BigIntegers and RSA in JavaScript

The jsbn library is a pure JavaScript implementation of arbitrary-precision integer arithmetic.

Demos

• RSA Encryption Demo - simple RSA encryption of a string with a public key
• RSA Cryptography Demo - more complete demo of RSA encryption, decryption, and key generation

Source Code

The API for the jsbn library closely resembles that of the java.math.BigInteger class in Java.

For example:

x = new BigInteger("abcd1234", 16);
y = new BigInteger("beef", 16);
z = x.mod(y);
alert(z.toString(16));

will print b60c.

• jsbn.js - basic BigInteger implementation, just enough for RSA encryption and not much more.
• jsbn2.js - the rest of the library, including most public BigInteger methods.
• rsa.js - implementation of RSA encryption, does not require jsbn2.js.
• rsa2.js - rest of RSA algorithm, including decryption and keygen.
• rng.js - rudimentary entropy collector and RNG interface, requires a PRNG backend to define prng_newstate().
• prng4.js - ARC4-based PRNG backend for rng.js, very small.
• base64.js - Base64 encoding and decoding routines.

Interoperability

The demo encrypts strings directly using PKCS#1 encryption-style padding (type 2), which is currently the only supported format. To show interoperability with a potential OpenSSL-based backend that decrypts strings, try the following on any system with the OpenSSL command line tool installed:

1. Generate a new public/private keypair:

   $ openssl genrsa -out key.pem
   Generating RSA private key, 512 bit long modulus
   ..++++++++++++
   ..............++++++++++++
   e is 65537 (0x10001)
   $

2. Extract the modulus from your key:

   $ openssl rsa -in key.pem -noout -modulus
   Modulus=DA3BB4C40E3C7E76F7DBDD8BF3DF0714CA39D3A0F7F9D7C2E4FEDF8C7B28C2875F7EB98950B22AE82D539C1ABC1AB550BA0B2D52E3EF7BDFB78A5E817D74BBDB
   $

3. Go to the RSA Encryption demo and paste the modulus value into the "Modulus (hex)" field at the bottom.

4. Make sure the value in the "Public exponent" field is "10001", or whatever value your public key uses.

5. Type in a short string (e.g. testing) into the "Plaintext (string)" field and click on "encrypt". The result should appear in the "Ciphertext" fields.

6. Copy the base64 version of the ciphertext and paste it as the input of the following command:

   $ openssl base64 -d | openssl rsautl -inkey key.pem -decrypt
   1JW24UMKntVhmmDilAYC1AjLxgiWHBzTzZsCVAejLjVri92abLHkSyLisVyAdYVr
   fiS7FchtI9vupe9JF/m3Kg==

   Hit ctrl-D or whatever your OS uses for end-of-file. Your original plaintext should appear:

   testing$

Performance

Since jsbn is pure JavaScript, its performance will depend on the hardware as well as the quality of the JavaScript execution environment, but will be considerably slower than native implementations in languages such as C/C++ or Java.

On a 1GHz Intel PC running Mozilla:

On similar hardware, running IE6:

Timing measurements, especially under IE, appear to have limited precision for faster operations.

History

Licensing

jsbn is released under a BSD license. See LICENSE for details.

Tom Wu