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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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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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 $l_{...
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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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What is the fixed point attribute of DES (when used with weak-keys)

I've heard in some lecture that the weak keys in DES implies the existence of $2^{32}$ fixed points in the algorithm, i.e. that $DES_k(x)=x$ for $2^{32}$ such $x$'s. I couldn't find anything about ...
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416 views

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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practically verifying block ciphers strength [closed]

How do we verify practically the strength of a block ciphers? Most of the security analysis of block ciphers is usually around theoretical analysis and proofs in some random oracle model etc. For ...
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184 views

implementing a cryptographic hash with a block cipher [duplicate]

How can a cryptographic hash function be implemented/created using the block cipher? I was wondering if we can use AES in cipher block chaining mode to implement it.
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Why the same characteristics cannot be used to recover all FEAL4 keys

I am trying to understand how differential Cryptanalysis works. Here I read how to use differential cryptoanalysis for cracking FEAL4 cipher. The big gotcha with this process is that you cannot ...
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317 views

How to perform benchmark of block/stream ciphers?

I would like to perform some benchmarking of different block and stream ciphers for general data (lossless and lossy data) encryption and decryption (with focus on power consumption). To get general ...
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127 views

How well analyzed are giant block length ciphers?

I have heard about ciphers that have a block length of 256 megabytes... That is huge. How well are they understood by the encryption community? I'm referring to PMC Ciphers. Has anyone used such a ...
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249 views

Is there a theoretical maximum useful keysize given the block-size?

Consider a block-cipher $F$ with a $N$-bit block-size and a $M$-bit key-size. That is, if $k$ is a $M$-bit key, $p$ is a $N$-bit block of plain-text and $c$ is a $N$-bit block of cipher-text, then: $$...
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DES generating the same ciphertext for different plaintexts and identical key

Is it possible to get the same output for DES while the inputs are different while using the same key? The key should not have to be a weak key. If yes could you give me a hint how to show how to do ...
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333 views

Block cipher does not provide security by itself

What is this statement all about? When I look at a block cipher it seems pretty hard to break it and the mathematics behind insure a good complexity. It may refer to key generation, initialization ...
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590 views

Is there a preferred way/standard on how to transmit an initialization vector for AES encryption

Is there a preferred way on how to transmit the initiliazation vector with witch AES encryption was made? I could imagine that, if the encrypted data is stored to a file, the IV is preppended at the ...
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18k views

AES256-CBC vs AES256-CTR in SSH [duplicate]

I used AES256-CBC to SSH to a remote server. Recently, it stopped working with the following message: ...
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638 views

Why do these Python XTEA implementations require different deltas?

I once needed to an XTEA snippet to from C++ to Python and it did not work properly. I then found a function on ActiveState.com and noticed that it looks almost exactly like mine, with one exception - ...
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137 views

Security of a parallelizable block cipher mode

This is the scheme of a parallelizable block cipher mode of operation: $IV$ is the initialization vector. $BN$ is the zero-based index number of a block in a stream of data. $BT$ is the tweak that ...
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223 views

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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224 views

What cryptanalysis is possible against two independent keystreams XORed against plaintext?

If a wise person was unsure about which commercial cryptography standards are truly secure from the fascist powers that be, it would seem the obvious option for companies and individuals is to now use ...
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194 views

Do I need to know the “mode of operation” to decrypt a message encrypted with a block cipher?

If I have received an AES encrypted message, and if I do know the key with which it was encrypted, do I also need to know the mode of operation with which it was encrypted in order to decrypt it? My ...
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87 views

How to break a 32 bit block cipher without key?

From what I know Skip32 is the only 32 bit block cipher and I heard its not secure. What do I do or which tool do I use to break a 32-bit block cipher? I did search for this but all I find is attacks ...
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183 views

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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502 views

Cryptography system by combining a block cipher and a stream cipher

I have to come up with a design for a cryptography system by combining a block cipher and a stream cipher. I can't seem to come up with anything on my own. Can someone point me in the right direction ...
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Correct way to read a given permutation cipher?

In some literature I am reading – where they are explaining the permutation cipher – they have two examples, which encrypt a small block of text. But they seem to contradict each other: First ...
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539 views

How do I calculate the maximum plain text length allowable for a certain cipher text length?

I am encrypting and storing sensitive values in the database. I have a set max length in the database, but in order to provide useful user feedback, I'd like to know the max input I should allow for a ...
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Block cipher block and key sizes

Is there any point in making block size less than key size in block cipher? For example, Russian GOST symmetric cipher uses 256-bit keys to encrypt 64-bit blocks. That means there are many keys which ...
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What information about the ciphertext can an attacker discern if a key is used too many times?

Suppose that I use Blowfish or XTEA (both secure block ciphers with 64-bit block size) in CTR mode. In these cases, I need to change the key after $2^{16}$ blocks to avoid leaking information with ...
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391 views

Why are twofish or other algorithms not NIST approved, are they still safe?

NIST has a total of 3 approved block ciphers on their website: AES, TDES and skipjack. I get why those are on there (though personally I find TDES a bit iffy) but from my understanding Twofish and ...
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285 views

Can we quantify strength of encryption scheme ?

Was wondering if we have any (or can we come up with) "units" for strength of encryption like in Physics. Most of our comparative analysis of encryption schemes are based on number of S boxes or ...
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200 views

Weak key schedule IDEA [closed]

Why was such a weak key schedule chosen for IDEA? The key schedule of IDEA works like this: Divide the key (128 bit) into 8 round keys, each 16 bit long. This are the first 8 "round" keys (6 keys per ...
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Process of dividing 168bit key into 56bit keys in TripleDES

I used TripleDES so many times but never thought about this. In TripleDES it requires 168bit ...
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343 views

What does it mean when two encrypted files, encrypted with the same method but with different keys, when xor'd together, produce a repeating pattern?

Let's say I have fileA and fileB, both encrypted somehow, but in the same method with different keys. If I xor them together, they will make a repeating pattern of 32 bytes length. What does that ...
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Is knowing the distribution of input domain only way to do cipher text only attacks?

Imagine there is a $n$ bit block cipher. The attacker has harvested some how all possible $2^n$ cipher texts for some given key. Without the knowledge of the distribution of input domain as in ...
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Using IV buffer after altered inside a Rijndael CBC Encryption/Decryption process as IV for next message?

When sending a block to be decrypted or encrypted, with RijndaelCBC, we input the data to decrypt/encrypt and an IV for syncing and to prevent identical outputs for identical inputs. This question is ...
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Removing Padded Value in Decrypted Message

How to remove padded value in Decrypted Message? I am using AES Algorithm. Let's take this as the decrypted message: "abcdefghijklmn " There is a 2 space in ...
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How can I protect against the failure of a block or symmetric cipher?

Can I protect against the failure of a block or symmetric cipher by chaining different techniques together? If so what implementation details should I be aware of? Are some combination of ciphers ...
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Tell where messages begin and end in a series of ciphertext blocks?

With block ciphers, I understand that the plaintext is first broken down into the right-sized blocks, padded, and then encrypted into the same number of cipher blocks. Once these blocks are sent over ...
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126 views

Definition of ciphertext security

If I got it right, chosen ciphertext security implies also CPA security. In other words, attacker can submit plaintexts to the challenger (along with ciphertexts). I do not understand why ...
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Is a tweakable block cipher still considered deterministic in nature?

"a block cipher is a deterministic algorithm operating on fixed-length groups of bits, called blocks, with an unvarying transformation that is specified by a symmetric key. - Wikipedia " But with a ...
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simple multiplication in GF(8)

I am trying to do multiplication in the GF($2^3$) defined by the irreducible minimum binary polynomial $X^3+X^2+1$. I want to multiply $A(x) * B(x)$ where $A(x) = x$ and $B(x) = x^2$. The ...
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Can AES decrypt with a wrong key?

The title is asking the question in layman terms, but what I really mean to ask is this: Let $x \in \lbrace 0,1 \rbrace^{128}$ (an arbitrary input block) and let $K \in \lbrace 0, 1 \rbrace^{128}$ (...
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How does the length of the plaintext affect the cipher strength?

Let the block length 64-bit, 256-bit key, cipher text accordingly - 64 bits. What is the strength of the block cipher, if any unknown attacks, which could reduce its strength. We can only brute force. ...
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3k views

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 64-...
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How is block cipher padding verified for consistancy?

Under what circumstances is a decryption routine able to tell that the padding of a message is invalid? If a cipher text block where to be randomly modified, what is the probability that the padding ...
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2k views

How can 3DES encryption use decryption in the middle step?

I have been reading about the 3DES algorithm and I cannot understand one part. In 3DES we do following operation: $$C=E[K_3,D[K_2,E[K_1,P]]].$$ Where $C$ is ciphertext, $P$ is plaintext, $E$ is DES ...
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Is there an avalanche effect using block cipher modes of operation?

I am studying cryptography and various block cipher modes of operation. Suppose I changed one bit in the last block of the message. Wouldn't that imply no change in all encrypted blocks except for ...
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Is this a secure keyed hashing construction?

I need a PRF for use with PBKDF2, however, the only thing I have is the AES block cipher primitive. I'm attempting to derive a keyed hash function from the block cipher. The design is based on the ...
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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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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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271 views

CBC with a fixed or random IV

I have two probably quite naive questions: 1) Why, exactly, is it so bad to have a fixed (or predictable) IV in CBC mode? An example would be great! 2) Given 1), why is a random IV better? And if ...