Lecture 3: Stream Ciphers, Random Numbers and the One Time Pad by Christof Paar
Why Cryptography Stack Exchange recommends it
Cryptography Stack Exchange users link it when explaining or questioning basic stream cipher concepts such as XOR encryption, key length, weak LCG-based generators and the danger of reusing one-time pad keys.
Recommended by 1 people across 1 different months, 2014 to 2024.
What it is
Lecture 3 of Christof Paar's Introduction to Cryptography course at Ruhr University Bochum. It introduces stream ciphers, random number generation (including LCGs) and the one-time pad, with XOR-based encryption worked through on the board.
What people said
Recommends Nick · 2024-04 · Cryptography Stack Exchange · on “How can LCG based stream ciphers be broken?”
I'm watching Christoph Paar's Introduction to Cryptography lecture series on youtube. In it he describes an example of an insecure stream cipher [this video] which uses XOR as the encryption and decryption functions and a Linear Congruential Generator as the source of the key material. The LCG function is defined as follows: Given $A,B,S_0 \in \mathbb{Z}_m$ , successive output symbols can be computed as \begin{equation} S_{i+1} \equiv A*S_{i}+B \mod m \end{equation} He illustrates a situation where an attacker has three consecutive ciphertext symbols $Y_1,Y_2,Y_3$ and knows their plaintext values $X_1,X_2,X_3$ . Using that information they can easily recover the the corresponding key symbols $S_1
Cites xorcoder · 2014-11 · Cryptography Stack Exchange · on “BruteForcer XOR (bfxor.exe) to attack 64-bit keys and longer”
@GuutBoy: "Just out of curiosity, is this kind of thing (reused OTP keys) a common mistake people make?" Yes it is a fairly common mistake. Professor Christof Paar of Bochum University in Germany mentioned in one of his Cryptography lectures such a classic case where Soviet Russian KGB agents ran out of OTP keys and re-used them to encrypt their secret spy messages. The British/US governments then could easily hack the Russian encryption by applying a comparitive frequency analysis on their ciphertexts and thus breaking the OTPs. See lecture: [this video]
Cites 4d4143 · 2021-03 · Cryptography Stack Exchange · on “How long must be the key of a stream cipher?”
I am watching Lecture 3 (Stream cipher, random numbers and one time pad) [this video] by Christof Paar and perhaps its something that will be explained in another lecture but: When he explains how XOR works, in the example he gives, plaintext X3.X2.X1 is encrypted with key S3.S2.S1 and becomes ciphertext Y3.Y2.Y1. So if the plaintext is N-bits long...the key has to also be N-bits long? If the answer is no and it can be an arbitrary number...wouldn't that be a block cipher? If the answer is yes, does the sending and receiver end have to agree on the quantity of bits to be transmitted beforehand? If this is random (ie. unknown beforehand), how can they know what key to use to encrypt and unencrypt