Martin Kochanski’s web site


Some aspects of public-key encryption

Traditional ("symmetric", "secret-key", or "single-key") cryptosystems are simple.  There is an algorithm that says what calculations to perform and a key (often a number) that controls how those calculations work.  The same key can be used to decrypt or encrypt data. 

Public-key ("asymmetric", "two-key") cryptosystems are different.  They have two keys, one for encryption and one for decryption, and it is not possible to derive one from the other.  So you could keep the encryption key secret but publish your decryption key in a directory, so that you could encrypt messages and everyone could decrypt them and be sure they were from you (digital signatures) or you could keep the decryption key secret and publish the encryption key so that people could send you secret messages even if you have never met them or made arrangements with them.

I got my degree at about the same time that public-key encryption was discovered, so I was involved in the field from the beginning.

A New Method of Serial Modular Multiplication

The big problem with many methods of public-key encryption is that they require large numbers of arithmetic operations on very large numbers (300 decimal digits or so).  Finding hardware designs that can do this kind of arithmetic fast is therefore quite important.  The technical paper cited here describes a new design that offers some advantages over existing ones.

Creating the FAP4 encryption chip

This paper is less technical.  It gives more of the history of the construction of an encryption chip based on the design described above.  It also explains in detail why arithmetic on large numbers is hard, and how it can be speeded up, in a way that anyone who did sums in primary school can understand and, I hope, enjoy.

The comedy of commercial encryption software

Every classic drama has some comic relief, and the comic relief in cryptography is provided by the activities of the commercial sector.  The weaknesses of the encryption packages are only matched by the feebleness of the public-relations excuses that are advanced when the weaknesses are pointed out.