This is the third of ten parts of the sci.crypt FAQ. The parts are mostly independent, but you should read the first part before the rest. We don't have the time to send out missing parts by mail, so don't ask. Notes such as ``[KAH67]'' refer to the reference list in the last part.

The sections of this FAQ are available via anonymous FTP to rtfm.mit.edu as /pub/usenet/news.answers/cryptography-faq/part[xx]. The Cryptography FAQ is posted to the newsgroups sci.crypt, talk.politics.crypto, sci.answers, and news.answers every 21 days.

Contents:

3.1. What is cryptology? Cryptography? Plaintext? Ciphertext? Encryption? Key?

3.2. What references can I start with to learn cryptology?

3.3. How does one go about cryptanalysis?

3.4. What is a brute-force search and what is its cryptographic relevance?

3.5. What are some properties satisfied by every strong cryptosystem?

3.6. If a cryptosystem is theoretically unbreakable, then is it

guaranteed analysis-proof in practice?

3.7. Why are many people still using cryptosystems that are

relatively easy to break?

3.8. What are the basic types of cryptanalytic `attacks'?

3.1. What is cryptology? Cryptography? Plaintext? Ciphertext? Encryption? Key?

The story begins: When Julius Caesar sent messages to his trusted

acquaintances, he didn't trust the messengers. So he replaced every A

by a D, every B by a E, and so on through the alphabet. Only someone

who knew the ``shift by 3'' rule could decipher his messages.

A cryptosystem or cipher system is a method of disguising messages so

that only certain people can see through the disguise. Cryptography is

the art of creating and using cryptosystems. Cryptanalysis is the art

of breaking cryptosystems---seeing through the disguise even when

you're not supposed to be able to. Cryptology is the study of both

cryptography and cryptanalysis.

The original message is called a plaintext. The disguised message is

called a ciphertext. Encryption means any procedure to convert

plaintext into ciphertext. Decryption means any procedure to convert

ciphertext into plaintext.

A cryptosystem is usually a whole collection of algorithms. The

algorithms are labelled; the labels are called keys. For instance,

Caesar probably used ``shift by n'' encryption for several different

values of n. It's natural to say that n is the key here.

The people who are supposed to be able to see through the disguise are

called recipients. Other people are enemies, opponents, interlopers,

eavesdroppers, or third parties.

3.2. What references can I start with to learn cryptology?

For an introduction to technical matter, the survey articles given

in part 10 are the best place to begin as they are, in general,

concise, authored by competent people, and well written. However,

these articles are mostly concerned with cryptology as it has

developed in the last 50 years or so, and are more abstract and

mathematical than historical. The Codebreakers by Kahn [KAH67] is

encyclopedic in its history and technical detail of cryptology up

to the mid-60's.

Introductory cryptanalysis can be learned from Gaines [GAI44] or

Sinkov [SIN66]. This is recommended especially for people who want

to devise their own encryption algorithms since it is a common

mistake to try to make a system before knowing how to break one.

The selection of an algorithm for the DES drew the attention of

many public researchers to problems in cryptology. Consequently

several textbooks and books to serve as texts have appeared. The

book of Denning [DEN82] gives a good introduction to a broad range

of security including encryption algorithms, database security,

access control, and formal models of security. Similar comments

apply to the books of Price & Davies [PRI84] and Pfleeger [PFL89].

The books of Konheim [KON81] and Meyer & Matyas [MEY82] are quite

technical books. Both Konheim and Meyer were directly involved in

the development of DES, and both books give a thorough analysis of

DES. Konheim's book is quite mathematical, with detailed analyses

of many classical cryptosystems. Meyer and Matyas concentrate on

modern cryptographic methods, especially pertaining to key management

and the integration of security facilities into computer systems and

networks. For more recent documentation on related areas, try

G. Simmons in [SIM91].

The books of Rueppel [RUE86] and Koblitz [KOB89] concentrate on

the application of number theory and algebra to cryptography.

3.3. How does one go about cryptanalysis?

Classical cryptanalysis involves an interesting combination of

analytical reasoning, application of mathematical tools, pattern

finding, patience, determination, and luck. The best available

textbooks on the subject are the Military Cryptanalytics series

[FRIE1]. It is clear that proficiency in cryptanalysis is, for

the most part, gained through the attempted solution of given

systems. Such experience is considered so valuable that some of the

cryptanalyses performed during WWII by the Allies are still

classified.

Modern public-key cryptanalysis may consist of factoring an integer,

or taking a discrete logarithm. These are not the traditional fare

of the cryptanalyst. Computational number theorists are some of the

most successful cryptanalysts against public key systems.

3.4. What is a brute-force search and what is its cryptographic relevance?

In a nutshell: If f(x) = y and you know y and can compute f, you can

find x by trying every possible x. That's brute-force search.

Example: Say a cryptanalyst has found a plaintext and a corresponding

ciphertext, but doesn't know the key. He can simply try encrypting the

plaintext using each possible key, until the ciphertext matches---or

decrypting the ciphertext to match the plaintext, whichever is faster.

Every well-designed cryptosystem has such a large key space that this

brute-force search is impractical.

Advances in technology sometimes change what is considered

practical. For example, DES, which has been in use for over 10 years

now, has 2^56, or about 10^17, possible keys. A computation with

this many operations was certainly unlikely for most users in the

mid-70's. The situation is very different today given the dramatic

decrease in cost per processor operation. Massively parallel

machines threaten the security of DES against brute force search.

Some scenarios are described by Garron and Outerbridge [GAR91].

One phase of a more sophisticated cryptanalysis may involve a

brute-force search of some manageably small space of possibilities.

3.5. What are some properties satisfied by every strong cryptosystem?

The security of a strong system resides with the secrecy of the key

rather than with the supposed secrecy of the algorithm.

A strong cryptosystem has a large keyspace, as mentioned above. It

has a reasonably large unicity distance; see question 8.8.

A strong cryptosystem will certainly produce ciphertext which appears

random to all standard statistical tests (see, for example, [CAE90]).

A strong cryptosystem will resist all known previous attacks. A

system which has never been subjected to scrutiny is suspect.

If a system passes all the tests mentioned above, is it necessarily

strong? Certainly not. Many weak cryptosystems looked good at first.

However, sometimes it is possible to show that a cryptosystem is

strong by mathematical proof. ``If Joe can break this system, then

he can also solve the well-known difficult problem of factoring

integers.'' See part 6. Failing that, it's a crap shoot.

3.6. If a cryptosystem is theoretically unbreakable, then is it

guaranteed analysis-proof in practice?

Cryptanalytic methods include what is known as ``practical

cryptanalysis'': the enemy doesn't have to just stare at your

ciphertext until he figures out the plaintext. For instance, he might

assume ``cribs''---stretches of probable plaintext. If the crib is

correct then he might be able to deduce the key and then decipher the

rest of the message. Or he might exploit ``isologs''---the same

plaintext enciphered in several cryptosystems or several keys. Thus

he might obtain solutions even when cryptanalytic theory says he

doesn't have a chance.

Sometimes, cryptosystems malfunction or are misused. The one-time pad,

for example, loses all security if it is used more than once! Even

chosen-plaintext attacks, where the enemy somehow feeds plaintext into

the encryptor until he can deduce the key, have been employed. See

[KAH67].

3.7. Why are many people still using cryptosystems that are

relatively easy to break?

Some don't know any better. Often amateurs think they can design

secure systems, and are not aware of what an expert cryptanalyst

could do. And sometimes there is insufficient motivation for anybody

to invest the work needed to crack a system.

3.8. What are the basic types of cryptanalytic `attacks'?

A standard cryptanalytic attack is to know some plaintext matching a

given piece of ciphertext and try to determine the key which maps one

to the other. This plaintext can be known because it is standard (a

standard greeting, a known header or trailer, ...) or because it is

guessed. If text is guessed to be in a message, its position is probably

not known, but a message is usually short enough that the cryptanalyst

can assume the known plaintext is in each possible position and do

attacks for each case in parallel. In this case, the known plaintext can

be something so common that it is almost guaranteed to be in a message.

A strong encryption algorithm will be unbreakable not only under known

plaintext (assuming the enemy knows all the plaintext for a given

ciphertext) but also under "adaptive chosen plaintext" -- an attack

making life much easier for the cryptanalyst. In this attack, the enemy

gets to choose what plaintext to use and gets to do this over and over,

choosing the plaintext for round N+1 only after analyzing the result of

round N.

For example, as far as we know, DES is reasonably strong even under an

adaptive chosen plaintext attack (the attack Biham and Shamir used). Of

course, we do not have access to the secrets of government cryptanalytic

services. Still, it is the working assumption that DES is reasonably

strong under known plaintext and triple-DES is very strong under all

attacks.

To summarize, the basic types of cryptanalytic attacks in order of

difficulty for the attacker, hardest first, are:

cyphertext only: the attacker has only the encoded message from which

to determine the plaintext, with no knowledge whatsoever of the

latter.

A cyphertext only attack is usually presumed to be possible, and

a code's resistance to it is considered the basis of its

cryptographic security.

known plaintext: the attacker has the plaintext and corresponding

cyphertext of an arbitrary message not of his choosing. The

particular message of the sender's is said to be `compromised'.

In some systems, one known cyphertext-plaintext pair will

compromise the overall system, both prior and subsequent

transmissions, and resistance to this is characteristic of a

secure code.

Under the following attacks, the attacker has the far less likely

or plausible ability to `trick' the sender into encrypting or

decrypting arbitrary plaintexts or cyphertexts. Codes that resist

these attacks are considered to have the utmost security.

chosen plaintext: the attacker has the capability to find the

cyphertext corresponding to an arbitrary plaintext message of his

choosing.

chosen cyphertext: the attacker can choose arbitrary cyphertext and

find the corresponding decrypted plaintext. This attack can show

in public key systems, where it may reveal the private key.

adaptive chosen plaintext: the attacker can determine the cyphertext

of chosen plaintexts in an interactive or iterative process based on

previous results. This is the general name for a method of attacking

product ciphers called `differential cryptanalysis'.

The next part of the FAQ gives the mathematical detail behind the

various types of cryptoanalytic attacks.