Welcome to Cryptology

1st Semester – Room B310

Syllabus

Cryptology• Is the science (and to some extent the

art) of building and analyzing different encryption-decryption methods.

(Spillman, 2005, p.3)

“Cryptanalysis is the science of discovering weaknesses in existing

methods so that the plaintext can be

recovered without knowledge of the key.”

“Cryptography is the science of building new

more powerful and efficient encryption-

decryption methods.”

Codes and Ciphers• Code – Substitution and no algorithm• Cipher – Uses an algorithm and key

Steganography – method used to hide information which conceal the existence of the ciphertext.

Example of Steganography

http://www.garykessler.net/library/fsc_stego.html

Cipher Evaluation• First General Principle of Cryptography• “The eavesdropper has knowledge of

the underlying algorithm used to encrypt data.”

(Spillman, 2005, p.4)

Kerckhoffs’ 6 fundamentals to any cryptographic algorithm

• The system should be unbreakable in practice if not theoretically unbreakable.

• Compromise of the system should not inconvenience the correspondents.

• The key should be easy to remember without notes and should be easy to change.

• The cryptograms should be transmissible by telegraphy.

• The apparatus or documents should be portable and operable by a single person.

• The system should be easy, neither requiring knowledge of a long list of rules nor involving mental strain.

(Spillman, 2005, p.4)

Economics• Ciphers do not have to be

“unbreakable” to be secure.• If (value of information) < (cost of

breaking the cipher) then it is secure.• If (time to break) > (lifetime of

information) then it is secure.

Cryptanalysis• Ethics• 3 methods of attack

– Ciphertext-only– Known-plaintext– Chosen-plaintext

Brief history of codes and ciphers• The Codebreaker, by David Kahn• Ancient Egyptian tombs• Hebrews• Greeks• Spartans• Arabs• Europe (post Dark Ages)

– Black Chambers– English Decyphering Branch

• MI-8• Bletchley Park• NSA

Skytale• tkdrhybmityisatlalhaneetetsaxhpraiaym

srmpwtelaasesnsousafsfgseoe.• This an example of skytale. This was

used by the Spartans for milatary message.

Classical and Contemporary Ciphers

Caesar Cipher• Shift letters by 3 positions

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

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

Plaintext

Ciphertext

Cryptology• Languages• Mathematics• Physics• Computer Science

Questions• What is the difference between a code

and a cipher? Why are codes rarely used today?

• Bob and Alice decide that because Eve can break a simple shift cipher they will use it twice—that is, they will create ciphertext by shifting every letter by 3. Then they will encipher the first ciphertext using a shift cipher with a key of 5. Is this a good idea? Why or why not?