Glossary - Cryptology Unlocked - page 492

Fail-stop signature A special type of digital signature where many private keys
belong to one public key. This helps to make provable successful attempts to
break the underlying asymmetric method and the forgery of a digital signature
(Section 6.6.5).
FEAL A block algorithm that was originally conceived as a DES substitute, but later
proved to be extraordinarily insecure (Section 5.7.3).
Feistel network A particularly simple structure of a product algorithm that uses only
one key-dependent function. The reversion of this function does not have to be
computed during encryption (Section 4.2).
Fingerprint The MD5 checksum of a public key, which is used, for example, to
verify the authenticity of this public key by phone. Fingerprints have become
known mainly from the PGP software package.
Firewall A computer (or program) used to protect an intranet from external attacks
based on some well-defined method. For example, a firewall may accept only
specific types of data packets, check on senders, etc.
Flat or non-linear key space All keys in a flat key space are equally strong. The oppo-
site is a non-linear key space. While the existence of weak keys is an undesirable
side effect, algorithms with non-linear key spaces are aimed at ensuring that peo-
ple unaware of the technique use only weak keys, making their ciphers particularly
easy to decrypt. Only people knowing the internals of the algorithm can encrypt
securely. This concept is even riskier than key escrow. Cryptologists are, therefore,
interested in proving that the key space is flat.
GnuPG (GNU Privacy Guard) An email encryption program available for free and
without license fee that implements the OpenPGP standard, in contrast to PGP
Version 5.0 and higher (Section 7.1.4).
Half block The left or right half of a block, i.e., the least significant or most significant
32 bits in a 64-bit block. Half blocks are important in Feistel networks.
Hash function A sort of mapping that calculates checksums for data streams such that
all possible function values occur roughly equally. The checksum is also called
hash sum or hash value . Hash functions are important in search algorithms, and
special hash functions are the one-way hash functions used in cryptography.
Hybrid method A cryptographic protocol where messages are encrypted by use of
a common symmetric method, whereas the corresponding (random) session key
is distributed by means of an asymmetric method. Hybrid methods have become
very popular, e.g., in PGP (Section 4.5.2).