diff --git a/src/core/config/Categories.json b/src/core/config/Categories.json
index aac00ca1c..07b2caeb5 100644
--- a/src/core/config/Categories.json
+++ b/src/core/config/Categories.json
@@ -222,6 +222,9 @@
             "Subtract",
             "Multiply",
             "Divide",
+            "Modular Exponentiation",
+            "Modular Inverse",
+            "Extended GCD",
             "Mean",
             "Median",
             "Standard Deviation",
diff --git a/src/core/lib/BigIntUtils.mjs b/src/core/lib/BigIntUtils.mjs
new file mode 100644
index 000000000..5ddd8786f
--- /dev/null
+++ b/src/core/lib/BigIntUtils.mjs
@@ -0,0 +1,73 @@
+/**
+ * @author p-leriche [philip.leriche@cantab.net]
+ * @copyright Crown Copyright 2025
+ * @license Apache-2.0
+ */
+
+import OperationError from "../errors/OperationError.mjs";
+
+/**
+ * Number theory utilities used by cryptographic operations.
+ *
+ * Currently provides:
+ *   - parseBigInt
+ *   - Extended Euclidean Algorithm
+ *   - Modular Exponentiation
+ *
+ * Additional algorithms may be added as required.
+ */
+
+/**
+ * parseBigInt helper operation
+ */
+export 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...)");
+}
+
+/**
+ * Extended Euclidean Algorithm
+ *
+ * Returns [g, x, y] such that:
+ *   a*x + b*y = g = gcd(a, b)
+ *
+ *   (Uses an iterative algorithm to avoid possible stack overflow)
+ */
+export function egcd(a, b) {
+    let oldR = a, r = b;
+    let oldS = 1n, s = 0n;
+    let oldT = 0n, t = 1n;
+
+    while (r !== 0n) {
+        const quotient = oldR / r;
+
+        [oldR, r] = [r, oldR - quotient * r];
+        [oldS, s] = [s, oldS - quotient * s];
+        [oldT, t] = [t, oldT - quotient * t];
+    }
+
+    // oldR is the gcd
+    // oldS and oldT are the BĂ©zout coefficients
+    return [oldR, oldS, oldT];
+}
+
+/**
+ * Modular exponentiation
+ */
+export 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;
+}
+
diff --git a/tests/node/index.mjs b/tests/node/index.mjs
index f6abba408..f872f8f48 100644
--- a/tests/node/index.mjs
+++ b/tests/node/index.mjs
@@ -23,6 +23,7 @@ import "./tests/Dish.mjs";
 import "./tests/NodeDish.mjs";
 import "./tests/Utils.mjs";
 import "./tests/Categories.mjs";
+import "./tests/lib/BigIntUtils.mjs";
 
 const testStatus = {
     allTestsPassing: true,
diff --git a/tests/node/tests/lib/BigIntUtils.mjs b/tests/node/tests/lib/BigIntUtils.mjs
new file mode 100644
index 000000000..d75a4893a
--- /dev/null
+++ b/tests/node/tests/lib/BigIntUtils.mjs
@@ -0,0 +1,150 @@
+import TestRegister from "../../../lib/TestRegister.mjs";
+import { parseBigInt, egcd, modPow } from "../../../../src/core/lib/BigIntUtils.mjs";
+import it from "../../assertionHandler.mjs";
+import assert from "assert";
+
+TestRegister.addApiTests([
+    // ===== parseBigInt tests =====
+    it("BigIntUtils: parseBigInt - decimal number", () => {
+        const value = parseBigInt("1", "test value");
+        assert.deepStrictEqual(value, BigInt("1"));
+    }),
+    
+    it("BigIntUtils: parseBigInt - large decimal", () => {
+        const value = parseBigInt("123456789012345678901234567890", "test value");
+        assert.deepStrictEqual(value, BigInt("123456789012345678901234567890"));
+    }),
+    
+    it("BigIntUtils: parseBigInt - hexadecimal lowercase", () => {
+        const value = parseBigInt("0xff", "test value");
+        assert.deepStrictEqual(value, BigInt("255"));
+    }),
+    
+    it("BigIntUtils: parseBigInt - hexadecimal uppercase", () => {
+        const value = parseBigInt("0xFF", "test value");
+        assert.deepStrictEqual(value, BigInt("255"));
+    }),
+    
+    it("BigIntUtils: parseBigInt - large hexadecimal", () => {
+        const value = parseBigInt("0x123456789ABCDEF", "test value");
+        assert.deepStrictEqual(value, BigInt("0x123456789ABCDEF"));
+    }),
+    
+    it("BigIntUtils: parseBigInt - whitespace trimming", () => {
+        const value = parseBigInt("  42  ", "test value");
+        assert.deepStrictEqual(value, BigInt("42"));
+    }),
+    
+    it("BigIntUtils: parseBigInt - invalid input (text)", () => {
+        assert.throws(() => parseBigInt("test", "test value"), {
+            name: "Error",
+            message: "test value must be decimal or hex (0x...)"
+        });
+    }),
+    
+    it("BigIntUtils: parseBigInt - invalid input (hex without prefix)", () => {
+        assert.throws(() => parseBigInt("FF", "test value"), {
+            name: "Error",
+            message: "test value must be decimal or hex (0x...)"
+        });
+    }),
+    
+    it("BigIntUtils: parseBigInt - invalid input (mixed)", () => {
+        assert.throws(() => parseBigInt("12abc", "test value"), {
+            name: "Error",
+            message: "test value must be decimal or hex (0x...)"
+        });
+    }),
+
+    // ===== egcd tests =====
+    it("BigIntUtils: egcd - basic coprime", () => {
+        const a = BigInt("36");
+        const b = BigInt("48");
+        const gcd = BigInt("12");
+        const bezout1 = BigInt("-1");
+        const bezout2 = BigInt("1");
+        assert.deepStrictEqual(egcd(a, b), [gcd, bezout1, bezout2]);
+    }),
+    
+    it("BigIntUtils: egcd - coprime numbers", () => {
+        const [g, x, y] = egcd(BigInt("3"), BigInt("11"));
+        assert.strictEqual(g, BigInt("1"));
+        // Verify Bézout identity: a*x + b*y = gcd
+        assert.strictEqual(BigInt("3") * x + BigInt("11") * y, g);
+    }),
+    
+    it("BigIntUtils: egcd - non-coprime numbers", () => {
+        const [g, x, y] = egcd(BigInt("240"), BigInt("46"));
+        assert.strictEqual(g, BigInt("2"));
+        // Verify Bézout identity
+        assert.strictEqual(BigInt("240") * x + BigInt("46") * y, g);
+    }),
+    
+    it("BigIntUtils: egcd - with zero", () => {
+        const [g, x, y] = egcd(BigInt("17"), BigInt("0"));
+        assert.strictEqual(g, BigInt("17"));
+        assert.strictEqual(x, BigInt("1"));
+        assert.strictEqual(y, BigInt("0"));
+    }),
+    
+    it("BigIntUtils: egcd - identical numbers", () => {
+        const [g, x, y] = egcd(BigInt("42"), BigInt("42"));
+        assert.strictEqual(g, BigInt("42"));
+        // Verify Bézout identity
+        assert.strictEqual(BigInt("42") * x + BigInt("42") * y, g);
+    }),
+    
+    it("BigIntUtils: egcd - large numbers", () => {
+        const a = BigInt("123456789012345678901234567890");
+        const b = BigInt("987654321098765432109876543210");
+        const [g, x, y] = egcd(a, b);
+        // Verify Bézout identity
+        assert.strictEqual(a * x + b * y, g);
+    }),
+
+    // ===== modPow tests =====
+    it("BigIntUtils: modPow - basic", () => {
+        // 2^10 mod 1000 = 1024 mod 1000 = 24
+        const result = modPow(BigInt("2"), BigInt("10"), BigInt("1000"));
+        assert.strictEqual(result, BigInt("24"));
+    }),
+    
+    it("BigIntUtils: modPow - RSA-like example", () => {
+        // Common RSA public exponent
+        const base = BigInt("123456789");
+        const exp = BigInt("65537");
+        const mod = BigInt("999999999999");
+        const result = modPow(base, exp, mod);
+        // Result should be less than modulus
+        assert(result < mod);
+        assert(result >= BigInt("0"));
+    }),
+    
+    it("BigIntUtils: modPow - exponent zero", () => {
+        // Any number^0 = 1
+        const result = modPow(BigInt("999"), BigInt("0"), BigInt("100"));
+        assert.strictEqual(result, BigInt("1"));
+    }),
+    
+    it("BigIntUtils: modPow - base zero", () => {
+        // 0^n = 0
+        const result = modPow(BigInt("0"), BigInt("5"), BigInt("100"));
+        assert.strictEqual(result, BigInt("0"));
+    }),
+    
+    it("BigIntUtils: modPow - large exponent", () => {
+        // Test with very large exponent (efficient algorithm should handle this)
+        const result = modPow(BigInt("3"), BigInt("1000000"), BigInt("1000000007"));
+        assert(result >= BigInt("0"));
+        assert(result < BigInt("1000000007"));
+    }),
+    
+    it("BigIntUtils: modPow - modular inverse verification", () => {
+        // If a*x . 1 (mod m), then modPow(a, 1, m) * x . 1 (mod m)
+        const a = BigInt("3");
+        const m = BigInt("11");
+        const x = BigInt("4"); // inverse of 3 mod 11
+        const result = modPow(a, BigInt("1"), m) * x % m;
+        assert.strictEqual(result, BigInt("1"));
+    }),
+]);