instruction stringlengths 32 27.2k |
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I'm trying to explain the concept of 1-out-of-2 oblivious transfer to folks who haven't seen this idea before. I can explain what properties it provides, but it's also helpful if I can show a simple protocol for 1-out-of-2 OT, to give people a feeling that this is achievable.
What's a simple protocol for 1-out-of-2 OT... |
I'm looking to do file encryption of a bunch of individual files, some small, some quite large. The files will be write-once/read-many, so I could use CBC, however, since the read-access needs to have seekability, XTS seems a better fit. One issue, as I understand it, is that XTS (or CBC for that matter) is malleable... |
Let's say I have an unknown string with the known sha256-hash of it. I was wondering if it was possible to now calculate the sha256 of the concatenation of the unknown string and "abc".
(In PHP: hash('sha256', $unknownString .'abc');)
I thought in order to do so "all I need" is to go from the hash that I know back to t... |
In an old cryptography FAQ, I found the following step described for determining a the length of the key a cipher was repeatedly XORed against:
Discover the length of the key by counting coincidences.
(See Gaines [GAI44], Sinkov [SIN66].) Trying each displacement of
the ciphertext against itself, count those byte... |
I'm new to the standards of encryption. I understand the math behind RSA really well. I'm wanting to design an application for private peer to peer communications using RSA and wanted to know what the best standard to use is. I want the highest level of security, regardless of efficiency or wait times.
|
As far as I can tell most one-way hashes apply some iterated encryption algorithm to the input data.
What would be the issues with a one-way hash based on some fixed large prime $p$ and a generator $g$ with $h(x) = g^x \bmod p$? Is it just that the methods used for modern hashes are even harder than the discrete logari... |
There's one thing that I've never quite understood about SSL (which perhaps means I don't understand SSL at all): how is SSL secure from rogue Certificate Authorities?
This is probably best elaborated as an example:
Suppose company B buys a certificate C from certificate authority A.
Now, normally I would go to the web... |
For a stream of packets, where each packet is individually encrypted with a block cipher, it's desirable to have each encrypted packet only valid for that position in the stream.
A message number would work, but there are many different ways of implementing it.
Given a construct using HMAC for integrity check (i.e. we ... |
What is the probability that two separate RSA public moduli are the same? For example, consider a 2048-bit modulus. The number seems to be huge, but the choice for prime factors p and q is much more restrictive: They both should be 1024-bit in size, not too close to $\sqrt{p*q}$ and the most significant bit should rem... |
I am actually checking use of EAX AEAD mode, and following EAX spec review (scheme definition for my part), my question is: what about the derivation of the authentication and encryption keys? In the described scheme the same input key $K$ appears used on all
OMAC (authentication) & CTR functions. Did I miss someth... |
I am trying to learn cryptanalysis on the Keccak hash function. One of the papers on zero sum distinguishers talks about Keccak having inverse degree of 3.
I am unable to figure this out: how do you get an inverse degree of 3? For example one of the permutations Chi in Keccak does the multiplication in GF(2). So I woul... |
I'm using TLS with a Diffie-Hellman key exchange to negotiate the symmetric keys.
How long should the exponents be?
|
Alice chooses
i) A large prime $p_A$ (say 200 to 300 digits)
ii) A primitive element $\alpha_A$ modulo $p_A$,
iii) A (possibly random) integer $d_A$ with $2 \le d_A \le p_A-2$.
Alice computes
iv) $\beta_A \equiv \alpha_A^{d_A} \pmod {p_A}$.
Alice's public key is $(p_A,\alpha_A,\beta_A)$. Her private key is $d_A$.
Wha... |
My correspondents and I would like to communicate privately by email. We used to employ PGP but that might have been shooting ourselves in the foot. PGP doesn't have forward secrecy. If any of our private keys are ever compromised (by law, hacking, or rubber hose), then all our past conversations can be immediately dec... |
I am curious of the details of how one would go about generating elliptic curve parameters. (I know standardized parameters exist, but I'm trying to understand both how they were generated and the general case.)
Let's also assume we want to work in a prime-order subgroup of size $O(2^n)$.
For simplicity, let's work ove... |
Currently I'm building a toy steganography system based in a web browser. Besides all the obvious issues with building a system requiring extreme privacy into a browser (this is a toy!), I was curious if there were any serious flaws to the basic flow of the system.
The idea is to take a binary buffer of any length as t... |
I've recently heard some people claiming that if the fact that P = NP is proven, most (all?) the current cryptographic algorithm considered secure like RSA will be unusable in secure systems.
My question is :
1. If P = NP is proven TODAY, will a RSA key (let say 3072 bits) be still enough to ensure a certain amount of ... |
It is being speculated that one of the ways that NSA leaker Edward Snowden may have created his "insurance policy" – distributing sensitive documents to various individuals with instructions to disseminate them should anything happen to him--is through the use of a partial key escrow.
The concept is described vaguely ... |
What are the "standard procedures in cryptanalysis" to analyze unknown ciphertext?
In other words: Are there any protocols, officially acknowledged checklists or something like that which represent a "standard" approach in cryptanalysis to analyze unidentified ciphertext? If yes, which ones would that be and what would... |
I am working on an occasionally connected app. To allow the users to authenticate I created a profile which is encrypted and placed on the users machine.
The first byte of the file contains the IV length followed by the IV and then encrypted using Aes where the key is the users password hashed using SHA256.
Within t... |
Given a message $M$ and a signature $S$, is it feasible to find a RSA public key $(n,e)$ such that $S$ verifies as a valid signature on $M$ (using this public key)?
What if we're given one public key $(n,e)$ where the signature verifies (presumably, the public key of the original signer), and we want to find a second p... |
This is a program that I made for encrypting text files. It uses a one time pad cipher to encrypt the files, but I do not know if there is any holes in my program that could be a vulnerability. Is my one time pad cipher secure?
import os
q = 1
while q == 1:
e = raw_input("file to encypt: ")
#This will open a fi... |
I know for a fact that DES is not a group, but are any of the Triple-Des versions a group? Why, or why not?
|
Scenario: a class that is not mandatory. The teacher wants a protocol that will satisfy the following requirements:
It must record each student's attendance.
A student X should not be able to say that another student Y went to the lecture.
The teacher should not be able to know who attended the lessons and who didn't ... |
In recent weeks the concept of (perfect) forward security/secrecy has
been mentioned a lot, primarily in the context of the shocking
revelations about NSA eavesdropping. As far as I'm aware, this concept
was introduced by C. G. Günther in [1]. Unfortunately [1] only sketches
the concept but does not give a security de... |
From the ECDH demo here, if I generate a private key for Alice I can get _
P = 1175846487558108474218546536054752289210804601041
Which gives the following public key point.
X = 583857549063195252150226340830731484791130788759
Y = 1195037839477751118658084226553581900533276838164
For ECDSA public key compression the a... |
I have a 64 bit integer. It has some sort of signature so that one can be sure it is valid.
Anyone may increase the integer, update the signature, and remove all trace of the old signature. But it is not permitted to reduce the integer.
Many updates are permitted.
Is there a scheme such that, given $n$ and publicly ver... |
In order to setup an authenticated shared secret key between two clients, I am faced with the choice between two possibilities:
Let users set a (low-entropy) shared password and then perform some password-authenticated key exchange that is resilient to low-entropy secrets (as described here, for example).
Generate a h... |
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 "small", I would imagine that it is possible using division polynomials. Is it a difficult problem if $n$ is large enough... |
After a lot of reading on the internet, i can't seem to figure out how Predicate Encryption works.
As i read, it consists of 4 algorithms:
Setup() - (outputs MSK - Master Secret Key and PK - Public Key)
Encrypt(PK, text) - (outputs CT - ciphertext)
GenKey(MSK, predicate) - (outputs a Token)
Query(Token, CT) - 0 if fal... |
I have an idea for a system that would be outsourcing some brute force calculations to many users in hopes of finding divisors of a number. However, there is a possibility that a given number would be prime, or that it would not have a divisor in a given range.
For example, a computer might be tasked to look for a divi... |
Background
Microsoft certifies Windows 7/8 as well as Server 2008 R2 and 2012 to be FIPS-140-2 compliant. Actually they certify just a small crypto core, bcrypt.dll (the library, which is unrelated to bcrypt, the key derivation function). Microsoft's FIPS-140-2 architecture is detailed here. Also, their FIPS-140-2 AES ... |
The Lamport-Diffie signature scheme is said to be quantum-resistant. Why is that? What would a quantum attempt to attack this signature scheme look like, and how does it fail?
|
I'm trying to learn more about the Subgroups implementation of Microsoft UProve. I'm unsure if they are Schnorr Groups or use a different foundation?
Can anyone point me to the technical reading required to understand the implementation? (anything appropriate for a determined, yet fledgling novice would be appreciat... |
I'm new to the field of cryptography, but I want to make the web a better web by setting up the sites that I host with Perfect Forward Secrecy. I have a list of questions regarding the setup of Perfect Forward Secrecy. Here it goes:
Can my choice of certificate authority hinder the use of Perfect Forward Secrecy?
Ho... |
I am trying to understand the design of Tor(onion router). I am reading the original paper on the 2nd generation onion routing system. Where, under integrity checking on streams, they say, "When Alice negotiates a key with a new hop, they each initialize a SHA-1 digest with a derivative of that key. Then they each incr... |
Forgive the very novice question, but...
Given an algorithm which, upon inputting an unchanging 4-character string, generates a variable 6-character string, but having no idea what the algorithm is other than it is at least partially based on the date, is it possible/practical to discover the algorithm based on 15 or 2... |
presume:
we have A and B integers, and C - product of multiplication of A and B, A and B are prime numbers (strong or usual)
G - is geometric mean of A and B (square root of C)
also, we have: if B > A => G/A = B/G (from geometric mean properties)
so: since G is float number, then GCD(G*10^x,C) > 1 and I can with trial... |
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 of checks and protocol tricks (e.g. requesting a lower bound on p / q)
Why do we need this flexibility? Do we add somethin... |
While reading NIST SP 800-90A, "Recommendation for Random Number
Generation Using Deterministic
Random Bit Generators", I noted that no stream cipher was included among the various specified solutions.
I also noted that, at least as far as I could tell from the list of other NIST SP 800 series publications, no stream c... |
When a block cipher key length is greater than the block length, is it the case that for some keys $k_0, k_1$ (with $k_0 \not= k_1$) and plaintext message $m$ that $E_{k_0}(m) = E_{k_1}(m)$?
|
When I first started learning cryptography, I had my first a-ha moment when I fully appreciated the value of a One Time Pad and XOR and all the functions that attempt to emulate that OTP's randomness (PRP/PRF).
Now that I'm learning about the math behind various lemmas, I see a recurring theme regarding "take the modul... |
I read about STS + variants being insecure in the SIGMA paper, which then proposes SIGMA as a replacement. Are the SIGMA variants still considered secure or are there some other protocol that's currently recommended to use?
|
If neither of the 'big two' of TLS Handshake and IKE are appropriate in a given situation, what alternative Authenticated Key Exchange (AKE) standards exist and are recommended?
Many protocols have been proposed in the literature over the past 20+ years, though only a small subset have made it into standards of some fo... |
to understand RSA better I am doing a little calculation by hand, this is what I got:
Choosing:
$p = 3\\
q = 5\\
n = 15\\
\varphi(p\cdot q) = 2 \cdot 4 = 8\\
e > \varphi(n) \implies e = 13\\
e \cdot d = 1 \pmod 8\\
13 \cdot d = 1 \pmod 8\\
(13 \cdot 5) \mod 8 = 1$
So there has to be $m \le n$ and I choose $m = 7$
Encry... |
Is it possible to send messages over TLS with out encryption?
If so which cipher suite is needed for this?
|
Recently NSA published two new algorithms, Simon and Speck. In the abstract they say
The aim of SIMON and SPECK is to fill the need for secure, flexible, and analyzable lightweight block ciphers.
so as the question title says, what does 'analyzable' mean? BTW I found the publication through Schneier's blog post about... |
How much stronger is RSA-2048 compared to RSA-1024? It is hard to imagine very big numbers. So what would be your way to explain the difference to someone who doesn't know much about cryptography?
|
Other considerations aside, is it possible to use DH with an established public key (together with fixed g, p, q) to safely authenticate a server instead of using some signing algorithm?
In other words, we will replace $SIG_S(Data)$ in some protocol with this scheme. The steps described below can be merged with whateve... |
Let us say there exists a permutation matrix $n \times n$ for some large $n$. Then, there are $n!$ possible permutations. If one to use brute-force to find the secret permutation among $n!$ possibilities, is there a parallel way to find such permutation?
thanks
|
I'm looking to fill in the gaps of my knowledge of lattices. Can anyone point me towards papers or books that introduce lattice crypto assuming a fairly solid math background?
Mods: Feel free to close this if it's not the proper format. I'm not exactly clear on the literature request policy.
|
I am reading the page 38 in this "Post Quantum Cryptography" book (Equations 8 and 9). My question is, why to compute the verification key $Y$, $f$ is applied $2^w-1$ times? Are there any security notions involved?
|
Imagine multiple parties executing a secure multiparty computation protocol to evaluate a function securely. Let Alice be one of the parties who is intended to get all the partial results and combine these partial results into complete result at the end of the protocol.
In practice , if Alice's machine is compromised ... |
I am currently investigating use of EAX mode for a dedicated application and following useful clarifications received from my previous post , to consider according to my applicable input security requirements (separate keys for encryption and authentication) either :
the Baseline solution where EAX is used in strict ... |
The situation involves a single party (single certificate) who would want to AES encrypt a file that they can later decrypt. Assume the EC certificate + EC keys have a purpose i.e. "File encryption" (similar to EFS perhaps), issued to a human ("Mr CSO") or an application ("My cat photo app"). Certs would be off existin... |
Are there Cryptography certifications variants of CISSP ( Certified Information Systems Security Professional)? Like a CISSP exams, but only for cryptography?
|
I'm designing a scheme for management of a small number of cryptographic keys (< 100 at any given time) and I had an idea: rather than storing all the keys, could I just store one master key and use a KDF to derive the particular key I need by varying the salt in a certain way?
For example, if I need the second encryp... |
I'm performing ElGamal encryption algorithm and using the additive homomorphic property so the product of two ciphertexts is the encryption of the sum of the plaintexts.
The problem is that I need to return the original plaintext sum values.
How could I get them using decryption?
Is there an algorithm I can follow, giv... |
How can we construct a (pseudo) random permutation from a (pseudo) random function ?
I've seen a method from Luby-Rackoff which allows to construct a PRP of length $2N$ from a PRF of input/output length $N$. Is there a method to keep the same length $N$ ?
I'm looking for a method that has better security bounds than tr... |
As generator $g$ is used in DH how do you find a combination of prime $p$ and $g$? eg: if we choose $p=23$ and its generator is $7$ (given in the book) how do we find the generator?
|
I have been using Asymmetric Encryption for a while, and I have come across an interesting concept, and it would be cool if it could be done.
Are there any Asymmetric Encryption Algorithms where the Encryption Key and the Decryption Key cannot be deciphered from one another?
|
When setting up a nonce/IV and counter (e.g. for AES-GCM-128), why are the nonce/IV and counter restricted to specific sections (e.g. random nonce/IV in first 8 bytes and counter in last 8 bytes)? This results in the last 8 bytes always starting off at a predictable value (0), reduces uniqueness (only 8 bytes of random... |
I am wondering whether the following form is a one way function with collision resistance:
$$ax^2+by^2 \mod p$$
where $a$ and $b$ are given, and $p$ is a prime number.
Since the QR (quadratic residue) $x^2 \mod p$ is one way, $ax^2 \mod p$ is also one way.
The same goes for $by^2$.
So, my guess is that the form $ax^2+b... |
I'm taking a Coursera cryptography course. The definition of semantic security is hard to understand. I tried to restate it with some slight changes (the word "efficient" was in the original definition). Have I gotten it right?
The adversary chooses two messages: $m_0, m_1$.
We encrypt one of these messages: $c \lef... |
For Java I'm only aware of the Bouncy Castle implementation of Salsa20. However, that version is significantly slower than AES-CTR and ARC4.
There are plenty of libraries in C, but are there any good implementations of this stream cipher in Java?
|
I am reading my lecture and I found this definition:
Let be $F$ be a family of hash functions, that is, a parameterized set
$$F=\{g_k:\{0,1\}^{*}\rightarrow\{0,1\}^{n}|k\in K\},$$
where $n\in \mathbb{N}$ and $K$ is a finite set.
Which could be an example of $F$?
|
The Snowden situation raises an intereting cryptograpic problem. At present, how can something be sent confidentially to Snowden?
Claim:
I have no particular political orientation. The above question is merely a cryptographic challenge. The meta assumption is…
You trust someone, denoted as Snowden, but Snowden does n... |
I'm trying to create a good scheme for generating passwords in my systems management scripts. What I'd like to do is have a secret key (let's say hello123), which I keep only on trusted management machines. I'd then like to combine that with a "password identifier" (for example testing-db-pass), and produce some ASCII ... |
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 signature from the private key $sk=\left(\begin{array}{rrrrrr}
x_{1,0} & x_{2,0} & \cdots & x_{l,0} \\
x_{1,1} & x_{2,1} &... |
When using PGP type public encrypted communication, would it be a good idea to generate a new
public for each message and attach that in the new public key in the reply to back to sender mail body? Idea is you start the communication with the known public key from a key server, but for each new message/chat message you... |
Can an encryption scheme (deterministic or not) be viewed as a pseudorandom permutation taking as input a message and returning a value of the same bit-encoding length?
|
One runtime platform provides an API that supplies PKCS#5 padding for block cipher modes such as ECB and CBC. These modes have been defined for the triple DES, AES and Blowfish block ciphers. The other platform API only provides PKCS#7 padding.
Are PKCS#5 padding and PKCS#7 padding compatible?
|
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 Why to use this algorithm $A'$ to prove the Theorem 1?, Why I cann't use other any algorithm (for example Grover, or Br... |
I was decrypting TLS encrypted data from wireshark using the openssl(TLS1.0 version) TLS logs which provides the complete key block which of size 136 bytes.Now i moved to TLS(1.2 version) in that Key block size is of 88 bytes only.Anybody please help me in this... Thanks in Advance
|
For a protocol where the server presents its signature to prove authenticity (e.g. IKE/SIGMA/STS with only one party authenticated), it's essential that the signing is extremely cheap, while verification can be almost arbitrarily expensive.
RSA appears to be the worst alternative, but what about DSA vs. ECDSA? Is there... |
What is the ideal system for an encryption scheme? For a pseudorandom permutation the ideal one is a random permutation, for a pseudorandom function the ideal one is a random function. For an encryption scheme do we have a well known application describing an ideal system for an encryption? Maybe the one-time pad?
|
I want to modify a standard block cipher in the following way. I replace each round key by a key picked at random. Is this block cipher as secure as the original one ?
Thank you.
EDIT
Some missing information related to the question, according to comment by OP to one of the answers:
My question is related to the key-... |
Is this encryption key sharing okay to use? Or is much better to use ECIES?
$G$ = base point
$a$ = Alice’s private key
$b$ = Bob’s private key
$A = aG$ = Alice’s public key
$B = bG$ = Bob’s public key
$C_x$ = x coordinate of $C$
Alice and Bob share an AES key:
Alice makes $C = aB$
Bob makes $C = bA$
The common encr... |
What is the relationship between an attack and an attack model?
For example, let $\Pi$ be the Lamport signature scheme. This signature has its security based on any one-way function. The Grover algorithm, an attack, inverts this function with complexity $\mathrm{O}(2^{n/3})$. Furthermore, there are algorithms that try... |
Given a random bit string $R=r_1r_2\cdots r_n$, let us encrypt each bit $c_i=E(r_i,k_i)$ where $k_i$ is taken from the stream of encryption keys. Suppose that an attacker can guess the correct $r_i$ from $c_i$ with probability $p>1/2$. Then, the probability that the attacker can recover correct $R$ from $C=(c_1,\cdots,... |
Suppose that tomorrow is July 7, 2050 and Snowden is still stayed in Sheremetyevo airport. Further, let us suppose that quantum computation and quantum commincation are available to everyone, and Snowden's laptops are also quantumly-made. My question is: Who can or how to say something confidentially to Snowden? (Pleas... |
I was reading up on CryptoCat's (pretty ridiculous) programming issues and how is dramatically reduced the key search space (link to bug synopsis)
One thing that caught my eye was
So 2^54.15 turns into 2^27.08
Unless I'm missing something, the impact of the fractional exponent is negligible. So, any ideas why the au... |
When using a “sponge function” to create a cryptographic hash, we can look at the flat sponge claim, which flattens the claimed success probabilities of all attacks using a single parameter: the claimed capacity cclaim
Is there any way to actually verify the probability of that claim - in this case: potential probabili... |
I was just looking at some NIST PRNG recommendations, specifically at Hash_DRBG. I read briefly through the algorithm, and even though it is not overly complex, it still seems unnecessary to me. I asked myself how I would implement a (cryptographically secure) PRNG on top of a Hash algorithm (SHA256 or SHA512), what ca... |
Is there a problem with signing a message like this?
p = the Elliptic Curve prime
G = base point
a = Alice’s private key
A = aG = Alice’s public key
e = hash(message)
Jx = x coordinate of J
In my target 32-bit microcontroller I am using p-256 and SHA256. Thus e will typically be in 1..p-1.
Alice computes:... |
Section 4.9.2 of FIPS PUB 140-2 specifies, amongst other things, a "Continuous random number generator test." Here are the relavant bits:
If each call to a[n] RNG produces blocks of n bits (where n > 15), [...] Each subsequent generation of an n-bit block shall be compared with the previously generated block. The te... |
From the original BEAST paper, it seems to me that you need the same session key when encrypting data, so you can check if the original plaintext block is the same as your chosen plaintext (which is impossible with different keys). However, in modes with PFS, the keys are different in each session, so doesn't this foil... |
I am only a beginner and doesn't know much about https. I have heard that they uses somekind of a key and is shared between them. If an attacker wishes to attack could he be able to capturebthe key and attack?
|
I am assuming there is a very simple way of working this out by brute force, but I am not sure if there is a better way.
I have a file of data that I wish to get (my data, generated by a machine). This file is encrypted by some method that I have no details of (and the company has folded).
However, as well as the encry... |
Is it possible for myself to change the key size (say from 2048 to 4096) on an EXISTING PGP / GPG key, and just republishing that key?
Or do I have to generate a new key all together?
I want to say that you have to create a new key pair, but I don't have enough documentation either way to support a claim.
|
In the section 3 (page 44) of the book "Post Quantum Cryptography", it says:
Then each one-time signature key must be generated twice, once for the MSS public key generation and once during the signing phase.
Also defined $\text{PRNG}$: $\text{Seed}_{\text{in}} \rightarrow (RAND, SEED_{\text{out}})$, where $\text{R... |
This paper on semi-generic algorithms considers
"non-standard properties of the employed hash function".
For BLS signatures whose main group is $G$, I'm curious what can
be shown when the hash function is a $G$-valued random oracle.
Is there a known semi-generic attack that is faster than just
ignoring the signing o... |
A Gap-CDH group is such that, given group elements $g, a = g^x, b = g^y$, it is hard to compute $g^{xy}$, but, given a group element $c$, easy to verify if $c = g^{xy}$. While such groups have been proposed, and it is obviously feasible to define a group where Decision Diffie-Hellman is demonstratively easy, exactly wh... |
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 document:
encrypt(privateKey, id)
id
Which requirenments should be fullfilled by the id, for the encrypt(privateKey, id)... |
Let's say that we have a key exchange of some sort, which leaves A and B sharing the long-term secret $S$. Then, A and B want to use AES-CTR for the communication, which leaves us with several possible strategies:
During the key exchange, A and B both generate and send nonces $N_a$ and $N_b$ which are mixed in the key... |
If I am not mistaken, plaintext RSA is not secure. So, I have read that padding and hashing is needed to make the RSA algorithm secure. However, I am confused on what does padding or armoring mean in this context. What does RSA-padding mean, how does it work?
For instance here I have read about this, but I do not reall... |
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 not choose the plaintext blocks ?
|
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 source:http://www.youtube.com/watch?v=NjedHm04ETM
move to 7min 40sec and this is the source.
|
In the section 2 (page 42) of the book "Post Quantum Cryptography", it says:
Then he generates the one-time signature $\sigma_{\text{OTS}}$ of the digest using the $s$-th one-time signature key $X_s$, $s\in\{0,\dots,2^H-1\}$.
My question: Is $s$ chosen in random form? Is it possible to have $s$ collisions to the si... |
I know: Never do your own crypto. But this question is just theoretical.
Assume you do a Diffie-Hellman Key Exchange with a server to produce a shared secret $x$. Then use a cryptographic hash function like SHA3 to generate a pseudorandom bitstream like this:
$$p_i = \operatorname{SHA3}(x||p_{i-1}).$$
To encrypt $i$th... |
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