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2answers
123 views

Deriving 2 keys using HKDF

I have an encryption scheme that uses a 256-bit master key, from which 2 separate keys (one for AES-256-CTR encryption and one for a HMAC-SHA256) are derived using HKDF. However, I'm not sure exactly ...
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1answer
142 views

SHA1 collision event probability after n iterations

There are String1 and String2 - some variables-strings. The probability of collision for different ...
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1answer
69 views

Non-commutitive and nonassociative algebraic structures in cryptography

Are there any cryptographic algorithms or primitives that have been developed and studied that make use of non-commutative or non-associative algebraic structures such as quaternion integers or ...
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1answer
42 views

Question about block erasure codes

I have a question about linear block erasure codes that are described in this paper. I briefly describe the idea behind the linear erasure codes and then I ask my question. Given a set $d=\langle x_i ...
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1answer
219 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 ...
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2answers
134 views

64 bit Elliptic Curve key?

For a simple proof of concept project i'm (attempting!) to do, i've started looking into openSSL elliptic curve cryptography. However instead of the standard key lengths, 160-512. I'm interested in ...
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0answers
66 views

Why don't we use H(m||k) as a MAC? [duplicate]

HMAC seems a bit complicated. Why can't we use $H(m||k)$ as a MAC? Unlike $H(k||m)$, length extension attacks won't work. Is there some other obvious attack?
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4answers
283 views

Does it make sense to have a 4000 bit long key?

My question is about the key length of a asymetric algorithm. How is it possible to memorize or remember a key that is about 4000 bits long?
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1answer
153 views

Simplified Fiat-Shamir example generates wrong output

I am trying to implement the Fiat-Shamir identification protocol, however the end results always fail to match. I am using algorithm's description from here. Preparation: Select 2 prime integers ...
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1answer
362 views

Certificate signature with SHA-1 and RSA: where do 1888 bits come from?

Plenty of X.509 Certificates use "PKCS#1 SHA1 with RSA encryption" as the Certificate Signature Algorithm for generating a 2048-bit signature. The SHA-1 hash function generates a hash value of 160 ...
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1answer
172 views

What are some restrictions when converting Montgomery Curves into Weierstrass Curves?

I want to represent a Montgomery Curve (curve25519) in Weierstrass form as a personal exercise. After doing some math and referencing the conversion equation at ...
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2answers
132 views

Proof for exponentiation in modular arithemtic

If $e$ is a natural number, then this is true: $$m^e \bmod\ n = (m\bmod\ n)^e\bmod\ n$$ This is often used when encrypting, especially with RSA, since one can avoid directly calculating $m^e$, ...
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2answers
248 views

Shamir's ID based signatures

I am currently implementing Shamir's ID based signature algorithm as proposed in Adi Shamir, Identity-Based Cryptosystems and Signature Schemes. Advances in Cryptology: Proceedings of CRYPTO 84, ...
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0answers
58 views

How to specify last $t$ bits are only sent when a signature is sent?

I have read that in cases where there is a limit on the amount of data sent over the network, one idea is to sent only $t$ bits of data rather than sending all the bits. For example, in the Schnorr ...
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1answer
99 views

Method to calculating e in RSA

For simplicity I choose two small primes for p and q. p=3 q=11 n=33 Φ(n)=20 Now we need ...
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1answer
187 views

Short length asymmetric encryption?

I want to send out short messages to the world which listeners could verify to be send from me. The way this is usually done is to encrypt the message with a private key where the readers decrypt it ...
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2answers
430 views

How do I calculate the private key in RSA?

Given $n=pq$ for $p,q$ known, I can calculate $\phi(n)$. $e$ is selected such that $\gcd (e,\phi(n)) = 1$. Using this, how do I calculate the RSA private key? Example: I have $n = 35$, with ...
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1answer
471 views

Why does plain RSA not work with big messages ($m>n$)?

From what I can remember, RSA is something like this: Generate 2 distinct prime numbers $p$ and $q$ that have similar bit length. Compute $n=pq$ and $\phi(n)=(p-1)(q-1)$ Compute $e$ such that ...
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1answer
74 views

What is the difference between a bijective random oracle and a random permutation?

Assume $S$ be a finite set $O$ be a random oracle from $S$ to $S$, such that $O$ is bijective $f$ be a random permutation of $S$ Is there any difference between $O$ and $f$? Does it makes any ...
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1answer
114 views

How to design a secure data container?

I want to design a secure data container that can be used to transfer encrypted data over a public network that can defeat common attacks. Data is already encrypted before entering this container so ...
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1answer
178 views

Is there proof to the relation between the gap Diffie-Hellman problem and the the Cha-Cheon signature scheme?

I am trying to prove that: "If the gap Diffie-Hellman problem is easy, then the Cha-Cheon signature scheme will be broken." Can you help me to prove it? Is there any proof to the relation between ...
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0answers
43 views

one-time key, zero IV in stream cipher (e.g. AES-CTR) [duplicate]

Say I am using a random key to construct a stream cipher with, say, AES in CTR mode, which will never be used to encrypt more than one data stream. Is there any advantage, no matter how small, to ...
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5answers
1k views

Using one-way hash functions as the encryption method

Suppose two parties want to communicate securely with each other (Bob and Alice) using a simple messaging system in English. There are approximately 180,000 currently used words in the English ...
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2answers
416 views

Quadratic residue problem on composite integers

Its believed that the quadratic residue modulo $n=p·q$ for large primes $p$ and $q$ is intractable, which forms the basis of some cryptosystems. However, it is solvable if the factors of $n$ are ...
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1answer
322 views

Is the salt value a secret or can it be stored in cleartext just like the IV? [duplicate]

From Wikipedia I read that the IV is used to randomise the mapping of cleartext to cipher text and that the IV is usually stored (in cleartext) together with the encrypted record. ...
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3answers
696 views

Hash function based on pseudorandom functions and security

Are there hash functions that make use of pseudorandom functions. Precisely, I'm looking for a specification of a hash function based on PRF (and based on the security of such a primitive).
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1answer
363 views

Homomorphic Encryption

Homomorphic Encryption (HE) which supports any function on ciphertexts is known as Fully Homomorphic Encryption (FHE), while Partially Homomorphic Encryption (PHE) includes encryption schemes that ...
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1answer
155 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 ...
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1answer
159 views

Securely generating passwords based on a secret and a public identifier

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 ...
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1answer
102 views

What type of groups does Microsoft's U-Prove use (Schnorr… etc?)

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 ...
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1answer
1k views

How long to bruteforce a RSA key [duplicate]

Suppose I have a 2048 bit RSA public key, and want to brute force the corresponding private key. I guess there are 2048^16 possible combinations? How long would this take me to brute force with an ...
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2answers
141 views

Does collision resistance stay when extending a hash function to a set domain?

Given a Cryptographic hash function $h$ for element $x$, let's extend it to sets via $H(S)=\prod_{x\in{S}}{h(x)}$. I am asking if the new hash $H$ (in domain of set) is still collision resistant? To ...
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2answers
295 views

Help me describe/identify this challenge-response protocol/algorithm?

My area of expertise is reverse engineering, specifically embedded systems. I do attack cryptographic systems, but this largely involves key recovery or exploiting the implementation. I was asked to ...
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1answer
189 views

Indistinguishability attack example

I want solve the next exercise. The author defined the experiment for the cryptosystem $\Pi$, the adversary $A$ and the security parameter $n$ as follows $\mathsf{PRIV_{EAV}}(\Pi,A,n)$ The ...
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1answer
215 views

chaining rsa with ecies

In an answer to a previous question it was suggested that one way to protect your asymmetrically encrypted AES-256 keys, from say a solution to prime factorization, would be to chain asymmetric ...
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1answer
2k views

decryption many time pad [duplicate]

I have eleven ciphertexts that were encrypted with the same key (which I don't know). I want to decrypt the last ciphertext. I read similar question like Many time pad attack but I can't solve my ...
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1answer
960 views

What is the probability of breaking the AES algorithm?

I am doing a project which requires the encryption to be done using AES. Is it really possible (technically) to crack AES? If yes, please tell me: What is the probability of breaking AES? How ...
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1answer
116 views

Security relevance of random factor in Paillier

In the Paillier cryptosystem [1] the encryption of $m \in \mathbb{Z}_N$ with randomness $r \in \mathbb{Z}_n^*$ is $c = g^m r^n \bmod{n^2}$. The additive-homomorphic property of the system shows that ...
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1answer
196 views

Scrypt as a KDF with one-time high-entropy input

I'm looking at using Scrypt as a KDF. Assume the following: the input will always be high-entropy random bytes generated by a CSPRNG the length of the input can vary from between 8 to 32 bytes the ...
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2answers
297 views

Store hashed email and compare hash values

I have a number of different systems sending me email addresses, but I don't actually need the underlying email, just a hash of the email address. I know I can compare hash values to find matches ...
1
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1answer
99 views

Two files encrypted with related data versus one file

I have some code that takes a list of credit-card numbers, splits the number into two chunks, one chunk is 8 characters long, the other chunk is the remainder. Each series of chunks is saved into ...
1
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1answer
244 views

Can Add and Multiply On Cipher Text achieve all operations?

A Fully homomorphic encryption scheme needs to support an evaluate function that can do add and multiply operations on cipher text. Can we do all kinds of complex operations on cipher text like ...
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1answer
632 views
1
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2answers
325 views

Are really, really long passwords any securer than short ones?

Just for fun, I want to encrypt a message that will take about 10 or so years to decrypt. My idea is to encrypt a message with AES-512 with a password one million decimal digits long. Knowing not ...
1
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1answer
335 views

How do unkeyed hash functions (for MDCs) provide security?

Unkeyed hash functions are, by definition, hash functions computed without a key. SHA-1 is an example. MDCs (message digest codes) are a subclass of unkeyed hash functions. How are unkeyed hash ...
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1answer
1k views

Blowfish: hex digits of pi used for s-boxes?

Preface: here is the official site for the Blowfish algorithm: http://www.schneier.com/blowfish.html The Blowfish algorithm uses an s-box, which consists of hex digits of pi (found here: ...
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1answer
181 views

Crack cryptographic hash functions using Toffoli gates?

Might it be possible that reversible combinatorial circuits (or some emulating software) are going to be used to get the input of the white box? I mean if I have constructed a proper reversible ...
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1answer
2k views

How can I implement the “Multiplication Modulo” and “Addition Modulo” operations in IDEA?

I am currently working on IDEA (International Data Encryption Algorithm), and I don't know how to perform Multiplication Modulo and Addition Modulo. This is how IDEA operates: IDEA operates on ...
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0answers
309 views

Stream Cipher : Never use stream cipher key more than once [duplicate]

Possible Duplicate: Taking advantage of one-time pad key reuse? How does one attack a two-time pad (i.e. one time pad with key reuse)? Denote C is cipher text, m is message, and PRG(k) is ...
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2answers
399 views

which of these is more secure (bcrypt vs srp)

OK, here's the two different ways I was thinking about making the authentication for the login thing to store the passwords securely. The first is the following. Client hashes password ...

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