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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Algebraic relationship between Original Key and Sub Key of DES

Does there exist any sorts of algebraic relationship between Original and Sub Key of DES? I understand that by solving K-Maps of 64-bit Input/48-bit Output, I can get 16 algebraic relationships ...
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Finding differentials and space complexity

Most of article about differential cryptanalysis present the generic method, the way to find sub keys. But I can't find a clear explanation about how to find the differentials used in examples. In ...
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What is the correct definition of the blowfish F-function?

The Blowfish cipher uses a so called F-function which uses S-Boxes (S[i], i=0,1,2,3) to ...
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Why does DES implement so much Cross Wiring?

I've been going through DES, and I find that most of the blocks simply Reshuffle the input, which may be termed as "Cross Wiring". This include IP, IP-1, PC-1, PC-2 and even Expansion (although this ...
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What is the difference between key size and block size (for AES)

We are working on AES and want to develop a website which should provide an encryption facility but we are not understanding the difference between key and the block size. More appropriately what does ...
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43 views

Is the IV value in Davies Meyer construction public?

I am learning the construction of Davies-Meyer to construct a hash function from block ciphers. The definition that I found is $$H_i=E_{m_i}(H_{i-1})\oplus H_{i-1}.$$ where $E_k$ is a block cipher ...
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Is it safe to use cipher block sizing instead of padding?

I'm using this kind of technique instead of padding of cipher buffer (here goes pseudo-code): ...
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121 views

AES Inverse Key Schedule

I have a 128-bit input-block and the corresponding cipher-block given. Additionally I have the last round-key given. Is it now possible to get (calculate) the associated cipher-key? I already ...
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121 views

Combining a block cipher with a (pseudo) OTP

One of the drawbacks of OTP is that it can require an inconveniently long key. Besides that, it doesn’t provide data authentication. Therefore, I have been working on an algorithm idea, which I hope ...
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57 views

Is it secure to encrypt random key with block cipher in ECB mode?

If we have a random key $K$ which is 256-bit, and have a block cipher $C$ which has a block size of 128-bit. Is it safe to encrypt $K$ with $C$ in ECB mode? This problem can be generalized to given a ...
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Differential Cryptanalysis

How do I find the differential characteristics using techniques Shrinking ? How do I extract the key on Differential Cryptanalysis using the technique Shrinking in Blockcipher Algorithm? Shrinking ...
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113 views

Muchomor cipher

I've been given a task to research and possibly implement so called 'Muchomor' cipher. Has anybody ever heard of this cipher? I've found only one article about it and it was in Russian. I'm quite ...
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77 views

How can there be AES-256-GCM, when GCM is defined for 128-sized blocks?

In both node.js and OpenSSL, I am seeing cipher "aes-256-gcm". However, in NIST Special Publication 800-38D, that defines GCM, I am reading: The underlying block cipher shall be approved, the ...
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Are there key collisions in AES-192, AES-256?

Suppose we have a single, fixed plaintext block $p$ of 128 bits. Call the set of all possible 128 bit blocks $B$. We could define fixed plaintext AES-128 as a function $f(k) \to B$ that takes a 128 ...
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228 views

Should Increment functions be near-constant time?

When operating a block cipher in CTR or GCM mode, should the function used to increment the counter or initialization vector be near-constant time? Here, I'm wondering about the "addition with carry" ...
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147 views

A self-decrypting block-cipher

If I have a block-cipher like for example AES, a plaintext $x$, we use different function to encrypt and decrypt : $\text{encrypt}(x) = y$ , $\text{decrypt}(y)=x$. Now, if we build a function with a ...
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414 views

Pseudo random generator vs block encryption, which is faster?

If one wishes to encrypt one can either use a pseudo random generator to generate a long sequence and then xor it with plaintext, similarly he can use the seed as key to encrypt the plaintext using ...
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171 views

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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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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240 views

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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50 views

Symmetric-like cyphers with several steps

Are there any good symmetric-ish cyphers that involve three or more steps with independent key material for each step? Formally, there should be a function $K(s)$ that returns a tuple $(k_1,..,k_n)$ ...
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49 views

XTS-AES key management

XTS-AES uses two keys $(k_1, k_2)$, which are obtained from a concatenated key $k$. Does this key $k$ remain same for the whole disk or changes per sector?
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911 views

Why is it OK for secretbox to leak the message length?

Most block cipher implementations expect users to pad so that the message matches the cipher block size. I understand the purpose of the padding is to make the input match the cipher block size. It ...
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324 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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44 views

Is there a standard prefix random padding scheme?

Is there a standardized padding scheme which is prefix instead of postfix and that uses random bytes except for the first byte which would be the length of the padding?
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243 views

Given a linear block-cipher,how can an attacker decrypt any plaintext value encrypted,using 128 chosen ciphertexts

Given the linear block cipher $\operatorname{LinearCipher}(k, p) = c$ $$\operatorname{LinearCipher}(k, p_1 \oplus p_2) = \operatorname{LinearCipher}(k, p_1) \oplus \operatorname{LinearCipher}(k, ...
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Encrypting a 160-bit plaintext into a 160 bit ciphertext with a 128-bit Block Cipher [closed]

So I have a kind of interesting issue. I have a field in my data store which must take exactly 160 bits of information. Some users will choose to make this data encrypted, some won't, so some of those ...
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Do we have to consider the cryptographic properties of the decryption, if we're only using the encryption?

Treyfer for example has slower diffusion in decryption. It has full diffusion between words in 2 rounds for encryption and 7 rounds for decryption. Do we have to consider the (bad) cryptographic ...
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Quantitatively measure the private information revealed by a matrix?

Suppose I have a private matrix $A$ that I want to conceal. I do the matrix multiplication $A^\prime =AK$ and $A^\prime$ is public. How can I quantitatively measure "how well" the matrix $A$ is ...
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279 views

XTEA: patterns in plain text

I'm using XTEA to encrypt passwords that I store in a database. (I need to be able to decrypt these passwords later to log in on another system.) But now I have noticed something: if I decrypt a ...
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How to propagate error in variable-sized message?

I want to design a scheme to encrypt a variable-length message with a secret key, to provide confidentiality. The message is a short human-readable text string with byte granularity, let's say under ...
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219 views

Encrypt with one cipher and decrypt with a different cipher?

I recently installed an android app called Secret Space Encryptor and think I found a serious flaw in it. Let me explain: This app provides a variety of different ciphers to use and I used Threefish ...
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187 views

Simple two round feistel chiffre

I am currently learning for a exam in cryptgraphy. I now try to understand the Feistel Cipher with two rounds. I do not have any information on what the functions are actually doing. The following ...
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2 round GOST_28147-89 cipher distinguisher

So, here is the scheme of how this will look graphically: The deal is to present distinguisher for this cipher. First of all here is my drafts: Way 1 Assume that distinguisher displays in ...
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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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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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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 a block cipher encrypt the message to itself (plaintext equal to ciphertext) [duplicate]

The result of the cipher should be indistinguishable from random, but can it happen that cipher produces the ciphertext that is equal to the plaintext as it also lies in the corresponding space? I ...
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235 views

Where is CFB-MAC defined?

I can find descriptions of using CFB mode to create a MAC and even implementations, but cannot find any standard or papers analyzing its security bounds. Is there a standard for CFB-MAC? Where can I ...
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How to encrypt/decrypt data in an SPI BIOS FLASH

I need some info/suggestions about the cryptographic algorithm that make me sure that the executed bios by the CPU is an authenticated bios. The check has to be performed by a FPGA. I have SPI Flash ...
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Encryption of 8 bit of block data

I'm looking for a cryptographic algorithm that allows encrypting only 8 bits of data in a block. Is there any such algorithm that is considered secure? The reason I'm asking is because I have to ...
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Why is it good to split a CTR-mode counter into nonce and counter?

When discussing the CTR mode of block ciphers, Wikipedia says the following: Simply adding or XORing the nonce and counter into a single value would completely break the security under a ...
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267 views

Is the GOST block cipher broken?

Is the GOST cipher broken or known to be weak and insecure? Please try to simplify your answer since I am a programmer and not a cryptographer and please consider this web page and tell me if the ...
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151 views

How AES treat string input to encrypt data and what will happen if the cipher key is less than that expected?

The official docs of AES have several example of how hexadecimal or byte input can be encrypted. I have some problem about the input text and cipher key. My question is: How can encrypt string input ...
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1answer
100 views

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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Is this commutative encryption protocol secure?

My assumptions are: encryption must be commutative man-in-the-middle is not an issue I would like to implement the following scheme between 2 parties ($A$ and $B$) to exchange a key: Lets $C = P ...
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One cipher to rule them all?

I learned from Dan Boneh's course that many cryptographic primitives (prngs, stream ciphers, hashes, hmac, key derivation functions) can be built from just one a block cipher or PRF. For example, the ...
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Reasons for components of AES

I started reading "Cryptography: theory and practice" by Stinson, and I am trying to understand the reason behind the choices for the different components of AES since they seem quite arbitrary. In ...
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Does XTS provide random write access?

I'm trying to find a sufficient encryption mode that supports random read and write access. It seems like XTS is a perfect fit, since in my use case authentication and integrity insurance are handled ...
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Double the key in block cipher - which approach is better?

Assume than you have a block cipher $E(k,m)$ for which the only attack exists has complexity of $2^{64}$. You consider to double the key size by either: $E'_1(k_1,k_2) := k_1 \oplus E(k_2,m)$ ...