0
votes
0answers
55 views

How to make a crypto-currency robust to 51% attacks?

For Example: In Bitcoin it is theoretically possible to double spend if you control > 50% of the computational power in the network. Also it is possible to apply "selfish mining" with far less than ...
0
votes
1answer
55 views

Is it secure to choose d in a RSA key pair?

An RSA key pair consists of the private key $(n,d)$ and a public key $(n,e)$ such that $de \equiv 1 \bmod{\lambda(n)} $. Usually one chooses a small $e$ and computes $d$ by inverting it modulo ...
6
votes
2answers
384 views

Besides key and ciphertext sizes what are other advantages of elliptic curve versions of various protocols?

There are elliptic curve variants of Diffie-Hellman, ElGamal, DSA and possibly other protocols/algorithms. I know that these elliptic curve variants have smaller key and ciphertext sizes which will ...
0
votes
1answer
207 views

Verify Signature Failed [closed]

I am writing a small module which will verify signed message. Message will be signed using private key in Smart Card. The inputs are: Signed Message Original Message Public Key / Certificate with ...
2
votes
0answers
106 views

Code-breaking software [closed]

Is there any (free) software available to check for standard encryption algorithms such as the Caesar cipher? Advanced encryption algorithms such as RSA, DES, AES, etc. are not necessary.
-2
votes
1answer
276 views

Encrypting with private key in Public Key InfraStructure [closed]

In Public key infrastructure, the MD5 of a piece of data is encrypted with the private key of a sender and this encrypted MD5 – along with the data – is again encrypted using an algorithm like AES or ...
3
votes
1answer
140 views

Difference between lightweight, online and low memory Authenticated Encryption schemes

What is the basic difference between the following types of Authenticated Encryption (AE) schemes? Lightweight Authenticated encryption Online Authenticated Encryption Authenticated Encryption for ...
2
votes
1answer
122 views

Perfect secrecy over Stirling numbers

For all $c_0\leftarrow m_0 \oplus k$ there exists a $k'$ such that $c_1 \leftarrow m_1 \oplus k'$, where $m_0 \neq m_1$ and $c_0 = c_1$. Assuming a truly random $k$, the first assignment is a ...
2
votes
1answer
56 views

Compare RFC 5246 SessionID re-use versus RFC 5077 Session Resumption?

Can you help me understand the differences, both algorithmic and practical, between RFC 5246 SessionID re-use and RFC 5077 Session Resumption? Both appear to be ways to nail up a second TLS session ...
12
votes
1answer
1k views

Is truncating a SHA512 hash to the first 160 bits as secure as using SHA1?

I am from a web development background (I don't know an awful lot about cryptography or how the algorithms themselves work), so I am asking this question in simple terms. Consider a hash of the word ...
2
votes
1answer
83 views

Security proof of FO(Fujisaki-Okamoto) hybrid encryption

The proof of FO hybrid encryption is hard to understand. $\:$ Especially, how does the challenger respond to the decryption queries when the challenger can only have some encryption queries? Can ...
1
vote
2answers
74 views

Do we need symmetric cryptosystems?

One question I had to answer in my crypto exam today was: Do we need symmetric cryptosystems? As it stands, that's probably a debatable question, so I'd like to reformulate this as: Are ...
2
votes
1answer
111 views

Explanations for the complexity values for second preimage attack on GOST?

I've been reading the article "A (second) preimage attack on the GOST hash function" by F. Mendel et al (link) and I'm having some difficulty to grasp some of the values of complexities/probabilities ...
3
votes
3answers
137 views

How useful is NIST's Randomness Beacon for cryptographic use?

NIST have just launched a new service called the NSANIST Randomness Beacon. It has been met with some initial skepticism. Perhaps the cryptography community would have used it before June 2013 when ...
6
votes
3answers
548 views

Lamport signature: How many signatures are need to forge a signature?

Lamport signature: Signing the message Note that now Alice's private key is used and should never be used again. The other 256 random numbers that she did not use for the signature she must never ...
2
votes
1answer
85 views

LFSR get output from characteristic polynomial?

Say you have a characteristic polynomial of an LFSR: $$f(X) = X^4 + X^3 + 1$$ How can I use this function f to get the output of the LFSR, given some initial state? Obviously I can create the LFSR ...
4
votes
1answer
122 views

Cryptanalysis of AES on SSD with TRIM enabled

Attack scenario: SSD physically retrieved from a computer that is turned off. The entire disk is encrypted with AES 256-bit (GNU/Linux, LUKS). The SSD has had TRIM enabled for several years and the ...
0
votes
0answers
76 views

m ∈ Zn \Z*n, RSA works but not secure

If you happen happen to have a message m ∈ Zn \ Z*n, RSA works but not secure. How likely is it going to happen? |n|=1024 bits |p| = 512 bits |q| = 512 bits. Thanks!
0
votes
1answer
86 views

Questions about hash functions

Let $H$ be a secure hash function (e.g. SHA256), is it possibile to find $H(M1)$ given $H(M1||M2)$ if the length of $M1$ and/or $M2$ is exact the block size of the hash (that is, 64 bytes for SHA ...
-1
votes
1answer
52 views

AES CBC MAC splicing attack

I'm trying to implement AES CBC MAC splicing attack in Python, the idea is: given a message M, its tag Tm (MAC(M) = T), a new message N and its tag Tn we build a message such as: M||N = (M1, ..., Mn, ...
0
votes
0answers
30 views

Encryption of numeric value using playfair

I am studying in CSE and in my recent exam paper, A question was asked as: Construct a playfair cipher for Plaintext: semester5 and key:technology. Generally as we are taught yet, there is no ...
2
votes
1answer
182 views

Is there a time-space tradeoff attack for breaking symmetrical cryptos?

Is there any known techniques for using time-space tradeoff for speeding up symmetrical crypto breaking? Kind of like rainbow tables speed up breaking hashes by using huge precomputed tables. Is ...
4
votes
4answers
270 views

Efficient Symmetric Mutual Entity Authentication Protocol

I am looking for an Authentication protocol for access control. Let's say both the client and server would share a symmetric secret (256 bit key) and the client wants to access the server. Access ...
3
votes
2answers
223 views

Can a “pattern” in a series of passwords be detected from their hashes (and maybe a single raw password)?

Let's say I'm a lazy user of a system with annoyingly frequent password change policies. I may have a "good" password I used initially that is only used for this system but since I have to change it ...
4
votes
2answers
139 views

Is PBKDF2-HMAC-SHA1 really broken?

I just read through this article which demonstrates practical (and seemingly trivial) collisions in PBKDF2-HMAC-SHA1, and provides a few examples of collisions. Am I missing something here? Is ...
0
votes
4answers
122 views

Construct an encryption algorithm using hash function [duplicate]

We know that Cryptographic hash function is generally a one-way function where we can't retrieve the original message. On the other hand, encryption is a two a function where we get the original ...
1
vote
1answer
67 views

How do I convert the definition of E-521 into a curve definition a la Bouncy Castle?

I am currently trying to create an ECCCurve for E-521. Unfortunately, it is not currently a named curve in the library I am using, so I will have to define it manually. I am using the definition of ...
-3
votes
1answer
274 views

How to calculate cycles per byte [closed]

Sometimes, websites and scientific papers that introduce and/or handle cryptographic algorithms also provide speed-analysis and tables comparing the individual performance of cryptographic ...
0
votes
0answers
96 views

Challenge–response authentication which can be done in head?

Say we have door where you have to say a password to pass. If theres someone standing nearby he would hear the password and could also use it. So therefore the doorman gives you a different number ...
0
votes
0answers
61 views

why it is not possible to find inverse of its own in diffie-hellman protocol? [duplicate]

In DH-Protocol bob calculate $R=g^b$, R as public parameter and Alice calculate $S=g^a$ & common key($K=g^{ab}$) using value $R^a$ then bob calculate common key $K= S^b=g^{ab}$ It can be ...
6
votes
1answer
799 views

What is a tweakable block cipher?

Pretty simple question - but I can't seem to find much information about it. What exactly is a tweakable block cipher? How do they differ from traditional block ciphers? What is the 'tweak'? Is it ...
0
votes
1answer
132 views

Hash function and outputs of arbitrary length

I know that Skein can output a digest of arbitrary size, with a kind of counter mode. Blocks of the resulting digest can be computed in parallel. What about Keccak ? I know that a sponge function can ...
4
votes
2answers
336 views

How secure is the AES master key if Round Keys are found

If an attacker finds some round key of AES256, is it possible to find the master key? How safe is the master key if an attackers finds multiple round keys?
4
votes
1answer
337 views

Is there a security analysis of CryptDB?

Its interesting to see critical thinking being applied to cryptDB in contrast to all the hoopla around it here . cryptDB is not a major theoretical breakthrough but potpourri of technologies to make ...
2
votes
0answers
69 views

Looking for a detailed example of proof by reduction

I'm looking for a very detailed example of proof by reduction. Say we have two or three protocols (that have been proven secure) and we construct a new protocol. We want to provide a proof of security ...
3
votes
1answer
91 views

Proofs by reduction and times of adversaries

I have some difficulties to understand, when we construct a reduction, how we determine the time for the constructed adversary to break a target security property. In general these details are not ...
2
votes
2answers
154 views

Pseudorandom functions

The usual case to distinguish a pseudorandom function from a random function is to assume that the adversary can choose the plaintext blocks. Is there another case (game) in which the adversary can ...
3
votes
2answers
148 views

Is prime regeneration necessary for every new session using a Diffie-Hellman key exchange?

As I understand it, bruteforcing a Diffie-Hellman generated secret key takes $P-1$ attempts to crack the shared secret, where $P$ is a very large prime used for modulus. If your $P$ is quite large, ...
4
votes
1answer
123 views

How can I tell how many bits of security a secure hash function has?

How can I tell how many bits of security a secure hash function has? For example, how would I calculate or tell how many bits of security a secure hash function with 160 bits output would have? ...
3
votes
1answer
95 views

Does re-iterate AES128 with different keys gives any benefict?

I'm not cryptography expert (well, I'm here after all!) but I have this doubt… Suppose I have 2 different keys: secretkey1 secretkey2 And a plain text: myReallySecretPlainText If I use AES128 ...
4
votes
1answer
138 views

I need a 64-bit cryptographic hash for 96 bits of data

I have a situation in which I need to combine a 32-bit datum, G, and a 64-bit datum, I, to produce a 64-bit datum. No two ...
3
votes
3answers
238 views

Third-party security issues with Shamir's secret sharing scheme

In Shamir's secret sharing scheme, we are trusting a third party who generates the secret polynomial. How can we ensure security here when we are involving a third party?
-1
votes
1answer
96 views

Why is it said that if we know few bits of pseudo random sequence we can predict the whole of it?

I am new to this cryptography and in the source from which i study it is said that if we know the first few bits of a pseudo random sequence we can predict the whole of it.How is it possible? The ...
1
vote
1answer
343 views

Is Curve25519-java secure?

I have only about 2 weeks of cryptography experience mostly in the form of questions on bitcoin.se. Is Curve25519-java up to date with current Curve25519 standards? Is Curve25519 itself secure? ...
3
votes
2answers
295 views

Signature based on public key cryptography and forgery

In the definition of existential unforgeability, there is no detail about the following questions. In general, can we suppose that a signer is also a possible adversary ? When generating a signature, ...
1
vote
0answers
35 views

Distributed authentication in low trust environment

I would like to inquire if there is any Method to achieve the following: There is a list $L$ of $n$ entities that are allowed to perform a certain action. $L$ is cryptographically signed by some ...
3
votes
2answers
164 views

Is a random oracle controled by the challenger?

When proving a Crypto scheme security under random oracle model, is the random oracle always controlled by the challenger? What if the Hash is only used by the adversary?
9
votes
4answers
6k views

What is the difference between known-plaintext attack and chosen-plaintext attack?

I am very confused between the concept of known-plaintext attack and chosen-plaintext attack. It seems to me that these two are the same thing, but it definitely is not. Can anyone explain to me how ...
2
votes
1answer
153 views

Is it true that for RSA with no padding, the length of data must be equal to the length of key?

The question pertains not in terms of security but computing operational functionality, as it how the computation is done. Is it true that for RSA with no padding, the length of data must be equal to ...
2
votes
1answer
129 views

counting points on elliptic curve

Given an elliptic curve with equation $y^2=x^3+ax+b$, and i want to find the number of points $(a,b)\in E(\mathbb{F}_p)$ where the polynomial has repeated roots, how do i do it? I have an intuition it ...

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