Operation Manual

62 NetWAYS/ISDN – Glossary
Hash algorithm
A hash algorithm is a function that yields a short value that is
practically unique for a given input. The value of the hash is
also called a “digest” of the input. One-way hash algorithms
are used in cryptography to create digital signatures for au-
thentication.
Characteristics of one-way hash algorithms:
– The input data can be of any length.
– The output is generally of a fixed length.
– The input data cannot be reconstituted from the out-
put.
– The algorithm must be sufficiently free of collision:
in other words, the probability of two different input
values yielding the same output must be very small.
Keyed hash functions:
Keyed hash functions are one-way hash algorithms that
use a key in addition to the variable input data. Keyed
hash functions are used to generate message authenti-
cation codes (MACs). Only those who hold the same key
can generate the same MAC from a given message. This
makes the hash algorithm still safer against collisions.
HDLC (High-Level Data Link Control)
A communications protocol standardized by ISO for data
packets over serial lines. HDLC is actually a structured set of
standards which define the means by which dissimilar
devices can communicate over data networks. HDLC is a bit-
oriented and hence code-independent data link protocol for
point-to-point and point-to-multipoint connections. HDLC is
also standardized by ITU-T (ITU = International Telecommuni-
cation Union; ITU-T = ITU Telecommunication Standardiza-
tion Sector). One version of this protocol, called LAP B, is
used in the data link layer of all X.25 networks. HDLC defines
frames in which the data blocks from the network layer are
encapsulated for transport over the physical link. According
to DIN 66221, an HDLC frame consists of the start-of-frame
flag, the address field, the control field, the data field, the
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