3
votes
0answers
126 views

Are there MD5 collisions for inputs of different length?

There are many examples of MD5 collisions (some of them can be found here Are there two known strings which have the same MD5 hash value?). But as far as I know two inputs should have the same length ...
2
votes
0answers
91 views

RSA: special parameters construction

I have to show that the following construction of $q$ allows to factorize $N$ : $p \in \mathbb{P}$, the set of primes $q \gets p \cdot \left((p - 1)^{-1} \bmod e\right) \bmod e$ $k=1$ while $q \not ...
0
votes
1answer
216 views

Is there a correct way to generate a symmetric key?

Looking in some cryptographic algorithms, I've realized that: The way the plain text is encrypted/decrypted is always specified, but what about the key? Every paper I've seen describing the algorithm ...
1
vote
0answers
159 views

Best way to implement secure client/server communication in Python

For a college programming assignment I have to implement a secure communication protocol between a server and one or more clients. Upon instantiation of the server program, a file is generated, call ...
2
votes
1answer
87 views

Key derivation design to avoid key leaks

I have the first function that generates keys from AES decryption (128-bit key). It takes seed as input: ...
1
vote
0answers
42 views

ABE Attributes Based Encryption

I have a question . How the system know the attributes of the users to be after that as a public key in ABE?
1
vote
1answer
176 views

SHA-256 “almost unique”?

I have seen numerous references on the internet of people describing SHA-256 as generating an "almost unique" hash. Exhibit A. there are more. Is there some mathematical basis to the almost ...
2
votes
0answers
321 views

Proper way to generate symmetric keys [duplicate]

I'm trying to educate my self in key management, how to store them, how to generate them, how to use them properly. I've been trying to find how you would generate a key for symmetric encryption (say ...
5
votes
0answers
225 views

How to break a columnar transposition cipher?

I'm currently studying for a cryptography exam. I've been given ciphertext that has been encrypted by a columnar transposition cipher. I've been given no shift key length or key word, the only thing I ...
3
votes
2answers
947 views

Generating unbiased numbers with a biased six sided die?

Eve takes a fair six-sided die and adds some heavy paint to the side of the die with the 6 on it. This results in a biased die that rolls a 6 with probability 2/7, and each other number (1-5) with ...
0
votes
1answer
43 views

What does CA certs contains? public key of site or CA?

I believe it contains public key but whose? the site like a bank or the certificate authority who has signed?
0
votes
0answers
22 views

Prove that complement of plaintext and the key, then ciphertext will be the complement of orig. ciphertext [duplicate]

I'm trying to prove in general terms the following: y = DESk(x) then.. y' = DESk'(x') I can plug in random variables for ...
1
vote
0answers
130 views

Weird base64-like (?) encoding [closed]

I have some encoded strings which are probably encoded in a base64-variant - except that there aren't only 64 chars, but all the (printable) ASCII chars between 33 and 126 (0x21 to 0x7E). I've ...
0
votes
1answer
231 views

Calculating the average key search time of DES

I am trying to understand how to calculate the average key search time given a specific scenario: Suppose we have a program that uses standard DES with 56 key bits, and we can test 10^6 keys per ...
0
votes
0answers
39 views

Indistinguishability through a chosen plaintext attack

I have a question about how to go about doing an IND-CPA style attack. I'm supposed to think of this as two black boxes, where I do not know which box contains which function. What would I do with the ...
1
vote
0answers
18 views

Understanding CPA and CCA Secure [duplicate]

CPA attack means the attacker can send in his own plaintext to the oracle and receives some sort of ciphertext back. CCA attack means the attacker can send in his own ciphertext to the oracle and ...
9
votes
2answers
348 views

How secure is a pronounceable password in terms of entropy?

There are some strong studies which support the use of pronounceable passwords and multiple tools which provide generation of such passwords. According to this question the entropy of a password ...
3
votes
2answers
628 views

Is it possible to apply hill-climbing algorithm to Diffie–Hellman?

If Eve intercepts all the values communicated between Alice and Bob in a Diffie–Hellman key exchange, and applies the hill-climbing algorithm to reverse the modulo function, how will the algorithm ...
0
votes
1answer
55 views

How can I prove I have a certain Facebook account by signature?

I would like to know how to prove I have certain Facebook account (or that I can login to that account).
2
votes
1answer
130 views

Bcrypt input length vs collisions

I am currently implementing oauth2 bearer token authentication and am storing the tokens bcrypt encrypted. My question is how long should the tokens be to achieve maximum security. E.g. at what length ...
0
votes
2answers
46 views

Certificate Path Verification of X509 Certificate

I have the trusted root certificate which I am validating using the self signed key. I also have the intermediate certificate, which i am validating using the root certificate. Now when i am ...
1
vote
1answer
59 views

Byte size of Diffie-Hellman public values

I have a scenario that uses RFC-3526's Diffie-Hellman Group 14 for 2048 bit key exchanges. From the document, it specifies the P (prime) value in hexadecimal value as: ...
0
votes
1answer
55 views

Is it safe to use MD5 in a proof of work system?

Here is the POW system: you are given a string and you need to compute a suffix so that first few bits of the MD5 value are all 0. I know MD5 is collision vulnerable, but is it safe to use MD5 here, ...
0
votes
3answers
97 views

Perfect hash function

Is a perfect hash function always bijective ? In Wiki "In mathematical terms, it is a total injective function" but it does not mean, that it is bijective.
0
votes
0answers
29 views

Is a timing side channel attack effective when the leaked information is related to the result of a hash?

I'm writing a function to act as a cryptographic hash of a number within a range that returns another number of the same range. The catch is that the result needs to have certain properties. I run a ...
2
votes
0answers
21 views

Does the length of the keyword used for keytable for Playfair matter?

So for Playfair, we choose a keyword and put that in the keytable with no duplicates. Then, we fill the rest of the table with alphabets in order that are not in the keyword without 'J'. Does the ...
1
vote
0answers
27 views

ephemeral-ephemeral ECDH and MITM attack [duplicate]

ephemeral-ephemeral ECDH is it susceptible to MITM attack ? if yes, can you please give me a detailed answer ( Workflow that can be followed by the attacker) and how we can prevent this attack ...
0
votes
2answers
72 views

hash functions preimage and second collision resistance [duplicate]

A hash function H is secure in terms of pre-image resis-tance. Does this imply that H has second pre-image (aka weak collision) resistance?
6
votes
3answers
195 views

Is there an upper bound to the private exponent in RSA?

In the RSA algorithm, we choose $p$ and $q$ as prime numbers and we select a value $e$ which is coprime to $\varphi(pq)=(p-1)(q-1)$. Then we calculate $d:=e^{-1}\bmod\varphi(pq)$. My question is: ...
4
votes
2answers
45 views

Does SRP also authenticate the server to the client?

The Secure Remote Password (SRP) protocol lets a client prove to a server that it knows a specific password without revealing that password to the server. The server stores a cryptographic verifier ...
0
votes
0answers
74 views

Perfect Secrecy of AES file Encryption

Why does AES file encryption not ensure perfect secrecy? I understand that $\Pr[E(K,M_1)=C_1] = \Pr[E(K,M_2)=C_2]$ given $M1\neq M2$ holds for perfect secrecy of a scheme. However, this seems to hold ...
2
votes
2answers
48 views

Is it secure to use TOTP with same key and different time-counters?

If I use TOTP to generate codes with different "expiration" time, using the same key for HMAC, will I create some kind of weakness?
1
vote
1answer
202 views

Is this variant of SRP for peer-to-peer authentication practical?

I'm interested in using a modified form of SRP as a peer-to-peer authentication method. Since neither side is acting as a host, one of the primary design goals for SRP (that the client doesn't need ...
1
vote
1answer
93 views

How is RSA able to prevent brute forcing using the public key?

Bob calculates a private and public key. Bob sends his public key to John. Jeff is a third party unwanted member and manages to snatch the public key mid-transfer. John encrypts his message ...
90
votes
4answers
57k views

Why is elliptic curve cryptography not widely used, compared to RSA?

I recently ran across elliptic curve crypto-systems: An Introduction to the Theory of Elliptic Curves (Brown University) Elliptic Curve Cryptography (Wikipedia) Performance analysis of identity ...
3
votes
2answers
111 views

Is reverse NTRU still secure?

I'm currently prototyping something with the NTRU encryption scheme but I wish to use it in "reverse" -- distribute private keys so anyone can decrypt, but keep the public keys secret and thus only ...
4
votes
3answers
138 views

Calculating $\mathbb F_{p^2}$-rational points of an elliptic curve defined over $\mathbb F_p$

How can I calculate points on an elliptic curve defined over $\mathbb F_p$, for example $y^2 \equiv x^3 + 1 \pmod p$, with coordinates in $\mathbb F_{p^2}$? (points might have complex number format in ...
8
votes
4answers
524 views

Can an encryption method be used as a hash function?

As I understand, there are a few requirements for a good hash function. Hard to find any message from a given hash Hard to find any 2 messages which give the same hash A single bit change in the ...
0
votes
2answers
179 views

How is message length involved in public/private-key encryption?

I've just read this question regarding why to sign only the hash of a message. I kind of understood the answer, but the question arisen is now: Why can I encrypt any text messages with the same ...
5
votes
1answer
90 views

How to exctract ECDH parameters from an OpenSSL-generated $G$?

I'm using ECDH for generating ECDH public parameters (p,a,b,G,n), I try to get this values using openssl ecparam -in cert.pem -text -noout For ...
1
vote
2answers
100 views

qkd - Does BB84 rely on a prearranged code?

First of all, sorry if the title is not that clear, I'll provide an in-depth explanation behind my question. So, basically I was reading about quantum cryptography and came across BB84. The protocol ...
2
votes
1answer
99 views

Stream Ciphers and cycling questions

For all PRNGs (including CSPRNGs) lets say there is a sequence x, y, z, then when the PRNG cycles back to x, then the sequence following it will be exactly the same as before. Is this true for all ...
6
votes
1answer
172 views

How can ECDSA signatures be shortened (to be used as a product key)?

So I made my own serial key generation software, using ECDSA, for use in my own applications and it works great so far! To keep the serial key short enough I use a 128 bit EC curve. My final signature ...
1
vote
1answer
44 views

Is this distributed OTP scheme secure?

This is for a distributed system: we don't want to have to maintain synchronization of random streams neither enforce uniqueness of pads because this would require some synchronization. Alice and Bob ...
2
votes
1answer
101 views

What is the difference between CPA and IND-CPA?

I am writing a paper and I got comments from the reviewer that I don’t really understand. Is there any difference between a CPA (Chosen Plaintext Attack) and IND-CPA (Indistinguishability under ...
8
votes
1answer
279 views

Quantum on hash

Perhaps this has been answered before. Grover's algorithm should result in a 256 bit hash being complexity 128 bits to crack. I was wondering, what if you had a 512 bit hash , and xor'd the lower 256 ...
0
votes
2answers
198 views

Cryptosystems used to generate public key certificate

Where can we find the cryptosystems used to generate public key certificate? Are the cryptosystems under the signature algorithm and signature hash algorithm? Do I need to analyze the packets ...
1
vote
1answer
136 views

Is it possible to create “non overlapping” RNGs?

For example, let's say I want two cryptographically strong RNGs (R1 and R2). And I want them to have the following property: R1 (resp. R2) will never generate the same random number twice. R1 (resp. ...
4
votes
3answers
749 views

When do we need composite order groups for bilinear maps and when prime order?

Why we need bilinear groups of composite order? What's the special security property of the composite order group in comparison with one of prime order? To put it in another way when do we need ...
1
vote
0answers
31 views

Why the group order has to be prime for pairing-based cryptography [duplicate]

I'm trying to get into pairing-based cryptography and I don't see why the group order of the group G in the pairing function e:G*G-> G_t has to be a prime number. I don't find an argument why ...

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