# All Questions

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### Probability of IV/key reuse

I generate randomly both an initialization vector (IV) and a symmetric key (key) to feed an authenticated encryption scheme. How much IV/key combinations can I generate until the same combination ...
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### Are the following schemes based on a pseudo-random permutation secure?

I am currently working on the following task: Let F be a pseudorandom permutation. Consider the encryption scheme for the message space $\{0, 1\}^n$ defined as follows: $Gen(1^n)$ chooses ...
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### Effective ways to find partial sha1 collision [migrated]

I need to find 2 different string and compare their hash value. Both string must contain "abc". I looking for the first eight character that are the same and stop. I have been running for more than 24 ...
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### How are security proofs done in ABE Schemes?

I have been studying several ABE schemes and I understand the security assumptions and the several types of security models used for the security game between the Challenger and the Adversary. What ...
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### Why does Carter-Wegman MAC have 2n-bit output?

The Coursera course on Cryptography 1 has a section about Carter-Wegman MAC. Here is the image of slide: Now my question is both $F$ and $S$ output $n$-bit strings. We XOR two $n$-bit strings and we ...
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### DTLS 1.2 Finished message

I am implementing DTLS 1.2 and using ECDHE_RSA_AES_128_GCM_SHA256 cipher suit. Below is my client finished message Can anyone explain this message? How many bytes for verify data? Is there ...
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### CPA on theoretical OTP with CBC mode

Body of question Note: this is THEORETICAL exercise. I understand OTP and CBC does not get along. I have to find a CPA on CBC enc-mode using OTP as the encryption algorithm, s.t a new key is ...
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### RSA Encryption problem for Discrete Math

I am doing practice problems for my upcoming final exam, and am having trouble with this RSA encryption problem. If any one could check to see if i did these correctly, it would be greatly ...
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### How to decrypt an RSA ciphertext given an oracle providing the lower 8 bits of decryptions?

I have access to an oracle that can encrypt and partially decrypt a number with RSA-1024 algorithm. For encryption: $$C = M^e\bmod n$$ But for decryption, result will ...
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### How do you prove an encryption scheme is IND-CPA or IND-CCA?

I know for proving a scheme is not IND-CPA or not IND-CCA you can construct an Adversary which has an unfair advantage in an experiment/game, but how would you prove a scheme is IND-CPA or IND-CCA?
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### How does hash function in Elgamal signature scheme prevent existential forgery attack?

I have heard that hashing the message M prevents an existential forgery attack. I was wondering how.
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### How to construct Hash function of type $H:\{0,1\}^x \times G \mapsto Z_p^*$?

In many cryptographic protocols design, I have seen the statement 'Choose a Hash function $H:\{0,1\}^x \times G \mapsto Z_p^*$ i.e a hash function which takes as input a binary string of finite ...
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### Is the following 4 digits code generator secure and non-deterministic?

In a C# WCF service, I need to send a 4 digits code by SMS to validate a user account/password. For security reasons, the code has to be non-deterministic (so that a malicious user/hacker should not ...
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### How is the block size of a block cipher related to how susceptible the block cipher is to a birthday attack?

For example what can you say about how susceptible Triple-DES is to a birthday attack due to the fact that it has a block size of 64-bits.
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### Secret construction

Given set of random strings $w =\{s_1, s_2, s_3 ... s_n\}$ are there any algorithms to construct single master secret $S$ from them? Subsequently we should be able to deconstruct the $S$ back to ...
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### tls 1.2 finished message stucture

I am implementing TLS 1.2 and using ECDHE_RSA_AES_128_GCM_SHA256 cipher suit. According to RFC 5246 finished messages have {verify_data[verify_data_length]} and verify_data = PRF(master_secret, ...
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### Security of randomized data dependant key schedules?

An example to demonstrate the point with two Feistel networks: Cipher A: \begin{align} roundkey = hash(key || counter)\\ roundkey = hash(key || roundkey)\\ left = left\oplus hash(roundkey || ...
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### Block Cipher vs PRP in CBC?

What is the difference between using a Block cipher and a Pseudorandom Permutation in Cipher Block Chaining mode, which is better?
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### Handshake SSLv2, SSLv3, TLS

Can someone confirm that SSLv2, SSLv3, and TLS have the same Handshake process (ClientHello/ServerHello, ..., ClientKeyExchange ...)? If not, can you tell me the main differences between theses three ...
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### Crack multiple One-Time-Pads with same key [duplicate]

I've got 12 pads, all with a length of 50 characters, except the last one. I also know, that the key has got a length of 50 and that the key has been re-used on all of the 12 pads. What do I have to ...
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### Properly storing an E-mail Password

I'm creating a messaging application that will use an outlook account to send an email. However, I'm unsure about the proper steps to store the e-mail password on the user's computer. Suppose I had ...
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### Difference between single-pass and two-pass MACs

I am having trouble understanding where the count of "passes" come in. What constitutes a pass in the context of message authentication codes? How are single-pass MACs achieved?
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### How to increment the counter value for CTR mode?

I am testing CTR mode on STM32 board. I want to know how the auto increment of counter happens and what is the method to follow if I have to increment the counter manually.
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### check user signed in group signature

I have $N$ users, all of them creating a group digital signature. Also I have user $A$. How can I check if user a signed or not signet in this signature? Is it possible ? Are there other methods how I ...
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### Is it safer to use timestamp in REST authentication?

I am working on a personal project to build a REST authentication system which authenticate clients via three parameters: ...
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### What security checks do I need for SRP-6a?

I'm implementing an SRP-6a login system. What security checks do I need to do? Note that the modulus is sent to the client at algorithm startup. Here is the list I have so far. I'm using the ...
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### How to break a Quagmire 3 cipher?

What would be a good way to go about attacking a Quagmire 3 cipher? I understand that it is polyalphabetic with a key and an indicator. I have started by taking the cipher text, taking every n-th ...
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### Statistical tests for PRNG that generates a sequence which is not binary

Is there a pratical application to PRNG that generates a sequence which is not a binary one? A ternary, quaternary sequence, for instance. If so, how can we test this? Is there any alternative test ...
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### Is this PRG secure?

$G$ is a safe PRG in range $\{0,1\}^n\rightarrow\{0,1\}^{n+1}$. Let us define $G'(S)=G(S\oplus G(S)_{1,...,n})$, s.t. $G(S)_{1,...,n}$ is the first n bits of $G(S)$. Is $G'(S)$ a secure PRG? ...
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### How to find HEXA value of String [closed]

This may be very easy thing, but i am just unable to understand this. I have a string "Hi", and in HEXA its written like this ...
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### Security of AES CTR with multiple messages containing the same known plain text

I have this scenario: The same plain text (that is part of an unkown bigger message) is being ecrypted multiple times using AES 128 CTR with a random key, the key is being transfered encrypted using ...
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### Conditions for a successful Differential Power Analysis attack

From this paper, there are two conditions for a successful DPA attack: i) there exists an intermediate variable in the implementation that is correlated with the power consumption and ii) ...
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### Binary fast exponentiation method

Evaluate $17^{93} \mod 23$ \begin{align}e &= 93\\ &= 1 × 2^6 + 0 × 2^5 + 1 × 2^4+ 1 × 2^3 + 1 × 2^2 + 0 × 2^1 + 1 × 2^0\\ &= |\ 1011101\ |_2 \end{align} Then we have: ...
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### DROWN - decrypting ServerVerify message is not enough?

I have read this article, but I can't understand why after decrypting ServerVerify, the client needs to send the export grade PMS again... When exploiting these flaws the attacker will send its ...