# Tagged Questions

A block cipher is an encryption algorithm which encrypts fixed-size blocks of plaintext to same-sized blocks of ciphertext. For good ciphers every bit of the ciphertext block depends on every bit of the plaintext block and every bit of the key.

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### Chance of finding the right key by exhaustively searching on half of the key space

An exhaustive search of half the key space requires $2^{n-1}$ work and provides the right answer 75% of the time. If the attacker finds the key, he knows the answer. If he doesn't find the key, he ...
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### Can I use a differential that can be traced through the whole cipher with 100% probability?

I'm trying to attack a very simple cipher using differential cryptanalysis as a way of becoming familiar with this method. The cipher is so simple that I've found a differential which I can trace ...
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### How to deal with a cipher with addition and XORs combined together?

I am trying to cryptanalyze a cipher where bitwise XOR and addition mod $2^n$ are combined together. I've been working on it for a long time but with no result - the main difficulty, which I am unable ...
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### How simple can the PRP used in the Even-Mansour scheme be?

For example, in the question “Is there any reason not to use Single-Key EM with AES and a constant key?” it is proposed to use AES with a fixed key as the permutation in a xor-permute-xor construction....
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### Does the Blowfish cipher work with OFB mode?

I'm trying to implement the Blowfish cipher in Output Feedback mode. OFB mode has two operations, XOR and the block cipher. XOR inputs two blocks of the same length and outputs one block of the ...
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### Proof for collision resistance on a block-based hash

I am designing an unkeyed hash based on the SIMON cipher. This is a follow up to “this” question. A quick summary: I have an ECC engine and the SIMON cipher in hardware, and it’s in an extremely ...
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### any post-shannon symmetric ciphers?

As far as I understand block ciphers use only simple operations to ensure good enough confusion and diffusion levels. meaning there is no math in it. "breaking a good cipher should require as much ...
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### Countering Cryptographic Attacks

By reading Camellia Design and Analysis document following is my understanding about few cryptographic attacks from designer point of view. Differential and Linear Crypt-analysis. Number of Active ...
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### How does blocksize affect security?

I have recently read about how blockciphers and how stream ciphers work, and although I think I finally got everything right, I'm still wondering how increasing blocksize increases security.
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### Can I use a block cipher in CTR mode to generate keys from a master key?

Suppose I have an AEAD mode based on a block cipher that requires two or more keys. In some cases, there is a complex proof that one key suffices. But finding such a proof is hard. An alternative ...
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### How to break a 32 bit block cipher without key?

From what I know Skip32 is the only 32 bit block cipher and I heard its not secure. What do I do or which tool do I use to break a 32-bit block cipher? I did search for this but all I find is attacks ...
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### tweakable block cipher and ideal block cipher

Can the set of block ciphers obtained from a tweakable block cipher (using distinct tweaks) be considered as the set of ideal block ciphers ?
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### Need for 3 rounds in Luby Rackoff theorem

I get that using Feistel network we can create a secure PRP from a secure PRF (in 3 rounds according to theorem). My question is why not just 2 rounds of Feistel network is sufficient. Is it that ...
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### Security of randomized data dependant key schedules?

An example to demonstrate the point with two Feistel networks: Cipher A: \begin{align} roundkey = hash(key || counter)\\ roundkey = hash(key || roundkey)\\ left = left\oplus hash(roundkey || right)\\...
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### How is the block size of a block cipher related to how susceptible the block cipher is to a birthday attack?

For example what can you say about how susceptible Triple-DES is to a birthday attack due to the fact that it has a block size of 64-bits.
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### Can RSA (theoretically) be used as a block cipher?

Assume AES gets a devastating attack making it unusable. Is it possible to use RSA in a secure way that can encrypt data in bulk? I can't see how RSA (assuming OAEP padding) can be used in normal ...
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### Why does Rabin cryptosystem split the message into key blocks?

Why does Rabin cryptosystem split the message into key blocks and not into plain text blocks like in Merkle Damgard Construction? Is it because it acts in a way similar to IV? Probably not, it doesn'...
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### Why was AES limited by NIST to a 128-bit block size, although Rijndael could support more? [duplicate]

Rijndael could support block sizes up to 256 bits; this seems to be useful in AEAD. Wht did NIST restrict AES to a 128-bit block size?