Cryptography Reference
In-Depth Information
Concerning practical security, Harel [97] provides a readable introduction to the
limitations of computational ability in modern computers, including reference to
complexity theory and cryptography. Talbot andWelsh [189] provide an introduction
to cryptography from a complexity theory perspective. Aspects of the wider
cryptographic process that we will only pay passing reference to are covered well by
other authors. Anderson [23] provides several examples of what can gowrong in real
systems when the wider process is not carefully considered. Another good article by
Anderson on this subject discusses why cryptosystems fail [22], as does the similar
essay by Schneier on security pitfalls in cryptography [169]. There aremany examples
of the 'gap' that can arise between the theory and practice of cryptography, see for
example Paterson and Yau [154]. With respect to implementation, McGraw [121] is
an excellent introduction to secure software development and Ferguson, Schneier
and Kohno [75] dedicate a chapter to implementation of cryptography.
The development of security models within which to evaluate the security of
cryptographic algorithms is an active area of research. A good introduction to the
most important formal provable security models for cryptographic algorithms can
be found in Katz and Lindell [105]. Evaluation of security products, including those
featuring cryptosystems, is a very difficult task. The most recognised framework for
such evaluation is the Common Criteria [51].
1
. The version of one-time pad that operates on alphabetic letters (associating
A
1, etc.) is being used and the ciphertext DDGEXC is intercepted:
(a) If the plaintext was BADGER then what was the key?
(b) If the plaintext was WOMBAT then what was the key?
(c) How many different plaintexts could have resulted in this ciphertext if the
plaintext is not necessarily a word in English?
(d) How many different plaintexts could have resulted in this ciphertext if the
plaintext is a word in English?
=
0, B
=
2
. Design a one-timed pad based on a Latin square that can be used to
encrypt the seven plaintexts: BUY TODAY, BUY TOMORROW, BUY THE DAY
AFTER TOMORROW, SELL TODAY, SELL TOMORROW, SELL THE DAY AFTER
TOMORROW, DO NOTHING.
3
. The following three squares have been suggested as potential one-time pads
for protecting four plaintexts:
P
1
P
2
P
3
P
4
K
1
2143
K
2
4314
K
3
1231
K
4
3422
P
1
P
2
P
3
P
4
K
1
2143
K
2
4312
K
3
1234
K
4
3421
P
1
P
2
P
3
P
4
K
1
2143
K
2
4312
K
3
3241
K
4
1423
