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Introduction to Applied Cryptography and Steganography

One can set up a reasonably secure wireless or wired network without knowing which ciphers are used and how the passwords are encrypted. This, however, is not an approach endorsed by us and discussed here. Hacking is about understanding, not blindly following instructions; pressing the buttons without knowing what goes on behind the scenes is a path that leads nowhere. Besides, security and quality of service are tightly interwoven, and as you will see later in this chapter, incorrect selection of the cipher and its implementation method can lead to a secure but sluggish and inefficient network. Although the achieved security enhancements are unlikely to be mentioned by the network users, low throughput and high delay would surely get reported to the IT team and, possibly, to management.

Before getting down to ciphers, modes, and protocols, let's get some definitions right.

Cryptography defines the art and science of transforming data into a sequence of bits that appears as random and meaningless to a side observer or attacker. The redundancy of data is also removed by compression data. However, whereas compressed data is easy to decompress, decrypting data requires a key that was used to bring the "randomness" to the plaintext. On a side track, because both encryption and compression increase the entropy of data compressed, encrypted data might actually expand in size after the compression, which makes compression unfeasible. If you have to implement both encryption and compression of data, apply the compression first.

Cryptanalysis is the reverse engineering of cryptographyattempts to identify weaknesses of various cryptographic algorithms and their implementations to exploit them. Any attempt at cryptanalysis is defined as an attack. Exhaustive key search (or brute-forcing) is not a form of cryptanalysis, but it is still an attack!

Cryptology encompasses both cryptography and cryptanalysis and looks at mathematical problems that underlie them.

Encrypting data provides data confidentiality, authentication, data integrity, and nonrepudiation services. Data availability could be affected by incorrect implementations of cryptographic services, for example when bandwidth consumption and packet delay are above the acceptable limit due to improperly implemented cryptographicChapter 8. Interestingly, similar covert channels can be employed to transmit highly confidential data over an insecure physical medium (wireless) as part of an advanced defense-in-depth strategy.

Running key ciphers involves a sequence of physical actions to obtain the key. For example, an agreed-on message might say bk10.3L.15.36.9, which states "The key is in a book on shelf 10, 3 books to the left, page 15, 36th line, 9th word." You open the book and the word is, of course, "Microsoft" (no pun intended!). Although running key ciphers can be reasonably secure, they aren't really applicable in network and host security.

Finally, there is a perfect encryption scheme that cannot be broken, no matter how much processing power is at the attacker's disposal. Ironically, this scheme is of very little use for IT security, just like running key ciphers. You probably gathered that we are talking about one-time pads. A one-time pad is a large matrix of truly random data. Originally it was a one-time tape for teletype transmission. Each pad is XORed with plaintext to encrypt it and is used only once on both communication ends. Irrecoverable destruction of the pad follows use. Such a data transmission scheme is perfectly secure from the cryptanalysis viewpoint, providing the entropy source for the pad is truly random. However, secure pad distribution and storage and senderreceiver synchronization prove a tremendously difficult task. Because the superpowers usually have sufficient resources to accomplish such a task, one-time pads were employed to secure the hotline between the Cold War giants and were frequently used by spies on both sides of the Iron Curtain. A Russian submarine radio operator in the movie K-19 Widowmaker appears to use a one-time pad to encrypt his message before the radio transmission takes place.

Looking back at the options just presented, we are left with two choices. One choice is continuing to fortify substitution and transposition ciphers until their cryptanalysis becomes computationally unfeasible. Another choice is to come up with novel encryption schemes different from classical methodologies described (we discuss this more when we come to asymmetric ciphers). Yet another choice is steganography. This chapter does not dwell on steganography because it is not widely used to secure wireless networks. However, stegtunnel from SYN ACK Labs (http://www.synacklabs.net/projects/stegtunnel/) is an interesting free tool one can employ for wireless traffic protection. If you have a particular interest in this subject, we suggest checking out a variety of online sources, such as http://www.cl.cam.ac.uk/~fapp2/steganography/ or http://www.jjtc.com/Steganography/, as well as books currently on the market (Information Hiding: Steganography and WatermarkingAttacks and Countermeasures by Johnson, Duric & Amp, 2000, Cluwer Academic Publishers, ISBN: 0792372042; Disappearing Cryptography: Information Hiding: Steganography; Watermarking by Wayner, 2002, Morgan Kaufmann, ISBN: 1558607692; and Information Hiding: Techniques for Steganography; Digital Watermarking by Katzenbeisser, 2000, Artech House Books, ISBN: 1580530354). Now it is time to return to the substitution and transposition ciphers we started with.

Before dealing with the modern-day substitution and transposition cipher offspring, there is a common misconception to deal with first. This misconception is that you have to be a brilliant mathematician to understand cryptography. As far as our experience goes, understanding what a function is, and understanding binary arithmetic, matrices, modular arithmetic, and Boolean logic operators, will get you by without significant problems. Some revision of the latter is, perhaps, a good idea. We find truth tables to be particularly good for Boolean logic memory refreshment:

NOT. NOT (!= in C) truth table is:
INPUT      OUTPUT
1           0
0           1
OR ( || in C, as in {if ((x>0) || (x<3)) y=10;} ) truth table is
A   B      A || B
1   1        1
1   0        1
0   1        1
0   0        0
AND ( && in C, as in {if ((x>0) && (x<3)) y=20;} ) truth table is
A   B      A && B
1   1        1
1   0        0
0   1        0
0   0        0
(remember subnetting ? IP && netmask !)
And finally, XOR (or eXclusive OR, ^= in C) truth table is
A   B      A ^= B
1   1        0
1   0        1
0   1        1
0   0        0
mention that:
a ^= a = 0
a ^= b ^= b = a
or if
p ^= k = c
c ^= k = p

In layman's terms, this is "XORing the same value twice restores the original value," pretty much like the double use of ROT13 shift cipher mentioned earlier. In fact, some software vendors implement XORing with a secret key as a form of encryption. This is a grave mistake, and that kind of "encryption" would not be more secure than ROT13. All one needs to do is discover the length of the key by counting coincidences of bytes in the ciphertext. Then the ciphertext can be shifted by that length and XORed with itself, efficiently removing the key.

However, XORing is used excessively by many strong ciphers as a part of their operation. When popular literature states that the key was "applied" to the plaintext, it actually means plaintext ^= key at some point. The main reason for this is because XORing the same data twice restores the original data, both encryption and decryption software can use exactly the same piece of code to perform these tasks.

So, how does one go about creating strong "product ciphers" on the basis of insecure substitution and permutation ciphers and XORing?