The Pohlig-Hellman exponentiation cipher is a symmetric-key cipher
that uses some of the same mathematical operations as the better-known
RSA and Diffie-Hellman public-key cryptosystems. First published in
1978, the Pohlig-Hellman cipher was never of practical importance due
to its slow speed compared to ciphers such as DES and AES. The
theoretical importance of the Pohlig- Hellman cipher comes from the
fact that it relies on the Discrete Logarithm Problem for its
resistance against known plain text attacks, as does RSA and several
other modern cryptosystems. For this reason, the Pohlig-Hellman
systemcan play a very important role pedagogically, since it also
shares many features in common with classical ciphers such as shift
ciphers and Hill ciphers. Thus, it allows the instructor to introduce
the important concepts of the discrete logarithm and known plain text
attacks separately from the more conceptually difficult idea of
public-key cryptography.