Encryption is used in communications systems to protect information being
transmitted over a channel from being intercepted and read by
unauthorised parties. This protection is achieved by converting the original
message (plain text) into an encoded form (cipher text) that
appears to be a random stream of symbols.
though a security-related encoding process (encryption) before entering the channel.
At the receiver, data passes through a corresponding security-related decoding
process (decryption). The security-related transformations used in encryption and decryption
are both controlled by some form of secret information (known
as a key). The distribution of this secret information is
usually the responsibility of a trusted third party.
One of the
simplest security-related transformations that can be used is the Caesar
cipher, which is attributed to Julius Caesar. A letter is
encoded by finding its location in the top row of
characters, and mapping it to the character in the same
position in the bottom row. For example, the letter ‘D’
would be encoded as ‘G’, the letter ‘Z’ as ‘C’.
The word "CANBERRA" is encoded as "FDQEHUUD". Ciphers of this
type are known as alphabetic ciphers.
The behaviour of the encryption
can be altered by changing the mapping between input and
output alphabets. The key specifies this mapping. The Caesar cipher
algorithm specifies that there is a fixed offset between plain
text and cipher text. The value of the offset is
Practical encryption systems, which are the subject of this
chapter, have much higher complexity than alphabetic ciphers. The major
reason for this is there are only a very limited
number of possible alphabetic ciphers, which makes it very easy
to break the cipher by testing all possibilities. Alphabetic ciphers
also tend to preserve the structure of the input data
in the cipher text. In the case of the Caesar
cipher above, each value of the input symbol is mapped
to one value of the output symbol (for example, ‘E’
is always mapped to ‘H’). By knowing the relative frequency
of letters used in English words, a cryptanalyst seeking to
break this cipher might easily guess that the most commonly
occurring symbol corresponds to the letter ‘E’.
GSM encryption uses
the GSM A5 algorithm.
Other topics in our resources on GSM related to GSM Encryption include: