All Questions

19
votes
6answers
12k views

Are there two known strings which have the same MD5 hash value?

Is there an example of two known strings which have the same MD5 hash value (representing a so-called "MD5 collision")?
5
votes
2answers
739 views

How do I solve this RSA instance for m?

How we can solve this equation and get the value of M? $$8 = M^{13} \mod 33$$ not a computer program, but a mathematical operation.
4
votes
3answers
297 views

What other one-way functions are used in cryptosystems?

For RSA and El Gamal (and most other public key cryptosystems), one of the key ideas is that factoring and finding discrete logarithms are hard. There are other systems that rely on certain properties ...
3
votes
1answer
990 views

Demonstrating the insecurity of an RSA signature encoding scheme

I'm working on problem 12.4 from Katz-Lindell. The problem is as follows: Given a public encoding function $\newcommand{\enc}{\operatorname{enc}}\enc$ and a textbook RSA signature scheme where ...
4
votes
2answers
319 views

Is it safe to store initial counter value for AES-CTR alongside with ciphertext?

The initial counter value can actually be thought of as a sort of IV, which I assume is safe to transmit with the ciphertext, but I really want to be sure that this is indeed safe.
5
votes
1answer
298 views

Why are the random exponents so much bigger in the Socialist Millionaire protocol versus Diffie-Hellman key exchange?

Section 8, Security considerations, of RFC3526, which defines groups used for Diffie-Hellman has a table recommending some random exponent sizes. In particular, it says: The strength of a key ...
3
votes
1answer
179 views

Does hash_df use binary or an ASCII hexadecimal representation for numbers to be passed to the hash function?

I'm implementing the HASH_DRBG algorithm. As per NIST-SP800 90 document for HASH DRBG Generation, section 10.4.1 "Derivation Function Using a Hash Function (Hash_df)", the Hash_df algorithm contains ...
12
votes
3answers
1k views

Why does the padding in Merkle–Damgård hash functions like MD5 contain the message length?

I understand the need for padding in MD5. But why do we append the message length to the padding? I heard it strengthens the hash but how? Please provide an example if possible and how it applies to ...
5
votes
2answers
5k views

Cracking plain RSA without private key

I was wondering whether plain RSA encryption can be cracked given: The public key A plaintext of a known cipher text(s). In other words the decrypted message from an encrypted one (but without ...
11
votes
2answers
879 views

Random oracle model proofs and programmability

Proving the security of a scheme with the random oracle model (ROM) involves two steps: first you prove that the scheme is secure in an idealized world where a random oracle exists, and then you ...
4
votes
1answer
639 views

Are there two-way encryption algorithms that include a work factor?

I recently learned about the hashing algorithm bcrypt, which allows you to specify a "work factor" for the hash which can be incremented to stay ahead of Moore's Law. I understand there are some other ...
-2
votes
2answers
951 views

How many possible input files are there who's length can be stored in 63-bits?

I'm trying to calculate how many possible combinations of files there can be using a signed 64-bit file length, but can't seem to find a formula (or I'm using the wrong keywords). For example, the ...
3
votes
1answer
588 views

Generalizing the conversion of Diffie-Hellman to El Gamal

How can we generalize the conversion of Diffie-Hellman to El Gamal public-key encryption scheme? The goals is to be eventually able to show that any 2-round key-exchange protocol can be converted into ...
2
votes
1answer
91 views

How do we find the last two correspondences in an otherwise known even permutation?

A secret even permutation $P$ of the set of non-negative integers less than $n$ is chosen. That might be a Feisltel cipher with a random key. We are given in sequence the $P(x)$ for $x$ from $0$ to ...
10
votes
2answers
2k views

Is HMAC-DRBG or Hash-DRBG stronger?

Out of the two deterministic random bit generators defined in section 10.1 of NIST SP 800-90A (i.e. based on hash functions), which one is cryptographically stronger? Hash-DRBG (Section 10.1.1) ...
3
votes
1answer
144 views

Is a using salt important when creating a hash data validator?

I am creating a service that will return an set of objects, which will be used by multiple systems. At the end of the process, one (or more) of the objects will be sent back to our system for ...
13
votes
2answers
403 views

Are safe primes $p=2^k \pm s$ with $s$ small less recommandable than others as a discrete log modulus?

I take the definition of safe prime as: a prime $p$ is safe when $(p-1)/2$ is prime. Safe primes of appropriate size are the standard choice for the modulus of cryptosystems related to the discrete ...
4
votes
1answer
199 views

What is the property of RSA where N=e?

In RSA, suppose we know that $e=N$ and we are given the value of $e$. ($N = p\cdot q$ for some large primes $p$ and $q$; $\gcd(e, \varphi(N) = 1)$ Can we calculate $d$ ($d = e^{-1} \mod ...
4
votes
2answers
389 views

What is the appropriate public key encryption for secure coin flipping?

I quote Bobby's question here since I encountered the same one... Random Coin Flip using ElGamal and a Trusted Party Consider the following protocol for two parties to flip a fair coin. Trusted ...
6
votes
3answers
2k views

How to generate a list of unique random strings?

How would you generate a list of distinct random strings, where all strings are alphanumeric and have a fixed length?
3
votes
2answers
987 views

Is it a good idea to use bitwise XOR on a set of MD5 sums?

I have designed an SQL aggregate function in Oracle that bitwise XORs all MD5 sums of the values stored in a column. For example, if my table is: ...
2
votes
1answer
539 views

Hash or encryption function for challenge-response protocol?

Say I have an authentication protocol where the shared secret is never transmitted. The server passes a challenge to the client and the client calculates a response using an algorithm where the ...
4
votes
5answers
913 views

Software implementation of a commutative cipher?

I've got an application (detailed below) that calls for the use of a cipher that is commutative. I've been doing some googling & reading, and there are two algorithms that seem to get mentioned ...
6
votes
1answer
627 views

Are there any specific requirements for the function $F$ in a Feistel cipher?

In a Feistel cipher (as shown below), it seems that the security of the cipher would be entirely dependent upon the function $F$. What requirements on $F$ are necessary for a secure Feistel cipher? Is ...
3
votes
2answers
3k views

How to distribute session keys in public key cryptography?

In public key cryptography we can also use session keys which are symmetric. How do the sender (say a server) provides this session key information to its clients? If the sender (here server) ...
12
votes
2answers
2k 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, ...
6
votes
2answers
192 views

Where is the proof of security of Diffie's cipher?

There is an apparently provably secure cipher that was proposed by Diffie, but enhanced by R.A. Rueppel. The scheme, which was mentioned in Applied Cryptography, works like this: Measure the length ...
4
votes
1answer
96 views

Is there any serious discussion about using blinding intermediaries in digital currency scenarios?

A digital currency system like Lucre (OpenTransaction) creates a coin by a mint blind signing the output of a hash function, which the payer then unblinds and pairs with the input to the has function. ...
10
votes
1answer
875 views

How can I make my cipher show the avalanche effect?

I am a beginner in cryptography. I designed an password based encryption-decryption algorithm, which uses a random salt and a password to encrypt a message. I'm using SHA-512 for hashing, matrix ...
8
votes
1answer
867 views

AES and perfect ciphers

I'm taking a crypto class this semester and after learning the definition of a perfect cipher. I started wondering how this definition applies to AES. Obviously AES isn't a perfect cipher, since the ...
7
votes
1answer
834 views

How can one show that an ElGamal-like signature verification scheme is valid?

For an ElGamal-like signature scheme, I am given two things: The signing function, the verification function. How can I show that the verification function is valid? Example 1: Signing: $s := ...
3
votes
1answer
338 views

ElGamal message signatures retrieving the secret value x

If the GCD(r, p-1) is small and the value k is used to sign a message using ElGamal is also small. Then the secret value of x can be determined. Why is this true? How would one retrieve x?
2
votes
1answer
665 views

Verilog simulation of Data Encryption Standard

I interested in developing DES(in verilog) for my college project.Can i get help in understanding the simulation results of DES encryption and decryption ? thanks in advance .
4
votes
2answers
381 views

How does the wider cryptographic community view non-abelian group based cryptography?

Is there perhaps some neural expository article on crypto systems based on non-abelian groups? I've gleaned that Anshel–Anshel–Goldfeld key exchange is the most well-known cryptographic algorithm ...
5
votes
3answers
1k views

Is a book cipher provably secure?

I've seen ciphers (usually in spy drama shows) that involve taking a book and writing down an index to individual characters. Essentially it's a keyed substitution cipher, where the key is the name ...
7
votes
3answers
964 views

Using a Non-Random IV with modes other than CBC

The weakness CWE-329 is an interesting problem with CBC mode. However, does this same weakness affect the other modes of operation that rely upon an IV such as: PCBC, CFB and OFB? My gut feeling is, ...
4
votes
3answers
2k views

How can I create a fixed length output in my hash function?

I've been recently looking into the creation (and theory) of hash functions, however I just can't figure out how to turn a message into something of a fixed length. At the moment, my theory of a hash ...
8
votes
3answers
1k views

Desirable S-box properties

What desirable properties should an S-box have? My current standard selection process is to just pick them at random and verify that they fit the following criteria: The probability that any random ...
6
votes
3answers
1k views

Security of simple xor and s-box cipher?

What weaknesses (or strengths) do block ciphers based on only key xor and s-box have when operating in CBC mode? A cipher's internal primitive might be a simple as this: $C = S[M \oplus k]$, where ...
0
votes
3answers
810 views

Does knowing common prefixes help crack blowfish?

I have strings that are if the form: {static data}{changing data} The beginning static data part is around 20 characters and is common to all strings. The last ...
6
votes
1answer
241 views

Message space in security definitions

What is the message space in the following example and how does a message space relate to a security definition? I mean, what difference does it make to such a definition if your message space is ...
5
votes
1answer
934 views

Example of CHI Square test on Caesar Cipher?

I am trying to get my head round the chi square test, when used with the Caesar cipher. I started off using this formula, $$ X = \sum_{i = 1}^k \frac{f_i · f'_i}{n · n'} $$ Where ...
5
votes
2answers
665 views

Is a RSA-signature of some identifying data a safe way to implement a license key?

I have this idea of implementing a license key: After the user downloads the program, he connects to a website and sends his Windows product ID. The website, then, sends this back to him with a ...
2
votes
3answers
413 views

Trying to find a different DES encryption system explanation

I need a mathematical explanation of what does the DES encryption system really do. This means I need more explanation than the one that offers FIPS, which is more an explanation for computer ...
3
votes
2answers
250 views

Can you help with that definition for a CCA?

The following is a definition taken from Introduction to Modern Cryptography by Katz and Lindell. I'm having a hard time understanding some basic concepts! Can you please help me? ...
4
votes
1answer
159 views

What are the consequences of a MAC tag collision?

I've seen some proofs of MAC security that are based on the extremely-unlikely event that two MAC tags collide (ie, they are equal for distinct messages). Suppose that this extremely unlikely event ...
4
votes
3answers
254 views

Derived Shared Key vs Distinct Keys?

I've seen a lot of 2-party applications that derive a shared key from distinct keys created by each party. Why is this technique employed? Would it not be better to use those two distinct keys for ...
9
votes
2answers
6k 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?
-1
votes
3answers
539 views

Encryption technique performance evaluation

I've come up with a light weight bit wise encryption algorithm and seems good. Ive tried it with text and image input. But I've the following questions I would like to ask What do you think of bit ...
5
votes
2answers
476 views

101: Advanced Access Content System (AACS) and Subset Difference techniques for Broadcast Encryption

I'm trying to get a grasp on AACS and Subset Difference for a project I'm working on and am having a hard time coming up with a technically valid layman explanation, let alone implementation. Is it ...

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