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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Does using a block cipher in CBC mode *twice* result in an all-or-nothing transform (AONT)?
Use a block cipher in CBC mode with a public or all-0 key. Twice.
Wrapping the last block into (xor) the first on the second round.
Every block will depend on all previous and all successive blocks. ...
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how 0/n split prevent BEAST attack against TLS?
I read that to mitigrate BEAST attack openSSL tried to inject empty TLS record before each real TLS record. and by doing that there is no opportunity to execute an attack but i dont understand way?
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Does any encryption/decryption algorithm supports linear decomposition?
I am not sure whether "linear decomposition" is appropriate to summary my question: We know that the traditional symmetric encryption/decryption algorithm (like AES, TDES) can be written as:
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How big of a threat are *diffusion model* based AIs to cryptographic systems?
The diffusion model, which is used by products like Midjourney and Dall-E, trains AI systems to de-noise (remove added randomness) from data to infer what the original de-noised data is. That would ...
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Encrypting random IV in CTR mode (no nonce!)
Use of plain random-IV's in CTR mode, without any special "nonces/counters" (or any "dedicated" bits!), can lead to problems with "partial overlaps", whereby attackers ...
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Given $i$ keyed-$PRP$ labels $\ell_{i,x}$ from a $2^{256} \times 2^{256}$ Sudoku (Latin-square), how difficult is it for an adversary to solve?
There's a keyed-permutation I'm playing with, $\ell_{i,x} = \pi_i(x_i)$, which is a bijection $X \leftrightarrow X$, where $|X| = 2^{256}$, and whose evaluations on plaintext inputs $x_i$ perfectly ...
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Attacking a one-round SPN
From Introduction to Modern Cryptography, 6.2.1 Substitution-Permutation Networks
A description of an optimized attack on one round SPN
A better attack is possible by noting that individual bits of ...
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May I use the same One-time pad key two times if I encrypt it with a block cipher using two different block cipher keys?
Let's suppose I have two 2GiB files and I want to encrypt them using One-time pad (Vernam cipher), but I don't want to store two big keys.
May I use only one key for the two files if I encrypt the One-...
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How CTR mode is encrypted?
I referred to the explanation of encryption mode from Wikipedia, but I'm confused now, and I went to find some books about Cryptography, but it didn't solve my confusion.
The CTR mode uses Counter to ...
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If I encrypt a plaintext with different keys for each block, will I have the same security as a one-time pad?
Let's suppose I encrypt an 1MiB plaintext with AES-256 in ECB mode but using different keys for each 16-byte block (I know this is weird but it's just an example).
Remembering that a different 256-bit ...
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Is it undesirable for authentication to require decrypting a ciphertext?
A couple years ago, I devised some primitives for block ciphers and block cipher modes of operation; I was partly inspired by CAESAR. What these designs all had in common is that the encryption/...
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Comparison of distinguishing attack against blockcipher and permutation
In a previous question, we explored various security definitions of encryption schemes, such as IND-CPA, IND-CCA{1,2,3,etc.}
These indistinguishability games can roughly translate to blockciphers, as ...
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Bit based division property
Bit based division property is a great method of Integral attack on block ciphers, which was invented by Y. Todo.
According to the paper Bit-Based Division Property and Application to Simon Family (in ...
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How many $S$-Boxes of length $3$ are there over $\mathbb{F}_{2}$?
Is there a simple way if counting the number of $S$-Boxes of length $\ell$ over $\mathbb{F}_{2}$? By $S$-box I mean an $S$-box satisfying the avalanche condition.
I mean it is quite easy to see that ...
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Does blockcipher decryption always offer equivalent security as blockcipher encryption? [duplicate]
I'm not sure if anyone explored this area before. Blockcipher encryption definitely fullfills IND-CCA, but what about its inverse? Is there a proof that shows blockcipher decryption is as secure as ...
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Cryptography particular block length valid message
I am new at cryptography and start to learn some terms. I just saw an question in the book and didn't got a clue about this question. Could someone can explain me the answer of this question? Because ...
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Is it fatal if a blockcipher uses a sbox with a fixed point at 0?
In a previous question, I asked how to implement arbitrary s-box in side-channel-free fashion. The code I posted in the question loops over all 256 values of a byte to avoid timing channel, and I want ...
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Can we still use 64-bit block ciphers?
After thinking a lot about 64-bit block ciphers, I ask a question here.
Most proposed block ciphers these days seem to have a block size of at least 128-bits.
Also, I know that when using a 64-bit ...
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Which encryption method support 256 bytes block size?
I am looking at some secret encryption method that apparently uses CBC and a block size of 256 bytes. I can do a chosen-ciphertext-attack. When I modify a single byte in the ciphertext a complete ...
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Rule 30 based block cipher
So I went ahead an wrote this little program. What it does is it takes a password for a seed and generates a (pseudo-random) bit-pattern using rule 30 from cellular automata. It then XORs our input ...
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If I encrypt two plaintexts with different keys, XOR the ciphertexts and send to an adversary, what can he/she do with what I sent?
Let's suppose I encrypt two blocks of 1MiB with AES-256 in CBC mode, each one using different keys, XOR the resulting ciphertexts and send this XORed block to an adversary. Remembering that the ...
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How does the plausible deniability used by TrueCrypt work mathematically?
I have been unable to find any mathematical explanations on how TrueCrypt's plausible deniability encryption works, when using TC containers.
Would someone be able to provide a mathematical ...
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In CTR block cipher mode of operation, can I reuse the nonce with another key for encrypting another plaintext if using different counters?
Let's suppose I encrypt a plaintext with one key and a nonce in CTR block cipher mode of operation.
Can I reuse the nonce with another key in another plaintext if I start the block counting with ...
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Is my domain separation in stream cipher and MAC accepted?
I would like to implement domain separation in encryption/decryption along with MAC.
The encryption algorithms consists of XChaCha20,XSalsa20,SM4 From China and MAC algorithms consist of HMAC-SHA512 ...
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What mode in EVP for AES-128 should I use for cryptanalysis of AES?
I've been trying to use AES from OpenSSL and got to know that I need to use the EVP-based implementations rather than the AES_*.
I need to use AES-128 ...
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modern version of a book cypher [closed]
I'm a computer programmer and I'm working on a truly unbreakable cypher and I keep going back to a book cypher (each letter of the message is referenced by a page,row,and column number in a random ...
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Resistance against timing attacks of AES candidates
It's difficult to implement AES securely and efficiently if the adversary can observe the timing and (approximate) location of memory accesses, unless you have dedicated hardware. The naive ...
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Vulnerability due to same IV, same key, same plaintext at same position in OFB
I am currently studying OFB mode, and one of the vulnerability mentioned for it is that if two different messages have a block at the same position in the ciphertext, and have same plaintext, the ...
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How to solve the problem of FHE ciphertext expansion?
FHE typically has large ciphertext expansion factor, meaning that the ratio
$$\frac{|\mathsf{Enc}_{pk}(m)|}{|m|}$$
is typically quite large --- in standard schemes it is $\omega(1)$. Even getting $\...
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Is the XOR two block ciphers still a block cipher?
Let's say I have a block cipher $E: K \times M \rightarrow C$.
Is the function defined by $\operatorname{Enc}(k_1,k_2,m) = E(k_1, m)\ \operatorname{XOR}\ E(k_2,m)$ guaranteed to have an inverse ...
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How is DES used with CBC?
I'm reading our school's notes for cryptography, and it does a good job explaining CBC and DES in detail, but it fails to provide good information on how they are related.
From what I've researched so ...
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Use same key k in deterministic counter mode to encrypt two different plaintext
What goes wrong if we use the same key k in deterministic counter mode to encrypt two different plaintext messages m0 /= m1.
It is not true that c0 = c1, because we XOR output of m0 with input of m1 ...
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What exactly is a "pass" when talking about hashing, ciphers and MAC algorithms?
I was very surprised when I said that hashing the same data twice was "double pass" and a comment came in that this wasn't the case if the hashing could be performed in parallel. This would ...
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By using a block cipher multiple times in a row is a 128-bit BC as secure as a 256-bit BC which uses his 128-key only as part of the message?
For given 128-bit numbers $S$ and $E$ we want to find a series of keys $k_i$ with
$$ E = BC(BC(BC(.....BC(S, k_1),k_2) ..k_n)$$
We can either use a 128-Bit blocksize block cipher similar to AES (ECB ...
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Increasing Block Length of Symmetric Cipher
Triple DES (using an existing cipher 'DES' of key length 56bits) provides security of 112bits (although it requires key material of 168bits).
Does there exist a mechanism to increase block length of ...
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Effcient collision attack for the Davies-Meyer compression function [closed]
We have a block cipher $E:\{0,1\}^{128}\times\{0,1\}^{128}\rightarrow\{0,1\}^{128}$.
We know that the PRP advantage of E is Adv$_E^{PRP}=t/2^{128}$ where $t$ is the time needed by the algorithm to get ...
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Are there any significant ways in which TLS could still be improved?
With TLS 1.3 supporting only secure, forward-secret cipher suites, are there any significant technological improvements that could still be made to the protocol?
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Could Blowfish be extended to larger block/key sizes if increasing the level of sum and XOR operations?
Blowfish encryption is made as follows:
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Why GCM is used more often than CTR?
I've seen (on Wikipedia) in the more recent version of TLS, the Counter-mode was not used. But the Galois-Counter-mode was used?
What is the advantage to use the GCM instead of using the Counter-mode?
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Pbox and S-Box values in AUT64
I am looking to get P-box and S-box from firmware
Decided to search the firmware for
Pbox - 8 nibbles from 0-7
i.e
pbox = [2, 0, 6, 5, 7, 4, 3, 1]
Found it :)
And ...
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Getting permutation box and S-box for AUT64 block cipher
Want to get 32 bits from 0x8000 address as 8 bytes (perm box)
...
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How to Ensure Security in a Gaming Peer to Peer Network
I've been recently thinking about building a c++ poker game that would let players to play over sockets in a peer to peer network. However I do not know how security would be ensured. I was thinking ...
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Identifying an encryption algorithm and/or writing an encryption function
I am currently trying to reverse engineer a piece of software that uses a seemingly-custom encryption algorithm.
After disassembling the decryption part of the code, I've come up with the following ...
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What is a block transposition cipher?
I was looking at the archives for the British national cipher challenge, and a modified version of an ADFGVX cipher came up twice (2003 2011), with block transposition instead of columnar ...
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Encrypting with CBC then XORing repeatedly a chunk of random data smaller than the plaintext: Is there a gain in security?
If I encrypt a 1MiB file with AES-CBC (or any other cipher) and XOR a 128KiB of (truly) unpredictable random data repeating until the end of file, will I have a security of 1048576-bits (128KiB*8)?
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Hash function collision resistance
I have a course work for university, and am not sure on my answer so if anyone could please take the time out to read the question and my answer to let me know if I'm going in the right direction that ...
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Can attacker create encryption of message m XOR 1^n given the ciphertext c, in nonce-based counter mode?
The following question is from Stanford cryptography course final exam paper.
Suppose an attacker intercepts a ciphertext c which is the encryption of a message m ε {0, 1}^n under nonce-based counter ...
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Using TEA to build a hash function
Background:
TEA uses a 128 bit master key $K_{0\ldots3}$. All odd rounds use $K_0$, $K_1$ as the round subkey, and all even rounds use $K_2$, $K_3$. One cycle of TEA applied to the block $A_i$,$B_i$ ...
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Modification of CFB Mode of Operation?
To encrypt plaintext $(P_1, P_2, P_3, ... P_n)$ Ciphertext Feedback Mode (CFB) works as follows :-
$$
C_0 = IV \\
C_i = E_K (C_{i-1}) \oplus P_i
$$
Lets define a modified version of CFB mode as ...
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Chosen Plain text attack [closed]
I have a course work for university, the question is:
Consider a symmetric encryption scheme with its encryption operation written as
$$C = E(K, R||P)$$
where $E$ is a block cipher encryption ...
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