Symmetric Cryptography Flashcards
Symmetric Cryptography
The same key is used to encrypt and decrypt the message. Always faster than Asymmetric but has an issue with key sharing.
Claude Shannon
in 1949 created the Theory of Secrecy Systems
Diffusion
Chnages to one character in the plain text affect multiple characters in the cipher text
Confusion
Attempts to make the relationship between the statistical frequencies between the cipher text and plain text as consistant as possible.
Avalanche
The term avalanche means a small chnage yields large effects in the output, like an avalanche.
Kerckhoff’s Principle
a cryptosystem should be secure even if everything about the system, except they key, is publicly known.
Symmetric Algorithm Expression
C = E(k,p)
Meaning the cipher text (C) is equal to the encryption function (E) with the key (k) and plaintext (p) being passed as parameters to that function. Decryption is also often expressed mathematically:
Substitution
Changing some part of the plain text for some matching part of the cipher text.
Transposition
Swapping blocks of cipher text.
Binary Math
Symmetric Ciphers often involve binary math. Three primary binanry operations.
AND
OR
XOR (exclusive OR)
Binary AND
asks if there is a one in BOTH the first and second number:
1101
1001
———–
1001
Binary OR
Asks if there is a 1 in the first number OR the second, OR in both:
1101
1001
———–
1101
Binary XOR
Asks if there is a 1 in one of the numbers, BUT NOT in both(exclusively in one and NOT the other).:
0100
1001
————
1101
Block Cipher
Encrypt the data in blocks (64 bit blocks are common although some like AES user larger blocks).
Stream Cipher
Encrypt the data as a stream, 1 bit at a time.
