# Questions tagged [block-cipher]

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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### Which among the following is a method of encryption where units of plain text are replaced with cipher text according to a fixed system? [closed]

Substitution Cipher DES AES All of the above Basically, I am confused between all three. Because all of them are symmetric. Yet, AES and DES are block ciphers. Any idea?
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### Why expand keys? Why not rather generate a longer key?

AES-128 takes a 128-bit key (four 32-bit words), and expands it to a 352-bit key schedule (11 words). Speck48/72 takes a 72-bit key (three 24-bit words), and expands it to a 528-bit key schedule (22 ...
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### How one can combine shift row and mix column to form a single matrix?

I am trying to combine the effect the shift row and mix column into a single matrix. The cipher that I am working with is skinny64 (untweaked version). I know that combining the effect of these two ...
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### Are Block Ciphers Pseudorandom functions? [duplicate]

I am reading this page on wikipedia on Key derivation function here where it states: In cryptography, a key derivation function (KDF) is a cryptographic algorithm that derives one or more secret keys ...
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### Private key encryption based on NP-complete problem

Over a decade ago, a question was asked on Stack Overflow, which basically asked if there were any encryption schemes that are reducible to an NP-complete problem, in the sense that breaking the ...
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### Can we combine two AES (with block size 128) to to a block cipher with a block size 192 which is collision-resistant?

Encrypting values with AES we only need about $2^{64}$ trials until we finding an already used value (some collision). This can be done these days with easy. To make this harder can we combine two AES ...
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### What is the security of multiple encryption using Even–Mansour scheme (XEX)?

As I know, the best attack on single or double Even-Mansour scheme is N/2, being N the key size (or size of one of the two keys used). I know that encrypting two times using this scheme is susceptible ...
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### Is there a common method for a virtual two way dictionary of statistically independent random values with a domain size of about 192bit? (like ECB BC)

Is there any commonly used dictionary/codebook like function $D$ which assigns for every value $r_i \in V=[0,s-1]$ with $s\approx 2^{200}$ a value $r_j$ also $\in V$ $$D(r_i) = r_j$$ and is self-...
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### An adversary does know the block cipher key and some bits of the message and the cipher (same length as key). Does this help him to find pairs? (ECB)

Given a $N$-bit AES or similar block cipher with his $N$-bit key $k$. We can encrypt a $N$-bit message $m$ to a $N$-bit cipher $c$ and decrypt it again. $$E(m,k) = c$$ $$D(c,k) = m$$ An adversary does ...
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### What kind of assumptions usually go into the design of block ciphers?

What are some standard assumptions made in showing the security of a block cipher? For example, is it commonly assumed that $P\not=NP$? To this end, are there any block ciphers whose security does ...
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### Does inserting an entry of 32/64-bits into a Feistel S-box would consume the same cycles per byte as inserting a single byte?

Blowfish splits a 32-bit word into 4 bytes and insert each byte as an entry in a S-box. Let's suppose I do the same but with an entire word, 32 or 64-bits. MARS block cipher does the same with 32-bit ...
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### Security of Blowfish with secret S-boxes

There are similar questions about secret S-box(es), but none of them about secret Blowfish (or any other canonical Feistel cipher) S-Boxes, only about secret AES S-Box. Let's suppose that instead of ...
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### 16-bit entry in a S-Box of a SPN block cipher: Would it have the same speed/performance than a 8-bit entry?

I learned that in Substitution–permutation block ciphers, the maximum size of S-Box set to receive an entry from a data is the max numeric count of the data, for example, if the entry is 8-bits the S-...
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### Is it possible to use words of bits as entries in S-Boxes bigger than 4/8 bits and achive similar security/speed in SPN block ciphers?

Blowfish splits a 32-bit word into 4 sets of 8-bits (1 byte) and use them as entries in its S-Boxes. Kuznyechick splits two 64-bits words in many nibbles (4-bits chunks), use them as entries in its S-...
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### In SPN block cipher, is each bit of the ciphertext dependent on each bit of the S-boxes set?

In substitution–permutation network block ciphers like AES, when encrypting a plaintext into a ciphertext, will each bit of the ciphertext be associated to each bit of the S-boxes set in the cipher? I ...
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### Encrypting with one key and decrypting with a related key

Given $n$-bit block cipher $E$ (and its inverse $E^{-1}$), define block cipher $E^\prime_k(m) = E_k(E_{f(k)}^{-1}(m))$ where $k,f(k) \in \{0,1\}^n$ and $\forall k:f(k) \ne k$. Under the ideal block ...
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### If keeping the S-Boxes secret in SPN ciphers increase they security, does using one different S-Box set per round in them increases even more? [duplicate]

I read in this paper that keeping the S-boxes secret in AES raise its security from 128-256-bits to 1812-1940-bits. Let's suppose I use one different S-Boxes set per single round. /\ Does this ...
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### Can the security margin of any Feistel block cipher be multiplied if I just multipy the number of rounds and use different random subkeys each round?

Kaweichel is a port of Blowfish for 64-bit architectures, its key size accepted is 1920-bytes, instead of 16 rounds it has 32 rounds, the double of Blowfish. ICE is a feistel block cipher that has a ...
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### Does the security margin of Kalyna block cipher double per for additional 4 rounds? Would be possible to create a version of it with larger key sizes?

I believe this question should be directed to the authors of Kalyna block cipher, but I think they cannot answer me because of the war they are facing in their country actually. I know that AES uses ...
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### What would be the disadvantages of very large block sizes in block ciphers?

Let's suppose someone creates a block cipher with 8192-bytes of the block size (65536-bits) or maybe 16384-bytes of the block size (131072-bits). What would be its disadvantages over a block cipher ...
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### OFB mode discussion

In OFB mode, I understand that a bit flip in $c_i$ for $i > 0$ only causes a bit flip in message block $m_i$. However, how is it possible for a bit flip in $c_0$ (i.e, in IV) will result in all the ...
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### Does random IVs in CTR block cipher mode combined with XOR allow to encrypt number of blocks up to the same numeric count of IV?

I read about CTR block cipher mode in Wikipedia: "If the IV/nonce is random, then they can be combined with the counter using any invertible operation (concatenation, addition, or XOR) to produce ...
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### If converting a SPN Block Cipher to a version with larger block size, should its S-boxes be enlarged too?

Let's suppose I want to convert a Substitution-permutation network block cipher like Kuznyechik to a version with a larger block size. Should its S-boxes be enlarged too in order to achieve similar ...
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### How to calculate/generate the inverse S-box of Kuznyechik block cipher?

Let's suppose I want to modify Kuznyechik block cipher by choosing a random S-box (taken from /dev/random for example). How can I calculate/generate the inverse S-box? Does anyone know the formula or ...
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### Security analysis of North Korean block cipher Jipsam1

The cipher is a modification to AES and is described on this website: The only difference between Jipsam1 and AES-256 is the S-box. Whereas in AES the S-box is public and constant, namely  \begin{...
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### How are keys shared safely for block / stream encryption?

I've picked up a grad level computer security class. I read a brief overview on block and stream encryption. I understand and have used PGP in the past and it make since to share your public key. ...
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### What is benefit of using substitution before permutation or permutation before substitution in a block cipher?

I am trying to understand the difference between the usage of permutation and substitution in a SP Network. Normally, in ciphers except DES, substitution appears to be done before permutation. But ...
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### Is F' = G(F)) a secure PRF given F and G are secure? [closed]

If we have a secure PRF 𝐹(𝑘, 𝑥) and a PRG G where 𝐺 : 𝒴 → 𝒴 × 𝒴 is a secure PRG. Is the PRF F'(k, x) = = 𝐺(𝐹(𝑘, 𝑥)) also secure?
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### How to understand the "Authenticity" and "Privacy" of CCM-mode encryption scheme?

CCM mode refers to CTR+ CBC-MAC encryption mode. Based on this paper, the adversary's advantages against the authenticity of CCM is: Eq(1) Authenticity: it should be infeasible for an adversary to ...
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### Is secure of RC6 cipher consider as weak secure? / how to increase safety in RC6 cipher?

I saw a paper said RC6 cipher is weak and weakness cause of using 'SINGLE CLASS OF WEAK KEYS' What is that mean? and could we solve this problem ? Thank you.
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### Ideal cipher vs Ideal encryption scheme

Ideal cipher is a random permutation for every key in its key space. And, ideal encryption scheme is the one which has perfect secrecy/indistinguishability. For an encryption scheme, random ...
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### Time and space complexity analyzation of meet and middle attack of triple DES

How can we analyze the time and space complexity of meet and middle attack on a Triple-DES?
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### Encrypting a random block with XTS and XORing into the plaintext: Would this encipherment scheme be secure?

I had an idea to make XTS mode parallelizable at CPU level. Let's suppose I take two AES-256 keys for XTS and encrypt a 512-byte block with this block cipher mode and XOR the output into the plaintext....
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### What percentage of the keyspace should be brute-forced to find an equivalent key if the block cipher has?

I have been reading the Hasty Pudding Cipher specification and the author make clear that the cipher has some equivalent keys if the keyspace is larger than 8192-bits: ...
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### The different bounds of PRP/PRF switching lemma

The PRP/PRF switching lemma is usually denoted as follows: I understand the proof of this version of the bound $\frac{q(q-1)}{2^{n+1}}$ and the game-playing technique behind it. However, I came ...
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