# 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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### Bit level permutation

Could anyone explain how secure is bit level permutation? What is the most serious threat against the security of this kind of cipher?
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### Security of a self-decrypting block-cipher?

If I have a block-cipher like for example AES, a plaintext $x$, we use different function to encrypt and decrypt : $\text{encrypt}(x) = y$ , $\text{decrypt}(y)=x$. Now, if we build a function with a ...
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### Building a cryptography system by combining a block cipher and a stream cipher?

I have to come up with a design for a cryptography system by combining a block cipher and a stream cipher. I can't seem to come up with anything on my own. Can someone point me in the right direction ...
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### Doubling the block length of a given block cipher

IMHO the main essence of a Feistel cipher of block length $n$ is the working of a non-linear function $F$ that is applied to the two halves of the block of length $n/2$. Now suppose a certain block ...
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### Why is it OK for secretbox to leak the message length?

Most block cipher implementations expect users to pad so that the message matches the cipher block size. I understand the purpose of the padding is to make the input match the cipher block size. It ...
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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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### Why use an Initialization Vector (IV)?

Why use an Initialization Vector (IV)? How are IV's used? What are the advantages/disadvantages of using an IV? Why use an IV instead of a longer key in which some section of the key is pubic? What ...
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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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### Is it practical to use a stream cipher in a block cipher mode?

My idea is to use a stream cipher encrypting 16 bytes at a time as a primative block cipher in a mode of operation such as CBC mode. Is this practical or useful in any way?
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### Can you create a strong blockcipher with small blocksize, given a strong blockcipher of conventional blocksize?

Suppose I want a strong 20-bit blockcipher. In other words, I want a function that takes a key (suppose the key is 128 bits), and implements a permutation from 20 bits to 20 bits. The set of ...
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### XSL on serpent and rijndael - which is most affected?

So I've often looked at serpent and thought it was a very strong contender in AES. Not so long ago I was looking for evidence as to why it didn't beat rijndael. So far, the closest I've got answering ...
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Twofish was an AES candidate and it uses $4 \times 4$ Matrix as MDS followed by a PHT. The Branch Number of MDS and PHT is 5 and 2 Respectively. from formula Branch~Number = Minimum~of~[... 2answers 65 views ### 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 ... 1answer 40 views ### 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 ? 2answers 606 views ### 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. 2answers 106 views ### 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 ... 3answers 42k views ### Difference between stream cipher and block cipher A typical stream cipher encrypts plaintext one byte at a time, although a stream cipher may be designed to operate on one bit at a time or on units larger than a byte at a time. A block cipher ... 2answers 64 views ### 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 ... 3answers 2k views ### How to publish a cipher (concept)? In the last months I was searching for a subject for my bachelor thesis. I came up with an idea for a new cipher concept that works by combining already known techniques in a (hopefully) new way. So ... 1answer 95 views ### 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. 0answers 25 views ### 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)\\... 1answer 65 views ### Block cipher mode of operation with beyond-birthday-bound security I am looking for block cipher modes of operation that are secure even when the number of blocks encrypted exceeds the birthday bound. 0answers 59 views ### Is 3 rounds more secure than 2 in this simplified DES? Consider the following simplification of DES: a plaintext message block of2n$bits of length is divided into two blocks of length$n$(left half$L_0$and right half$L_1$). In other words, the ... 3answers 177 views ### How to prove the security of block ciphers I see very often proofs of security for asymmetric crypto algorithms, for instance, using reductions to known hard problems, or game based proofs... In the field of protocols (like authentication) it ... 1answer 82 views ### How would an attacker perform an exhaustive key search on a block cipher using ECB mode? Do you always assume from Kerchoff's principle, that the attacker has access to everything but the decryption key? That is, am I to assume that to perform an exhaustive key search, the attacker has a ... 3answers 370 views ### What is the fastest block cipher in the (Intel) world? In AES competition,Rijndael cipher obtained first rank and become Standard and Serpent, Twofish, RC6 and MARS obtained another ranks and I know that Serpent took second rank because it was slower than ... 0answers 100 views ### Deoxys: disadvantages vs. AES? The CAESAR candidate Deoxys is based on a tweakable block cipher that seems to be a strict improvement over AES, in that it is a tweakable cipher resistant to related-key attack. The mode proposed ... 0answers 250 views ### Simple two round feistel chiffre I am currently learning for a exam in cryptgraphy. I now try to understand the Feistel Cipher with two rounds. I do not have any information on what the functions are actually doing. The following ... 2answers 188 views ### How secure would hand ciphers be using a Block Cipher Mode? I've been looking into different ciphers that require very minimal computation power (calculator, deck of cards, index cards and a pencil), i.e. hand ciphers. For the most part all methods typically ... 1answer 84 views ### Known Plaintext Attack against 3-round SPECK48/96 I am attempting to break 3-round SPECK48/96 using a known-plaintext attack, provided with 10 PT-CT pairs. Here is a diagram: I have seemed to almost figure this out, but I cannot figure out how the ... 1answer 268 views ### HW acceleration for Camellia cipher (x86_64/AES-NI/AVX2) Modern processors support hardware acceleration for various crypto functions such as AES directly, or general vector operations which can be used in crypto ... 3answers 198 views ### Why does DES implement so much Cross Wiring? I've been going through DES, and I find that most of the blocks simply Reshuffle the input, which may be termed as "Cross Wiring". This include IP, IP-1, PC-1, PC-2 and even Expansion (although this ... 0answers 57 views ### 2 round GOST_28147-89 cipher distinguisher So, here is the scheme of how this will look graphically: The deal is to present distinguisher for this cipher. First of all here is my drafts: Way 1 Assume that distinguisher displays in ... 1answer 47 views ### How do substution-permutation networks work? I understand what's going on in higher level. But i don't understand what's really going on in page 3 and 4 of this slide. Specifically, How S0 and S1 is used? What goes in what goes out? How ... 2answers 165 views ### How to crack unknown 8-bit encryption algorithm? Can anyone tell me how to crack the encryption of 8 bit algorithm, when you have some clue that CBC mode is used, but the algorithm is secret. You also have some encrypted data (chosen plain text), ... 3answers 213 views ### Generating random non-repeating numbers from a small group I read this Q&A which gave a clever solution: feed an incrementing index into a block key cipher as a way of producing non-repeating random numbers. The problem is the block size of all the good ... 2answers 690 views ### Encryption of 8 bit of block data I'm looking for a cryptographic algorithm that allows encrypting only 8 bits of data in a block. Is there any such algorithm that is considered secure? The reason I'm asking is because I have to ... 1answer 79 views ### How to use AES-CBC to password-encrypt a small block of stored data I have a small block of data (16 bytes) that I am required to encrypt, store (in an unsecure, visible location). Later I have to retrieve and decrypt it. I must use AES-CBC and a user-entered ... 2answers 103 views ### Block cipher block and key sizes Is there any point in making block size less than key size in block cipher? For example, Russian GOST symmetric cipher uses 256-bit keys to encrypt 64-bit blocks. That means there are many keys which ... 0answers 110 views ### How secure is$\operatorname{AES256}_{\operatorname{ECB}}(\operatorname{ChaCha20}(\text{plaintext}))\$?

Suppose I encrypt a bytestream using ChaCha20, and then encrypt the resulting ciphertext using AES in ECB mode. How secure is the combination?
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### One cipher to rule them all?

I learned from Dan Boneh's course that many cryptographic primitives (prngs, stream ciphers, hashes, hmac, key derivation functions) can be built from just one a block cipher or PRF. For example, the ...
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### A block cipher whose key changes after each block

I have been reading about various block cipher modes of operation, and all of them seem to treat the block cipher (including it's key and key schedule) as a sort of immutable black box. I would like ...