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Cryptography also sometimes called cryptology (originally from Greek meaning “study of secret”) is a science studying a techniques of securing communication between two(or even more) parties as well decrypting a secured communication. General cryptography is trying to secure message while assuming that potential attacker know the algorithm used to encrypt the message in opposition of stenography that is trying to hide used algorithm itself.

Basic terms

  • open/plain message - original message to be secured by encryption
  • ciphertext/cryptogram - encrypted message
  • encryption - process of securing message
  • decryption - process of obtaining original message from secured message
  • cryptography - transformation of open message to encrypted form and (usually) vise versa using known key(s)
  • cryptoanalyses - transformation of encrypted message to open message without knowing the key(s) that has been used for encryption; this is also sometimes referred as codebreaking in some literature
  • cryptology - both cryptography and cryptoanalyses
  • attacker - a third party entity interested in breaking the cipher to get open message

Classification of classic ciphers

Steganographic techniques

Stega means in Greek hidden and graph stands for writing - “Hidden writing”. Open message is hidden in another meaningful resource. It is popular to hide information into another text which has been mostly used in history or as mask over picture. It can be said that steganography puts subliminal information into another form of information to confuse the possible third party reader/attacker. In modern use of steganography it is common to work on bit level. In such case there can be some bits written as usual, some written from left to write and some added as dummy bits (useless information to confuse attacker); to successfully decrypt the message you need to know what bits to read in what order (that is the key of cipher). The disadvantage of steganographic encryption techniques is the meaningless information overhead. The encoded text is usually significantly bigger as can be seen in example.

Try to read the last word of each line in this reference example of message encrypted by steganography:

Dear George, 3rd March Greetings to all at Oxford. Many thanks for your
letter and for the Summer examination package.
All Entry Forms and Fees Forms should be ready
for final dispatch to the Syndicate by Friday
20th or at the very least, I’m told, by the 21st.
Admin has improved here, though there’s room
for improvement still; just give us all two or three
more years and we’ll really show you! Please
don’t let these wretched 16+ proposals destroy
your basic O and A pattern. Certainly this
sort of change, if implemented immediately,
would bring chaos.

Substitutional techniques

In these techniques every character or group of characters of alphabet is substituted by unique symbol or symbols (keep on mind that not only characters can be used in informatics meaning of alphabet, language and grammar). Advantage of this approach is that you can encode message to the same size ciphertext and the encrypting algorithm will probably have low asymptotic notation (that means it encryption/decryption will be usually faster then other techniques). A disadvantage of these techniques is relatively easy breaking the cipher text by frequency analyses.

Frequency analyses

is attack on weak ciphertext encrypted by substitutional algorithm. As first step histogram of character occurrence in ciphertext. Then histogram is compared to characters occurrence histogram of plain text language (we expect that attacker knows language of open text, e.g. English) and then trying to substituted characters with similar occurrence. For example if English open text is encrypted by weak cipher it is likely that the most occurring ciphertext character will be 'e' like it is in English. Whole words can also be analysed instead of characters. Usually frequency analyses is used in addition to another techniques on stronger ciphers.

Some well known substitutional ciphers: Ceasar cipher First known algorithmic encryption in human history. It was first used and probably inveted by Julius Ceasar. He used it to send secure message to his generals. In this technique every character translated into another character with fixed distance between them. For example if we would use translation of 2 we could encrypt message “HELLO WORLD” into ciphertext “JGNNQ YQTNF”. The key of this cypher is the size of translation. Disadvantage of this technique is weakness to frequency analyses and even brute force attack as there is only so much possible keys as there is characters in alphabet (twenty six when using English alphabet). The algorithm can be easily modified to endure such attacks by adding to each character translation for example position of character in word. For example if we use again translation of 2 but we add the position of character(or any other variable) in word starting from one, we will encode “HELLO WORLD” into “KIOPT XQUPI”. As you can see in this modification two characters 'L' was not translated into same making frequency analyses quite useless.

Monoalphabetic substitutions

In this technique whole alphabet is substituted by permutation of this alphabet and the permutation index is key itself. That means there is 26! (=4*10^26)key in English alphabet. The security is much better than Ceasar cipher and was unbreakable in the first thousand year AD. The frequency of characters in cipher text is the same as in plain text so it solvable by frequency analyses. This can be prevented for example by encoding pairs of character or even whole words instead of single characters. Advanced variant of this technique can be also seen in Dreyfus telegram cipher - named after French officer Alfred Dreyfus who was giving classified information to Germany in 1894.

Playfair - also known as Diagraphic substitution This technique was invented by Charles Weatston in 1854. Lets assume we want to encode an English plain text. We will write whole alphabet excluding letter “j”(or any other rarely used character in alphabet) into grid(diagram) 5×5 characters. This grid of characters is the cipher key. Then we encode pairs of characters by this rules:

  • when both letters are in the same row we will replace them by the character on the right of both characters
  • when both letters are in the same column we will replace them by the character below both characters
  • when both letters are the same we will ignore one of them and create new pair
  • when non of above is true we will replace first letter by letter in same row but in column of second letter and vise versa
  • when the length of open message is odd, last character is not translated; the same goes for excluded character

Example: Grid (key):

B C D F G
R S O T N
H I K Q U
P A L M E
V W X Y Z

"HELLO WORLD" encryption results in "UPLO XSOPD":
HE -> UP; LO -> XK; WO -> XS; RL -> OP; D -> D

The advantage against monoalphabetic substitution is much more keys (26 x 26 = 676 diagram) and character encryption dependent of neighbor characters (as pairs of letters are encoded) which makes frequency analyses hard to use (as well as other kind of analyses) as we would need a long text to get relevant results(few hundreds characters). This technique was widely used for many years in twenty century, even by USA and GB during World War I.

Polyalphabetic substitutions

This is basically usage of multiple different substitution algorithms for parts of open message.


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