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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Decryption formula of Plaintext Block Chaining (PBC) mode of a block cipher

Considering that $C_n = E_k(M_n) \oplus M_{n-1}$ formula is used in encryption side of this mode of block cipher, and knowing that instead of M0 a randomly chosen initialization vector (IV) is used in ...
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How are keys mapped to cipher texts in block ciphers with large block sizes?

What I understand so far: For n-bit block size, there are 2n plaintexts (P) there are P! possible keys The bits necessary to represent each of the possible mappings is log2(P!) rounded up Basically, ...
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How are SHACAL-2 ciphers calculated?

I am trying to learn about the underlying functionality of the different hash functions (currently SHA) and I'm pretty stuck even after watching a Stanford video about it. One method of hashing is by ...
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How many additions modulo $2^k$ and multiplications in $\mathbb F_{2^k}$ are needed to resist cryptanalysis?

Consider a $k$-bit block cipher with $r$ rounds, and key composed of $r$ subkeys $K_i\in\{0,1\}^k-\{0^k\}$ (that is, non-zero $k$-bit bitstrings), for $i\in[0,r)$. Plaintext is $P=S_0\in\{0,1\}^k$, ...
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Looking for encryption function given decryption function (2048-bit ciphertext and key, addition and subtraction operations)

I have a decryption algorithm for 256-byte (2048-bit) blocks and want to find the matching encryption algorithm. What I know about the algorithm and have: 256-byte ciphertext (I have multiple ...
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Why don't we use Blowfish if it hasn't been cracked?

Since Blowfish is old, well-audited, and has no published attacks, why are we using AES instead? I know that Bruce Schneier said that Blowfish is insecure and told people to transition to Twofish, but ...
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Mathematical and empirical analysis of AES

Hello everyone and happy new year in advance. I'm doing a work about the analysis of the AES algorithm, and I have some doubts about this. First, I need to do the mathematical analysis of the ...
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How is it possible to prove that Shanon entropy does not change applying any bijective function to X? [duplicate]

I was wondering how is it possible to mathematically prove that Shannon entropy does not change when applying any bijective function to X? For instance H(X) = H(f(X)) assuming that f is invertible and ...
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Usage of block and stream ciphers

I see that in many applications stream ciphers are common than block cipher - whether is Salsa20, AES-GCM, etc. In which cases would a stream cipher be preferred over a block cipher and vice versa? ...
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Stream cipher and block cipher usage in network data encryption

Stream ciphers are usually used for streaming data encryption, such as voice, and also for low layers data encryption. For example, MACSec uses AES-GCM encryption. In L3 ad L4 security protocols (...
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How do salt and initialization vector differ?

Reading through the NodeJS API docs I came across IV for the first time. From what I can tell it has the same role as a salt in password hashing? Are they equivalent?
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CMAC - how is ambiguity resolved?

To avoid the dummy padding block, CMAC uses 2 separate keys K1 & K2. If padding is required, the final block is padded & then XORed with K1. If no padding is required, then it's XORed with K2. ...
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Which block ciphers are considered staples for constructing hash functions?

I've been looking all over the internet for an answer, and I must not know how to search because I've been really surprised that I haven't been able to find one. I'd also love to know what makes a ...
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Unconditionally secure block cipher for a finite number of blocks

I am wondering if someone can point me toward a block cipher that provides unconditional security for only a bounded number of encryptions. For example, you can encrypt $h > 1$ length $n>1$ ...
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Are there any tools to analyze cryptographic algorithms performance?

How can I analyze a cryptographic algorithm performance to make a table and charts comparing them. I have seen several papers but there's no paper explaining how it is exactly done. As comparing data ...
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Deterministic Counter Mode with a PRF. What does evaluate at a point mean?

This is from Dan Boneh's Lecture where he talks about operating a PRF (AES, DES) in Deterministic Counter Mode. Dan Boneh says What we could do is we could use what's called a deterministic counter ...
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Do Korea and Japan have their own national hashing standard?

I know China has SM3 and SM4 commercial cryptographic hash and block cipher standards, and I know Japan has Camellia block cipher and Korea has SEED. Do Japan and Korea have an industry standard for ...
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Encrypt a big file in blocks with AES-GCM: how many nonce do we need?

I need to encrypt big files using AES-GCM, potentially 10 GB or more. For memory (RAM) reasons, I need to processs them by blocks (let's say 16 MB), rather than doing ...
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Probability of false positive key matching two plaintext/ciphertext pairs

Given a keyspace of $ 2^{80} $ and plaintext space of $2^{64}$. And two plaintext and ciphertext pairs $(x_1, y_1)$ , $(x_2, y_2)$. Now we have $2^{80}/2^{64} = 2^{16}$ keys that encrypt $x_1$ to $y_1$...
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Brute force attack on a block cipher with length n [duplicate]

Consider a block cipher whose block length is 𝑛. 𝑁 = 2𝑛 is number of possibilities. Imagine we have 𝑑 plain text-cipher text pairs { 𝑃𝑖, 𝐢𝑖 = 𝐸𝐾(𝑃𝑖) } , where the key 𝐾 selects one of ...
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The modes of encryption preserve the format

Can anyone give me some documentation on the various format-preserved encryption modes?
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Symmetric Crypto Primitives derived from Rijndael Block Cipher

Rijndael Block Cipher supports multiple block/ key lengths and a subset of these was selected as AES. What other cryptographic primitives are derived from Rijndael or make use of its round function/ ...
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Avalanche effect SBOX

It is possible to achieve the avalanche effect in an SPN or in a Feistel network that use a single round of an SPN in his function (like DES) using SBOX that receive 2 bit in input and output 2 bit?
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Block cipher swap key and message

Say I use AES. If I insert the message the way the key normally is used and vice-versa, would this be equivalent secure to the intended encryption?
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Formally, what is AES?

AES is supposed to be a symmetric key block cipher. The theoretical counterpart to this is a pseudorandom permutation. I'd like to say that AES is a PRP (well, supposedly at least), but that doesn't ...
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Block Cipher Mode of Operations

I am studying for a test and I don't seem to understand the Block Cipher and its different modes of operations especially when it comes to encrypting and decryption. Could anyone please help solve ...
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How to securely encrypt/decrypt data with a maximum chunk size?

I am aware of the different modes so that a "raw" block cipher such as AES (ECB) can be used to encrypt more than a single block of data (CBC, OFB, CTR, etc) and modes such as AES-GCM which ...
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Correctness of Propagating Cipher Block Chaining mode of operation (PCBC)

I was looking at the Wikipedia article about Block cipher mode of operation and I was wondering how can you from the encryption $C_i = E_K(P_i \oplus P_{i-1}\oplus C_{i-1})$ get the decryption $P_i = ...
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OFB output jth-block expression clarification

Stalling says: OFB encryption can be expressed as Cj = PjβŠ•E(K, Oj-1) where Oj-1 = E(K, Oj-2) Some thought should convince you that we can rewrite the encryption expression as: Cj = PjβŠ•E(K, [Cj-1 βŠ•Pj-...
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AES Encryption vs Decryption

We recently learned about AES in class and it was heavily emphasized that encryption and decryption in AES are not the same because the functions are not reversible. Why then, when we got to block ...
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Why are iterated substitution-permutation product ciphers only used on fixed-size plaintext blocks?

Another way to ask this question: when block ciphers were invented, why did they add the restriction that the block cipher would be restricted to blocks of a fixed size? From my laymen's point of view,...
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Why is confusion & diffusion not required PKI encryption schemes like Elgamal encryption?

The Elgamal encryption scheme is a Multiplicative Mask with the shared key. It doesn't seem to have any confusion or diffusion. So why is it not necessary here as compared to Block Ciphers?
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In what paper was OCB2 introduced?

I've been working through "Cryptanalysis of OCB2: Attacks on Authenticity and Confidentiality" 1 from earlier this year. My knowledge of OCB2 is pretty shallow though, and so I'm keen to ...
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How Do I Decipher This Ciphertext? [closed]

Ciphertext:1rUHjDnJs09VNvxLiWZzL7PXeKUHvxFVG5GT59WdS6nvOZ1UypVbKlQTcl4orHFShQMBCFGH8+4+SlFDDv4+dUIcmd3zyaBnsXKCb9lMuGmMJkslgPFvgPKNWGsSVu4BhI6/42dGG3q0Lca465xeDaEKTj3Yan/59/+...
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Re-encrypt modified file section with AES-CTR

I am trying to encrypt a file using AES/CTR/NoPadding. The goal is to re-encrypt the new/changed blocks and leave the old ones unchanged then generate a new MAC. My thinking is that when the file ...
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Can XEX or XTS modes be used with only one tweak? (part 2)

This question is similar to Can XEX or XTS modes be used with only one tweak? except I am interested to know if one can instead use $X = E_{k}(0) \otimes \alpha^j \\ C = E_{k}(P \oplus X) \oplus X$ in ...
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Can I derive IV, if I know Encrypted Txt, Key and Encryption Method?

I want to know is there any way to derive the initialization vector, if I know all the other details. AES SHA 256bits CBC pkcs5padding Input: Hello Key: 123 Output: xORGCDL88sa8cUrK8JNSCQ== Now, how ...
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Is there a universal construction for Davies-Meyer hash functions?

My understanding is that there exists no strong pseudorandom permutation for which the Davies-Meyer construction is known to yield a provably collision-resistant hash function. Were I mean provably ...
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AES in terms of Diffusion and confusion [duplicate]

I will like to get a better explanation if anyone can give on the roles of matrix multiplication (mixColumn) and S-box operations in AES regarding the diffusion-confusion properties. I know that if a ...
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Parity attack on block cipher

My misunderstanding is all about the parity attack mentioned in "Cryptography Engineering by Ferguson, Schneier and Kohno". Most modern block ciphers have a 128-bit block size, but they ...
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DES : Is Double Encryption using same key twice equivalent to single DES using different key but of same size

If we encrypt a plain text using Double DES but with same key K1, Can we get the same result as if we would encrypt the plain text using single DES but with different key, say K2. What I'm asking is ...
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Decrypting an AES string

I'm dealing with a website that loads a javascript file which eventually sets an AES hash/value (whatever it's called) as a cookie. The file is heavily obfuscated making it hard to understand certain ...
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Security of block cipher PRP(kβŠ•m)βŠ•k

Let $\mathcal S=\{0,1\}^n$ be the set of bitstrinsg of $n$ bits (for security parameter $n$). Let $P$ be a public Pseudo-Random Permutation of $\mathcal S$, efficiently computable in both directions. ...
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blockcipher-based MACs with variable length that are UF-CMA secure?

There are may papers and notes explain why CBC-MAC is only UF-CMA secure for fix length messages. Are there any other blockcipher-based (blockcipher and hash combined included) MACs that is UF-CMA ...
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What is Associated Data in AEAD?

I am trying to understand what is AEAD exactly. I am application developer not a cryptographer. I have been searching online for a while watched a bunch of Youtube videos, read through a couple of ...
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Ciphers with identical round keys

Ciphers like PRINCE or LED seem to use identical round keys. Does anybody know other cipher (may include hash functions, MACs or AEADs) that has the same property?
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Question about IND-CPA with block cipher mode with random IV (CTR$ mutation)

I know for a fact that CTR random is IND-CPA secure due to if an adversary want to break it, it will have to run a long loop where the $$\mathit{Adv}^{ind-cpa}_{CTR~random} = C(2^{n},q) - 0$$ However, ...
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Can combining two secure block ciphers be insecure?

Let's say we have two secure block ciphers FOO and BAR. Will a new block cipher built by combining them into a single new block cipher result in any insecurity? $$\begin{align} \operatorname{BAZ}_K(x)&...
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Randomness Testing of Block Ciphers with 256-bit Block/ Key length

NIST explained the process of "Randomness Testing of the AES Candidate Algorithms" in the document. The document explains the generation of 9 datasets for AES candidates with 128-bit block ...
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OpenSSL aes-128-cfb does not produce expected size

I try to implement aes-128-cfb myself for learning. To confirm whether my implementation is correct, I want to verify it with a well-known library such as OpenSSL. But then I noticed something strange ...

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