About jsotp

jsotp is a JavaScript implementation of a one-time password generator/calculator for the OTP and S/Key systems. It supports MD4, MD5, and SHA-1 hashing methods, by way of code written by Paul Johnston. It also supports RIPEMD-160, through some code that I wrote. This calculator was inspired by Harry Mantakos's Java applet OTP calculator (available at his site). If there's already a Java applet version, then why a JavaScript version? Because in regard to web browsers, JavaScript is still by far the most universally available programming language (I was actually very surprised that I couldn't find an existing JavaScript-based calculator). Also, I've added some enhancements that make jsotp basically the only thing you need for generating OTP and S/Key passwords: it runs in most semi-modern web browsers, can generate passwords for all of the hash methods commonly supported, and can generate single passwords, as well as lists of passwords suitable for printing. Another benefit is that if you're using jsotp remotely, it's relatively easy for you to verify that the copy you're using doesn't do anything funny, since you can easily view the source from within your browser.

If you're so inclined, jsotp can also be used as a divination tool. Just type in your question as the seed, enter your current favorite not-too-big number, and flail on the keyboard for a bit. Finally, pick your lucky hashing method to see what fate has in store for you today. It usually doesn't make much sense, but then again, what in life does?

Feel free to examine the source and host a copy of your own for better security. If you intend for other people also to use your copy, however, please at least retain a link back to the jsotp site.

Questions, comments, bug reports, etc? Email me.

What are OTP and S/Key, and why use them?

OTP and S/Key are one-time password systems. Unsurprisingly, OTP also stands for "one-time password". In these systems, you generate a sequence of hashes by repeatedly applying a secure one-way hash on a random seed value prepended to your secret passphrase. Then you can use the hashes as one-time passwords, in backwards order; since the hashes are one-way, knowing a hash later in the sequence will not help you compute an earlier hash in a reasonable amount of time. Note that since the hashes are just random-looking 64-bit numbers, they are usually converted to a six-word format to make it easier for the user to keep track of what he's typing. Also, while OTP generators are required to show the words in all uppercase, the server should not care what case they are in, so feel free to type an OTP in all lowercase.

OTP is basically a somewhat updated (and mostly compatible) version of the S/Key system created at Bellcore (which also owns the trademark on the name S/Key). OTP is described in RFC 2289, and S/Key is described in RFC 1760. You can also find some additional documents related to OTP authentication here. OTP and S/Key have generally not been all that useful since the advent of SSH, but I think they remain useful in two situations:

You want to log into a remote system from a computer that has neither a native SSH client nor a browser that supports running a Java applet SSH client (either because Java isn't installed, or the version is too old), or the connection is slow and you just don't feel like downloading a large applet. In this case, as long as you do have a telnet client (which comes with most operating systems), an OTP system can be a good alternative.
You want to log into a remote system from another computer, but you suspect (or are just paranoid) that this computer may log keystrokes or something. Having an SSH client available obviously doesn't help with this problem, but having a precomputed list of one-time passwords does (though having such a list ready when you need it does require a little advance planning). You can use jsotp to generate however many passwords you think you might need (make sure you do it from a trusted computer!), then print them out and keep them safely with you.
OTP's can be used fruitfully with other network services (such as email, FTP, etc.), assuming the particular daemon you're using can be configured to accept OTP's. For example, if the SSH daemon on the remote end is configured to accept OTP's, then you can use an OTP in conjunction with SSH to gain end-to-end encryption. However, if you think keystrokes are being logged, then it's not such a stretch to think that the local SSH client may be capturing incoming data too, so it's still risky to type or display anything sensitive. Also, there are more nefarious, active attacks that OTP's would not provide any protection against; for example, the local SSH client could conceivably be trojaned to allow an attacker to send commands of his choice to the system you've logged into.

The source

jsotp is normally stripped of comments and whatnot to make it a tiny bit faster to download. There are also versions that include only one particular hashing method (MD4, MD5, SHA-1, RIPEMD-160) that provide a more noticeable decrease in page size (these contain all code inline, so you can just save the page without copying any other source files). And while it's not all that interesting, you can find the version with comments retained as otp.js in this directory.

License

jsotp is available under a BSD-style license. The HTML scaffolding on this site also falls under the same license. As mentioned previously, feel free to mirror whatever pages you need on your own webspace for better security (and again, note that the single-hash versions contain all code inline, so you can just save the page directly).

Jeremy Lin, Dec. 2003, Mar. 2004.