]
+ * @copyright Crown Copyright 2025
+ * @license Apache-2.0
+ */
+
+import Operation from "../Operation.mjs";
+import OperationError from "../errors/OperationError.mjs";
+
+/* ---------- helper functions ---------- */
+
+/**
+ * modPow helper operation
+ */
+function modPow(base, exponent, modulus) {
+    let result = 1n;
+    base %= modulus;
+
+    while (exponent > 0n) {
+        if (exponent & 1n) {
+            result = (result * base) % modulus;
+        }
+        base = (base * base) % modulus;
+        exponent >>= 1n;
+    }
+
+    return result;
+}
+
+/**
+ * parseBigInt helper operation
+ */
+function parseBigInt(value, param) {
+    const v = value.trim();
+
+    if (/^0x[0-9a-f]+$/i.test(v)) return BigInt(v);
+    if (/^[0-9]+$/.test(v)) return BigInt(v);
+
+    throw new OperationError(param + " must be decimal or hex (0x...)");
+}
+
+/* ---------- operation class ---------- */
+
+
+/**
+ * Modular Exponentiation operation
+ */
+class ModularExponentiation extends Operation {
+
+    /**
+     * ModularExponentiation constructor
+     */
+    constructor() {
+        super();
+
+        this.name = "Modular Exponentiation";
+        this.module = "Crypto";
+        this.description = "Performs modular exponentiation, as used in Diffie-Hellman and RSA.<br><br>" +
+            "Computes Base ^ Exponent mod Modulus.<br><br>" +
+            "<b>Input handling:</b> If <i>either</i> Base <i>or</i> Exponent is left blank, " +
+            "its value is taken from the Input field.";
+        this.infoURL = "https://wikipedia.org/wiki/Modular_exponentiation";
+        this.inputType = "string";
+        this.outputType = "string";
+        this.args = [
+            {
+                name: "Base",
+                type: "string",
+                value: ""
+            },
+            {
+                name: "Modulus",
+                type: "string",
+                value: "1"
+            },
+            {
+                name: "Exponent",
+                type: "string",
+                value: ""
+            }
+        ];
+    }
+
+    /**
+      * @param {string} input
+      * @param {Object[]} args
+      * @returns {string}
+      */
+    run(input, args) {
+        const [baseStr, modStr, expStr] = args;
+
+        // Trim everything so "" and "   " count as empty
+        const baseParam = baseStr?.trim();
+        const expParam  = expStr?.trim();
+        const modParam  = modStr?.trim();
+        const inputVal  = input?.trim();
+
+        const mod = modParam;
+        if (!mod) {
+            throw new OperationError("Modulus must be defined");
+        }
+
+        // Base *or* Exponent (but not both) are taken from the Input
+        // if their boxes are empty.
+        let base, exp;
+        if (baseParam && expParam) {
+            // Case 1: base and exponent both given as parameters
+            base = baseParam;
+            exp  = expParam;
+        } else if (!baseParam && expParam) {
+            // Case 2: base missing - take from input
+            base = inputVal;
+            exp  = expParam;
+            if (!base) {
+                throw new OperationError("Base must be defined");
+            }
+        } else if (baseParam && !expParam) {
+            // Case 3: exponent missing - take from input
+            base = baseParam;
+            exp  = inputVal;
+            if (!exp) {
+                throw new OperationError("Exponent must be defined");
+            }
+        } else if (!inputVal) {
+            // Case 4: base and exponent both missing
+            throw new OperationError("Base and Exponent must be defined");
+        } else throw new OperationError("Ambiguous input: specify either Base or Exponent when using Input");
+
+        // Parse numbers
+        const baseBI = parseBigInt(base, "Base");
+        const expBI  = parseBigInt(exp, "Exponent");
+        const modBI  = parseBigInt(mod, "Modulus");
+
+        // Check for invalid modulus (parseBigInt eliminates negatives)
+        if (modBI === 0n) {
+            throw new OperationError("Modulus must be greater than zero");
+        }
+
+        return modPow(baseBI, expBI, modBI).toString();
+    }
+}
+
+export default ModularExponentiation;