COMFIC- Stream Ciphers

Stream Ciphers

Encrypt each byte/bit in turn

Generate a pseudorandom key stream and use as key for XOR cipher

One time pad needs truly random key- so stream can be insecure

Based

a XOR cipher where key is as long as message

Synchronous stream ciphers

Password into stream cipher- outcome is long sequence of random numbers

Plaintext- same length as stream of random numbers generated

XOR these against each other to get cipher text

How to decrypt:

Generate stream from password, xor with ciphertext

Complicated version:

Plain text is series of packets

Encrypt by making stream cipher match

End up with encrypted packets

But packets can get lost, end up XORing with wrong stream, so wouldn’t get plain text

Synchronous as doesn’t handle lost messages

Avoid; tag each

Stream cipher per packet, seq number, password would be gareth1, gareth2 for example

LSFR

With known plaintext and LSFR can calculate state and hence future steam

If unkown LFSR, Berlekamp-Massey algorithm

Linear Feedback Shift Register

Used for GSM phone calls- 2g

RC4

Pass phrase dictates how you “shuffle deck”

Tells you what to swap based on passphrase for each “card in deck”

Swap 2 cards, add together, use index for next swap

Weakness

Sometimes uses same key

First 3 bytes of keysteam corelated to just the first 3 bytes of key

Second output byte is biased to zero with probability of 1/128

WEP

Packets encrypted with RC4

Sequence number in WEP is 24 bits

Keys relate, so a number of packets means it Is vulnerable

Goals of a cipher

Complex relationship between plain text and cipher text

Shouldn’t be a one to one mapping of plain to cipher