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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Block Cipher Without Key Schedule?

I'm designing a cryptography assignment for a college security course, and one of the problems is to perform a simple meet-in-the-middle attack on a 2-round, 2-key cipher. Since we want this to be a ...
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Have these compositions of block ciphers the same security?

I'm interested by the compositions of the block cipher DES, instantiated with independent keys $k_1$, $k_2$ and $k_3$. Are these 3 compositions equivalent in terms of security? ...
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Multi-target attacks on AES-CTR with a random nonce

128-bit block ciphers are vulnerable to multi-target attacks where the attacker seeks to attack a collection of keys instead of a single key. A simple example: Generate keys $k_1, k_2, ...
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What information should I send along with an AES ciphertext and is there any standard to do that?

While using some crypto implementations I got to see that many times a comment is added to the ciphertext. For example: ...
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Coding of unsigned int to prevent guessing next ID

Suppose we are assigning records an unsigned integer ID from a N-bit space (say 32-bit) in a sequential manner. Is there a way we can code this ID before showing it to the public such that someone ...
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Which block cipher mode(s) is most appropriate in these applications?

I have a few questions I'm stuck on for a cryptography course I'm doing, any help would be much appreciated. The first application is for decryption on a multicore processor. I assume any of ECB, ...
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How do I construct variable length input (VIL) in a block cipher mode?

I've just been skimming NIST's "Five Confidentiality Modes". I understand those block cipher modes are used to securely encrypt input longer that the block cipher length. However, the common block ...
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215 views

When is it safe to not use authenticated encryption?

I have read the post Why should I use Authenticated Encryption instead of just encryption? When is it safe to not use authenticated encryption? I assumed hard drive volume encryption or per file ...
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AES block cipher modes of operation

have a project in which I have to implement an en/de-cryption structure using a standard AES block of 128-bits in VHDL and I think I'm a bit confused. So I'd like to ask some questions about AES and ...
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SIMON implementation, decryption issues

I've been trying to implement the new SIMON block cipher in Ruby as a personal challenge. The spec didn't include a pseudocode version of the decryption but I thought I had it figured out. The code I ...
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Okay to use OpenSSL to encrypt then sign a message?

I am planning on encrypting a message then signing the message so the recipient can authenticate the source. Reading this article has me concerned that I'm doing something wrong. My current plan is ...
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298 views

OCB - brute force against unknown IV

I read that brute force attacks against a plaintext encrypted in OCB with unkown key and IV has approximately the same complexity as an attack where only the key is unknown. Why is that ? Is there a ...
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711 views

KeeLoq showing that decryption is indeed the inverse of encryption

In some text I am reading, there is an exercise asking to show that KeeLoq decryption function is the inverse of the encryption function. Details about KeeLoq are given in the Wikipedia article. As I ...
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Is simple XOR with secret key using DHE secure?

Client and server have given generator 3 and modulus which is 256 bit prime same all the time for all clients and server (simply hardcoded). They both generate a private exponent and send result to ...
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63 views

Fixed-points in block ciphers; why don't they indicate a vulnerability?

I've seen a few questions here on fixed-points in ciphers, most asking about the possibility and existence. Most of the answers however pointed out that fixed points are not exactly a security threat. ...
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71 views

Use of the IV for the CBC Block Cipher in TLS1.2

I'm struggling to understand at what point a message is encrypted when looking at RFC 5246. The spec gives the generic block cipher in section 6.2.3.2 as: ...
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Can CFB mode of block cipher be used as a commutative encryption?

Can CFB mode of block cipher be used as a commutative encryption? Is it possible to use CFB mode of AES block cipher to create commutative encryption? With CTR it is possible but not secure.
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Block Ciphers - Standardize plain text

I need to use AES-128 to encrypt a plain text about 720 bits. Is it correct to say that – in this case – the plain text will be divided in 5 blocks of 128, the result will be equal 640 bits, and the ...
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132 views

Why must the sender and receiver be synchronised in synchronous stream ciphers?

I would like to know why the sender and receiver must be synchronized in synchronous stream ciphers as said in synchronous stream cipher. Why is there no synchronization between the sender and ...
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226 views

Block size vs key size confusion

I understand that if a block cipher has $k$-bit keys and $n$-bit input/output blocks, then if $k>n$, we can expect one message-ciphertext pair to narrow us down (I think?) to $2^{k-n}$ possible ...
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Is it possible to generate a secure permutation F over 32-bit integers even if F(0) … F(n) is public knowledge?

I'm currently interested in the problem of generating random-looking URLs from sequential database IDs, like how they do it in link shorteners. One way to do this is to encrypt the sequential database ...
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114 views

Galois field step in XEX mode

I'm reading through the XEX mode spec, and I want to make sure I've understood the $\alpha^j$ step correctly: $X = E_K(I) \oplus \alpha^j$ $C = E_K(M \oplus X) \oplus X$ where: $M$ is the ...
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Pseudorandom permutations on domains of arbitrary (i.e. not a power of 2) size

The Feistel cipher algorithm produces a pseudorandom permutation with domain size equal to $2^{2m}$ (for some $m \in \mathbb{Z}^+$). That is, a one-to-one function $\pi$ of the set $\{0, 1, 2, \dots, ...
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249 views

XOR with constant key in CBC mode

I have a hypothetical encryption scheme where somebody uses the one-time pad in CBC mode. That is, the block cipher is $E(k, m) :=k⊕m$, and that block cipher is used in CBC mode. Now, I am assuming ...
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Security Level Estimate when Cascading/Compositing Ciphers?

Way back when, DES had a theoretical security level 56-bits. 2-key TDEA provides about 80-bits of security, and 3-key TDEA provides about 112-bits of security. Obviously, those security levels are not ...
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Block Cipher Mode Amicable to Fast Key Change/Rotation Like XOR?

I'm working with documents encrypted in long term storage. When a client requests the document, the document should be delivered encrypted under a distinct key (distinct from the long term storage ...
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563 views

Serpent cipher : Osvik S-Boxes confusion and test vectors

I'm having hard time with the implementation of the S-Boxes by Osvik found in this paper: Speeding up Serpent. At the end of the paper, all the s-boxes are given and then, I just implement them. ...
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Making counter (CTR) mode robust against application state loss

Counter (CTR) mode, which is a block cipher mode of operation, has some desirable qualities (no padding, parallel encryption and decryption), but at the cost of failing badly when non-unique counter ...
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How does DES decryption work? Is it the same as encryption or the reverse? [duplicate]

If DES decryption is the same as encryption done in reverse order, then how can the reversed S-Box convert 4 bits into 6 bits?
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Block cipher with key longer than block size

When a block cipher key length is greater than the block length, is it the case that for some keys $k_0, k_1$ (with $k_0 \not= k_1$) and plaintext message $m$ that $E_{k_0}(m) = E_{k_1}(m)$?
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Why are the Davies-Meyer and Miyaguchi-Preneel constructions secure?

The Davies-Meyer compression function $h(H, m) = E_m(H) \oplus H$ is said to be secure. So too is the Miyaguchi-Preneel compression function $h(H, m) = E_m(H) \oplus m \oplus H$. Why are these ...
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Encrypt array of int for individual retrieval

I have limited exposure to cryptographic terminology, so please bear with me. My end goal is to encrypt integer IDs, before transmitting them to a web client in a list of search results, in a way ...
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297 views

Deciphering “easy” ciphers without hints

I've been keen on IT Security for a long time now and I've learned a lot about networking & security. However trying a "decryption challenge" I'm lacking what I think is basic ...
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OMAC : Padding on empty string

I started to implement some MAC since last week with the specifications given here. I'm currently testing the OMAC (one-key CBC) with test vectors. In the OMAC specifications at page 4, they explain ...
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257 views

Generalizing a CBC-MAC collision with a common final message block

I found this question from a textbook but I don't really understand what it is asking: Suppose $c$ is $1$ block long, $a$ and $b$ are strings that are a multiple of the block length, and ...
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Stream vs Block cipher: Synchronising and the receiver

I am learning cryptography at school and I was wondering: How are the blocks/streams after decryption synchronized at the receiver to form one long meaningful message? Am I right by saying that ...
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Given a linear block-cipher,how can an attacker decrypt any plaintext value encrypted,using 128 chosen ciphertexts

Given the linear block cipher $LinearCipher(k, p) = c$ $$LinearCipher(k, p_1 \oplus p_2) = LinearCipher(k, p_1) \oplus LinearCipher(k, p_2)$$ where $k$ and $p$ are 128-bits. If an attacker uses a ...
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Repeatable crypto

Is there a safe way to do crypto that will always produce the same result for a given input? My use case is transferring deltas of huge files, to a backup server. The backup server has no knowledge ...
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Chosen Ciphertext Attack with a theoretical decryption machine

Say we have an encryption algorithm that encrypts data blocks of 128 bits size, and makes them cipher blocks $C = E(P)$ without chaining. Also assume there is a linearity rule for XOR: For every pair ...
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Analyzing ECB's and CBC's resistence to a known-plaintext attack

I'm trying to analyze the strength of a block cipher with CBC mode or with ECB mode on the scenario of an exhaustive search attack with knowledge of pairs of plaintext – ciphertext (known plaintext ...
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Pseudorandom Functions with different input and output lengths

I am working on a problem found in my Cryptography textbook that goes as follows: Let F be a pseudorandom function such that for $k \in \{0,1\}^n$, function $F_k$ maps $l_{in}(n)$-inputs to ...
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Malleability attacks against encryption without authentication

Suppose there is a message that is encrypted with AES-128-CBC. The message is as follows, new lines are used to delimit the 16 byte boundary for each block: ...
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Is the 1st Encrypted Block Less Secure Than Subsequent Ones?

It's my understanding that Electronic Code Book (ECB) produces similar cypher text for similar plaintext inputs which is not a good thing. To get around that, Cypher Block Chaining (CBC) can be used ...
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Is it safe to encrypt random data using ECB mode?

If I have 48 random bytes, is it safe to encrypt them using ECB mode? Or is there still a good reason to use another mode like CTR, CBC, etc.? Context is this: I am creating something like a TLS ...
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Is my pseudo-random initialization vector secure?

How can I know if I am generating a secure pseudorandom initialization vector? Currently I am planning to generate a pseudo-random initialization Vector using current date and time - is this secure ...
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Known plaintext, unknown 128 bit block cipher

I have an encrypted configuration file from an embedded device which I'm trying to decrypt. The file seems to be encrypted in 128bit blocks, as changing a single option causes a 16 byte block to ...
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I read ECB is bad because the same plaintext outputs the same ciphertext. Isn't that a requirement of a cipher?

In the Wikipedia article on Block Ciphers, it says about the Electronic Code Block mode: The disadvantage of this method is that identical plaintext blocks are encrypted into identical ...
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Secure use-cases of block cipher with 64-bit block size

In what cases can we use a weak block cipher like DES ? More precisely, Are there specific situation in which a weaken block cipher can still be used, for instance for certain types of plaintext ?
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salsa20 is invertible, useless in CTR mode?

I'm reading the salsa20 spec (sorry for the newbie question, i'm really a newbie here). Every paragraph ends with : "this operation is invertible", I suppose the whole salsa20 algorithm is. If I use ...
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How is this affine function a pair wise independent permutation?

Can anybody explain how the below is true ? $I_n$ denotes the set of all n-bit strings, ${\{0,1\}}^n$. (..) Let $A$ be a finite field then the permutation $f_{(a,b)}(x) = a \cdot x + b$, ...