# All Questions

1answer
240 views

### Probability of Repeated Encrypted Block In Output Feedback (OFB) Mode

Is there any probability/chances of repeated encrypted block in Output Feedback (OFB) mode? Is there any existing documentation/studies about that? If it is existing, can you please provide me a link ...
2answers
3k views

### Replay attack in ECB mode

How do replay attacks work in ECB mode? I found this in Wikipedia: "ECB mode can also make protocols without integrity protection even more susceptible to replay attacks, since each block gets ...
2answers
1k views

1answer
1k views

### Generating a strong unique Initialization Vector

How can I determine if I am generating a unique and strong Initialization Vector? If my mode is generating Keystream? Is there any scientific explanation in generating a unique and strong ...
1answer
153 views

### Why does it matter for a signature scheme to be without random oracles?

There is a profusion of articles proposing signature schemes without random oracles (see for yourself). What does that mean, and why does it matter?
1answer
237 views

### Length of data to hash for PGP

I have finally managed to verify some simple PGP signed message blocks. However, I discovered that for some reason, my implementation limits me to verifying data that is 9-16 bytes long. no less. no ...
4answers
2k views

### How can I validate a hashed password if all I have is another hash?

The Scenario I have a client-side web application that bounces requests against a server-side API. For the sake of simplicity, every request must pass a username and password. This is similar to ...
2answers
241 views

### What is the advantage of an attacker over breaking a 4 digits PIN?

When a hardware system is protected by a 4 digits password, what is the advantage of the attacker into breaking that system? Isn't it $10*10*10*10=10^{4}$? If $\frac{1}{10^{4}}$ is the cost of such ...
8answers
5k views

### Is it possible to make time-locked encrytion algorithm?

I'm not sure if what I'm asking is even a valid question but here goes. Would it be possible to add a mechanism to an encryption algorithm that would mean it had to be a certain time of the day or a ...
2answers
163 views

### Abstracting primitives and modes of operation

I am developing a symmetric crypto library and have reached a roadblock. Looking at block ciphers, it is quite obvious that all block ciphers are trivially abstractable as a simple primitive ...
4answers
686 views

### Asymmetric algorithm to generate compact unique messages that can be validated

I have a cryptographic problem with the following characteristics: I need to generate a set of relatively short messages; say 20 bytes in length The contents of the messages themselves is not ...
2answers
1k views

### Capacity of Advanced Encryption Standard in terms of File Encryption

what is the capacity of AES in terms of File Encryption? is it really good to encrypt a large files in AES? ex. I am encrypting a 8GB of File... is it still good to used AES? is it still good to used ...
2answers
618 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 \tag{1}....
2answers
573 views

### Are Stream Ciphers Less Secure?

This is by no means a scientific observation, but it seems to me that stream ciphers receive a lot less attention than block ciphers. Is there any reason for this? (Is it because block ciphers are ...
2answers
11k views

### Are there any known collisions for the SHA (1 & 2) family of hash functions?

Are there any known collisions for the hash functions SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512? By that, I mean are there known values of $a$ and $b$ where $F(a) = F(b)$ and $a ≠ b$?
2answers
14k views

### How much computing resource is required to brute-force RSA?

It's been over 30 years since Rivest, Shamir and Adleman first publicly described their algorithm for public-key cryptography; and the intelligence community is thought to have known about it for ...
1answer
739 views

Can Anybody tell me why the expectation value of the Index of Coincidence is the following? For a cipher text string of length $L$, where $n$ is the number of alphabet characters, $k_r = 1/n$, and $... 1answer 173 views ### Does it make sense to use slow hashes in digital signatures? For storing passwords, it seems that the use of intentionally slow hash functions (like PBKDF2 and bcrypt) is now recommended. This is because most hashes are designed to be fast, which also makes ... 3answers 1k views ### Getting started [closed] I'm looking for a good place to start in cryptography and places to go to get free books etc on the topic. I have been looking online but I always get stuck at some point or another. I need something ... 2answers 240 views ### Byte location of plaintext vs ciphertext when using a block cipher or stream cipher I know that when you use a stream cipher you are just XOR'ing the plain text with the PRNG stream, such that you know if a specific byte is in position 3 of the plain text it will still be in ... 1answer 2k views ### What is the difference between these AES encryption methods I am using AES encryption (Rijindael) with Base-64 encoding in Obj-C and VB. I am currently using the following two projects to achieve this: Obj-C: https://github.com/dev5tec/FBEncryptor/tree/... 2answers 228 views ### Is there a way to compare the 923 bit pairing based key with RSA or AES, etc I've see many articles, most of them basically the same, praising Fujitsu for cracking what is referred to as a 923 bit pairing based encryption. I understand that in comparing RSA to AES you've got ... 1answer 740 views ### bcrypt - collision-resistance against chosen salt and work factor? Would it be difficult to find$\:$cost,salt,input,password0,password1$\:$such that bcrypt(cost,salt,password0,input) = bcrypt(workfactor,salt,password1,input) and$\:$password0 != password1$\:$... 1answer 3k views ### Use of nonces in Kerberos and Needham–Schroeder protocols Can you please explain me what is the reason of using nonces in the Needham-Schroeder and in the Kerberos protocols for Key Transport. In particular : Can you explain me the use of$N_A$? And ... 2answers 5k views ### What do the magic numbers 0x5c and 0x36 in the opad/ipad calc in HMAC do? Wikipedia lists the following pseudocode for HMAC: ... 3answers 3k views ### Do I have to recompute all hashes if I change the work factor in bcrypt? The well-known article about why we should use bcrypt for hashing passwords mentions the work factor - some parameter to the algorithm that determines how long one hashing should be in terms of number ... 1answer 401 views ### Chaum undeniable signature - justification for probability of misleading the verifier? Can anyone explain me in details why does the following statement hold true in Chaum and van Antwerpen scheme for undeniable signature? The probability that a dishonest signer is able to ... 4answers 6k views ### What is the recommended replacement for MD5? Since MD5 is broken for purposes of security, what hash should I be using now for secure applications? 1answer 4k views ### Why must IV/key-pairs not be reused in CTR mode? Many sources mention that IVs must not be reused with the same key in CTR mode, for encrypting 2 different pieces of data, because that totally destroys security - but I haven't found an explanation ... 2answers 667 views ### How can I solve the discrete logarithm modulo 2q+1 if I can solve it in the subgroup of order q? As part of my cryptography course I came across an exercise that neither me or my friends could figure out. The problem statement is as follows: Let$p$be a large prime of the form$p = 2q + 1$... 1answer 1k views ### Partial collisions for md5 Let$h$be a bitstring and let$P(h, n)$be the n-bit prefix of$h$. A partial collision of length$n$, for a hash function$H$is a pair$(x,y)$, such that$P(H(x),n)=P(H(y),n)$. What is known about ... 1answer 245 views ### RSA Signature - Multiple Use Weakness I cite from Fundamentals of Computer Security (Chapter 7 on Digital Signature, Paragraph 7.3 on RSA Signatures, page 289): Multiple uses of the RSA Signature scheme tend to weaken it. The way out ... 2answers 565 views ### Are there standards for transport layer security using only symmetric keys? I have an embedded system with AES-128 implemented in hardware. There is very little flash/RAM and the (8-bit) CPU runs relatively slowly. Public key crypto isn't a viable option. This system is ... 1answer 267 views ### Why are there$ signs in my passwd file?

I am trying to get access to my eReader and I managed to get the passwd file. ...
2answers
668 views

### Is ECB mode secure if plaintexts guaranteed to be unique?

I've got a scenario where I need to encrypt many small (16-byte) plaintexts. I want to use AES-128 in ECB mode. Notably, each plaintext is guaranteed to be unique, though each may differ by only a few ...
2answers
170 views

### Can you distinguish encryption methods from the output?

Suppose that you see all IP communication packets between two computers for a day, as they pass through a connecting router, and this amounts to 10MB. You additionally know that all data transmission ...
1answer
363 views

### Does Identity-Based Encryption actually solve any problem?

Identity based encryption schemes [*] seem to have great potential in high-latency Delay-Tolerant and mobile, ad-hoc networks since they apparently seem to avoid the need for key negotiation and ...
2answers
3k views

### Digital Signatures, Standard Hash Functions and MACs

I'm studying Hash functions and Digital Signatures in sequence, and came up with some doubts about their usage. First of all: What is the difference between hashing a document and signing it? And ...
1answer
660 views

### Goldreich-Goldwasser-Micali Construction

The Goldreich-Goldwasser-Micali construction allows to build a (cryptographically secure) pseudo-random function from of a (cryptographically secure) pseudo-random generator. More formally, let \$G: \...
1answer
534 views

### Is symmetric encryption vulnerable to plain-text-attacks?

Imagine that Bob sends a message to Alice for symmetric encryption to send to Charlie. (Only Alice and Charlie know the key.) Alice sends the encrypted message back to Bob to send to Charlie. Can Bob ...
3answers
310 views

### Are there any differences between PIR, oblivious transfer and differential privacy?

I am trying to make a taxonomy of different purposes of some cryptographic protocols. Generally speaking the purpose of PIR, oblivious transfer and differential privacy it sounds like being invented ...
3answers
2k views

### Is this password migration strategy secure?

I want to upgrade the security of some existing databases of users' authentication tokens strictly for the purpose of making sure that if the database is stolen, attackers will not be able to guess ...

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