Computer Security Cryptography

Cryptography Now and Before  In the past – mainly used for confidentiality  Today –Still used for confidentiality –Data integrity –Source authentication –Non-repudiation

Definitions  Cryptography –Hiding the meaning of communication  Cipher –Transforms characters or bits into an unreadable format  Cryptographic Algorithm –Procedure that turns readable data into an unreadable format (usually through mathematical formulas

More Definitions  Cryptanalysis –Science of studying and breaking encryption mechanisms  Cryptology –Study of cryptography and cryptanalysis  Key Clustering –When two keys generate the same ciphertext from the same plaintext

MORE Definitions  Cryptosystem –Encompasses all of the components for encryption and decryption  Plaintext –Readable format / decrypted  Ciphertext –Unreadable format / encrypted  Work Factor –Time, effort, and resources necessary to break a cryptosystem (should be too high for compromise)

Symmetric Cryptography  Two instances of the same key –One key used for encryption and decryption –Sender/receiver use same key (public/shared)  Key distribution a problem  Secure 2 nd channel needed or offline  Does not implement non-repudiation

Historical Symmetric Keys  Hieroglyphics –Recorded in use in 2000 B.C. –1 st known type of cryptography –Only certain people knew the symbols –Used to glorify the life of deceased

Scytale Cipher  Paper or leather was wrapped around a rod  Message was written on paper and delivered by messenger  Sender and receiver had to have rod of same diameter  Was used around 400 B.C.  Transposition Cipher

Caesar Cipher  Symmetric cipher  Keys shifted ‘n’ number of letters  Also referred to as a shift cipher  If one alphabet is used – monoalphabetic substitution  If two alphabets are used – polyalphabetic substitution

Example  Decode the following messages:  OP CPOVT QPJOUT PO FYBN

Vigenere Cipher  Polyalphabetic substitution cipher  Proposed by court of King Henry III  An extension of the Caesar Cipher  Strength is no frequency analysis can be performed

Enigma Machine  Rotor cipher used polyalphabetic substitution  Employed in WWII  Symmetric – original setting of rotors and how rotors moved must be the same  Sender entered characters and the rotors moved

Vernam Cipher  One-time pad –Only method that is completely secure –Still used today for backups encryption method –Senders uses one-time pad to encrypt and receiver uses twin to decrypt

Vernam  Deemed unbreakable if: –Pad is truly random values –Used only once –Securely distributed –Securely stored –Pad is at least as long as message

Key & Algorithm Relationship  Key – long string of values  Algorithm – group of mathematical equations that can be used for the encryption process  Used together – key values are used by the algorithm to indicate which equations to use, in what order, and with what values

Breaking Cryptosystems  Brute Force –Attempts all possible combinations of a given key space to derive the key  Takes ciphertext, applies key, and sees if understandable plaintext is derived –How many possible keys?  40-bit key = 1 trillion  56-bit key = 72 quadrillion –Easily breakable due to Moore’s Law –In 1998, broke DES in three days with 1536 PCs running at 40 MHz

Frequency Analysis  Patterns in ciphertext are identified and matched back to plaintext  For example, the most common letters in the English language are “iron seat”  Allows attackers to reverse-engineer encryption process

Characteristics of Strong Algorithms  Confusion –Complexity of the process to increase the workfactor of reverse engineering –Interceptor should not be able to predict what changing one character will do to ciphertext  Diffusion –Component going through an encryption should have MANY things take place –Change should affect many part of the ciphertext

Kerckhoff’s Principle  Only secrecy involved with cryptography should be the key  Should not base security on figuring out the algorithm  Algorithms should be publicly known  Government does NOT agree