2
votes
0answers
181 views

john the ripper, usages [closed]

As evident from the site of john the ripper, JTR example, john can be used for finding password form a unix machine passwd file. can JtR be used to crack password ...
2
votes
3answers
141 views

Combating traffic analysis over request-response protocols

Suppose I am to design a request-response protocol (similar HTTP). For the sake of simplicity let us assume that this is a "chat" protocol where the client can only perform two actions: Contribute a ...
2
votes
3answers
458 views

Questions about BCrypt and PBKDF2

I would ask you some questions about users' password storage in a login system. What algorithm do you suggest me to use? bcrypt or PBKDF2? I was looking for a way to implement them client-side and ...
2
votes
2answers
188 views

How to construct a variable length IND-CPA cipher from a fixed length one?

Suppose we can construct a secure (IND-CPA) encryption scheme for fixed length messages. I was wondering if there is a natural way to extend this construction for messages of variable length such that ...
1
vote
5answers
815 views

Using one-way hash functions as the encryption method

Suppose two parties want to communicate securely with each other (Bob and Alice) using a simple messaging system in English. There are approximately 180,000 currently used words in the English ...
1
vote
4answers
662 views

Would this simple encrypted chat program be feasible using One Time Pads?

Lets say I want to build a simple chat program that has encryption that is impossible to crack for anyone, even a theoretical government with a massive super computer. Ok here's my idea. You'll need ...
1
vote
4answers
253 views

Does it make sense to have a 4000 bit long key?

My question is about the key length of a asymetric algorithm. How is it possible to memorize or remember a key that is about 4000 bits long?
1
vote
2answers
482 views

RSA problem if i choose two specific small prime numbers?

I know that rsa normally is made to make it with big integers, but i trying to make the algorithm myself (with the help of wikipedia) and I´m making it with small numbers to track the problems easily. ...
1
vote
3answers
233 views

One-Time Pad Distribution of Keys Between Two Parties

Consider the One-Time Pad (OTP). Suppose two parties, A and B, generate a completely random secret key in a real-life meeting, and they keep this secret. Now A wants to send some message to B using ...
1
vote
4answers
131 views

Are there use cases where a signature itself needs to be signed?

I'm writing some code to digitally sign certain files (JSON, if you must know), and I'm trying to understand the use cases, not being a crypto guy myself. I understand there are use cases where ...
1
vote
3answers
943 views

How to use HMAC-SHA1, HMAC-SHA2 in conjunction with AES

I understand how to compute HMAC-SHA1, HMAC-SHA2 etcetera. Please explain why/how is the resulting HMAC value is used in conjunction with AES for encryption/decryption and signing/verifying. In ...
1
vote
3answers
388 views

Can PBKDF2 be used to create an XOR cipher key to encrypt random plaintext?

I want to encrypt a truly random plaintext (a key file) based on a user password. I'll use PBKDF2-HMAC-SHA256 to generate an encryption key from the password, but I'm wondering if I even need to ...
1
vote
1answer
191 views

Why is “multiplying” $g^x$ and $g^y$ not possible?

The computational Diffie-Hellman problem states that for a cyclic group $G$ of order $p$ and a generator $g$, it is hard to find the value $g^{xy}$ given only $g^x$ and $g^y$ (but easy if either $x$ ...
1
vote
4answers
497 views

RSA with composite numbers

It has been said that RSA uses a modulus product of two prime numbers for security reasons. But does RSA even work correctly if we allow composite integers instead? I think that the answer is "NO". ...
1
vote
2answers
132 views

SHA-1 colission resistance proof

I'm searching Why the SHA-1 is collision resistance? Anybody could help me with the proof or say me where I can find this proof?
1
vote
2answers
173 views

What security flaw exists in MAC authentication scenario?

What security flaw exists (if any) in the solution of this non-encrypted authentication scenario? An unencrypted message passes one-way from computer A to B. Its transmission is viewable by the ...
1
vote
3answers
333 views

Does it make sense to block RSA keys under 1024 bits?

A paper I found (http://eprint.iacr.org/2012/064.pdf, which was submitted to a cryptography conference about two years ago), is based on the analysis of some 7.1 million 1024-bit RSA keys published ...
1
vote
3answers
850 views

Why crypto hash functions must be collision resistant and how to find resistant?

Why cryptographic hash functions must be collision-free and is there any methods to evaluate whether a function is not resistant to collision? Thanks,
1
vote
2answers
814 views

Custom crypto library in C

I will be making (in the near future) an app that will contain some pretty heavy crypto. As it is possible that my app will be ported to different devices (initially it will be iOs only, but Mac, ...
1
vote
4answers
830 views

“Padless” One-time-Pad encryption

I have just read about the perfect security of an OTP encryption and what came to my mind was that what if the Pad used for encryption/decryption did not have to be transported separately from the ...
1
vote
2answers
328 views

Is there an algorithm for factoring N, which is just as simple as this one, but faster?

I found a simple algorithm for factoring semiprime numbers, you can read about it in Factoring Semiprimes and Possible Implications for RSA. It basically works like this: You reverse the digits in ...
1
vote
1answer
4k views

How to solve MixColumns

I can't really understand MixColumns in Advanced Encryption Standard, can anyone help me how to do this? I found some topic in the internet about MixColumns, but I still have a lot of question to ...
1
vote
2answers
164 views

Is there a secret sharing scheme which allows sharing of a secret (one-out-of-n)

Is there a secret sharing scheme where the knowledge of just one share is sufficient to find the secret, in other words a (one-out-of-n) sharing scheme ? Plz I need to know if it is possible to make ...
1
vote
2answers
120 views

Parallel hash construction

I'm wondering why hash constructions aren't parallel like CTR mode for encryption? For example: We have ideal block cipher E, where we encrypt message block with counter and xor output block with ...
1
vote
2answers
525 views

AES key padding

Is the initial AES key (expanded to the key schedule) byte padded if less than e.g. 16 bytes? Is there a safe way of determining if decryption was successful (i.e. used the correct key)?
1
vote
2answers
251 views

Secure Hash Function based on AES

How secure would a hash function be which appends an extra block of 16 zeroed out bytes to the end of the message and then AES-encrypts it with a well-known password (say the first 128 bits of pi) ...
1
vote
3answers
307 views

How does one provide a secure and authentic communication channel?

Let assume two participants Alice and Bob who perform a given protocol which is a sequence of messages exchange between them. My question is: How can I provide a secure and authentic communication ...
1
vote
1answer
129 views

MAC using a modified CBC mode of operation

I'd like to know whether the following modified CBC-MAC is secure (given a fixed IV). Source: an old course on Network Security Are there some obvious attacks I can't see?
1
vote
2answers
142 views

Decryption issue with self-coded AES-128

I have written my own AES-128bit implementation in Java, till yesterday I thought it would be alright. (I was working on the encryption) Now I’m trying to decrypt my encrypted cleartext, but this ...
1
vote
3answers
243 views

random access stream cipher

Some time ago I was working in a pet project for a type of database engine. And I thought about securing the data in disk with encryption.. and since the main code was already developed.. I had a few ...
1
vote
3answers
268 views

Extracting only the entropy

(I cleaned up the question a bit, It was previously titled "extracting entropy without hashing" .. thus some of the comments / answers) I'd like to extract the entropy of data without injecting ...
1
vote
1answer
1k views

What is the meaning of “trapdoor” in cryptography?

I do not really understand the meaning of a "trapdoor" in cryptography, so here are my questions: What is the meaning of trapdoor and how can I convert a word or string using a trapdoor in ...
1
vote
2answers
210 views

How resilient to attackers with extreme resources available is this encryption method?

How could the following scheme be made more resilient to attacks capable of utilizing massive amounts of resources? Assume all random values are cryptographically secure. Key derivation: scrypt ...
1
vote
1answer
143 views

What parts of AES encryption result are safe to pass on the network

I'm using AES to encrypt the message. both sender and receiver know the passphrase, but only sender knows IV and salt I assume that receiver also needs to know IV and salt to decypher text; is it ...
1
vote
2answers
276 views

Help me describe/identify this challenge-response protocol/algorithm?

My area of expertise is reverse engineering, specifically embedded systems. I do attack cryptographic systems, but this largely involves key recovery or exploiting the implementation. I was asked to ...
1
vote
2answers
266 views

What's the difference between AES and earlier block ciphers?

Is there any difference except speed and key and block size from user's point of view? Or does AES have something that ideal block cipher doesn't have?
1
vote
2answers
205 views

ElGamal: Generation of “g” value?

I haven't been able to find a clear explanation on this (and I'm probably just confused in general). I have the following parameters of ElGamal signatures: $p$ = safe prime $q = (p-1)/2$ $g$ = an ...
1
vote
1answer
244 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 ...
1
vote
2answers
434 views

Tools for modelling and analysis of cryptographic protocols

I am designing some cryptographic protocols and I am new to it. Are there any well-known tools that can be used to model and design these protocols? And also verify or analyze their validity? If not ...
1
vote
1answer
130 views

Slow hash algorithm that does not require salt?

I usually choose bcrypt when hashing as I can set the work factor to slow down the calculations of the hash. However, I have to keep a copy of the hash in order to verify it (since it includes the ...
1
vote
2answers
183 views

Subgroups generators with respect to group generators of composite order

If I have a group $\mathcal{G}$ of order $N=npq$ and subgroups $\mathcal{G_n,G_p,G_q}$ of order $n$, $p$, $g$ respectively and if $g$ is a generator of $\mathcal{G}$ why then $g^{nq}$ is a generator ...
1
vote
2answers
451 views

Is Common Name encoded in the certificate?

When I make a certificate like so cd /etc/openvpn/easy-rsa/2.0/ source ./vars . /etc/openvpn/easy-rsa/2.0/build-key client1 Then ...
1
vote
2answers
155 views

How to encrypt a file for random access

I am a programmer familiar with C/C++ and Python. Lately, we got an assignment from a team asking us to work on one of their projects: we need to make a small program to encrypt a large file using ...
1
vote
2answers
188 views

Given $g$, $b$, $g^{ab}$, is finding $g^a$ a hard problem?

As in the title, given $g$, $g^{ab}$ are big elements in a prime group $Z_p$ and $b$ in prime group $Z_r$ ($p > r$, $g$ is one generator of $Z_p$). $a$ is unknown and also in $Z_r$, is finding ...
1
vote
1answer
320 views

Fastest random number generator

I just read High performance physical random number generator by K.H. Tsoi, K.H. Leung and P.H.W. Leong. It proposes a physical RNG of speed 400 Mbps. Is this the fastest random number generator at ...
1
vote
2answers
81 views

Preimage of SHA512 with a known suffix

Is it possible to figure out what $arg1 is if I know both $arg2 and $arg3 in addition to ...
1
vote
1answer
67 views

Is it secure to choose d in a RSA key pair?

An RSA key pair consists of the private key $(n,d)$ and a public key $(n,e)$ such that $de \equiv 1 \bmod{\lambda(n)} $. Usually one chooses a small $e$ and computes $d$ by inverting it modulo ...
1
vote
2answers
113 views

Standard or guidance for entropy collection

I need to write some high-level security requirements which include a random number generator suitable for use in cryptography. I don't need something very formal, this isn't a security certification ...
1
vote
2answers
168 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 - ...
1
vote
2answers
250 views

RSA performance

I was wondering what is the performance bottleneck in RSA algorithm? Is it the size of message, calculating the modular inverse, exponentiation? Say I have a fixed value of n to encrypt and decrypt ...

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