Just some quotations from above website:
One of the biggest reasons I think the class was so good was its focus on offense. I don’t really understand how defensive security people can try to defend stuff without understanding offense… yet the crypto classes I’d taken before tried to do exactly that. How was I supposed to understand why things needed to be done a certain way if I don’t know how it can break? Crypto books have been the same way – every crypto book I’ve read before (e.g. Bruce Schneier books) don’t seem to give much page space to offense. Dan brings the attacker’s perspective into every lecture, and I have a much better understanding of practical cryptography because of it.
Week 4 – CBC with IV
Problem:
An attacker intercepts the following ciphertext (hex encoded):
1
| 20814804c1767293b99f1d9cab3bc3e7 ac1e37bfb15599e5f40eef805488281d |
He knows that the plaintext is the ASCII encoding of the message “Pay Bob 100$” (excluding the quotes). He also knows that the cipher used is CBC encryption with a random IV using AES as the underlying block cipher. Show that the attacker can change the ciphertext so that it will decrypt to “Pay Bob 500$”. What is the resulting ciphertext (hex encoded)? This shows that CBC provides no integrity.
Solution:
This is insecure because the first message block is xored with the random IV
20814804c1767293b99f1d9cab3bc3e7 ac1e37bfb15599e5f40eef805488281d
P a y B o b 1 0 0 $
P a y B o b 1 0 0 $
9th char
0xb9 decrypts to 1
0xb9 xor ascii (1 xor 5)
0xb9 xor 0x31 xor 0x35
= 0xbd
0xb9 decrypts to 1
0xb9 xor ascii (1 xor 5)
0xb9 xor 0x31 xor 0x35
= 0xbd
20814804c1767293bd9f1d9cab3bc3e7 ac1e37bfb15599e5f40eef805488281d
