3
votes
2answers
2k views

How to perform file encryption using 128-Bit AES?

I am confused, how can I encrypt a file using 128 Bit Advanced Encryption Standard? Do I need only to encrypt the file name and it's content or is there something that I need to do to encrypt it? Is ...
3
votes
2answers
270 views

Feedback on rolling my own entropy gatherer

First of all, I don't recommend doing this. This was something I created when I didn't know better and didn't have a solution available to me. Long ago I created my own entropy gather for a ...
3
votes
2answers
278 views

Why not use an algorithm's code rather than data itself for one time pads?

Instead of using actual data for one time pads, what would be different if the code for an algorithm was exchanged so that to acquire the pad one would execute the algorithm and use its output? Why ...
3
votes
4answers
892 views

Webapp password storage: Salting a hash vs multiple hashes?

For security's sake, of course it's blasphemous to store passwords in plain-text; using a hash function and then doing a re-hash and comparison is considered much better. But, if bad guys steal your ...
3
votes
2answers
384 views

Are there public slow-but-strong algorithms out there that resist brute-force attacks better?

I'm reading that AES uses 4x4 bytes (4*4*8 = 256 bits key) matrix for performances matters (since it's a requirement for common standard encryption algorithms), but are there implementations with ...
3
votes
3answers
537 views

Is public-key cryptography the only option in this scenario?

Two parties: a client and a server are to a agree on a symmetric key. Both the client and the server are aware of a master password. The way this is currently done ...
3
votes
3answers
133 views

How useful is NIST's Randomness Beacon for cryptographic use?

NIST have just launched a new service called the NSANIST Randomness Beacon. It has been met with some initial skepticism. Perhaps the cryptography community would have used it before June 2013 when ...
3
votes
2answers
176 views

Avoiding overflow when encrypting with RSA

When encrypting with RSA one calculates $ m^e \pmod n $ by doing the following: m^e % n Where $m$ is what we encrypt. Often $e$ is a very big number to make it ...
3
votes
2answers
207 views

DES — Can I recover the key when I have both ciphertext and the plaintext?

Given a message and DES encrypted form of said message, is it possible to efficiently compute the key used to encrypt the data?
3
votes
3answers
289 views

Timing Attacks on ECDSA, ECDHE, AES and SHA2

Are there any known timing attacks (both practical and theoretical) on any implementations of the following? ECDSA (I'm aware of this one - are there any applicable to prime fields?), ECDHE (again, ...
3
votes
3answers
185 views

How to attack a fixed LCG with partial output?

To show some colleague programmers exactly how broken C's rand() is (at least on Windows) I decided to break it. So everyone knows the exact parameters, MSVC's ...
3
votes
2answers
968 views

Why is AES not a Feistel cipher?

I am studying for an exam right now. And I wanted to make sure I got this point correct. AES is not a Feistel cipher because the operations in AES are not invertible. Is the above statement ...
3
votes
2answers
406 views

How is the key shared in symmetric key cryptography?

Symmetric key cryptography is an encryption system in which the sender and receiver of a message share a single, common key that is used to encrypt and decrypt the message. Is the key public or it is ...
3
votes
2answers
140 views

Can you really insert the text you want in one-time pad?

The Wikipedia article "One-time pad ~ Authentication" says : For example, an attacker who knows that the message contains "meet jane and me tomorrow at three thirty pm" at a particular point can ...
3
votes
1answer
418 views

CBC-MAC , fixed length, all blocks returned

CBC-MAC, with fixed length message. Is it safe to return all ciphered blocks instead of the last? My intuition says it is less secure, since is gives an attacker more information. But how could one ...
3
votes
2answers
751 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 ...
3
votes
2answers
342 views

Theoretical pi-based stream cipher

Let's pretend that all digits of pi are known and arbitrarily long sequences of digits are trivial to get. Further, some mathematician proves that there are no patterns in pi. We could create a stream ...
3
votes
1answer
345 views

What's the main difference between Pohlig-Hellman and RSA?

Both Pohlig-Hellman and RSA perform encryption and decryption by exponentiation modulo some integer ($p$ prime for PH, $n$ composite for RSA). They both use a key $e$ as the exponent to encrypt a ...
3
votes
2answers
2k views

Is it possible to obtain AES-128 key from a known ciphertext-plaintext pair?

I have a file, which was encrypted with AES-128 in ECB mode. I know the format of the original file and know that all files in this format have the same headers. So, I have an encrypted block and the ...
3
votes
3answers
271 views

How to encrypt data and know it will be secure for at least a few decades?

I want to encrypt a small amount (few kilobytes) of data and be fairly confident it will be secure against brute force decryption and advances in cryptanalysis for at least a a few decades (or more). ...
3
votes
1answer
222 views

What is the name of this kind of protocol

There is a communication protocol that I believe creates the equivalent of a one time pad, with the downside that the secret message must be transferred multiple times. The protocol is so simple that ...
3
votes
2answers
220 views

Can a “pattern” in a series of passwords be detected from their hashes (and maybe a single raw password)?

Let's say I'm a lazy user of a system with annoyingly frequent password change policies. I may have a "good" password I used initially that is only used for this system but since I have to change it ...
3
votes
2answers
268 views

Is there any recent cryptographic algorithm especially designed for low-level processors?

Most modern algorithms require relatively large amount of resources. Is there any recent (and freely usable) encryption/decryption algorithm which is specially designed for low-level microcontrollers ...
3
votes
3answers
464 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 ...
3
votes
2answers
327 views

Key space size when either of two public keys are valid for authentication?

If for authentication a user can own either A OR B public key instead of just one specific key is that equivalent to halving the key space. i.e. it it theoretically twice as easy to brute force and ...
3
votes
2answers
1k views

Decrypting DES with decrypted and encrypted data

I got two 8-byte strings. One which is decrypted is: FF FF FF FF FF FF FF FF and one which is encrypted is: ...
3
votes
2answers
175 views

Protocol to generate Client Certificates at the start of a SSL session automatically?

A more secure form of 'cookie' could be created for SSL communications through the following method. The client generates and requests the server to sign a certificate. Then the client authenticates ...
3
votes
2answers
132 views

Do deterministic secret sharing schemes exist?

Are there algorithms for secure secret sharing such that the algorithm depends solely on the value being secured, relying on no randomness in its calculations. If there aren't, is such an algorithm ...
3
votes
1answer
189 views

Practical (and secure) PRGs

Does anyone know if there exists a PRG construction which takes as input an (RSA-sized) integer seed and outputs a fixed number of bits? There are number-theoretic PRGs such as the RSA, ...
3
votes
2answers
109 views

What was the problem in the SSL version 2.0?

What is the main reason (attacks) the appearance of the protocol SSL version 2.0 to replace version 3.0?
3
votes
2answers
362 views

In RSA, why is it important choosing e so that it is coprime to φ(n)?

When choosing the public exponent e, it is stressed that $e$ must be coprime to $\phi(n)$, i.e. $\gcd(\phi(n), e) = 1$. I know that a common choice is to have $e = 3$ (which requires a good padding ...
3
votes
2answers
302 views

Why is sharing the seed and using SecureRandom “deterministically” so bad?

I've recently been reading an article about "using cryptography to store credentials", and I would love it someone explained this in more detail… Up until Android 4.1, Android used a SecureRandom ...
3
votes
2answers
137 views

Isn't the deterministic property of (EC)DSA a security problem?

From what I read in Wikipedia's article "Elliptic Curve DSA ~ Security", the "deterministic" generation of $k$ can help prevent certain type of attacks (such as the one happened to Sony). However, ...
3
votes
1answer
113 views

Is this really the correct way to do an IND-CPA test?

In a description of IND-CPA (indistinguishability under the chosen plaintext attack), I have been reading the following, simple test: The adversary can generate as many messages as he wants. Then, ...
3
votes
1answer
101 views

How difficult is it to find the “pre-image” of a block cipher?

Say that I have $$ C_1 = AES_{k_1}(M_1) $$ How difficult would it be to find a key, K2 and plaintext M2 $$ C_2 = AES_{k_2}(M_2) $$ such that $$ C_1 == C_2 $$ How would using a block cipher ...
3
votes
2answers
81 views

Can CBC-encrypted files be modified in-place?

When changing a (small) part of the plaintext of a CBC-encrypted file, how much of the file will have to be rewritten? My suspicion is that everything after the modified block will have to be ...
3
votes
2answers
249 views

Are block ciphers used in public key crypto?

I was reading about block ciphers and most articles state they are being used in symmetric key cryptography. Are they also being used in public key cryptography? if not, what alternative pkc use? ...
3
votes
4answers
349 views

Tamper-proofing log files

Problem Overview I want to securely store log files so the contents are secret, and they can't be modified without detection. The files will be encrypted using authenticated encryption (AES in GCM ...
3
votes
2answers
334 views

Simple RC4 key generation scheme

I would like to ask you about your opinion on the following scheme of combining a master key and a nonce and stretching the result to the full 256-bytes RC4 key. Master key $K_m$ is a pre-shared ...
3
votes
4answers
415 views

Is there a practical security difference between XXX-bit encryption?

I know I'm treading in dangerous waters asking this - my comprehension of cryptography math is sorely lacking. On the flip side it gives me massive admiration for what many of you are able to do. ...
3
votes
2answers
425 views

Why does the recommended key size between symmetric and assymetric encryption differ greatly?

In various articles it is mentioned that for secure communications, the recommended key sizes are 128-bit key size for symmetric encryption (which makes it $2^{128}$ possible keys?) and 2048-bit key ...
3
votes
3answers
491 views

A proof-of-work random number generation system for Pokémon [closed]

(The original question that was here was considered too confusing, unclear, and rambling. You can still view it in the edit history, but the content is no longer useful by itself, and cannot be ...
3
votes
3answers
144 views

The meaning of “scheme”

This question is a bit different from other questions here, but I think it is suitable to correctly understand the terminology of cryptography. Consider the following two sets of terms: Encryption ...
3
votes
2answers
2k views

How many keys does the Playfair Cipher have?

I was just studying the Playfair cipher and from what I've understood, it is just a slightly better version of a Caesar cipher, in that it isn't actually mono-alphabetic but rather the 'digrams' are ...
3
votes
3answers
645 views

Is it possible to pick your Ed25519 public key?

Is it possible to generate an Ed25519 keypair that has a very similar public key as another keypair (fooling a casual visual comparison) or is this as hard as solving one of SHA-512 or the discrete ...
3
votes
3answers
2k views

Is it fair to assume that SHA1 collisions won't occur on a set of <100k strings

I'm building a system that has to take file paths, and generate a unique name for each one. I'm planning on using SHA1 as the hash function. My question is: do I have to deal with possible collisions ...
3
votes
3answers
278 views

How much bigger does a precomputed lookup table get when salt is added?

I am trying to wrap my head around the benefits of salt in cryptography. http://en.wikipedia.org/wiki/Salt_(cryptography) I understand that adding salt makes it harder to precompute a table. But ...
3
votes
2answers
434 views

How do public key rings work in cryptographic applications?

I am wondering if there are any links to articles or resources available online, or explanations that you can give, that would help me to understand the concept of a public key ring, and why I might ...
3
votes
1answer
1k views

RSA: If n=35, show that e will equal d

Show that if $n = 35$ is used as an RSA modulus then the encryption exponent $e$ always equals the decryption exponent $d$? What I have so far: $n = 35$ Therefore $p = 5$ and $q = 7$ or vice versa, ...
3
votes
2answers
774 views

Is it a good idea to use bitwise XOR on a set of MD5 sums?

I have designed an SQL aggregate function in Oracle that bitwise XORs all MD5 sums of the values stored in a column. For example, if my table is: ...

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