6
votes
3answers
282 views

Why is a MAC needed?

I agree that for certain encryption systems or modes of operation, a MAC is indispensible. The best example are probably stream ciphers (and therefore also block ciphers in OFB or CTR mode) that ...
4
votes
1answer
308 views

Memory-hard operations in work-factor hash functions

I'm playing around with work-factor hash functions, and I'm looking for a memory-hard operation to make it resistant to GPU / parallel hardware attacks. I considered a very large (i.e. 64K) s-box that ...
1
vote
1answer
151 views

Trying to find an algorithm to share portions of a key with multiple people

Sorry if this question's a bit basic, but I don't know of any way to ask it concisely enough for a search... What I'm looking for is a proven means of doing the following: Create x number of ...
4
votes
2answers
1k views

Why should the RSA private exponent have the same size as the modulus?

Consider the generation of an RSA key pair with a given modulus size $n$ and a known, small public exponent $e$ (typically $e = 3$ or $e = 65537$). A common method is to generate two random primes ...
2
votes
1answer
470 views

Is there a standard for OpenSSL-interoperable AES encryption?

Many AES-encrypted things (files, strings, database entries, etc.) start with "Salted__" ("U2FsdGVkX1" in base64). I hear it's some sort of OpenSSL interoperability thing a b c. Is there some ...
9
votes
1answer
3k views

How does a birthday attack on a hashing algorithm work?

A "normal", brute-force attack on a cryptographic hashing algorithm $H$ should have a complexity of about $2^{n}$ for a hash algorithm with an output length of $n$ bits. That means it takes about ...
7
votes
3answers
858 views

Is this a secure implementation of password reset email?

I am redesigning a password reset email mechanism because the existing implementation scares the hell out of me. My goal is to generate reset codes that are: Expired Tamper Resistant Single Use ...
12
votes
3answers
1k views

Can you create a strong blockcipher with small blocksize, given a strong blockcipher of conventional blocksize?

Suppose I want a strong 20-bit blockcipher. In other words, I want a function that takes a key (suppose the key is 128 bits), and implements a permutation from 20 bits to 20 bits. The set of ...
3
votes
1answer
350 views

Is truncating a hashed private key with SHA-1 safe to use as the symmetric key for AES for data at rest?

I realize this is mixing the purposes between asymmetric and symmetric crypto, but I was wondering if it is safe to use a hashed, truncated private key (asymmetric) as the symmetric key for encrypting ...
3
votes
2answers
281 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 ...
8
votes
2answers
2k views

Use of salt to hash a password

In a few implementations of hashed passwords, I have seen that the length of the random salt is chosen to be, say, 10 or "some constant". Is there any specific reason why the salt is chosen to have a ...
2
votes
1answer
328 views

Is there difference between Algebraic Homomorphic Encryption and Fully Homomorphic Encryption Schemes?

Is there difference between Algebraic Homomorphic Encryption and Fully Homomorphic Encryption Schemes?
0
votes
1answer
183 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 ...
3
votes
2answers
1k views

How can I break REDSHIRT / REDSHIRT2 encryption?

Recently, a user on Gaming.SE asked a question about whether the user password in the video game Uplink could be modified after being initially set. The game does not contain an option to change the ...
1
vote
1answer
330 views

Other than brute force, are there any attacks on Threefish-512 using only a single known plaintext block?

As per title, other than brute force, are there any attacks on Threefish-512 using only a single plaintext block? Are there any attacks like this in any other cipher?
4
votes
4answers
1k views

What is an efficient random number generation algorithm

I have been looking for the algorithm that generates random number and this algorithm has to be more secure. I am going to use this algorithm to generate the salt that will be used in PBKDF2. ...
7
votes
2answers
571 views

Is bcrypt better than GnupPG's iterated+salted hashing method?

GnuPG has slow hash built-in in form of iterated+salted S2K. Does it have disadvantages in comparance with bcrypt or scrypt? Is GnuPG's slow hash method easily automated in GPUs?
2
votes
4answers
329 views

Looking for cryptographic secure hash algorithm(s) that produces identical root hash for differently sliced hash list

I have a scenario similar to the one described in Wikipedia: hash list, but with a twist. I'm looking for a cryptographically secure hash function that would create the same root hash for the same ...
0
votes
1answer
2k views

Is MAC better than digital signature?

MACs differ from digital signatures in the sense that MAC values are both generated and verified using a shares secret key. Does this in any way put MAC on a disadvantage as compared to digital ...
2
votes
1answer
238 views

Conforming Randomness To An Alphabet

Imagine that we're trying to create a function to generate a random string conforming to a user-supplied alphabet. That way, users can generate random strings with given characters. Something like: ...
2
votes
1answer
333 views

Using “Additional Authenticated Data” as a secondary key

In implementing a cipher in GCM or CCM mode, you are provided the option to add "Additional Authenticated Data" (AAD). This AAD is required for decrypting the cipher text, and seems to be used when ...
2
votes
3answers
145 views

Is it possible to ensure security with zero pre-shared information?

Is it possible to secure a communications channel against both passive (sniffing) and active (injecting / MitM) attackers without either legitimate party knowing any pre-shared information? I know ...
2
votes
1answer
200 views

Signing 14 bytes of data

for an embedded device I need to sign a 14-byte string and want to verify that string on the device. Since there is already an AES-Library on the device, I thought about using the following scheme: ...
1
vote
1answer
332 views

RC4 S-Box and Keystream

I'm studying the RC4 algorithm and I have the following questions: On all questions assume that an expanded (2048-bit) key is used, and that the first 4096 bytes of the KeystreamIm are discarded. ...
1
vote
1answer
751 views
7
votes
1answer
2k views

why do we need Diffie Hellman?

my question from stackoverflow: http://stackoverflow.com/questions/11374592/why-do-we-need-diffie-hellman Diffie–Hellman offers secure key exchange only if sides are authenticated. for ...
4
votes
1answer
185 views

How to communicate authentication tag for GCM?

I have written some code to do AES in GCM. I currently manually append the tag property to the ciphertext, is this the proper way to communicate the authentication tag?
5
votes
1answer
1k views

Padding methods for block ciphers - PKCS7 vs ANSI X.923

I was looking through block cipher padding methods, and found two good candidates: ANSI X.923 - pad with zeros, then a final byte for the padding length, e.g. ...
8
votes
3answers
1k views

Modern integer factorization software

What are the modern software packages that can be used to factoring large numbers into primes. By modern I mean developed and made public within the last 5 years. I'm interested in things that are ...
10
votes
2answers
9k views

Signatures: RSA compared to ECDSA

I'm signing very small messages using RSA, and the signature and public key are added to every message, which requires a lot of space compared to the actual content. I'm considering switching to ...
1
vote
1answer
712 views

AES GCM implementation in c#

I am implementing an AES cipher in GCM mode in c# and have a few questions. My code is based on the code found here for reference. I'll copy in the relevant portions. ...
0
votes
1answer
135 views

Combatting traffic shape analysis with spurious packets

I was reading a question about combatting traffic analysis, and a thought occured. If I send random junk messages periodically, would that defeat traffic analysis? The messages would contain some ...
4
votes
2answers
2k views

RC4 Keylength Limits

While reading the Wiki page on RC4 I noticed that the key size must be in the range of 40–2,048 bits. Why is that? Is there a reason it can't have a lower or higher length?
17
votes
3answers
358 views

What differentiates a password hash from a cryptographic hash besides speed?

I understand that password hashes like bcrypt have the principal property of taking a long time to run, but I'm wondering what if anything about password hashes make them superior to merely running a ...
2
votes
2answers
267 views

Untraceable communication protocol

I am doing a research about secure communication protocols. I would be interested to know whether a protocol exists such that it grants that the two end-points taking part to the communication cannot ...
2
votes
2answers
311 views

Encryption with private key?

we normally always encrypt by public key and decrypt with private key. If i encrypt with private key, then its still secure as normal PKI ? i mean known-plain-text will not take private key on the ...
3
votes
1answer
354 views

RSA algorithm's license free or paid?

I checked RSA's patent application, which was registered in 1983. As patents don't last more than 20 years, it seems to me it should be free. But my friend said to use RSA I have to buy a license from ...
4
votes
1answer
302 views

Using a derived key for CMAC

Consider the following authenticate-and-encrypt scheme that uses AES-128 in CBC mode for encryption and AES-128 - based CMAC for authentication: Two keys are derived from the master key k (16 byte): ...
4
votes
1answer
1k views

Is this design of client side encryption secure?

I want to build a secure file storage web application. Users should be sure that server doesn't know how to decrypt files so encryption should take place at client side (i.e. in Javascript) and TLS ...
1
vote
0answers
181 views

Proper formatting of symmetric algorithm secret key

Given this description from RFC 4880 sec 5.1: The value "m" in the above formulas is derived from the session key as follows. First, the session key is prefixed with a one-octet algorithm ...
4
votes
3answers
1k views

Why does CBC decryption with a wrong IV still give readable results?

While developing some code that uses the .NET AesManaged algorithm, I made some mistakes but was surprised at the results. My encryption was correct. I was generating a random IV block and writing ...
7
votes
4answers
4k views

How can one securely generate an asymmetric key pair from a short passphrase?

Background info: I am planning on making a filehost with which one can encrypt and upload files. To protect the data against any form of hacking, I'd like not to know the encryption key ($K$) used for ...
4
votes
1answer
324 views

How can one share information using the 'host-proof' paradigm?

I am attempting to make a web-based secure password management and sharing utility, both as an academic exercise and to fully understand and feel safe about using it. I really like the idea of a ...
10
votes
2answers
842 views

Encryption scheme for social-network-like data sharing data via untrusted server?

I am thinking quite a lot lately abut the problem of secure, privacy-preserving social networking. Distributing the network among trusted, preferably self-hosted servers (like Diaspora, GNU Social ...
3
votes
3answers
412 views

Multiple Hash Functions that work in either nesting

Are there any hashing functions that, if two are used in conjunction (with the same salts) will return the same response regardless of ordering? I.e. are there hash-functions $H_1$, $H_2$ such that ...
5
votes
1answer
317 views

I need an opinion of encryption method I thought of in High school

First, I'm really not into cryptography, but have some basic knowledge. This was a thought experiment (and later exercise for my programming skills), but even though it was long time ago and I tried ...
2
votes
2answers
290 views

Is the Blum Blum Shub PRNG suitable to create initialization vectors?

Is it a good idea to use a Blum Blum Shub Generator to create my initialization Vector (IV) in AES-OFB (Output Feedback Mode)?
1
vote
0answers
128 views

Setting protocol parameters to achieve concrete security

Background One issue with modern security proofs is that they are usually asymptotic. In other words, such proofs are usually formulated as follows: For any polynomial-time adversary $\mathcal A$, we ...
4
votes
2answers
160 views

Does a high exponent compensate for a low degree of certainty?

If a RSA certificate is created with a low degree of certainty, does the value of the exponent compensate for this? I'm asking because certain implementations of key generation software hide the ...
2
votes
1answer
273 views

Vulnerabilities of encrypting data with known regularities

I need to encrypt data that has a pre-defined header and footer structure, and furthermore the data follows pre-defined patterns. The header and footer structure follow a defined structure but cannot ...

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