11
votes
5answers
691 views

How do I construct a 256-bit hash function from 128-bit AES?

I would like to generate a 256-bit hash on a microcontroller that has a 128-bit (only) AES engine. How can I construct a 256-bit hash function from a 128-bit cipher?
11
votes
2answers
477 views

Why use a 1-2 Oblivious Transfer instead of a 1 out of n Oblivious Transfer?

When initiating an oblivious transfer, why would someone use a 1-2 oblivious transfer rather than going for an 1 out of n oblivious transfer? Perhaps a slight time overhead for the extra message ...
11
votes
2answers
2k views

Is using a predictable IV with CFB mode safe or not?

While writing this answer, I noted that NIST SP 800-38A says that (emphasis mine): "For the CBC and CFB modes, the IVs must be unpredictable. In particular, for any given plaintext, it must not be ...
11
votes
3answers
1k views

How well does scrypt perform on different architectures / OSes?

The scrypt algorithm seems to be a prominent feature in the "CPU friendly" Bitcoin clones for the proof-of-labor part. I've heard claims that it's relatively slow on Windows and/or Intel compared to ...
11
votes
3answers
675 views

What is the best way to put a backdoor in an encryption system?

How can you put a backdoor into an encryption algorithm? Are there any techniques that can be used to reduce the time it takes to break a key? I am looking for practical examples encryption schemes ...
11
votes
1answer
571 views

Compressing EC private keys

For reasonable security, EC private keys are typically 256-bits. Shorter EC private keys are not sufficiently secure. However, shorter symmetric keys (128-bits, for example) are comparably secure. I ...
11
votes
1answer
743 views

Security of N bit HMAC

Lets say that I am using 128 bit HMAC. How many operations are needed to find a "non secure" message. Is a birthday attack possible?
11
votes
3answers
1k views

Are any of the major asymmetric ciphers distinguishable (EG, RSA, ECC)?

Related to "Is it possible to derive the encryption method from encrypted text?". Given ciphertexts generated by any of the major asymmetric ciphers (RSA, ElGamal, ECC, etc..) can these ciphertexts ...
11
votes
1answer
9k views

How is SHA1 different from MD5?

On the surface, SHA1 and MD5 look pretty similar. Their diagrams include chunks of bits, bit rotation, xor and special functions. Their implementations are roughly the same length (at least the ones ...
11
votes
1answer
2k views

Three-way hash collision

According to the birthday paradox we need approximately $O(|T|^{1/2})$ samples from the tag-space to find a collision for a hash function $h:K\times M \to T$. But how many samples are needed to find a ...
11
votes
1answer
8k views

Calculating RSA private exponent when given public exponent and the modulus factors using extended euclid

When given $p = 5, q = 11, N = 55$ and $e = 17$, I'm trying to compute the RSA private key $d$. I can calculate $\varphi(N) = 40$, but my lecturer then says to use the extended Euclidean algorithm to ...
11
votes
2answers
398 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 ...
11
votes
4answers
982 views

Can you make a hash out of a stream cipher?

A comment on another question made me wonder about something: Assume you're on a rather constrained platform — say, a low-end embedded device — with no built-in crypto capabilities, ...
11
votes
2answers
1k 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 ...
11
votes
1answer
7k views

How to choose a padding mode with AES

Depending on the framework you are using, there are various padding modes that can be used with AES encryption. For example, with .NET we can choose PKCS7, ISO10126, ANSIX923, Zeros or None. I ...
11
votes
1answer
644 views

What is the theoretical and practical status of mental poker?

I'm able to find a lot of scattered papers on the development of mental poker since RSA proposed the initial solution but no recent report (i.e. after 2005) on what is the status of the problem, eg: ...
11
votes
2answers
12k views

What's the fundamental difference between Diffie-Hellman and RSA?

What is the difference in the purpose of DH and RSA? Aren't they both public-key encryption?
11
votes
1answer
2k views

How to break an arbitrary XOR and Rotation based encryption?

I heard encryption based purely on XOR and Rotation is inherently weak. The paper Rotational Cryptanalysis of ARX says: It is also easy to prove that omitting addition or rotation is devastating, ...
11
votes
2answers
592 views

Proving knowledge of a preimage of a hash without disclosing it?

We consider a public hash function $H$, assumed collision-resistant and preimage-resistant (for both first and second preimage), similar in construction to SHA-1 or SHA-256. Alice discloses a value ...
11
votes
2answers
630 views

Why is the salt used only once in PBKDF2, while the password is used often?

The purpose of PBKDF2 is to create a derived key (DK) from a master password (PW) and a salt, often using a function like HMAC-SHA256. I have read that the salt should be as random as possible. But ...
11
votes
1answer
1k views

Duration for attacking Two-Key Triple-DES Encryption using all RAM ever built?

I am considering attacks on Two-Key Triple-DES Encryption assuming $2^{32}$ known plaintext/ciphertext pairs (that's a mere 32 Giga Bytes of ciphertext) by the method devised by Paul C. van Oorschot ...
11
votes
1answer
330 views

Choice of multiplication polynomial in Rijndael s-box affine mapping

The Rijndael specification details the design choices for the s-box in section 7.2. They describe the choice of affine mapping as follows: We have chosen an affine mapping that has a very simple ...
10
votes
5answers
3k views

Galois fields in cryptography

I don't really understand Galois fields, but I've noticed they're used a lot in crypto. I tried to read into them, but quickly got lost in the mess of heiroglyphs and alien terms. I understand they're ...
10
votes
4answers
1k views

What tests can I do to ensure my PRNG is working correctly?

In the past I have used the Chi-squared test to check the statistical randomness of my generator. Is this a good test to use? Are there other tests?
10
votes
6answers
948 views

Why does PBKDF2 xor the iterations of the hash function together?

The definition of PBKDF2 states that I obtain a derived key (1) by calling a pseudorandom function a bunch of times recursively: $U_1 = PRF(password, salt)$ $U_2 = PRF(password, U_1)$ … $U_n ...
10
votes
3answers
826 views

Entropy of two concatenated random values

In this Intel blog posting, the author claims: The amount of work required to brute-force predict a random value that has n bits of entropy is $O(2^n)$. If you concatenate two values together, the ...
10
votes
4answers
2k views

Is compressing data prior to encryption necessary to reduce plaintext redundancy?

As explained in William Stallings' Book, in PGP encryption is done after compression, since it reduces redundancy. I couldn't relate encryption strength with redundancy. Could anyone explain more on ...
10
votes
5answers
1k views

Why should I make my cipher public?

As I understand it, the less people know about the internals of my protocol or cipher, the more secure the protocol is. However Kerckhoffs's principle states that A cryptosystem should be secure ...
10
votes
3answers
2k views

In what way is XXTEA really vulnerable?

I'm looking at using the XXTEA algorithm to encrypt a small amount of data (say, less than 32KB) in the context of a software licensing algorithm. That is, we wish to make it difficult (not ...
10
votes
3answers
633 views

Bitcoin Research

I have recently been assigned to advise a student on a senior thesis in math. She has taken linear algebra, introductory real analysis, basic cryptography, and abstract algebra. Her interest is in ...
10
votes
3answers
5k views

What is the relation between RSA & Fermat's little theorem?

I came across this while refreshing my cryptography brain cells. From the RSA algorithm I understand that it somehow depends on the fact that, given a large number (A) it is computationally ...
10
votes
7answers
2k views

How exactly is “true randomness” defined in the realms of cryptography?

Especially in relation to stream ciphers, I frequently read about (sometimes theoretical, sometimes practical) attacks that are able to "distinguish a ciphertext from a truly random stream". What's ...
10
votes
4answers
1k views

Properties of PRNG / Hashes

There are a lot of quite elaborate PRNG's out there (e.g. Mersenne Twister et.al.), and they have some important properties, especially when it comes to crypto applications. So, I was wondering how ...
10
votes
3answers
2k views

MIT says: mathematical theory behind encryption is wrong. What are the consequences?

A friend shared with me the following link: Encryption is less secure than we thought I'm not a security expert and could understand great part the article, except the section about noise, but what I ...
10
votes
2answers
5k views

Are there any known collisions for the SHA-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 $a ≠ b$?
10
votes
2answers
777 views

Why RSA can't handle numbers above 76?

I'm going to encrypt the characters Zhu, and decrypt them using RSA. I'm using the public key $\{e, n\}$ and private key $\{d, n\}$. The values of $e$, $d$ and $p$ ...
10
votes
1answer
8k views

HMAC vs MAC functions

I've read definitions of MAC and HMAC, but can't say I've completely grasped the differences. What are principle differences? When to use one and when the other?(Typical Use Cases)
10
votes
4answers
10k views

What is the difference between known-plaintext attack and chosen-plaintext attack?

I am very confused between the concept of known-plaintext attack and chosen-plaintext attack. It seems to me that these two are the same thing, but it definitely is not. Can anyone explain to me how ...
10
votes
3answers
3k views

What is the most secure hand cipher?

By "hand cipher", I mean a symmetric cipher for which encryption and decryption can can both be performed with a pencil on graph paper, consuming about 10-20 seconds per character by a proficient ...
10
votes
2answers
4k views

Are there practical upper limits of RSA key lengths?

Suppose one wanted to use RSA encryption for the sole purpose of sending key bits for use in symmetric crypto systems, a dedicated key exchange system so to speak. And say you didn't think that the ...
10
votes
5answers
349 views

Tactics available to help prove security of a new system?

I believe that the accepted tactic to "prove" a system as secure is to allow the crypto-community to review it and if no vulnerabilities are found over a long period of time (5 or 6 years), then a new ...
10
votes
9answers
475 views

Physical analogue for MACs

What would be a good analogue with which to describe Message Authentication Codes to a person who has little to no understanding of cryptography? For instance, a vault is a reasonable analogue for ...
10
votes
2answers
2k views

How is XOR used for encryption?

I am a programmer, so when I hear XOR, I think about the bitwise operator (e.g. 0110 ^ 1110 = 1000). The mention of "XOR" comes ...
10
votes
1answer
11k views

How does RSA signature verification work?

I understand how the RSA algorithm works for encryption and decryption purposes but I don't get how signing is done. Here's what I (think) I know and is common practice: If I have a message that I ...
10
votes
3answers
1k views

PBKDF2 for key diversification

I am looking for a secure key diversification function to create individual AES keys for a local smart card deployment. The keys need to be derived from a secret master key and the smart card serial ...
10
votes
3answers
565 views

How to construct a good PRF from a block cipher?

We want to explicitly construct a good (as tentatively defined below) Pseudo-Random Function $F$ with $b$-bit input and output, from (preferably just) one Pseudo-Random Permutation $E$ of $b$-bit, as ...
10
votes
2answers
7k views

How does a chosen ciphertext attack work, with a simple example?

Can someone please explain - using a simple example - how a chosen ciphertext attack works?
10
votes
1answer
3k views

Why is it insecure to use a randomized IV for CBC-MAC instead of an all-zero IV?

A fixed length CBC-MAC uses an all-zero block as the initialization vector. Suppose that we used a randomized IV instead, and sent the IV along with the tag. So if the message $m$ will be $m = b_1 || ...
10
votes
2answers
3k views

How can I calculate the SHA-256 “midstate”?

Recently I've been trying to implement some Bitcoin-related code, and I've stumbled upon a weird concept, a SHA-256 "midstate". Some explanation is given here. The general concept is that Bitcoin ...
10
votes
3answers
487 views

Are these emerging threats against AES affecting your designs?

Recentally, an attack on AES was discovered which reduces its computationally complexity, by a very slight amount. The first key recovery attack on the full AES-128 with computational complexity ...

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