# Tagged Questions

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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### How exactly was the finalist chosen in the NIST AES competition?

I was just reading the Stick Figure Guide to AES and came across an interesting table explaining how the winner was chosen: Unfortunately the NIST site is down so I can't gain further information ...
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### Block cipher and parity of permutation

Can anyone explain the following text passage to me? Most real-world block ciphers build even permutations, because it's hard to build odd ones using small operations (32 bit) on larger (128 ...
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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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### Statistical saturation attack on block ciphers

I was wondering if anyone around here could give me some explanation on this type of attacks. Pretty much the only thing that I could find is A Statistical Saturation Attack against the Block Cipher ...
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### GCM encryption for 256-bit and 512-bit block ciphers

The Galois/Counter Mode of Operation (GCM) This paper describe using GCM mode with 128-bit and 64-bit block cipher. Can we use this mode for 256-bit and 512-bit block cipher?
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### 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 (i.e....
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### Which characters to take into account when calculating unicity distance?

I need to calculate the unicity distance of a block cipher. Assuming I know that the plaintext is an English message I considered using 53 as the size of the alphabet, since the message could contain ...
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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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### how to calculate non linearity of AES S-box?

S-box has to satisfy different design criteria. How to calculate non-linearity , propagation criteria for an AES S-box?
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### 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 confidentiality-...
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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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### Why are modes of operation used, what attacks do they prevent?

I know you always need to use a mode of operation when using a block cipher, AES for example, and Wikipedia has a good explanation for what modes of operation are Now I know if i do not use a mode of ...
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### AES key/ciphertext space sizes

This is giving me a brain ache now... If I have AES-128, block is 128 bit, then every plaintext (128-bit) can be encrypted to some ciphertext that is also 128-bit. This is the block size. But: 128-bit ...
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### How to secure a mental poker protocol? [closed]

I would like to implement a mental poker protocol in a secure fashion. How should I go about that without (preferably) infringing on the Mental Poker Framework patent?
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### Would this method allow fast authenticated encryption using only a single encryption and RNG operation per block? [closed]

The most promising of the 4 approaches described here (version 4) has been posted as a separate question. This post should be considered for archiving and historical purposes only. Disclaimer: the ...
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### Why use an Initialization Vector (IV)?

Why use an Initialization Vector (IV)? How are IV's used? What are the advantages/disadvantages of using an IV? Why use an IV instead of a longer key in which some section of the key is pubic? What ...
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### Why do block ciphers need a non-linear component (like an S-box)?

Why is there a requirement of "Non-Linear functions" as a component of many popular block ciphers (e.g. the S-box in DES or 3DES)? How does it make the cipher more secure? The only intuition I have ...
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### AES in ECB mode weakness

In a project that I'm currently working on, we are encrypting some data using AES with ECB mode in a database. Each piece of data being encrypted is very small, no more than 10 characters long. Very ...
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### CBC - a canonical mode, even though there are streaming modes

Why is CBC considered the canonical mode when there are streaming modes available such as CFB and OFB? One thing that I can think of is that in CBC you can easliy do range-based decryption. All you ...
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### How to calculate an IV when I have a shared private key?

I'm working with a server/client system that has securely negotiated a private key. I need to encrypt/decrypt messages passing in both directions which are typically between 100 bytes to 100 KB. I'm ...
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### Modes of operation that allow padding oracle attacks

It seems to me that padding oracle attacks are mainly a concern for users of CBC mode encryption. Question: are any other modes of operation vulnerable to padding oracle attacks? And if so, why? ...
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### Why is CAMELLIA suddenly so widely used?

When nowadays I point my browser to https sites, the cipher that is on most occasions used is Camellia. My browsers (Chrome and Firefox) seem to prefer it, even when AES is available. Is that not ...
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What weaknesses (or strengths) do block ciphers based on only key xor and s-box have when operating in CBC mode? A cipher's internal primitive might be a simple as this: $C = S[M \oplus k]$, where $... 3answers 1k views ### Are there any secure commutative ciphers? This answer lists two commutative cipher algorithms - Pohlig-Hellman and SRA. However, they don't appear to be too secure. My question is, here there any commutative ciphers out there that are secure ... 2answers 90 views ### Twofish MDS and PHT Twofish was an AES candidate and it uses$4 \times 4$Matrix as MDS followed by a PHT. The Branch Number of MDS and PHT is 5 and 2 Respectively. from formula$Branch~Number = Minimum~of~[...
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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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### Difference between Rijndael 128 / 256 blocksize implementations? (and impact of block size in general)

Can anyone shed some light onto the advantages/disadvantages of using Rijndael with 256-bit block size, as opposed to the 128-bit (AES) implementation? (please note: I'm not referring to key-size here)...
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### Hash function from narrower block cipher operated in CBC-encryption mode?

I am trying to build a public hash function (thus collision-resistant and preimage-resistant, and more generally behaving like a random oracle), with input a message $M$ of fixed size $|M|=m\cdot b$ ...
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### Security of Pohlig-Hellman exponentation cipher?

I am looking into implementing Pohlig-Hellman exponentation cipher and I would like to know how secure that algorithm is? I am guessing it's security relates greatly to the prime number used in it. ...
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### Is a small size block cipher usable?

Consider a block cipher with a key of size 128 bits but a small block size, say 32 bits. Is this kind of block cipher secure? I would like to know to what extent I can use a small block cipher, like ...
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### Is TEA considered secure?

Wikipedia claims that the best attack on the surprisingly simple TEA block cipher, that isn't a related-key attack, has a time complexity of $2^{121.5}$. So despite how unsophisticated the cipher ...
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### AES and perfect ciphers

I'm taking a crypto class this semester and after learning the definition of a perfect cipher. I started wondering how this definition applies to AES. Obviously AES isn't a perfect cipher, since the ...
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### Can a commutative block cipher be indistinguishable from a random one, assuming a random permutation of keyspace?

Define a commutative block cipher with keyspace the finite set $K$, and message space the finite set $S$, to be an application \begin{align} E:K\times S&\mapsto S\\ (k,x)&\mapsto E(k,x)\text{...
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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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### Slow one-way pseudo-random permutation?

I'm looking for a slow one-way pseudo-random permutation; or in other words a block cipher $E_K: P\in\{0,1\}^b\mapsto C\in\{0,1\}^b$ with moderate block size $b\approx 64$ bits, wide key $K$, ...
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### What is the most computationally efficient way of generating pseudo-random permutations?

I have an application in which I need to create up to J randomly shuffled-copies of an array of length N. Then I will have millions or even billions of iterations such that, in each iteration, I will ...
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### Keeping IV secret for AES CFB mode

I'm developing a security/encryption software and I'm using AES CFB (block size: 16 and key size: 32 bytes). I want to know, if I also keep IV (32 bytes) secret like the key itself (32 bytes), would ...
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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.. [2b] [28] [ab] [09] [7e] [ae] [f7] [cf] [15] [d2] [15] [4f] [16] [a6] [88] [3c] ...
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### What are the advantages of CBC over ECB?

From this question I understand that, for a block cipher, using CBC is better than ECB. It seems that if one only has part of the cipher text, then decryption is difficult because the decryption ...
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### 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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### Is a strong block cipher usable as a strong sponge function?

From the literature, it looks like the security proofs of sponge functions depend on how well they approximate a random permutation, Since a block cipher also ideally behaves like a random permutation ...
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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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### How to find which padding method is used in Block cipher(AES) encyption?

I have a cipher text which is encrypted using AES128 Algorithm. I want to know the following things to decrypt the cipher. IV (Initialization Vector) Padding Method Encryption Key How do I find ...
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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 ...