In the simplest of substitution ciphers
, the key is nothing more than a shift in the alphabet where A=B, B=C, C=D, and so on; resulting in a chart which looks like this:
Cryptanalysis of the Playfair cipher is much more difficult than normal simple substitution ciphers
, because digraphs (pairs of letters) are being substituted instead of monographs (single letters) .
The author begins a description of various symmetric-key cryptographic algorithms that includes basic substitution ciphers
, cryptanalysis and other encryption standards.
and code-books in which whole words substituted for others dominated cryptography from Caesar's time through the nineteenth century.
Simple substitution ciphers
like these are easy to solve, even if the code is not known, as shall be shown subsequently.
About 1,000 years ago, simple substitution ciphers
were familiar enough for their weaknesses to be understood and for people to begin exploring more complex systems intended to counter those weaknesses.
If the cryptanalyst knows which language the cipher was written in and has enough cipher text to work with, simple substitution ciphers
can often be solved easily.
One of the early substitution ciphers
is credited to Julius Caesar.
Such ciphers are harder to break than either transposition or substitution ciphers
alone (see "Unscramblilng the ABC's of Cryptography," Communications News, February 1980, page 48).
In a related move, the company's iPower Business Unit launched its own Web site along with CryptoSolver, an interactive game that invites participants to solve encrypted messages based on the concepts of substitution ciphers
and the alphabetic frequency table.
In a November 1970 article, Ross Eckler cited a finding in communications theory that even simple substitution ciphers
of length 27 or greater have an extremely high probability of a unique solution.
The undergraduate textbook introduces number theory, modular arithmetic, substitution ciphers
, the Euclidean algorithm, and the mathematical basis for an exponential cipher.