# All Questions

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### 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 ...
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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 ... 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 ... 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 ... 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: ... 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? 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 ... 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 ... 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 ... 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 ... 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? 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 ... 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 ... 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 ... 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 ... 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 ... 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 ... 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 ... 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). 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 ... 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 ... 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: ... 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, ... 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. 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 ... 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 ... 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 ... 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? 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)$. My question is: ... 2answers 45 views ### Does SRP also authenticate the server to the client? The Secure Remote Password (SRP) protocol lets a client prove to a server that it knows a specific password without revealing that password to the server. The server stores a cryptographic verifier ... 0answers 74 views ### Perfect Secrecy of AES file Encryption Why does AES file encryption not ensure perfect secrecy? I understand that$\Pr[E(K,M_1)=C_1] = \Pr[E(K,M_2)=C_2]$given$M1\neq M2$holds for perfect secrecy of a scheme. However, this seems to hold ... 2answers 48 views ### Is it secure to use TOTP with same key and different time-counters? If I use TOTP to generate codes with different "expiration" time, using the same key for HMAC, will I create some kind of weakness? 1answer 202 views ### Is this variant of SRP for peer-to-peer authentication practical? I'm interested in using a modified form of SRP as a peer-to-peer authentication method. Since neither side is acting as a host, one of the primary design goals for SRP (that the client doesn't need ... 1answer 93 views ### How is RSA able to prevent brute forcing using the public key? Bob calculates a private and public key. Bob sends his public key to John. Jeff is a third party unwanted member and manages to snatch the public key mid-transfer. John encrypts his message ... 4answers 57k views ### Why is elliptic curve cryptography not widely used, compared to RSA? I recently ran across elliptic curve crypto-systems: An Introduction to the Theory of Elliptic Curves (Brown University) Elliptic Curve Cryptography (Wikipedia) Performance analysis of identity ... 2answers 111 views ### Is reverse NTRU still secure? I'm currently prototyping something with the NTRU encryption scheme but I wish to use it in "reverse" -- distribute private keys so anyone can decrypt, but keep the public keys secret and thus only ... 3answers 138 views ### Calculating$\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}$? (points might have complex number format in ... 4answers 524 views ### Can an encryption method be used as a hash function? As I understand, there are a few requirements for a good hash function. Hard to find any message from a given hash Hard to find any 2 messages which give the same hash A single bit change in the ... 2answers 179 views ### How is message length involved in public/private-key encryption? I've just read this question regarding why to sign only the hash of a message. I kind of understood the answer, but the question arisen is now: Why can I encrypt any text messages with the same ... 1answer 90 views ### How to exctract ECDH parameters from an OpenSSL-generated$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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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. ...