diff --git a/lib/BigNumber.js b/lib/BigNumber.js
deleted file mode 100644
index 8423f5d..0000000
--- a/lib/BigNumber.js
+++ /dev/null
@@ -1,66 +0,0 @@
-'use strict';
-
-const { jsbn: { BigInteger } } = require('node-forge');
-
-class BigNumber {
-	constructor(value, radix = 10) {
-		if (value === null || value === undefined)
-			throw new Error('value is missing.');
-
-		this._n = new BigInteger(null);
-
-		if (value instanceof BigNumber)
-			this._n = value._n;
-		else if (value instanceof BigInteger)
-			this._n = value;
-		else if (typeof value === 'number')
-			this._n.fromInt(value);
-		else if (typeof value === 'string')
-			this._n.fromString(value, radix);
-		else if (Buffer.isBuffer(value))
-			this._n.fromString(value.toString('hex'), 16);
-		else if (Array.isArray(value))
-			this._n.fromString(Buffer.from(value).toString('hex'), 16);
-		else
-			throw new TypeError('Unsupported value type.');
-	}
-
-	toBEBuffer(length = undefined) {
-		const arr = this._n.toByteArray();
-		if (length && arr.length > length)
-			throw new Error('Number out of range.');
-		while (arr.length < length)
-			arr.unshift(0);
-		return Buffer.from(arr);
-	}
-
-	toBuffer() {
-		return Buffer.from(this._n.toByteArray());
-	}
-
-	toString(radix = 10) {
-		return this._n.toString(radix);
-	}
-
-	and(num) {
-		return new BigNumber(this._n.and(new BigNumber(num)._n));
-	}
-
-	mul(num) {
-		return new BigNumber(this._n.multiply(new BigNumber(num)._n));
-	}
-
-	mod(num) {
-		return new BigNumber(this._n.mod(new BigNumber(num)._n));
-	}
-
-	shrn(num) {
-		return new BigNumber(this._n.shiftRight(new BigNumber(num)._n));
-	}
-
-	static fromBuffer(buf) {
-		return new BigNumber(buf.toString('hex'), 16);
-	}
-}
-
-module.exports = BigNumber;
diff --git a/test/unit/BigNumber.js b/test/unit/BigNumber.js
new file mode 100644
index 0000000..0b95bca
--- /dev/null
+++ b/test/unit/BigNumber.js
@@ -0,0 +1,48 @@
+'use strict';
+
+/* eslint-env node, mocha */
+
+const { assert } = require('chai');
+const { jsbn: { BigInteger } } = require('node-forge');
+
+const BigNumber = require('../../lib/crypto/BigNumber');
+
+const types = [
+	{ name: 'BigNumber', value: new BigNumber(11) },
+	{ name: 'BigInteger', value: new BigInteger('11') },
+	{ name: 'string with default radix', value: '11' },
+	{ name: 'string with radix 16', value: '0b', radix: 16 },
+	{ name: 'Buffer', value: Buffer.from('0b', 'hex') },
+	{ name: 'Array', value: [11] },
+];
+
+describe('BigNumber', () => {
+	describe('creating an instance', () => {
+		it('should throw with no value given', () => assert.throws(() => new BigNumber()));
+		it('should throw wrong value type', () => assert.throws(() => new BigNumber({})));
+
+		for (const { name, value, radix } of types) {
+			let instance;
+			describe(`out of ${name}`, () => {
+				it('create instance', () => assert.doesNotThrow(() => {
+					instance = new BigNumber(value, radix);
+				}));
+				it('toString with radix 10', () => assert.equal(instance.toString(), '11'));
+				it('toString with radix 16', () => assert.equal(instance.toString(16), '0b'));
+				it('toBuffer', () => assert.equal(instance.toBuffer().toString('hex'), '0b'));
+				it('toBEBuffer without length', () => assert.equal(instance.toBEBuffer().toString('hex'), '0b'));
+				it('toBEBuffer with length', () => assert.equal(instance.toBEBuffer(4).toString('hex'), '0000000b'));
+			});
+		}
+	});
+	describe('exports', () => {
+		it('toBEBuffer with too short length should throw', () => assert.throw(() => new BigNumber(837462187362).toBEBuffer(1)));
+	});
+	describe('operators', () => {
+		const num = new BigNumber(1);
+		it('and', () => assert.equal(num.and(1), 1));
+		it('mul', () => assert.equal(num.mul(2), 2));
+		it('mod', () => assert.equal(num.mod(1), 0));
+		it('shrn', () => assert.equal(num.shrn(1), 0));
+	});
+});