Glossary - Cryptology Unlocked - page 495

One-time password A cryptographic protocol (Section 6.5) enabling authentication
over insecure lines, used without asymmetric methods. A password is used only
once so that intercepting one-time passwords is worthless. One-time passwords
are important in home banking or when somebody works at their own computer
from third-party computers.
One-way hash function A sort of mapping that creates checksums for data streams,
where no data can be reconstructed from a given checksum at a reasonable cost.
An additional requirement is non-collision (Section 6.3.1).
OpenPGP A standard developed on the basis of PGP Version 5.0 and higher for secure
email communication. Other than in PGP, it is also implemented in GnuPG and
other products (see Section 7.1.4).
Passphrase A long character string used instead of traditional passwords six or eight
characters long. Passphrases can be sentences (including punctuation marks and
blanks), or phrases. Passphrases should be preferred over passwords since they
offer more security. Passphrases are used, for example, in PGP.
Permutation Mathematically, the mapping of a biunique (one-to-one transformation)
mapping of a finite set onto itself. Colloquially, an arrangement of a finite number
or character set. Transposition ciphers (Section 2.2.4) are actually block algo-
rithms, where plaintext blocks are encrypted by permutation. Extremely vulnerable
to differential cryptanalysis, if used by itself (Section 4.4.2).
PGP (Pretty Good Privacy) The most popular free program used to encrypt files and
email (see Section 7.1).
Plaintext A piece of readable text to be encrypted.
Plaintext attack A cryptanalytic method where a small part of the plaintext is normally
known.
Playfair method A special digram cipher (see Section 2.3).
Polyalphabetic substitution A special substitution cipher, where the substitution rule
depends on the position in the text. Examples are the Enigma and the Vigenere
cipher.
Polygraphic substitution A special substitution cipher, where several characters
together are substituted based on a large table. Corresponds to current block
algorithms with a typical block length of two characters (then called digram
substitution ) or three characters (then called trigram substitution ).