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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Message lengths with AES CTR mode?

Im working on my own AES implementation as a personal educational exercise (not for production use), and a book im using as a resource states: In the case of CTR, we are merely XORing the output ...
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314 views

Break double encryption

Let $E_k$ : {0,1}$^l$ be a block cipher encryption function with block-size $l$ and key-length $n$. In class, we saw that a double encryption with two independent keys $E{}'_{k_1k_2}(x)$ = ...
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924 views

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 CBC really dead?

I developed a p2p-app in C# which sends and receives encrypted text messages (50KB). For encryption, my app uses AES 128 bit in CBC cipher mode. For each message it uses a new randomly-generated IV. ...
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Will varying plaintext compensate for a fixed initialisation vector?

This is a follow-up question to Relative merits of AES ECB and CBC modes for securing data at rest. I need to store encrypted Personal Account Numbers (PANs) in a database. The only encryption option ...
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Is there any general description language for block ciphers?

In page 9 (16 in the PDF) of Final report of European project IST-1999-12324 (NESSIE), it is mentioned that: The NESSIE project is also developing a new generic tool to analyze block ciphers ...
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630 views

Does AES have any fixed-points?

Is there any pair of 128-bit strings M and K such that AES$_K$(M) = M? If yes, how do I go about (efficiently) finding such a pair?
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436 views

Practical uses of Manipulation Detection Code (MDC) and IGE

I've just skimmed over most of the paper "On Message Integrity in Symmetric Encryption". I've included (the last) part of the abstract below: We show that generic compositions of ...
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Is Blowfish strong enough for VPN encryption?

I'm looking at an OpenVPN connection between two sites configured to use 128 bit Blowfish in CBC mode, and trying to figure out how to assess the strength, but I just don't know enough of the maths. ...
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Is it possible to construct a secure block cipher of size $2n$ given a secure block cipher of size $n$?

Given, say, the Blowfish block cipher, which is considered secure but only has a 64-bit block size, can we construct a secure block cipher of 128-bit block size? Say we run the key through two KDFs, ...
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818 views

Can you explain “weak keys” for DES?

A weak key for DES is a key $K$ such that $DES_{k_1}(DES_{k_2}(x))=x$ for all $x$. I don't get why are the 4 keys $k_1||k_2$: $1^{112}$, $0^{112}$, $0^{56}||1^{56}$, $1^{56}||0^{56}$ considered as ...
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211 views

CTR mode nonce with aggressive key rotation policy

I'm writing a small encryption library which will be required to use the shortest possible amount of overhead for describing ciphertexts. I want to use CTR mode with a 6-byte random nonce. Normally ...
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What is a block cipher?

I have read about block ciphers, but I do not understand the topic completely. Is a block cipher a way to send data or a way to encrypt data? How is the data divided into multiply blocks before it ...
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511 views

Which block cipher modes of operation allow a predictable IV?

Recently I found out that in the modes CBC and PCBC the IV may be passed in cleartext but never must be predictable. However for this part of my app I rather have the IV be predictable and unique ...
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331 views

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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279 views

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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512 views

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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163 views

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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201 views

OFB or CTR mode without an IV

I'm reading the SSL specs here. The interesting thing seems to be that RC4 is not re-keyed with a new IV for each message. The stream cipher state is simply carried over to the next record. Why is ...
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247 views

Why is $h(H, m) = E(m, H) \oplus m$ insecure?

I am taking a cryptography class on Coursera. I learned that the compression function $h(H, m) = E_m(H) \oplus m$ is insecure (even though other variants like Davies­-Meyer or Miyaguchi-Preneel are ...
2
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1answer
103 views

Using UMAC with stream cypher

I understand that most stream ciphers, due to being applied with a simple XOR, are specially fragile against data tampering, and must be used with some MAC mechanism. So I am investigating the use of ...
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329 views

Can I build a secure tweakable block cipher from a normal one by adding key and tweak?

Let (E,D) be a secure block cipher. Consider the following tweakable block cipher: ...
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879 views

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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How to use RCON In Key Expansion of 128 Bit Advanced Encryption Standard

I have a question about RCON here is my illustration... this is the 128 bit key.. ...
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How can I implement the “Multiplication Modulo” and “Addition Modulo” operations in IDEA?

I am currently working on IDEA (International Data Encryption Algorithm), and I don't know how to perform Multiplication Modulo and Addition Modulo. This is how IDEA operates: IDEA operates on ...