2
votes
1answer
310 views

inverse problem about scalar multiplication on elliptic curve

Let $E$ be an elliptic curve over a finite field $F_p$. Given $n$ be a positive integer and $Q$ be a point on $E$, assume that $Q=nP$, how can we find this $P$? We can assume that $n|p-1$. If $n$ is ...
2
votes
2answers
735 views

Combining LFSRs for Stream Ciphers: Why do we need high non-linearity?

Linear Feedback Shift Registers (LFSRs) can be excellent (efficient, fast, and with good statistial properties) pseudo-random generators. Many stream ciphers are based on LFSRs and one of the possible ...
2
votes
2answers
335 views

Why is $h(H, m) = E(m, H) \oplus m$ insecure?

I am taking a cryptography class on Coursera. I learned that the compression function $h(H, m) = E_m(H) \oplus m$ is insecure (even though other variants like Davies­-Meyer or Miyaguchi-Preneel are ...
2
votes
0answers
139 views

Correct way to truncate data to a range [duplicate]

2 hours ago I thought I had this figured out, but now I am doubting myself and want someone to validate my algorithm. I want to take a stream of k trusted random ...
2
votes
2answers
2k views

Even passwords are vulnerable to hash collision attacks?

As stated in this page large documents hashed using md5 maybe vulnerable to collision attacks. My question is even passwords of 6-30 character are vulnerable to such hash collision attacks? If yes, is ...
2
votes
4answers
707 views

How to perform Multiplicative Inverse Modulo in IDEA

How to perform the Multiplicative Inverse Modulo in International Data Encryption Algorithm? I don't understand on how to perform it… For example, let's say I have a value of ...
2
votes
1answer
195 views

Is there an efficient way to hide the encrypted plaintext length with a block cipher?

In block cipher modes of operation for encryption on input of a plaintext of $N$ blocks (We assume that the input size is always a multiple of the blockcipher mode: $N·16$ bytes) the size of the ...
2
votes
3answers
218 views

Why is Bcrypt called a Key Derivation Function?

I am trying to understand why is bcrypt called a Key Derivation Function? I looked up the details of Ekfblowfish on Usenix article here: ...
2
votes
1answer
299 views

Questions about proof of correct encryption in the Paillier cryptosystem

In the Paillier cryptosystem [1] the encryption of $m \in \mathbb{Z}_N$ with randomness $r \in \mathbb{Z}_n^*$ is $c = g^m r^n \bmod{n^2}$. A proof of correct encryption could look like presented in ...
2
votes
0answers
121 views

How hard is a known prefix hash preimage attack?

Suppose the attacker knows $X, Z$ such that $H(X || Y) = Z$ If bit-length(Y) < 60 then a brute force attack is possible. What if ...
2
votes
1answer
117 views

Using UMAC with stream cypher

I understand that most stream ciphers, due to being applied with a simple XOR, are specially fragile against data tampering, and must be used with some MAC mechanism. So I am investigating the use of ...
2
votes
0answers
176 views

Why not use CBC, not send IV, and assume first plaintext block is garbage? [duplicate]

Possible Duplicate: Using CBC with fixed IV Forgive me if this has been asked before, I looked but haven't seen any answers that suit. Is there any danger in using CBC, encrypting a ...
2
votes
2answers
511 views

How can I add more rounds to AES?

I'd like to understand a bit more about the AES key schedule if anyone would mind explaining that. For instance, why is the way the schedule generated (with recursive xors and occasional ...
2
votes
1answer
197 views

Messages of different lengths and one-time computationally-secret

I'm preparing myself to exam, but I have a lot of troubles with rigorous proofs. Let $\Pi=(Gen,Enc,Dec)$ be an efficient secret-key encryption scheme that is not fixed-length. That is, for any $n$ ...
2
votes
1answer
352 views

Increase number of rounds for SPN and Feistel ciphers

Read a post on Schneiers blog (and again 2011) about increasing the number of rounds for AES from to "AES-128 at 16 rounds, AES-192 at 20 rounds, and AES-256 at 28 rounds" to raise the security. ...
2
votes
2answers
678 views

Avalanche noise RNG for one-time pad use

I came across this little HRNG widget and was really intrigued as I have been looking for a decent but afordable source for truly random bits to use in a one-time pad. The question is, would a HRNG ...
2
votes
1answer
354 views

implications of SSH server key compromission when authenticating users against a public key

Friday we had a disagreement with a colleague of mine about the implications of SSH server key compromission The question was stated as this : What could a hacker do while provided with the SSH ...
2
votes
1answer
512 views

Is there another resource for Carter-Wegman-style message authentication?

I'm wondering if there are other resources that cover Carter-Wegman style message authentication, besides the sources themselves. Is there an online text or a book that covers their ideas? I'd ...
2
votes
2answers
583 views

Can I make a cipher (ex: Vigenère) harder to break?

The Vigenère cipher can relatively easy be broken when the key size is small compared to the size of the message. One first finds the length of the key, and then uses frequency analysis to actually ...
2
votes
4answers
662 views

What is the actual difference between security through obscurity and true encryption?

In an abstract sense, aren't both the same? Don't their definitions boil down to the following? security through obscurity: trying to make sure some information cannot be obtained without knowing ...
2
votes
1answer
358 views

Generating a cryptographically secure, many-time use, symmetric encryption key

I need to generate a 256 bit encryption key described by the adjectives in the title. Currently I intend to create the key using this RNG. Is this a secure manner of creating the key, given that it ...
2
votes
1answer
573 views

Is there a standard for OpenSSL-interoperable AES encryption?

Many AES-encrypted things (files, strings, database entries, etc.) start with "Salted__" ("U2FsdGVkX1" in base64). I hear it's some sort of OpenSSL interoperability thing a b c. Is there some ...
2
votes
1answer
444 views

Several questions about Paillier cryptosystem

I have several questions concerning the original Paillier cryptosystem as described in Paillier, Pascal (1999). "Public-Key Cryptosystems Based on Composite Degree Residuosity Classes". EUROCRYPT. ...
2
votes
0answers
200 views

How to properly format a PGP Message Block?

As sort of a follow up to my (currently) unanswered question, if it is correct, how do i properly calculate and format the packet that actually stores the data being encrypted? I thought I knew what I ...
2
votes
2answers
839 views

Capacity of Advanced Encryption Standard in terms of File Encryption

what is the capacity of AES in terms of File Encryption? is it really good to encrypt a large files in AES? ex. I am encrypting a 8GB of File... is it still good to used AES? is it still good to used ...
2
votes
4answers
1k views

Is my pseudo-random initialization vector secure?

How can I know if I am generating a secure pseudorandom initialization vector? Currently I am planning to generate a pseudo-random initialization Vector using current date and time - is this secure ...
2
votes
1answer
9k views

How to use RCON In Key Expansion of 128 Bit Advanced Encryption Standard

I have a question about RCON here is my illustration... this is the 128 bit key.. ...
2
votes
3answers
558 views

reverse of md5sum

This might be out of ignorance, I apologize, but how complex of a problem might it be to generate a file of size N whose md5sum is X? For example, ...
2
votes
1answer
478 views

Offline anonymous electronic money systems and their cryptographical base

What anonymous offline electronic money systems exist and what are they based on? I know only one currently - eCash, based on RSA blind signatures.
2
votes
4answers
354 views

Looking for cryptographic secure hash algorithm(s) that produces identical root hash for differently sliced hash list

I have a scenario similar to the one described in Wikipedia: hash list, but with a twist. I'm looking for a cryptographically secure hash function that would create the same root hash for the same ...
1
vote
1answer
69 views

Is there a scheme that allows to use RSA based challenge response protocol limited to signing only 8 bytes to create a secure signature of any data?

I have a tiny device that has a challenge-response authentication mechanism where the device signs 8 bytes of any data with it's internal 2048 bit RSA key. I would like to be able to certify any data ...
1
vote
2answers
109 views

Severity of Cooking NIST P Curve Constants

Bruce Schneier and Gregory Maxwell have both stated that they believe the constants chosen for NIST's P curves (i.e. P-256r) are cooked. DJB has put together a detailed list of red flags but, outside ...
1
vote
1answer
34 views

Could this alternative hash based MAC construction be as, or even more secure than an HMAC?

Begin hashing (Key||Message) Encrypt the hash state (or some part of it, such as the first 128 or 256 bits), with the key. Add the encrypted hash state to the ...
1
vote
3answers
126 views

Is what i'm trying to do safe?

What both parties have: Server Master Secret (MS) Server supported versions (Svers) Agreed upon version (ver) Client Master Secret (MS) Client supported versions (Cvers) Agreed upon version (Ver) ...
1
vote
1answer
169 views

Where can I learn the basics of AES?

I was tasked with making an expo about AES and its steps, but I still can't grasp it, especially when it comes to "key schedule". Does anyone know some good material to start learning? Or can someone ...
1
vote
1answer
54 views

Rabin-Williams, blinding and size of Integer r?

I was reading the paper Breaking the Rabin-Williams digital signature system implementation in the Crypto++ library. The library uses blinding, but it was not enough to stop key recovery. But my ...
1
vote
2answers
126 views

Is computing roots moduli a composite $N$ a hard problem without knowing the factorization of $N$?

Suppose that we are given $\mathbb{Z}_{N}$ and an element $x^u \in \mathbb{Z}_{N}$ with $u \in (0,l]$ where $l$ is the bit-size of $N$. Is it difficult to recover $x$ by knowing $u$ without knowing ...
1
vote
1answer
60 views

Why are modes of operation used, what attacks do they prevent?

I know you always need to use a mode of operation when using a block cipher, AES for example, and Wikipedia has a good explanation for what modes of operation are Now I know if i do not use a mode of ...
1
vote
1answer
103 views

Koblitz encoding a message to a point, what is the “associated auxiliary base parameter”?

I am looking at the Koblitz method for encoding a message as an elliptic curve point. The first step given in the paper I'm reading is: "Choose an elliptic curve and its associated auxiliary base ...
1
vote
1answer
83 views

How does ccrypt work?

On many Linux/GNU distributions CCrypt (ccrypt) by Peter Selinger is available as a command line utility to encrypt files. How does the protocol work and is it ...
1
vote
1answer
113 views

ccrypt and its security

I would like to know how secure is ccrypt to encrypt files (like images, videos and other non-plain text data) using single password of 20+ chars length. It uses Rijndael with 256-bit block size and ...
1
vote
3answers
201 views

RSA Key generation: How is multiplicative inverse computed?

As per Wikipedia, RSA keygen goes like this: Choose two distinct prime numbers p and q. Compute $n = pq$. Compute $\varphi(n) = (p − 1)(q − 1)$. Choose an integer e such that $1 < e < ...
1
vote
1answer
65 views

How bad would it be to reuse the random blinding factor in a scheme like Paillier?

A secure and somewhat fast way to "re-encrypt" (refresh? anonymise?) a Paillier ciphertext, $c$, is to multiply it by an exponentiated random value: $c \gets c \cdot r^n \mod n^2$ (with $r \in ...
1
vote
3answers
203 views

Point decompression on an elliptic curve

I'm programming an elliptic curve cryptosystem and I'm having difficulty with decompressing points. The following information is from my project specification as to my understanding: Given a point ...
1
vote
0answers
138 views

Existential Forgery on textbook RSA signatures

Can an adversary who haven't seen the message before forge the signature of that message? The adversary has seen $σ_1 = (m_1)^d \bmod N$ and $σ_2 = (m_2)^d \bmod N$ He forges $m' = m_1\cdot m_2$ $σ' ...
1
vote
1answer
128 views

How can CBC-MAC be secure when message length is fixed?

I was doing some self-initiated knowledge gathering about digests, signatures and hmacs and I ran across the fact that you can use CBC as a MAC, but if the message size is not fixed; then it is not ...
1
vote
1answer
135 views

File format of an $2048$ bits RSA public key

From here, there are $14$ bytes specifying the file format of the key. I am still quite confused about the format bytes. For example, the following public key 30 82 01 0a 02 82 01 01 00 8e a3 d1 c7 ...
1
vote
2answers
242 views

RSA Decryption given n, e, and phi(n)

my cryptography professor gave us this problem for extra credit a while back and I attempted it but I didn't get it correct. I have gone back to it, but I'm even more lost now than I was the first ...
1
vote
1answer
62 views

Is equal length of primes in Paillier cryptosystem is mandate for security reasons?

In continuation to this question about length of primes , I am in doubt about the restriction on length of primes itself . In Paillier cryptosystem , equal length of primes are used . My doubt is ...
1
vote
1answer
172 views

Security of chameleon hash functions

With the knowledge of the trapdoor, we are able to compute the same digest for differents messages. I've read that given two messages $m$, $m'$ and two numbers $r$, $r'$ such that ...

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