# All Questions

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### RSA with modulus n=p²q

I've been wondering what would happen if the RSA modulus would be $n=p^2q$ instead of $n=pq$. I feel like there is an obvious security flaw (as there is with most modifications). Would this choise of ...
195 views

### MAC using a modified CBC mode of operation

I'd like to know whether the following modified CBC-MAC is secure (given a fixed IV). Source: an old course on Network Security Are there some obvious attacks I can't see?
106 views

### Is my implementation of a PRG at least intuitively secure?

I've devised a PRG using Bezier curves , constructed with Bernstein polynomials, and I was wondering if you guys could help me with some suggestions or constructive criticism. Basicly I take a random ...
95 views

### Publicly Verifiable Authenticator

I am working on a Block Storage system, where I have the need to encrypt and authenticate blocks by a client, but also want the server to be able to detect bit corruption while minimizing the work ...
124 views

### Lowest number challenge scheme

Suppose Alice knows a secret number $a$, and Bob knows a secret number $b$. Is there a simple way for Alice and Bob to know who has the lowest number, without Alice & Bob exchanging their numbers ...
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### Validity of authentication scheme?

Here's an authentication scheme I had in mind that combines the simplicity of HTTP Digest Authentication and the security of real crypto primitives. I have in mind the user-level authentication, ie ...
309 views

### PGP message integrity : How can receiver of an email check the integrity using the sender's public key?

PGP message integrity: Basically, only private key can decrypt message encrypted with public key. But in the message integrity service in PGP does the reverse, where the Hash of the message if ...
495 views

### How do we compute IV+1 in CTR mode?

This may be a very simple question, but I have not found info about this in any place. My question is about CTR mode: When it says $IV+1$… How do we make this operation? For example, if I have the IV ...
308 views

### Use AES-256 Or AES-CTR-256 For One Block?

I am have one block of data to encrypt that is 32-bytes long. Is it better to use the AES-256 primitive for this single block (not a stream) or AES-256-CTR, and also would there be any cons or pros to ...
146 views

### understanding the proof of knowledge

Recently I've been reading the paper “A New Family of Implicitly Authenticated Diffie-Hellman Protocols”. It's very hard for me to go further. Especially when it comes to the proof of knowledge. ...
71 views

### AES multiple devices to server communication

I have multiple devices connecting to a server, whose communication needs to be encrypted. At the moment I have an AES library doing the job (possibly inadequately): I am using a database of keys on ...
572 views

### Is there a generic attack on encrypted CRC32 when used as a MAC?

I am examining a protocol that uses CRC32 as a MAC (see note 1) the weaknesses of this method but I would nevertheless like to see if it is just weak or actually relatively easily to break. The ...
214 views

### AES-CTR-256 Sub Keys With Encrypted Sub Key Index

I was working with some encryption and I decided to put a spin. To preface this would be used to encrypt multiple files. You start with a key file generated from random data. For example you could ...
208 views

### Allowing multiple password reset tokens, and a question on random number predictability

I have a few requirements for a password reset system, along with some questions at the end as to its security. Requirements The aim is to provide a password reset mechanism satisfying the following ...
239 views

### Security equivalent to Diffie–Hellman problem?

I've been doing the security proof for one of my Theorem. Basically, given $g^a$, $g^b$, $g^{cb}$, $g$ and $c$ as known values. Is the problem of computing $g^{acb^{-1}}$ equivalent to the Diffie ...
221 views

### Spoofing protocol nonce

Amy and Betty have a shared key $k$, and the protocol below is to provide a mutual authentication for both Betty and Amy. A sends B : $n_a$ B sends A : $n_b \;\|\; E(k, n_a)$ A sends B : $E(k, n_b)$ ...
147 views

### Rate my idea: is that a stronger GPG-like algorithm?

I came up with a simple idea but I don't know if it is really secure or not. I know that you shouldn't create any algorithm on your own. I'm just curious, that's all. Algorithm explanation: First ...
83 views

### Process of dividing 168bit key into 56bit keys in TripleDES

I used TripleDES so many times but never thought about this. In TripleDES it requires 168bit ...
107 views

### Turning PRPs in PRGs with a counter

I am following the Coursera Cryptography I course and I have the following question, I am a bit perplexed by the statement, in week 2 lecture "What are block cyphers?" that a counter-mode PRF is a ...
418 views

### Can a EC private key be derived from a public key?

I understand that the public key does not expose the private key. That is not the question. The question is: Given a EC public key, can a different, but plausible and functional private key be ...
418 views

### KDF based on HMAC-SHA-256

Is KDF based on HMAC-SHA-256(Hashed Message Authentication Code, Secure Hash Algorithm) algorithm a suitable option to generate symmetric key from the secret key? What is the basic funda, behind ...
204 views

### Difference between “ECDH with cofactor key” and “ECDH without cofactor key”?

I need to use “ECDH with a cofactor key” for generating symmetric key. I have a fair idea on how ECDH works, but I don’t understand the cofactor part. What is the difference between ”ECDH with a ...
269 views

### Importance of salt when deriving an encryption key

I'm very curious to know this and a bit confuses too: Suppose, I have two files encrypted using AES-128bit with keys PBKDF2-derived from the same password and the same salt. If an attacker does ...
71 views

### Are HMACs based on hashes with larger bit-lengths also more secure?

When doing encrypt-then-mac, I can choose to use a hmac as the MAC. For example, I could use a hash like SHA-256 or SHA-512 (by using it as a keyed hash) to create that HMAC. Does it increase ...
2k views

### Is it safe to seed a random number generator from system time?

It seemed to me that the Bouncy Castle SecureRandom class for C#/.NET only uses DateTime.Now.Ticks as its seed by default. I ...
153 views

### Advantages of making streamcipher stream depend on plaintext?

Most descriptions that explain how streamciphers work (like the one on Wikipedia), tend to describe a model that boils down to a simple “$ciphertext = plaintext \oplus stream$”, where the stream is ...
255 views

### OCB nonce usage

I've just come across this piece of code in Bouncy Castle's implementation of OCB Mode: ...
434 views

### Polynomial representation of the affine part of the AES S-box

I have been reading up on the polynomial representation of the AES Sbox (PDF: “Essential Algebraic Structure Within the AES” by Murphy and Robshaw) and I can't get it to produce the correct output. ...
619 views

### Is TripleDES 168bit vulnerable to Differential Cryptanalysis?

I was reading this paper. In the table-1 of the paper it says TripleDES (showing both 128bit and 168bit keysize) is vulnerable to Differential Cryptanalysis. It also says that AES (showing all three ...
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### Attacks on Full TripleDES [closed]

I have been reading about attacks and many of them work on specific rounds of TripleDES but not on full TripleDES. Is there any attack except brute force that works on full TripleDES? (theoretical)
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### Is it possible to deduce the value of a string when given its hash and multiples hashes of the string + other known strings?

To be clear, I have access to $H(S)$ and as many $H(S||K)$ as I want. Every string $K$ is known. Can I use the extra information to my advantage and try to deduce $S$, without requiring bruteforce? Or ...
172 views

### Storing encryption key

It's my first time using stackexchange! So, I am doing a password manager in Java. But now I am VERY confused because of the encryption. There will be the program login, where the user types its ...
1k views

### Modes of operation that allow padding oracle attacks

It seems to me that padding oracle attacks are mainly a concern for users of CBC mode encryption. Question: are any other modes of operation vulnerable to padding oracle attacks? And if so, why? ...
599 views

### How secure is using a pad (using xor) on a encrypted data, for the purpose of obfuscating/hiding the underlying encryption?

Edit: I know this isn't really a one time pad (as pointed out), though I don't really know what to call it aside from just a pad, I guess. I have been reading around (mostly various stackexchange ...
214 views

### Given $g$, $b$, $g^{ab}$, is finding $g^a$ a hard problem?

As in the title, given $g$, $g^{ab}$ are big elements in a prime group $Z_p$ and $b$ in prime group $Z_r$ ($p > r$, $g$ is one generator of $Z_p$). $a$ is unknown and also in $Z_r$, is finding ...
187 views

### Generalized Merkle Signatures, SHA-3 and Sakura

Sakura specifies a tree hash mode for e.g. Keccak (SHA-3). Are there any reasons to use the Sakura tree hash mode in a Generalized Merkle Signature Scheme? It seems to me Sakura primarily solves a ...
2k views

### How long does it take to crack PBKDF2?

How much time will it take to crack PBKDF2 while using a 9 character password? I'm not specifying any specific system or platform. If a brute force attack is made using the best ever super computer ...
232 views

### How to find which padding method is used in Block cipher(AES) encyption?

I have a cipher text which is encrypted using AES128 Algorithm. I want to know the following things to decrypt the cipher. IV (Initialization Vector) Padding Method Encryption Key How do I find ...
658 views

### Can I crack an AES string if I have all these parameters?

This is for a challenge at followthewhiterabbit.trustpilot.com: Knowns: The algorithm is AES (Rijndael) Blocksize: 128 Keysize: 256 You only need to find the first 6 bytes of the ...
785 views

### Using GMAC for Authentication without encrypting the message

I am pretty new to cryptography and have a sample of GCM mode of operation for AES. PFB the code ...
1k views

### Does having a known plaintext prefix weaken AES256? [duplicate]

I plan on storing data, encrypted with AES256, where the first dozen bytes are known plaintext. ('Hello World!' for example.) Does having this known plaintext prefix weaken AES256? I do not care ...
183 views

### Collecting entropy: can I use timing of reading as a separate source from the reading itself?

I'm implementing an entropy gathering program. For that I collect 2 bits from two gpio-pins connected to a random source. I whiten this stream of bits using the von Neumann decorrelation algorithm. ...
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### How to calculate complexity of ElGamal cryptosystem?

How to calculate time and space complexity of ElGamal encryption and decryption as there are two exponentiation operation during encryption and one during decryption? Here is my code for encryption: ...
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### Decrypting an encrypted text out of order

Does there exist a cipher that can be encrypted twice with two different keys and then decrypted twice using either key first? For example: ...
1k views

### Ciphertext indistinguishability (IND-CPA) for symmetric cryptography

My understanding are these steps are necessary for symmetric cryptography to be IND-CPA: The adversary submits two distinct $M_0$, $M_1$ plain-texts to the challenger. The challenger selects one of ...
175 views

### Attack on DSA modification with bad hash function

Given a (bad) modification of DSA where the hash function is the identity ($H(m)=m$), I am now supposed to design an attack. So $(m, (r,s))$ are given and I should be able to create a legit signature ...
129 views

### A question about elliptic curves and finite fields in bilinear pairings

Based on what mentioned in the paper "Pairings For Cryptographers" http://www.sciencedirect.com/science/article/pii/S0166218X08000449 the two inputs of a pairing map are two members of two additive ...
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### Protocol: Coin Flipping over phone

Protocol: Coin Flipping over telephone Hypothesis: Alice and Bob have agreed: a one to one function $f$ which is easy to compute $f(x)$ from $x$ and while given any value $f(x)$ it is nearly ...