# All Questions

454 views

### Is the 1st Encrypted Block Less Secure Than Subsequent Ones?

It's my understanding that Electronic Code Book (ECB) produces similar cypher text for similar plaintext inputs which is not a good thing. To get around that, Cypher Block Chaining (CBC) can be used ...
225 views

### Maximum length in bits of the product n=pq

If p and q are two 1024-bit prime numbers, would the maximum length in bits of product $n=pq$ be ${1024}^2$ or $2^{1024}$? I think it is ${1048576}={1024}^2$? Also, would the least significant bit ...
328 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 ...
331 views

### Sharing an encrypted file between a group of people

Is there any way to share an encrypted file with a group? In the other words, can I encrypt a file with a public key so that each person in a group decrypt it with his/her private key? Is there any ...
283 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 $x$ ...
787 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 ...
186 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?
7k views

### How to decode an OTP message?

First off, I'm extremely sorry if this comes off as a "homework dump", but to clarify, it isn't. I don't want the answer, I want to know how to get to the answer. My cryptography professor gave us an ...
324 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 ...
1k 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, ...
1k views

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 ...
610 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 ...
1k views

### Which attacks can be avoided by the use of OFB instead of ECB?

For a file encryption program, I was told to use Output Feedback mode (OFB) instead of ECB (Electronic code book) mode. Which attacks can I avoid by this choice?
346 views

### Sharing a secret key between many users

I would like to know if there is any security issue in sharing a secret key (of an asymmetric cryptography) between several users. I know that this secret key is no longer "secret", so it's no longer ...
122 views

### Loss of data due to encryption

Could any kind of encryption method (AES, etc.) lead to data loss like in lossy compression? I'm interested if data (lossy compressed, lossless compressed, uncompressed) that is encrypted and ...
883 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 ...
351 views

### Different padding rules for Merkle–Damgård and Keccak/sponge function

We learned that length padding was used in Merkle–Damgård where after padding with zeros another block is added that contains the initial length of the input. This is supposed to prevent same hash ...
145 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 ...
820 views

### 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 ...
879 views

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)?
343 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) ...
543 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 ...
106 views

### Is it safe to use XOR to encrypt data key using master key

I am implementing a hierarchical key system with a randomly generated master key and many randomly generated data keys that should be encrypted using the master key. I am wondering what the best way ...
128 views

### Is it possible to create “non overlapping” RNGs?

For example, let's say I want two cryptographically strong RNGs (R1 and R2). And I want them to have the following property: R1 (resp. R2) will never generate the same random number twice. R1 (resp. ...
84 views

### Concerns about use of MAC in my specification

Here is the pseudocode for a routine I am designing: ...
240 views

### Is encryption time greater than decryption time?

Is encryption time always greater than decryption time for all encryption techniques (AES / DES / RSA / DNA), or is there a mistake somewhere in my implementation?
143 views

### Are modified implementations of cryptographic algorithms a good idea?

Are modified implementations of standard cryptographic algorithms better that the standard versions? Since the algorithm is not standard, is it harder to break? Is it safer?
199 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?
232 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 ...
261 views

### Does MAC provide origin authentication? Why not just use symmetric encryption?

Let's assume Bob and Alice has a secret key for MAC. If MAC is constructed by the key and the message, that means that both Alice and Bob can construct the exact same message and MAC. So, Bob can ...
412 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 ...
226 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 ...
367 views

### why AKS is so slow in practice? [closed]

This note (http://maths-people.anu.edu.au/~brent/pd/primality4.pdf) states that AKS is not practical. However, it is known that AKS runs in polynomial-time, and I cannot understand where the slowness ...
178 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 ...
377 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 ...
350 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?
419 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 ...
512 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 ...
161 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 ...
820 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 ...
51 views

### Clarification regarding multiple modular exponentiation

If the base is same and exponents are different, for example: $R_1=b^x\bmod{p}$; $R_2=b^y\bmod{p}$; $R_3=b^z\bmod{p}$; ($p$ is large prime (2048 bit); $x$, $y$ and $z$ - 160 bit integers)) To ...
283 views

### Message exchange with textbook RSA

I am working on this exercise but can't work out the answer to the question b. I have solved the question (a) by decrypting the ciphertext $C = 2$ using the formula:  M = C^d \bmod n = 2^{17} \bmod ...
191 views

### Why can't we use a hash with no collision to compress data reliably?

According to wikipedia, a hash maps digital data of arbitrary size to digital data of fixed size. For all practical measures, a hash is a unique signature of a big chunk of data. But there is such a ...
406 views

### Trying to understand the use of ECC in TLS certificates

I was looking the certificate of this website: https://www.cloudflare.com that has an ECC based certification. I'm just curious to know if is possible to understand which elliptic curve is used and ...
129 views

### Severity of Cooking NIST P Curve Constants

Bruce Schneier and Gregory Maxwell have both stated that they believe the constants chosen for NIST's P curves (i.e. P-256r) are cooked. DJB has put together a detailed list of red flags but, outside ...
483 views

### What are words in a HASH algorithms

From wikipedia: "They differ in the word size; SHA-256 uses 32-bit words where SHA-512 uses 64-bit words." In this context what do they mean with words? is it the same as a 32 bit (or 64 bit) block ...
106 views

### How to attack the Two Time Pad?

I encrypt 2 random keys with the same 'one' time pad. You can get both encrypted keys. You don't know anything besides the size. Is there a way to attack the two time pad in this scenario? And what ...
178 views

### Cipher text only attacks on deterministic fully homomorphic encryption schemes

If we have encryptions of additive and multiplicative identities in the corpus of cipher text of a deterministic fully homomorphic encryption (FHE) scheme, I guess we can break it. If the FHE scheme ...