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S1L6 - Cryptographic Protocols & Analysis (Cryptanalitic Attacks …
S1L6 - Cryptographic Protocols & Analysis
Basics of Cryptography
Cryptographic algorthm called CIPHER, mathematical function used for encrypting/decrypting
Key is a secret piece of info used in conjunction with cipher to produce cipher text. More bits more complex to crack
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Key-Based Algorithm
Symmetric
Ek can be calculated from Dk and vice-versa
Most cases Ek = Dk
Require sender and receiver agree on key before comm.
Public-key (Asymmetric)
Ek and Dk are different
Dk cannot be used to calculate Ek
Ek public to all only specific people with Dk can decrpypt
sometimes encrypt with Dk and decrpyt with Ek (used in digital signatures)
Cryptanalysis
Recovering plaintext message without access to the key
Cryptanalitic Attacks
Assumes that cryptanalyst has knowledge of cipher
Ciphertext-only Attack
ciphertext should decrypt as many as possible
or better, deduce key in order to decrypt messages encrypted with same key
Known-plaintext Attack
Has both ciphertext and plaintext. Should deduce keys used or algo to decrypt new messages with same key
Chosen-plaintext Attack
Not only has plaintext and associated ciphertext but
also can choose plaintext to encrypt.
Can encrypt blocks of plaintext to deduce info about kety
Adaptive-chosen-plaintext Attack
can also modify choice based on previous encryption results
Security of Algorithms
probable safe:
cost to break > value of encrypted data
time to break > time encrypted data must remain secret
Amt if data encrypted with single key < amt of data necessary to break algo
Categories of breaking
Total Break - finds key
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Global Deduction - finds alternate algorithm
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Instance Deduction (Local) - finds plaintext of intercepted ciphertext
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Information deduction - gains some info about key/plaintext.
Substitution Ciphers
Simple Substitution Cipher
each character of plaintext substituted with corresponding character from ciphertext
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Caesar Cipher - 3 to the right modulo 26
ROT13 - Letter rotated by 13 places. Double encryption restores
Homophobic Substitution Cipher
same as simple except one character can map to one or several ciphertext characters
Polygram Substitution Cipher
Blocks of characters are encrypted in groups.
Ployalphabetic Sunstitution Cipher
Multiple Simple substitution ciphers.
Use each key as Caesar Cipher Key
Transposition Ciphers
Plaintext remains the same but order of characters is shuffled around
Simple Columnar Transpositional cipher
Plaintext is written horizontally on fixed width and ciphertext read off vertically
One-Time Pads
