Security - Telenor

1.2 Cryptanalysis
While cryptography denotes the search for methods
and algorithms to protect communication
against adversaries, cryptanalysis finds weaknesses
in these algorithms (breaks them) to facilitate
eavesdropping or counterfeiting. Cryptanalysis
is not necessarily “evil”; your friendly
cryptanalyst can disclose weaknesses in your
systems and propose countermeasures. Much
cryptologic research is concerned with finding
weak points in existing cryptosystems, and then
modify them to withstand these attacks. A frequently
used method for testing the security
of computer systems is employing a team of
experts with knowledge of security “holes”
(tiger team), and let them try to break into the
system.
In earlier times, cryptosystems usually depended
on the language in which the plaintext was written.
Cryptanalysts employed knowledge of the
message language, including the structure, frequencies
of letters and words, and the relationships
between vowels and consonants. The use
of computers for cryptanalysis has generally
made such systems obsolete.
Analysis and breaking of modern cryptosystems
require advanced mathematical/statistical methods
and major computer resources. Typically
Terabytes of storage space and operations
counted in thousands of MIPS-years are necessary;
see e.g. the account of the breaking of RSA
keys in Chapter 3.5.
If cryptography is used in a communication system
with a significant number of users, one must
assume that the details of a cipher cannot be kept
secret. If the security is based on the secrecy of
the cipher, the system is said to rely on “security
through obscurity”, a rather derogatory term
within the crypto “community”. This does not
imply that all secret ciphers are insecure, although
this is a common misunderstanding.
Obviously, it is much more difficult to cryptanalyze
an unknown cipher than a known one, and
cryptosystems that protect matters of national
security are generally kept secret.
While considering the security of a cipher, secret
or public, it is therefore assumed that an attacker
knows all details of the cipher, except the actual
key in use. This principle was first stated by the
Dutch/French philologist Auguste Kerckhoffs
in his seminal book La cryptographie militaire
(1883), and is known as Kerckhoffs’s principle.
In addition, a common assumption is that an
attacker can generate an arbitrary number of
corresponding pairs of plaintext and ciphertext
for a given key. This is called a known plaintext
attack, or a chosen plaintext attack if the attacker
can choose which plaintexts to encrypt.
Telektronikk 3.2000
Alice
Key channel
Eve
Bob
The one-time-pad system, where the key and the
message have the same length, was described by
the American engineer Gilbert S. Vernam in
1917. If the key is truly random (e.g. resulting
from fair coinflips), such a cipher is perfect, as
was shown mathematically by Claude Shannon
[9] in 1949. The main drawback with this cipher
is that the key must be at least as long as the
message, and it must be used only once (hence
the name). Because of the key exchange problem,
one-time-pad systems are used only in environments
where security is paramount and the
messages are rather short. As an example, a onetime-pad
system developed by the Norwegian
telephone manufacturer STK was used to protect
the “hot line” between Washington D.C. and
Moscow in the 1960s.
2 Some History
The history of cryptography is probably is old as
the history of writing itself. Of course, during
most of the history of writing, the knowledge of
writing was enough – there was no need to encrypt
messages (or encrypt messages further),
because hardly anyone could read them in the
first place. There are a few examples of Egyptian
scribes playing around with the hieroglyphic
symbols, possibly to attract the curiosity of the
readers, much like rebuses. (Considering the
problems modern linguists encountered trying to
read hieroglyphs, one is tempted to say that they
were thoroughly encrypted in the first place.)
2.1 The Caesar Cipher
One of the most well-known ciphers in history
is the Caesar cipher, probably employed by the
Roman emperor Gaius Julius Caesar. Each letter
in the alphabet is “moved” three places to the
right, so that A is replaced by E, B with F, etc.
This is an example of a simple substitution
cipher, as each letter in the plaintext is replaced
with another letter (monoalphabetic substitution.)
The Caesar cipher is a simplified case of the
linear congruence cipher. If we identify the
English letters with the numbers 0–25, encryption
and decryption is defined as follows: (We
assume the reader is familiar with the modulo
notation.)
Figure 1 The fundamental
cryptographic objective
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