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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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 $m\in\{0,1\}^n$ under nonce-based ...
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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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Block cipher decryption
I have a course work for university.
The question is:
My solution to the question is:
P||R = D(K,C)
However, in the question it doesn't say we are given R so ...
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Can we achieve IND-CCA without a MAC?
In modern cryptography, IND-CPA is the lowest security we want. We want at least IND-CCAx security from encryption mode. Their relation can be found in
2007 Authenticated Encryption: Relations among ...
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Would an encryption-only block cipher be useful at all?
I recently implemented AES block cipher, encryption side only, to be used in QUIC parsing (QUIC uses GCM mode). There are other modes than GCM that use only encryption: for example CTR, OFB, and CFB.
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Is the following attack on a DES-based commitment scheme valid?
I've encountered the following question while practising for an upcoming exam in Cryptography and would like to know if I'm on the right track:
The following is a suggestion for a DES-based ...
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Do symmetric and skew symmetric key schedules have any known weaknesses?
A symmetric key schedule is a key schedule of the form $(k_1,\dots,k_{n-1},k_n,k_{n-1},\dots,k_1)$ or of the form $(k_1,\dots,k_{n-1},k_n,k_n,k_{n-1},\dots,k_1)$. A skew symmetric key schedule is a ...
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Adversary's advantages against mode of operation instantiated with random permutation and block cipher relation
I'm reading an article "A Tweakable Enciphering Mode" by Halevi and Rogavay and wondering about one statement. On page 6 there are 3 inequalities:
$$
\begin{equation}
\begin{split}
\...
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Why do we need nonce in the CTR mode given that Deterministic counter mode is semantically secure?
In the CTR mode, a nonce, a counter value and the key are needed to form the input of the PRF. However, the Deterministic counter mentioned in Dan Boneh's slides (https://crypto.stanford.edu/~dabo/...
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Does TLS convert individual messages to ciphertext? If the network is breached, is the data exposed?
As far as I understand it, TLS creates a tunnel or channel or secure connection between the parties right? Where everything is protected? But hypothetically if the network gets breached, would the raw ...
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Provably secure and Practical ciphers
Are there any ciphers that are
provably secure (reducable to a hard problem (including like factorization, not necessarily to NP-Complete or harder) )
are practical (not necessarily for too much ...
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Are small domain keyed permutations inherently unsafe?
In the accepted answer to a question in Computer Science Stack Exchange, it was mentioned that a block cipher in counter mode makes a random number generator with a large bitspace, which visits every ...
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Derive independent values using block cipher
Suppose having an arbitrary $GF(2^n)$ element $x$. Its distribution is unknown.
The task is to derive two $GF(2^n)$ elements $y$ and $z$, that have uniform distribution and are independent from each ...
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Why Block Ciphers
I fail to understand what block ciphers can do that stream ciphers cannot.
Also aren't they construct able from one another.
prg(stream cipher) -> prf -> prp(block cipher)
Any practical example ...
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Which block cipher modes of AES-256 are quantum safe? Is OCB mode quantum safe?
I was wondering if I have to keep my data safe in an age when quantum computers become a reality. Which AES block cipher modes will be secure? Will AES-OCB will be quantum safe if used in a PGP like ...
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What is the lightest cipher that provides AES like security?
The lightest, or fastest cipher, that is well known and quite secure like AES. Is ChaCha20 a good option? Are there any better alternatives that are faster but still as secure?
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No decryption algorithm for Hummingbird-2?
I stumbled upon hummingbird-2 encryption algorithm when I was looking for a lightweight encryption module that can be used in resorce-restrained environments like Network-on-Chips.
A problem I faced ...
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XChaCha20 vs AES 128 security and speed
Is XChaCha20 with 256 bit key and 192 bit nonce more secure than 128 bit key 128 bit key nonce AES, due to using larger key/nonce sizes? Isn't it supposed to be faster due to being a stream cipher? (...
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Use blake2 permutation as a block cipher
Blake2(1024 bit state) permutation (compression function core) is basically a modified ChaCha permutation. One of the modifications is to mix in message bits (key schedule?). Given that blake2 is a ...
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Cipher feedback mode is susceptible to replay attacks?
In a Korean examination, the correct answer was "Cipher feedback mode is known to be susceptible to replay attacks". But when I read the Wikipedia page, it only mentions that for the ECB ...
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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 ...