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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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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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 encryption/...
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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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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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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 ...
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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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Combining a keystream generator with a block cipher
I understand that:
A stream cipher's security is diluted if the same key is used more than once. This is fixed by using an additional initialization vector in the keystream's computation.
A block ...
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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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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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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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Block cipher to encrypt 8 bytes/64 bits - Use 64 or 128 bit cipher?
Here's the problem: I have a transport stream that I have been requested to encrypt between the transmitter and receiver. The data size is at most 8 bytes so it seems I can only use ECB as I only have ...
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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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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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Block cipher fixed points (plaintext equal to ciphertext)
A block cipher is a bijective map from the set of possible plaintexts to the set of ciphertexts, which are the same size and might as well be considered the same thing: $\theta: S\to S$. In this there ...
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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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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 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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Recovering key length from transposition block cipher ciphertext
First of all I would like to say hello to all StackExchange users. So far I've been stalking the forum for some time, now it's time to leave the shadows.
In my master thesis I'm writing about ...
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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 chosen-...
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Block-cipher based vs Hash based MAC
When it comes to MAC algorithms one can have many choices. CBC-MAC, CMAC, PMAC etc are some MAC algorithms that rely on block ciphers (ex AES) to generate a MAC. On the other hand HMAC is very popular ...
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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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Encrypt-Mix-Encrypt: Full Diffusion?
I've read "A Parallelizable Enciphering Mode" by Halevi and Rogaway about the encrypt-mix-encrypt mode for ciphers and was asking myself if this mode provides "full" diffusion.
So if an attacker ...
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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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Understanding Meet-in-the-Middle attack on block ciphers
I understand how to implement a Meet-in-the-Middle attack on 2DES/3DES, by computing for one half all possible keys and storing the outcome and then subsequently do the same for the other half but ...
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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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How to prove Security of Onion Layers of encryption?
CryptDB has Onion layers of Encryption to provide wider functionality from weaker forms of encryption. How do we prove such things are indeed secure ? Intuitively It seems ok. Are there any parallels ...
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Why is a Feistel network bijective?
EDIT: Fixed for clarity of intention
I was reading "The FFX Mode of Operation for Format-Preserving Encryption" when I came across Figure 1 (see below). It's obvious why it's reversible (top-down ...
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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:
$E'_1(k_1,k_2) := k_1 \oplus E(k_2,m)$
$E'_2(...
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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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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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Block Cipher Modes
I have a question here asking the following:
Why do block ciphers need the use of blocking modes?
To encrypt messages larger than the size of the block.
To avoid having the same block ...
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Calculate the security of a cascade cipher
I’m reading “The Security of Multiple Encryption in the Ideal Cipher Model” and I’m having a hard time understanding something. How do I calculate the bounds of cascade cipher from the following ...
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Are keys generated by the user or block cipher algorithms themselves? [closed]
We know block cipher algorithms. In the block cipher algorithms, Key Scheduling is the one crucial part. I see that in some simulation programs of the AES,DES,TripleDES encryption algorithms. In those,...
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How are subkeys, in Feistel block ciphers, derived for each round?
I'm bit confused about the subkeys that are used in Feistel ciphers. I know that all the subkeys ki are derived from the main key K, but how?
Say I have a simplified Feistel block cipher of i rounds, ...
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Is CBC theoretically harder to brute force when compared with ECB?
Theoretically – is CBC mode harder to bruteforce when compared with ECB when assuming:
we ignore computational cost for the XOR operations, and
the IV is made public?
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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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Strength of CBC with Ciphertext Stealing
Is CBC with Ciphertext Stealing (CTS) considerably weaker than CBC with padding such as PKCS7?
I would imagine the most common situation where CTS is necessary would be due to some size constraint of ...
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Security of (cryptographic) padding compared to other methods for blockcompletion for block ciphers
As said in the heading, I want to know how the security of different padding methods, e.g. ANSI X.923, ISO 10126 and PKCS7, is compared to other methods to reach the needed block size, like ciphertext ...
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Padding methods for block ciphers - PKCS7 vs ANSI X.923
I was looking through block cipher padding methods, and found two good candidates:
ANSI X.923 - pad with zeros, then a final byte for the padding length, e.g. ...
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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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Turning a 64 bit block cipher into a 128 bit block cipher
There are quite a few block cipher modes of operation that require 128 bits. There are also modes of operation where a higher block size than 128, e.g. a block size of 256 bit would even be practical.
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security in format preserving encryption
In format preserving encryption an attacker knows the format and data type of the plain text. The format of the plain text and cipher text are same. usually cipher text are unreadable form. then how ...
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Distinguishing attack on stream cipher created from a block cipher in counter mode
A block cipher in counter mode creates a key stream by:
$$z_i = E_K(IV || i)$$
Where $IV$ is some chosen initialisation vector (with some length $l$ less that $n$ bits) and $i$ is the binary ...
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Can AES be considered as Perfectly Secure?
I am studying about the AES encryption standard, and as I know from the theory there is no known weaknesses today about this block-cipher.
I would like to find out or at least to understand what "...
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Operation modes of block ciphers how are used?
I am studying about different operation modes of block ciphers my question isn't something specific to that, but is how these operation modes are used in cryptography?
I have this picture here (OFB-...