# All Questions

1k views

### Fastest multiplication algorithm for efficient exponentiation in C++?

First I apologize if this question is better tailored for SO, but since I'm using the method for crypto stuff, I thought I'd ask anybody that might know here. Karatsuba algorithm does a pretty good ...
567 views

### Digital signatures in SSL/TLS-like protocols

The TLS protocol includes an optional Server Key Exchange message that is signed by the server. It seems to me that this message is digitally signed only to prove to the client that the message ...
2k views

### From hash to Cryptographic hash

After reading some excellent papers on SipHash, I understood that good non-cryptographic hashes such as MurmurHash and CityHash are not secure for MAC usage, due to a certain type of DDos attack ...
283 views

### Is the following key stretching algorithm as memory hard as I think it is?

I'm having some fun designing a key stretching algorithm that can be implemented in pure Python. It's built entirely out of the standard library's hash functions in an attempt to at least wrest some ...
373 views

### Computational indistinguishability and example of non polynomial algorithm

The wikipedia page on computational indistinguishability says that two ensembles are not distinguishable if "any non-uniform probabilistic polynomial time algorithm A" cannot tell them apart. To help ...
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### Is storing original file size in an encrypted file header a bad idea?

I'm encrypting a file using AES-256 in CBC mode. I'm padding to the 16-byte input multiple by using the PKCS#7 limit. The problem I'm currently having is that if I'm transferring or reading the ...
2k views

### Validating successful decryption in AES

I have a program which uses AES-256 in CBC mode to encrypt and decrypt files. As I have quickly realized, AES will even use an incorrect passphrase to decrypt data, which leaves me with no way to ...
51 views

### Is there a way to analyse xxHash and tell, either by human analysis, or with an automated tool, that this hash function is or is not cryptographic? [duplicate]

After reading some excellent papers on SipHash, I understood that good non-cryptographic hashes such as MurmurHash and CityHash are not secure for Internet usage, due to a certain type of DDos attack ...
2k views

### 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 ...
232 views

### Why is Blum-Goldwasser not IND-CCA-2 secure?

Consider the Blum-Goldwasser encryption scheme as described on Wikipedia. I was told that it was not IND-CCA-2 secure. I heard there was malleabilty, probably it has to do with XOR-ing. But I do not ...
1k views

### How does Output Feedback mode use the initialization vector?

How can I process Initialization Vector inside the Block Cipher Encryption box (using for example AES) together with the Key? Will I encrypt the Initialization Vector like a block of plaintext? So ...
2k views

### Use of CBC-AES-256 to encrypt usernames

I came across some code in our project that encrypts usernames (limited to 50 chars, most are less than 10) using the following parameters: IV size is 16 bytes Key size is 32 bytes Algorithm is ...
508 views

### AES encryption with shared IV

Sorry about the horrible title, didn't now what to say. I'm trying to avoid having to send an IV with every message purely to keep packet sizes down, so I thought of a method to establish a "IV ...
544 views

### Number of states in a LFSR

Do all $2^{\ell}$ (where $\ell$ is the bit length of the shift register) states always occur in a LFSR or can I choose my taps badly so some states are skipped and the period is shortened? If so is ...
2k views

### Cryptanalysis of Linear Feedback Shift Registers

It is well known that simple m-sequence linear feedback shift registers have a linear algebraic structure and therefore the generator seed can easily be deduced using the Berlekamp-Massey algorithm. ...
283 views

### LFSR dynamic mutation

In normal LFSR, the state is a function of the initial seed, taps positions and time, nothing else. I've seen a modification of LFSR that works like this: ...
761 views

### Linear Feedback Shift Register Taps

Linear feedback shift register tap charts are availale for registers of length 3 to 168. Does anyone have a chart for register lengths from 168 to 256 or beyond? Thank you.
579 views

### How does the cyclic attack on RSA work?

I am trying to get the idea of cyclic attacks againts assymetric RSA encryption. Taken from Handbook of applied cryptography . Let $k$ be a positive integer such that c^{(e^{k})} = c\mod n ...
925 views

### How to break AES CBC with PKCS#5 padding?

If I have a string encoded with AES CBC with PKCS#5 padding, how can one possibly decrypt/crack it? What are the approaches that one can take to break its security?
172 views

### ASCII Message in TV show? [closed]

So I was watching a popular series tonight (Person Of Interest) when suddenly during a frame change I noticed a very quick blue screen with writing. At the time I never thought anything of it as it ...
2k views

### Why programming languages don't provide simple encryption methods?

In c#, there is a namespace called System.Security.Cryptography and there are many classes in this namespace, which help developers encrypt and decrypt data. ...
823 views

### Is AES reducible to an NP-complete problem?

Is breaking AES NP-hard? Can the security of AES be reduced to a NP-complete problem? If it is reducible, what does the reduction look like? If it is not reducible, why do we assume it is secure?
3k views

### How and why can a decryption program tell me that a key is incorrect?

I have noticed that some programs used for file encryption will tell you if an entered key is wrong when you try to decrypt. It seems (to me at least) that this would mean that the key somehow is ...
374 views

### Measuring Shannon's diffusion

Shannon's idea of diffusion is fundamental to cryptography. Besides being a descriptive idea, is there any work on measuring or expressing it? Saying something like "System A has more diffusion than ...
2k views

### Can one reduce the size of ECDSA-like signatures?

Using $n$-bit ECDSA, a signature has a size of $2·n$. It is possible to recover the public key from this signature, which shows that there is a publicly visible redundancy in the signature. Is ...
370 views

### Is semantic security important in a hybrid cryptosystem?

RSA doesn't provide semantic security when used unmodified, and neither does the commonly used PKCS#1 v.1.5 padding scheme for encryption. Is this a problem for hybrid cryptosystems at all? My ...
237 views

### MD5 implementation

As a personal project, I want to implement MD5 on an FPGA, but I have some doubts about the specifics of the implementation. My first source of how the algorithm is implemented was the RFC 1321, where ...
162 views

### Can the hash of one message be used to make it easier to find the hash of a very similar message?

Background: I am trying to get an understanding of using a hash of a passphrase as a secret. Example: ...
215 views

### Request for 1024-bit primes $p$ , subgroup $q$ and subgroup generator $g$

I need to find a prime $p$ of $1024$ bits with a $160$ bit sub group size $q$, such that $q|p-1$ , and $g$ is the generator of the sub group size $q$. I'm looking for the numeric values of $p$ , $q$ ...
976 views

### Can we replace the XOR operation in DES with some other operation?

Can we replace the XOR operation in the DES algorithm with some other operation? If so, does it work for both encryption and decryption?
198 views

### Name for identical operations for encryption and decryption

Is there a name for the property of a cryptographic algorithm that the operations for encryption and decryption are identical, i.e. $E_k(x) = D_k(x)$ and accordingly $E_k(E_k(x)) = x$? An example ...
179 views

### How to bring little changes in DES algorithm

I thought of implementing DES modes of operation as a project...But my guide asked to change or to modify the DES algorithm in such a way to overcome drawbacks of DES...So is there any way to do ...
350 views

### Two step encryption

Is there any asymmetric cryptography algorithm which will allow recursive encryption. ...
1k views

### What are rounds in the context of ciphers?

Another recent question on security margin included in its answer the discussion of how many rounds are broken so far. What is the definition of a round?
3k views

### How to check the strength of an encryption algorithm?

I asking for advice on how to check strength of an encryption algorithm.
472 views

### What is 'security margin'?

I have been academic papers about Rijndael, Serpent, and Twofish, and there is this term that is vague to me. I cannot find a tangible definition in google. Can someone briefly define, describe, ...
984 views

### Hashing a password with SHA256 on the client then bcrypt on the server

I'm trying to implement a cryptographically secured storage site (not Mega, or anything similar) and am trying to prevent the user's password from ever touching the server. The password is used to ...
210 views

### Secure way to compare two secret numbers without disclosing them?

For example, two people want to know who is the older, without disclosing their age. Is there a secure way to do this without a third party involved?
373 views

### AES implementation in java that allows key of 320-bit length [closed]

I need an implementation of AES(Rijndael) in java that allows use of a 320-bit key. Please suggest a reliable implementation
117 views

### Tell me about non-MD5/SHA1 key fingerprints

I understand there are specifications for public-key fingerprints that use modern hash algorithms like SHA2. What are they?
241 views

### Obtaining a key that's using ice algorithm

I have a text file that has been processed by the SNOW steganography tool that uses ICE encryption. However, I do not know the key that would enable me to decrypt and retrieve the message hidden. ...
246 views

### What are alternatives to the random oracle model for modelling hash functions? [closed]

I was looking for more realistic alternatives to the ROM for describing hash functions in theoretical proofs. I came across the common reference string model (where hash functions can be modeled as ...
621 views

### RSA primes vs. largest known primes

In the context of a new largest (mersenne) prime number being found this week - The largest known prime number is now 2^57,885,161 − 1, and it took 5 years to find ...
3k views

### AES-CTR vulnerability to cryptographic oracle

I am new to cryptographic issues and from what I googled so far I could not retrieve the information I need. Consider the use of AES-128 in CTR mode. Let M be the set of possible plaintexts, for ...
429 views

### Source for examples with broken cryptography

I've heard again and again that many crypto systems have been broken in the past for one reason or another and that it is best to use one that has been peer reviewed, etc etc. However, I've yet to see ...
429 views

### FIPS 140 compliance for encrypted files

Does FIPS 140-2 require specific key sizes, or does it have specific requirements for the data? In other words, if I encrypt a file, how I can check whether it is FIPS-compliant or not? Does only the ...
175 views

### Is such a crypto-system available?

$E(k_1, pt) = c_1, E(k_2, c_1) = c_2, D(k_{new}, c_2) = pt$, where $k_{new} = f(k_1, k_2).$ Sharing $k_{new}$ and $k_2$ should reveal no information about $k_1$. Clarifications: Being able to ...
556 views

### What are some approaches to attack an triple key polyalphabetic cipher when the IV key and PT key are known?

The cipher is an triple key polyalphabetic cipher also known as an Quagmire IV by the ACA. An example would be here: Quagmire IV example and info (pardon the ads) I posses the IV key and the PT key. ...
Alice is communicating with $n$ people. We do not assume that the $n$ people don't trust each other or trust Alice. Alice doesn't trust them either, and needs to assume that they will not form a ...