Questions & Answers about Yarrow

What is Yarrow?

Yarrow is a high-performance, high-security, pseudo-random number generator
(PRNG) for Windows, Windows NT, and UNIX. It can provide random numbers for a
variety of cryptographic applications: encryption, signatures, integrity, etc.

Why is a PRNG important?

PRNGs are used everywhere in cryptography. In fact, it is hard to imagine a
well-designed cryptographic application that doesn't use random numbers. Random
numbers are used to generate session keys, initialization vectors, salts to be
hashed with passwords, unique parameters in digital signature operations,
random initialization for public-key generation, and nonces in different
protocols. If the random numbers in any of these applications are insecure,
than the entire application is insecure. Algorithms and protocols can't cover
for bad random numbers.

What's wrong with all the other PRNGs?

Over the years, Counterpane Systems has broken proprietary PRNGs for clients.
Recently, we published the first extensive cryptanalysis of PRNGs at Fast
Software Encryption 98. (Copies of the paper can be found at: http://
www.counterpane.com/pseudorandom_number.html.) In the paper, we analyzed the
PRNGs in X9.17, DSA, RSAREF, and CryptoLib. We found that these PRNGs are not
secure for all applications. We have also analyzed PRNGs similar to the one
found in PGP, and will publish that work in a future paper. We believe that
current PRNG design is simply ad hoc, without any strong security arguments or
analysis.

Why did we write Yarrow?

We wrote Yarrow because we wanted to build a secure PRNG based on the research
we have done.

Why is Yarrow free?

No one would use Yarrow if it were not free. We have worked with enough
cryptographic products and security companies to know what can and cannot be
sold. No one would pay money to use Yarrow as long as they believed they could
do nearly as well for free. If Yarrow is ever to be widely used, it must be
free.

Is Yarrow exportable?

Yes. Yarrow is not an encryption algorithm, and thus not subject to the U.S.
export regulations on encryption. Of course, if someone uses Yarrow to build an
encryption product, that product may not be exportable.

Where did the name Yarrow come from?

Yarrow is a plant: a flowering perennial with distinctive flat flowerheads and
lacy leaves. Yarrow stalks have been used for divination in China since the
Hsia dynasty, in the second millenium B.C. The fortuneteller would divide a set
of 50 stalks into piles, then repeatedly use modulo arithmetic to generate two
random bits. Yarrow stalks are still used for fortunetelling in China, but with
a greatly simplified method: shake a container of 100 numbered yarrow stalks
until one stalk comes out.

Copyright Counterpane Internet Security, Inc., 2001