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encrypted messages simply by replacing each letter with the letter three places further on in
the alphabet (A became D, B became E, etc.). The word ECHELON would thus become
HFKHORQ. The encryption algorithm thus consists of the shifting of letters within the
alphabet, and the key in this particular case is the instruction to move the letters three places
in the alphabet. Both encryption and decryption are done in the same way: by moving letters
three places: a symmetrical process. Nowadays this type of process would not provide
protection for as much as a second!
A good encryption system may perfectly well be publicly known and still be regarded as
secure. For this purpose, however, the number of possible keys needs to be so large that it is
not possible to try all the keys (known as a brute force attack) in a reasonable time, even
using computers. However, a large number of possible keys does not necessarily imply secure
encryption if the method results in an encrypted text which gives clues to its decryption (e.g.
the frequency of particular letters)1. Caesarean encryption is thus an insecure system for both
reasons. Because it uses simple substitution, the varying frequency of letters in a language
means that the procedure can quickly be cracked; moreover, since there are only 26 letters in
the alphabet, there are only 25 possible letter shifts and thus only 25 possible keys. In this
case, then, the codebreaker could very quickly find the key by trying all the possibilities and
decipher the text.
We will now consider what a secure system should look like.
11.2. Security of encryption systems
11.2.1. Meaning of security in encryption: general observations
If an encryption system is required to be secure , this may mean one of two things. Either it
may be essential and susceptible of mathematical proof that the message is impossible to
decipher without the key. Or it may be sufficient for the code to be unbreakable at the present
state of technology and thus in all probability to meet the security requirement for far longer
than the critical period during which the message needs to be kept secret.
1
cf. Leiberich, Vom diplomatischen Code zur Falltürfunktion Hundert Jahre Kryptographie in Deutschland
[From diplomatic code to trap-door function a hundred years of cryptography in Germany], Spektrum der
Wissenschaft June 1999, p. 26 et seq.
DT\437638EN.doc 76/92 PE 300.153
EN
11.2.2. Absolute security: the one-time pad
At present the only absolutely secure method is the one-time pad. This system was developed
towards the end of the First World War1, but was also used later for the telex hot-line between
Moscow and Washington. The concept consists of a key comprising a non-repeating row of
completely random letters. Both sender and recipient encrypt using these rows, and destroy
the key as soon as it has been used once. Since there is no internal order within the key, it is
impossible for a cryptoanalyst to break the code. This can be mathematically proven.2
The drawback to this process is that it is not easy to generate large numbers of these random
keys3, and that it is difficult and impractical to find a secure means of distributing the key. In
normal business transactions, therefore, this method is not used.
11.2.3. Relative security at the present state of technology
11.2.3.1. The use of decryption and encryption machines
Even before the invention of the one-time pad, cryptographic processes were developed which
could generate a large number of keys and thus produce coded texts which contained as few
regularities in the text as possible and thus few starting-points for codebreaking. In order to
make these methods sufficiently fast for practical application, machines were developed for
encryption and decryption. The most spectacular of these was probably Enigma4, used by
Germany in the Second World War. The small army of decryption experts working at
Bletchley Park in England succeeded in cracking the Enigma code by means of special
machines known as bombs . Both the Enigma machine and the bombs were mechanical in
operation.
11.2.3.2. Use of computers in cryptography
The invention of the computer represented a breakthrough in cryptography, since its power
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