Coursera cryptography notes

https://www.coursera.org/learn/crypto

W1S4
1. If you use same pad to encrypt multiple messages m1,m2 - an
attacker can XOR resulting CTs C1,C2 = m1 XOR m2 from which one can
recover easily the original messages since there is plenty of
redundancy in English esp ASCII.

2. Real world failures of the 2 time pad - Project Venona (US vs
Russia - 1941-46). MS-PPTP (Windows NT) wherein both server and client
used the same key to encrypt messages. Also 802.11b WEP - IV || k is
used to encrypt a frame. Length of IV is 24 bits. So after 16M frames,
encrypting key gets recycled. So 2 diff msgs encrypted with same key.
Also if you reset router, IV gets reset to 0 - so it will get recycled
faster than normal. Also IV goes like this - 0,1,2,3 so all the keys
are closely related. The PRG used by WEP is RC4 which was demonstrated
to fail after 10^6 frames.

3. Disk encryption fail -
4. OTP is malleable. If attacker has access to CT, he can XOR it with
some pattern to modify the resulting message.