diff --git a/lib/crypto/BigNumber.js b/lib/crypto/BigNumber.js
new file mode 100644
index 0000000..fda2e84
--- /dev/null
+++ b/lib/crypto/BigNumber.js
@@ -0,0 +1,65 @@
+'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 (length && arr.length < length)
+			arr.unshift(0);
+		return Buffer.from(arr);
+	}
+
+	toBuffer() {
+		return Buffer.from(this._n.toByteArray());
+	}
+
+	toString(radix = 10) {
+		const result = this._n.toString(radix);
+		if (radix === 16)
+			return result.padStart(2, '0');
+		return result;
+	}
+
+	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));
+	}
+}
+
+module.exports = BigNumber;
diff --git a/lib/crypto/Crypto.js b/lib/crypto/Crypto.js
index 7a6db0e..a39fd75 100644
--- a/lib/crypto/Crypto.js
+++ b/lib/crypto/Crypto.js
@@ -2,7 +2,7 @@
 
 const crypto = require('crypto');
 
-const BigNumber = require('../BigNumber.js');
+const BigNumber = require('./BigNumber.js');
 const mgf1 = require('./MGF1');
 
 const modPow = (base, power, mod) => {
@@ -27,7 +27,7 @@ const emsaPSS = (msg, salt) => {
 	const dbMask = mgf1.generate(mTickHash, db.length);
 	const maskedDb = mgf1.xor(db, dbMask);
 
-	let maskedDbMsb = mgf1.rjust(BigNumber.fromBuffer(maskedDb.slice(0, 1)).toString(2), 8, '0');
+	let maskedDbMsb = mgf1.rjust(new BigNumber(maskedDb.slice(0, 1)).toString(2), 8, '0');
 
 
 	maskedDbMsb = `0${maskedDbMsb.substr(1)}`;
diff --git a/lib/crypto/MGF1.js b/lib/crypto/MGF1.js
index 0a7aeed..f986a5c 100644
--- a/lib/crypto/MGF1.js
+++ b/lib/crypto/MGF1.js
@@ -2,7 +2,7 @@
 
 const crypto = require('crypto');
 
-const BigNumber = require('../BigNumber.js');
+const BigNumber = require('./BigNumber.js');
 
 const MFG_LEN = 32;