# All Questions

228 views

### Finding where I am in a linear recurrence relation

Suppose I have a linear recurrence relation $$a(n) = c_1 a(n-1) + \dots + c_k a(n-k) + d,$$ where the constants $c_1,\dots,c_k,d$ are given and the initial values $a(0),\dots,a(k-1)$ are given as ...
475 views

### Practical uses of Manipulation Detection Code (MDC) and IGE

I've just skimmed over most of the paper "On Message Integrity in Symmetric Encryption". I've included (the last) part of the abstract below: We show that generic compositions of ...
332 views

### Derive a key from public data using a secret master key

Users in my system will be given a device that contains a 128-bit AES user key. The user key will be derived from 64 bits of public data related to the user, which is unique but very predictable (i.e. ...
330 views

### understanding forking lemma

Every time when I read a paper that has digital signature, when it comes to prove the security of a digital signature scheme, many chances that the author will use the forking lemma. The forking ...
237 views

### Clipper chip (SKIPJACK) key exchange protocol

According to the Wikipedia page for NSA (emphasis mine): However, NSA's Fortezza hardware-based encryption cards, created for the Clipper project, are still used within government, and NSA ...
236 views

### Question on monoalphabetic substitution

Question: A stream of cipher operates on a data stream of 6-bit characters using a simple mono-alphabetic substitution technique. Estimate and explain the number of different substitution ...
242 views

### CTR mode nonce with aggressive key rotation policy

I'm writing a small encryption library which will be required to use the shortest possible amount of overhead for describing ciphertexts. I want to use CTR mode with a 6-byte random nonce. Normally ...
274 views

### Explain the 'Breaking 104 bit WEP in less than 60 seconds' paper

Could you explain the 'Breaking 104 bit WEP in less than 60 seconds' paper, without skipping too much technical details. Could you an answer questions, like: how is '1.24/256' a significant ...
826 views

### Why elgamal is still discussed

As I understand, if two users have a shared key, it is possible to encrypt a message using symmetric key encryption. So when a secret key is shared through Diffie-Hellman asymmetric key exchange, it ...
485 views

### What is the purpose of using different hash functions for the leaves and internals of a hash tree?

I just learned that the THEX hash tree specification which is widely used in P2P requires that two different hash functions be used: one for the leaf nodes (hashes of input data) and one for the ...
1k views

### Is Wiener's attack on RSA extendable to larger keys with low hamming weight?

Using small private exponents with RSA improves performance. However, it has been shown (Wiener, 1990) that if $\log d \leq \frac14 \log N$, the private exponent $d$ can be reconstructed from the ...
448 views

### Digital Signature size requirement

What is an approximate length for a digital signature (SHA) given the following? Using RFID tags, size ranging from 100 to 2000 bytes (8000 bits), our project is setting aside a portion for a digital ...
145 views

### Security of a 3-party key exchange protocol

I am trying to find attacks on a theoretical protocol or prove its security. An initiator wants to establish a shared key with a responder with help of a trusted server. We have the following roles: ...
250 views

### Why is feed-forward mechanism used in hash functions?

The compression function of SHA-1 when used in Davies-Meyer mode adds its input to the chaining values at the final step. For the first message block, the IV is used as the input and in the next step, ...
198 views

### Why use variable p, q, g for Diffie-Hellman?

In the book Cryptographic Engineering, it is said that fixing p, q, g for a key negotiation protocol based on DH is a bad idea (page 228 1st ed). But allowing for flexible p, q and g requires a lot ...
84 views

### Lamport-Diffie + attempting to invert OWF

I am studying Lamport-Diffie signature scheme and in my lecture present the follow algorithm for attempting to invert the one way function $f$, where $f$ is used to compute the public key of this ...
213 views

### Did Merkle invent cryptographic hashing?

Chapter II of Merkle's 1979 PhD thesis is titled "One Way Hash Functions." The chapter appears to be the first reference to cryptographic hashing. The chapter has no references. Is there an earlier ...
97 views

### Merkle path authentication

In point 4 of the Algorithm 4.2 (Classic Merkle Tree Traversal) of *"Post_Quantum_Cryptography.pdf"* shows *"Stack_h.update(2)"* each stack receives two updates. I am trying understand to understand ...
126 views

### Lamport-Diffie + Security Proof

I am studying Lamport-Diffie signature scheme. In the lecture present the algorithm $A'$ for attempting to invert the one way function $f$, where $f$ is used to compute the public key. My question is ...
2k views

### Algorithm/Technique for Steganography [closed]

What is the most used Algorithm/Technique in Steganography? I made some research about the algorithm/technique and I think Least Significat Bit(LSB) is the most used algorithm, is there any more ...
176 views

### Finding a collision for a hash function

I'm trying to find a collision for the following (modified) Merkle-Damgard hash function. Suppose we already have a hash function $h : \mathbb{Z}_2^{2·n} \to \mathbb{Z}_2^n$ for fixed length bit ...
1k views

### Is a second preimage attack on MD5 feasible?

What's the practical status of MD5 w.r.t. second-preimage? Integrity of a piece of data is protected by an MD5 hash, itself assumed genuine. The data (and thus the hash) is known to the adversary. ...
297 views

### Why is H(message||secret_key) not vulnerable to length-extension attack?

Given a Merkle-Damgård hash function $H$, I know that an attacker can forge a message protected by a MAC computed as $H(\textrm{secret_key}||\textrm{message})$. Why can't he perform the same ...
112 views

### Abstracting primitives and modes of operation

I am developing a symmetric crypto library and have reached a roadblock. Looking at block ciphers, it is quite obvious that all block ciphers are trivially abstractable as a simple primitive ...
286 views

### Does the GCM authentication tag need to be protected?

I am just learning about the GCM mode of AES and I'd like to start using it in my software. However, I have a question about its use. As I understand it the output of a GCM encryption operation is ...
161 views

### How to safely sign a 6 characters String?

I have to decide, whether a document, which I am given, was correctly signed by a private key. my Ideas is to sign the document id using RSA with the private key and to print the following onto the ...
1k views

### How can I remove my personal data from my PGP public key?

According to this Q&A-discussion it is possible to remove all personal data (name and mail address) that is attached to a public key. What steps do I have to follow in order to remove all ...
2k views

### How are primes generated for RSA?

As I understand it, the RSA algorithm is based on finding two large primes (p and q) and multiplying them. The security aspect is based on the fact that it's difficult to factor it back into p and q. ...
85 views

### How to choose the appropriate public (i, m) and private (j, m) keys?

I studied some encryption and decryption and I have found some very interesting problem to solve on the internet. I hope I am writing to right site - there are so many in StackExchange otherwise I ...
430 views

### What does “running in polynomial time” really mean?

I'm currently learning private-key cryptography. I've been able to see that perfect secrecy is achievable if no assumption is made about the computational power of the attacker. However, perfect ...
231 views

### Which encryption method used to crypt this text? [closed]

This is encrypted text: http://pastebin.com/bsHQ3Xvs It starts with <~ and ends with ~> What's the name of this encryption method?
134 views

### Approaches to decoding data of an unknown type [closed]

This is a high level question. One area of computer science that I'm working on getting better at is encoding and decoding data. My background is that of a self-taught web engineer who is trying to ...
706 views

### Okay to use OpenSSL to encrypt then sign a message?

I am planning on encrypting a message then signing the message so the recipient can authenticate the source. Reading this article has me concerned that I'm doing something wrong. My current plan is ...
482 views

### Questions about BCrypt and PBKDF2

I would ask you some questions about users' password storage in a login system. What algorithm do you suggest me to use? bcrypt or PBKDF2? I was looking for a way to implement them client-side and ...
106 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 ...
499 views

### What changed in PKCS#1 v2.2, and why?

PKCS#1 is one of the most used (de-facto) standard for real-world use of RSA. That's for good reasons: PKCS#1 is well thought, versatile, understandable, has been relatively stable for over two ...
554 views

### How did LinkedIn “salt” all their passwords?

First, just to make sure I understand "salting" correctly: You randomly generate a string to append to the password before hashing it, so as to increase its length and make precomputed tables much ...
554 views

### Reduction of Integer factorization to Discrete logarithm problem

I was reading Eric Bach paper entitles "Discrete logarithms and factoring", in which he states the following reductions: solving the integer factorization problem suffices to solve the discrete ...
176 views

### Why work in a subgroup QR(n) of an RSA group $Z^*_n$?

I sometimes read in papers that a (sub-)group generator $g$ is taken from $\mathrm{QR}(n)$ instead of $\mathbb{Z}^*_n$, where $n = p \cdot q$ and $p$ and $q$ are prime. Is there a reason for this? ...
139 views

### split up sha256 hash and compare indiv block results

We have a h/w accelerator in our SoC and it can do sha256. The restrictions are it's input descriptors (which defines the input packet and len) must not straddle 64K boundary. When it does we can ...
475 views

### Can two cipher letters per plaintext letter easily defeat character frequency analysis?

For a class 5 years ago I wrote a paper about "defeating character frequency analysis by using two cipher letters per plaintext letter" ...
488 views

### Sending TLS messages with out encryption using openssl code

Is it possible to send messages over TLS with out encryption? If so which cipher suite is needed for this?
98 views

### Hardness of CDH in different groups

What is the difference of the CDH problem in different groups? In particular, given a group $\mathbb{G}_1$ of order $q$ that is a subgroup of $\mathbb{Z}_q^*$, $q$ prime, and another group ...
190 views

### How long does it take to extract a key from a FIPS-140 Level 2 device?

How long does it take to extract a key from a FIPS-140 Level 2 device? What records are there of successful extraction? How much did the first extraction from a given device cost? How much would a ...
203 views

### Efficiency of finding sub group order vs factorization

Suppose you got a prime $p = 2\mathbb\Pi_{i=0}^{n-1}q_i+1$, where $2^{k-1} \lt q_i \lt 2^k$ for some $k$ and all $0 \le i \lt n$, and that you also got a generator $g$ of one of the prime order sub ...
253 views

### Ed25519 choice of private key implementation

This answer to another question describes the different values chosen as private key and public key by various Ed25516 implementations. What are the advantages / disadvantages of each? (Table ...
357 views

### Solve a system of non linear equations over GF

I have the following set of equations: $$M_{1}=\frac{y_1-y_0}{x_1-x_0}$$ $$M_{2}=\frac{y_2-y_0}{x_2-x_0}$$ $M_1, M_2, x_1, y_1, x_2, y_2,$ are known and they are chosen from a $GF(2^m)$. I want ...
198 views

### Is the polynomial reconstruction problem really “intractable”?

In the family of the believed (experts haven't come yet with a solution) intractable problems in cryptography belongs the Polynomial Reconstruction problem in which an attacker is given $n$ points in ...