Author: Josef Gruska
Josef Gruska is a Professor of Computer Science at Masaryk University
in Brno, at the Slovak Academy of Sciences, Bratislava and at Commenius
University in Bratislava. He has held numerous positions and visiting professorships
across North America and Europe, including the European Association of
Theoretical Computer Science (EATCS) . He was founding Chair of the IFIP
Specialist Group on Foundations of Computer Science.
Cryptosystems and Cryptology
Cryptography is the art of providing secure communication
over insecure channels
Cryptoanalysis is the art of breaking into such
communications
Cryptology is the combined art of cryptography and cryptoanalysis.
A cryptosystem is called a secret-key cryptosystem if some secret piece of information, the key, has to be agreed upon ahead of time between two parties the want or need to communicate through the cryptosystem. CAESER, POLYBIOS and HILL are examples.
Public-Key Cryptosystems
The key observation leading to public-key cryptography is that whoever
encrypts a plaintext does not need to be able to decrypt the resulting
cryptotext. In such a case each user U can choose a private and a public
key as well as an encryption and a decryption algorithm, make the encryption
algorithm public and keep secret the decryption algorithm. In such
a case anybody can send messages to U, and U is the only one capable of
decrypting them using his secret key and his decryption algorithm.
Examples: Trapdoor one-way functions, Knapsack cryptosystems and RSA
cryptosystems.
Digital Signatures
Digital signatures are one of the most fundamental and important inventions
of modern cryptography. They help to verify that a received message that
claims to be from a certain user is really from this user and that not
anybody else pretending to be this user has sent it.
A public-key cryptosystem, in which plaintext and cryptotext space
are the same and each user makes his encryption function public and keeps
his decryption function secret, can be used for this purpose, for e.g.
the RSA cryptosystem. However not all public-key cryptosystems are equally
good for this. In addition there are some signatures-only (crypto)schemes.