Cryptography Reference
In-Depth Information
If a more detailed history of cryptography is of interest then Kahn [104] provides
the definitive chronicle of wartime cryptology. Several topics such as Hodges [99],
Paterson [155] and Smith [179] focus on the vital role that cryptography, and
cryptographers, played during the Second World War. An impressive web resource
on historical cryptosystems is provided by Dirk Rijmenants [162] and includes several
simulators, including one for the Enigma machine.
Cryptography, both historical and contemporary, has proved an attractive subject
for more mainstream media and entertainment. Foremost amongst these has been
Dan Brown, whose topics often contain references to cryptography. The most
famous is TheDa Vinci Code [41], whichdoes not really contain any true cryptography
but does feature a 'cryptographic heroine' Sophie Neveu who was notably (for
our purposes) trained at Royal Holloway and thus would surely have benefitted
enormously from this topic! Dan Brown's Digital Fortress [42] features an impressive
machine which is claimed to have the power to break all known cryptosystems (we
discussed the practicality of this in Chapter 1). Other popular novels concerning
cryptography include Enigma by Robert Harris [98] and Cryptonomicon by Neal
Stephenson [184]. Cryptography has also been the focal subject of several films,
including Enigma and Sneakers.
For those who want to 'play' further with historical cryptosystems of the type
considered in this chapter then there are several topics devoted to cryptographic
puzzles, such as Gardner [87]. By far the best way of further pursuing these is,
however, to use an online tool. We cannot recommend highly enough the free open
source e-learning application CrypTool [52], which is easy to install and use. CrypTool
has implementations of all the historical cryptosystems that we discussed in this
chapter. It is possible not just to encrypt and decrypt using these cryptosystems,
but also to conduct cryptanalysis through letter frequency analysis software, run
Vigenère Cipher cryptanalysis tools, and much more. We will also recommend
CrypTool activities in subsequent chapters, since it is just as valuable a tool for
experimenting with contemporary cryptography.
1
. The Caesar Cipher is our first historical cryptosystem and has numerous
flaws.
(a) Decrypt the Caesar Cipher ciphertext: IT STY ZXJ RJ YT JSHWDUY DTZW
UFXXBTWI
(b) The Caesar Cipher has an inadequate keyspace. What other problems does
it have that make it insecure?
(c) The Caesar Cipher is often described as having a fixed shift of three
positions. Explain why in this case the Caesar Cipher is not really a
cryptosystem at all.
(d) Is ROT13 a cryptosystem?
