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4
votes
1answer
657 views

Is there are a metric based on collisions to compare bad hash functions?

Bad hash functions are not so perfect as in the "general collision probability" hypothesis... And a general concept of "collision resistence" not need the constraint of independence between the ...
14
votes
2answers
440 views

Does GCM (or GHASH) only provide 64-bit security against forgeries?

In a recent comment a doubt was voiced about my answer, which claims GCM to requires $2^{128}$ for a successful forgery. The doubt was that the square root needs to be taken meaning the security would ...
10
votes
2answers
4k views

How bad it is using the same IV twice with AES/GCM?

I understand that initialization vectors (IV) should not be used twice when using AES/GCM. I am using a counter as an initialization vector. Every time I send out a new packet (I am developing an UDP ...
7
votes
1answer
1k views

Solving Vigenère Encryption

I'm trying to crack a cypher that I believe is Vigenère encrypted and I'm currently stuck. I calculated the key length by finding repeated sequences in the cypher and calculating the the common ...
6
votes
3answers
2k views

A simple block cipher based on the SHA-256 hash function [duplicate]

I've come up with this little routine for doing encryption using the SHA-2 (in this case SHA-256) hash function. As such it is a block cipher with a 256 bit (32 byte) block size and an arbitrary key ...
81
votes
6answers
73k views

How can I generate large prime numbers for RSA?

What is the currently industry-standard algorithm used to generate large prime numbers to be used in RSA encryption? I'm aware that I can find any number of articles on the Internet that explain how ...
25
votes
5answers
46k views

What is the lowest level of mathematics required in order to understand how encryption algorithms work?

What mathematical fields of knowledge would be required in order to get a good understanding of encryption algorithms? Is it basic algebra, or is there a "higher education" mathematical field ...
33
votes
2answers
13k views

How does order-preserving encryption work?

Order-preserving encryption (OPE) is, apparently, a method of encrypting data so that it's possible to make efficient inequality comparisons on the encrypted items without decrypting them. I've been ...
29
votes
4answers
26k 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 ...
26
votes
1answer
35k views

What is Attribute Based Encryption?

Can someone explain what attribute based encryption is? I was searching for a book or something that can help me in this regard but so far I have found none. Google also returns practically nothing ...
30
votes
5answers
12k views

Any practical uses of machine learning for cryptography?

I am about to go study for my masters in machine learning, data mining and high performance computing, but have recently become very interested in cryptography after taking Dan Boneh's Cryptography ...
28
votes
2answers
12k views

Using the same secret key for encryption and authentication in a Encrypt-then-MAC scheme

Is it a weakness to use a single shared secret for protecting messages using a Encrypt-then-MAC scheme? Assuming a system is using AES-256-CBC and a SHA1-HMAC and the same secret key for both ...
41
votes
3answers
26k views

Why is public-key encryption so much less efficient than secret-key encryption?

I'm currently reading Cryptography Engineering. After giving a high level explanation of the difference between secret-key encryption and public-key encryption, the book says: So why do we bother ...
27
votes
5answers
46k views

Why is padding used for RSA encryption given that it is not a block cipher?

In AES we use some padded bytes at end of message to fit 128/256 byte blocks. But as RSA is not a block cipher why is padding used? Can the message size be any byte length (is the encrypting agent ...
48
votes
5answers
17k views

Who uses Dual_EC_DRBG?

Recent news articles have suggested that the NSA may be involved in trying to influence the cryptography in public standards or commercially deployed software, to enable the NSA to decrypt the ...
20
votes
3answers
7k views

What is the ideal cipher model?

What is the ideal cipher model? What assumptions does it make about a block cipher? How does it relate to assuming that my block cipher is a pseudo-random permutation (PRP)? When is the ideal ...
21
votes
2answers
6k views

Deriving Keys for Symmetric Encryption and Authentication

So here's the concept. Rather than storing 2 keys and using a random IV, which presents its own problems (key rotation, ensuring no key is used in more than 2^32 cycles, sharing the keys, etc), is it ...
29
votes
8answers
29k views

Simply put, what does “perfect secrecy” mean?

I would like to ask for a clear (but maybe not so deep) explanation of what the term "perfect secrecy" means. As far as I have researched and understood, it has to do with probabilities of assuming ...
21
votes
2answers
31k views

How can Cipher Block Chaining (CBC) in SSL be attacked?

I am trying to understand how CBC-mode in SSL/TLS can be attacked. I have been looking around online but all examples and explanations are very hard to understand and follow. Can you give a simple ...
20
votes
2answers
4k views

Using same keypair for Diffie-Hellman and signing

Are there any security risks using a single key-pair for both key-exchange and signing? I'm mainly interested in using Curve25519 for key-exchange and Ed25519 for signing. But similar combinations, ...
13
votes
2answers
1k views

How to derive two keys from one password

What is the best way to generate two independent symmetric keys from one user-entered password or passphrase? Would using both scrypt and pbkdf2 achieve this?
13
votes
2answers
5k views

Is there any strong enough pen-and-paper or mind cipher?

Some ciphers are talked about at “Is there a secure cryptosystem that can be performed mentally?”, but (at the time of writing) I don't see an answer. Are they strong enough, or are non-computer ...
19
votes
1answer
6k views

Could we break MD5 entirely in the future?

Even of today MD5 is (sadly) still heavyly used in some applications. Even big tools like ApacheMD5. But even today there are more then enough MD5 hashes which are still not cracked. According to ...
23
votes
1answer
20k views

Is AES-256 a post-quantum secure cipher or not?

We know Grover's algorithm speedup brute-force attacks two time faster in block ciphers (e.g brute-forcing 128 bit keys take $2^{64}$ operations not $2^{128}$). That explains why we are using 256 bit ...
19
votes
3answers
4k views

Plain text size limits for AES-GCM mode just 64GB?

Based on NIST SP 800-38D section 5.2.1.1, it seems that the maximum length of plaintext is 2^39-256 bits ~ 64 GB. We've got 100+GB files in genomics that need to be GCM encrypted so are concerned ...
11
votes
1answer
12k views

how to use common modulus attack?

I am struck with the following problem: Let Alice, Bob, Chris and Eve communicate over a public network. They encrypt all messages they send using RSA system. Bob and Chris have the RSA modulus $...
24
votes
2answers
9k views

Why is MixColumns omitted from the last round of AES?

All rounds of AES (and Rijndael) have a MixColumns step, save the last round which omits it. DES has a similar feature where the last round differs slightly. The rationale, if I recall correctly, ...
17
votes
2answers
4k views

Is there a feasible method by which NIST ECC curves over prime fields could be intentionally rigged?

The NIST elliptic curves P-192, P-224, P-256, P-384, and P-521, prescribed in FIPS 186-4 appendix D.1.2, are generated according to a well defined process, but using an arbitrary random-looking seed ...
11
votes
3answers
5k views

One time pad: why is it useless in practice?

The symmetric cryptosystem one-time pad (OTP) seems to be very beautiful since it is perfectly secret according to Shannon. Many books, however, point out the main drawback: one must create a secret ...
10
votes
5answers
2k views

How to prove the security of the PRNG?

Are there any realties tests or criterias that prove the security of the PRNG? What kind of tests or criteria?
10
votes
1answer
3k views

How to salt PBKDF2, when generating both an AES key and a HMAC key for Encrypt then MAC?

When using Encrypt-then-MAC with AES and HMAC by password, and given 128 bits of payload with the ciphertext to store a random salt, which would be more secure: Using PBKDF2 with then entire 128 bit ...
9
votes
1answer
3k views

Why can ECC key sizes be smaller than RSA keys for similar security?

I understand how ECC is based on the discrete log problem and RSA on integer factorization. I've read several references that show how a solution to either of these problems can typically be adapted ...
23
votes
2answers
4k views

Is H(k||length||x) a secure MAC construction?

If $H$ is a typical secure hash function, then $(k,x) \mapsto H(k \mathbin\| x)$ is not a secure MAC construction, because given a known plaintext $x_1$ and its MAC $m_1$, an attacker can extend $k \...
17
votes
3answers
14k views

Is AES in CBC mode secure if a known and/or fixed IV is used?

I have a need to encrypt credentials for a third-party app used by a secured internal app. Over on ITSec.SE, I was helpfully shown a scheme to encrypt the third-party credentials based on a hash of ...
16
votes
2answers
3k views

Can I select a large random prime using this procedure?

Say I want a random 1024-bit prime $p$. The obviously-correct way to do this is select a random 1024-bit number and test its primality with the usual well-known tests. But suppose instead that I do ...
20
votes
2answers
2k views

Is (AES-)GCM parallelizable?

I recentely faced the issue of random access decryption while AES-GCM was being used. I said this person that the underlying CTR should allow parallelization but I have no idea how authentication ...
10
votes
1answer
1k views

How random is the shared secret in the Diffie Hellman key agreement

How random is the value $ZZ$ in the DH protocol? This question was triggered by this somewhat naïve implementation in I2P shown by Sergei at Stackoverflow. Obviously $ZZ$ is distinguishable from a ...
8
votes
2answers
879 views

Is it okay to use an HMAC of the plaintext and a (possibly distinct) key as the IV for symmetric cryptography?

I was thinking of how to create an IV for a block cipher that doesn't require stored state, and I came up with the idea of using an HMAC of the (padded) plaintext and a (possibly distinct) key as the ...
18
votes
2answers
1k views

Can I use HMAC-SHA1 in counter mode to make a stream cipher?

Say I have an embedded device which is only capable of doing HMAC-SHA1 transformations (that device is, in fact, a YubiKey). Would it be secure to feed it like a block cipher in counter mode to get a ...
9
votes
1answer
1k views

In textbook RSA with low public exponent, how big does a random message needs to be?

Assume RSA with a public modulus $N$ of $n$ bits, a small odd public exponent $e$, plaintext $M$ a random non-negative integer less than $2^m$ for some integer parameter $m$, with $M\mapsto C=M^e\bmod ...
8
votes
2answers
3k 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?
6
votes
1answer
2k views

Bit Flipping Attack on CBC Mode

To perform a bit flipping attack, the previous block is modified by using XOR. This results in an altered plaintext. However, now the ciphertext of the previous block is altered, hence it will result ...
5
votes
1answer
1k views

Decrypting Ciphertext with partial Key Fragment using LFSR and Berlekamp-Massey

Edit Does the state register (the LFSR) always have to remain with 16 bits (I'm assuming yes). If so, are we shifting the register right by one (lfsr>>1) ...
3
votes
1answer
548 views

ECDH security when no KDF is used

Let's suppose our device performs ECDH with a fixed, unknown, private key $\text{prv}$. It accepts as input any point $Q$ lying in the proper subgroup of the proper elliptic curve, then computes: $P =...
134
votes
6answers
59k views

Why can't we reverse hashes?

First off, I know hashes are 1 way. There are an infinite number of inputs that can result in the same hash output. Why can't we take a hash and convert it to an equivalent string that can be hashed ...
88
votes
8answers
26k views

How is CipherCloud doing homomorphic encryption?

Much of the literature and latest papers suggest that homomorphic encryption is still not practical yet. How is CipherCloud able to achieve this? Does anyone have an idea? Their website does not ...
75
votes
9answers
105k views

Should I use ECB or CBC encryption mode for my block cipher?

Can someone tell me which mode out of ECB and CBC is better, and how to decide which mode to use? Are there any other modes which are better?
27
votes
4answers
26k views

Recommended skills for a job in cryptology [closed]

First let me apologize if this is an ill posed question. Let me also note that I do not in any way seek a comprehensive answer, simply your thoughts on what makes for a valuable asset to a company ...
84
votes
11answers
9k views

Is modern encryption needlessly complicated?

RSA, DES, AES, etc., all use (relatively) complicated mathematics to encrypt some message with some key. For each of these methods, there have been several documented vulnerabilities found over the ...
82
votes
5answers
35k views

Is AES-256 weaker than 192 and 128 bit versions?

From a paper via Schneier on Security's Another AES Attack (emphasis mine): In the case of AES-128, there is no known attack which is faster than the 2128 complexity of exhaustive search. However, ...

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