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0

the effective key length is different with brute force, vs. meet in the middle attacks. As far as my understanding goes.

29

This is simply saying that if a cryptosystem has a functional composition that is $$h_{k}(x) = f_{k_1}(g_{k_2}(x))$$ then you can find a key for single encryption that works as the double encryption. For example: consider the permutation cipher where a permutation is a key. The permutations are forming a group, named permutation group, under the ...

3

"In a good cipher system, each bit of the ciphertext should depend on all bits of the plaintext" This property is also called the "avalanche effect" and is usually an explicit design guidance when designing novel block ciphers whereby flipping any input bit gives about a 50% chance to also flipping all output bits, ensuring maximal dependence of all outputs ...

6

The other answers are great, but I wanted to call out an interesting feature of modern cryptography. Your question asked: I thought, why even use AES or DES or any other complex way for encrypting data when there are far simpler ways, like just generating a random (pseudorandom) bitstream, and XOR'ing it with the data or secret message? In point of fact, ...

2

...random enough.. Let's just focus on that as it forms the nub of your question. Yes,$$\text{random (pseudorandom) bitstream} \oplus \text{secret message}$$ is very simple, works and is in common usage. But the first term in this encryption function masks great (and necessary) complexity. In order to be secure, the bitstream must comprise independent ...

9

We use more complex encryption algorithms than XOR with a random or pseudo-random keystream for a number of reasons: In order to get a short secret key in symmetric encryption. XOR with a true random stream (One Time Pad) requires storing or/and transfering a secret keystream the size of the data to encipher, which is utterly impractical. Replacing the ...

4

You could use a one-time pad, which does grant confidentiality when properly employed, but then you would have many non-trivial problems: Your message or messages would have to be quite short because generating letters in a truly random manner usually takes a bit of time and effort (rolling dice). One-time pads only have limited practical use. You cannot ...

1

What you're describing would be a One-Time-Pad. The OTP would actually be perfectly secure, but the first problem is in the issue that you have to generate a bit stream using a cryptographically secure pseudorandom number generator (CSPRNG) instead of a usual pseudorandom number generator. But even that doesn't pose a particularly large problem. Keep also in ...

1

update: The key is transmitted openly via some modulation. Modulation is security by obscurity. In the real world, we assume that the attacker knows everything but the key, i.e. words we live under Kerckhoffs principles. Since you have one-way transmission, you can use RSA-KEM to transmit the message with the key. Better use ...

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