2
votes
2answers
145 views

GGM PRG construction: Why do we need to change keys all the time with underlying PRG

When constructing a PRF that has n bit input using the GGM PRG, why do we always have to recursively run the PRG using its previous output as a seed key n times. Instead, why don't we run the PRG n ...
2
votes
1answer
192 views

How does the birthday attack work in AUTH and UF-CMA games?

In the AUTH and UF-CMA games, an adversary is required to forge a ciphertext or message/tag pair to win the game. Given an encryption scheme $E$ and a PRF $F$, let $\hat{E} = C || F_k(C)$ and $C = ...
8
votes
1answer
508 views

Are there any authoritative definitions of “key stretching”?

This is mostly a terminology question, but I suppose that it is best asked and answered here. After browsing the Internet I have come across a fair number of completely different definitions of the ...
0
votes
1answer
142 views

Can one detect if two pairs of elements in Zp have the same exponential relation?

Suppose that $p$ is a safe prime of 2048 bits ($p = 2q + 1$, and $q$ is prime). Suppose that one is given two pairs $(x_1, y_1)$ and $(x_2, y_2)$ such that: $y_1 = x_1^{r_1} \pmod p$ $y_2 = ...
5
votes
2answers
4k views

In RSA, do I calculate d from e or e from d?

In the original paper introducing RSA, it is implied that one should first choose $d$ and then calulate $e$ from $p$, $q$, and $d$. However, I have found in other places (such as the wikipedia article ...
1
vote
2answers
275 views

Amateur question: two-way RSA?

Is this a sensible scheme for 2-way client/server communication? A client connects to a server. The server and client both generate RSA keys, and send over the public ones to each other. If the client ...
1
vote
1answer
175 views

Preimage resistance hash in digital signature

I'm studying about preimage resistance property of the hash functions. In particularly I'm reading as the missing of this property can be fatal in digital signatures that use RSA. Further details: ...
1
vote
0answers
70 views

Why is it impractical to generate a semiprime dictionary? [duplicate]

This might be a very simple question. However, I am just learning the concept, so just excuse me. I am wondering why there is not any attempt to generate all semiprime numbers? (as an dict. attack to ...
2
votes
1answer
455 views

length extension attack on an MD5 hash of a text file of about 10K Bytes?

How much computing time on a typical desktop computer would it take to find a new length and new data to extend a text file of about 10K Bytes with a given MD5 hash?
5
votes
1answer
206 views

Can Poly1305 be used with block ciphers running in CTR mode?

Is the use of Poly1305 limited to stream ciphers? (note, I'm not talking about Poly1305-AES )? Can it be used with block ciphers running in CTR mode? If so, what other considerations/limitations are ...
4
votes
1answer
340 views

Given a private RSA key, how do we get the public key?

Is it possible to pre-choose a private RSA key, then obtain a public key from it?
3
votes
1answer
441 views

Difference entity authentication and implicit key authentication

From the Handbook of Applied Cryptography, in discussions of key sharing algorithms, I see definitions: Implicit key authentication is when one party is assured that no other aside from a ...
2
votes
1answer
815 views

Finding CRC collisions for specific divisor

My current textbook (Information Security: Principles and Practice by Mark Stamp) discusses how to determine the CRC of data via long-division, using XOR instead of subtraction to determine the ...
1
vote
2answers
113 views

Differential privacy definition

Differential privacy defines "privacy" of a mechanism $A$ as the "closeness" of the two distribution $Pr[A(D) \in S]$ and $Pr[A(D') \in S]$ where $D,D'$ differ in one element. And the distance between ...
3
votes
1answer
108 views

Can substrings of a long string be efficiently authenticated?

Suppose one has a (strong) shared secret key and wishes to authenticate a very long string in a way that will allow the verifier to verify small substrings without reading much of the stored data and ...
1
vote
1answer
125 views

What kind of cryptography should i use?

I have a trusted third part A that issues an access token (xml file) to an untrusted client C that uses this token to log into an untrusted server S and access to the authorized files. I want only ...
5
votes
3answers
316 views

What are some good references for the implementation weaknesses in RSA

I'm pretty sure I understand textbook RSA. Choose p and q, large primes, and compute n=pq. Choose e such that $gcd(\phi(n),e)=1.$ Publish n and e. Compute d such that $de=1$ (mod $\phi(n)$). To ...
3
votes
1answer
193 views

What's efficient MPC protocol for determining if sum's bigger than y?

My secure multi-party computation (MPC) in need is simply to determine if a sum of two private variable is bigger than a given value $y$, as $f(x_0, x_1) = [(x_0 + x_1) > y]$ in which the value ...
3
votes
1answer
420 views

Can you identify the public key used to encrypt something?

If I encrypt a string with a public key, does the encrypted ciphertext reveal the public key I used to encrypt it? Basically, I don't want anyone to know who the ciphertext is addressed to. I'm ...
1
vote
1answer
2k views

How long to bruteforce a RSA key [duplicate]

Suppose I have a 2048 bit RSA public key, and want to brute force the corresponding private key. I guess there are 2048^16 possible combinations? How long would this take me to brute force with an ...
6
votes
2answers
333 views

When using HMAC, does key encoding matter?

Given: H is a good hash function with block size L. K is a key of length >= L (recommended by RFC 2104). Khex and Kbase64 are ASCII encodings of K. In the HMAC algorithm, is there a good reason to ...
-1
votes
2answers
336 views

Is triple des similiar to RSA in that they message size is limited to the key size?

Is triple des similiar to rsa in that the message size you can encrypt is limited (unlike AES)? Yes you can break the message size into parts and apply it, but I'm not interested in doing that so I ...
4
votes
1answer
75 views

“proof of access” schemes

What is the state of the art way when implementing a scheme challenging a party to prove they have access to certain data? What I'm looking for something along the lines of Give them ...
0
votes
1answer
133 views

Trying to finalize my understanding about password storing

am sorry, an a beginner, so please forgive me if i make mistakes, so here is what I understood: PBKDF2: uses HMAC, so, the message is the password, and the key that crypts the message is the salt? or ...
2
votes
2answers
144 views

How to compare two datasets „anonymously”?

Ok, I hope this question makes some sense because I am not so sure how to word it any differently… Imagine the following situation: There are 10 defined colors (blue, orange, yellow etc.) There are ...
3
votes
2answers
180 views

Is this OTP scheme safe?

I have a message $m_1$ and I want to encrypt it by XORing it with two keys $k_1$ and $k_2$: $$c_1 = m_1 \oplus k_1 \oplus k_2$$ So far so good. Now I was wondering if I could create a "fake" result ...
1
vote
0answers
71 views

Password checking algorithm

I'm trying to reverse engineer key exchange protocol and faced with the following problem: Protocol is based on Diffie-Hellman key exchange method: g - generator m - modulus Alice: a - private key $A ...
3
votes
2answers
397 views

What does “message schedule” mean in SHA-256?

I am trying to understand the sha-256 algorithm from FIPS 180-2. I understood the padding and parsing of the message string. However after that it states (page 15): For $i = 1$ to $N$: { ...
3
votes
3answers
305 views

Block cipher fixed points

A block cipher is a bijective map from the set of possible plaintexts to the set of ciphertexts, which are the same size and might as well be considered the same thing: $\theta: S\to S$. In this there ...
1
vote
2answers
102 views

Prepend information about encryption implementation to the stored data

I want to preface this by saying i will not be using any of this code/information in a live project, this is only for learning/fun (so I welcome some speculation) I was looking at how the PHP crypt() ...
2
votes
1answer
2k views

Bcrypt VS Scrypt, Again?

I found this article on SO, but the answer was short, and the subject was closed, What i don't understand, is: Which one of the two limits memory usage to avoid custom Hardware to break the password? ...
1
vote
2answers
145 views

Does collision resistance stay when extending a hash function to a set domain?

Given a Cryptographic hash function $h$ for element $x$, let's extend it to sets via $H(S)=\prod_{x\in{S}}{h(x)}$. I am asking if the new hash $H$ (in domain of set) is still collision resistant? To ...
1
vote
0answers
70 views

Will D-Wave's quantum computers ruin classical encryption? [duplicate]

D-Wave has commercially available 512-qbit quantum computers now. A lot of big names are taking it seriously. Google, NASA, and USRA have joined forces to start a quantum AI lab. How far are we ...
3
votes
1answer
217 views

In ECDSA, how many field operations are used for signature verification?

I am wondering about the computational cost of ECDSA signature verification, in term of multiplications in the base field; and, as an aside, in term of (much cheaper) additions. To make things ...
8
votes
1answer
209 views

Why use $(r,s)$ instead of $(r,s^{-1})$ as DSA signature?

A DSA signature consists of two scalars $(r,s)$. When signing $s$ is generated as: $s=k^{-1}(H(m)+xr) \mod q$ The signature is $(r,s)$ When verifying $s$ is only used to compute $w = s^{-1}$. So ...
5
votes
1answer
325 views

What do recent announcements about solving the DLP in $GF(2^{6120})$ mean for RSA

After just reading the post Do recent announcements about solving the DLP in $GF(2^{6120})$ apply to schemes proposed for cryptographic use? I was a bit confused. DSA, ElGamal and others are based on ...
9
votes
2answers
874 views

Why are bitwise rotations used in cryptography?

Any understanding I have of cryptography stops right around the cipher level. As such, I'm just curious as to why bit shifts and moreover circular bit shift are so prevalent in cryptography.
1
vote
1answer
119 views

How can I split a message in parts of similar size or smaller?

I have a 130-160 characters message that I need to split in say, 3 parts, and be able to reconstruct it by recovering all 3 parts. I also need that these parts are type-able, meaning that they can't ...
2
votes
2answers
257 views

IV Security Clarification

After doing lots of reading on SO and other websites relating to AES cryptography, I am trying to understand the security issues surrounding IV's. There seems to be a lot of confusion and ...
1
vote
1answer
394 views

When making public key fingerprints - is a sha1 hash still a good idea?

I'm thinking about trying to save some space (and readability) when referencing 2k and 4k public keys (millions of them) by storing the fingerprint in some places instead of the full public key. ...
3
votes
3answers
684 views

If RSA is limited to 117-200 bytes or so, is that a very limited use case?

Am I missing something, or is RSA very very limiting when it comes to ecrypting data when it comes to the actual message size? I have read that you can only encrypt a message of around 117 to 200 ...
3
votes
2answers
480 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
1answer
44 views

Distributing blocks with validation and non-dependant list generation

Problem Suppose I have a system of nodes that can communicate with a parent node, but not among each other. Suppose then a file on the parent node is split up into blocks and divided among the ...
1
vote
1answer
387 views

How is text converted to a number for RSA? [duplicate]

According to http://en.wikipedia.org/wiki/RSA_%28algorithm%29#Key_generation the key length is the number of bits in n. So how can a message of many megabytes (millions of bits) be modded by a 1024 ...
2
votes
1answer
306 views

Encrypt-then-MAC paradigm

I read that the Encrypt-then-MAC paradigm is provably secure. From what I understand, when using for example AES for encryption and HMAC_SHA256 for MAC generation (and the keys $K_1 \neq K_2$), this ...
2
votes
2answers
238 views

HMAC and assumptions on the cryptographic hash

According to Wikipedia, a cryptographic hash function has the following properties: Pre-image resistance: Given $h$, it's difficult to find any message $m$ such that $h = H(m)$. Second pre-image ...
1
vote
1answer
234 views

ECC Point Multiplication of Product

I can calculate $Q = a\,b\,G$ in several ways: $Q = a \, (b \, G)$ or $Q = b \, (a \, G)$. These give the same result, as expected. But if I do $c = (a \, b) \bmod n$ where $a \, b$ is much greater ...
1
vote
3answers
1k views

Why crypto hash functions must be collision resistant and how to find resistant?

Why cryptographic hash functions must be collision-free and is there any methods to evaluate whether a function is not resistant to collision? Thanks,
1
vote
2answers
229 views

Is entropy affected by time?

I'm quite new to cryptographic systems, and today we discussed entropy at work. Since entropy is used to ensure the difficulty for an attacker to break in, I was wondering if the more time the high ...
1
vote
1answer
66 views

Why does the server in S/KEY authentication only store a single password?

I've been reading about the S/KEY One-Time Password system on wikipedia here and was wondering why the server only stores a single password and not the list of one-time passwords like the client does. ...

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