6
votes
1answer
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 ...
6
votes
1answer
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 ...
1
vote
2answers
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. ...
6
votes
1answer
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 ...
3
votes
1answer
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 ...
1
vote
1answer
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 ...
4
votes
2answers
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 ...
3
votes
1answer
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 ...
4
votes
1answer
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 ...
7
votes
2answers
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 ...
15
votes
4answers
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 ...
2
votes
1answer
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 ...
2
votes
0answers
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: ...
4
votes
1answer
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, ...
6
votes
1answer
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 ...
1
vote
2answers
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 ...
6
votes
1answer
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 ...
1
vote
0answers
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 ...
1
vote
1answer
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 ...
-3
votes
2answers
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 ...
2
votes
1answer
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 ...
8
votes
1answer
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. ...
4
votes
1answer
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 ...
6
votes
1answer
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 ...
5
votes
1answer
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 ...
0
votes
0answers
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 ...
4
votes
2answers
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 ...
26
votes
3answers
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. ...
0
votes
1answer
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 ...
9
votes
1answer
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 ...
-3
votes
1answer
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?
-1
votes
0answers
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 ...
3
votes
1answer
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 ...
2
votes
3answers
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 ...
2
votes
1answer
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 ...
9
votes
1answer
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 ...
4
votes
2answers
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 ...
1
vote
1answer
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 ...
4
votes
1answer
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? ...
1
vote
1answer
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 ...
4
votes
1answer
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" ...
1
vote
1answer
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?
4
votes
1answer
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 ...
4
votes
2answers
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 ...
3
votes
2answers
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 ...
2
votes
1answer
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 ...
2
votes
1answer
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 ...
3
votes
2answers
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 ...
5
votes
4answers
2k views

How does Scrypt use Salsa?

Bcrypt uses Blowfish to encrypt a derived key from the passphrase, and Blowfish is a cryptographic algorithm, but here it is said that: Note that Salsa20/8 Core is not a cryptographic hash ...
3
votes
1answer
624 views

Extract private RSA key from USB cryptographic token using Bardou et al. attack (variant of “million message attack”)

There is a side channel attack on tamper-resistant USB cryptographic tokens using padding-oracle, described by Bardou, Focardi, Kawamoto, Simionato, Steel and Tsay, titled "Efficient padding oracle ...

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