All Questions

59 views

Does it make sense to stretch non-keys?

I was thinking about stretching salt. Key-stretching is good against brute force guessing and assumes your system isn't compromised. Salting a one-way makes it so you have to compute * n for each ...
172 views

Using the same RSA keypair to sign and encrypt

The RSA signature operation is basically the same as encrypting with the private key. In particular, both operations use the same kind of keys. Is it safe to use the same RSA keypair both for ...
83 views

Is an elliptic curve over $\mathbb{F}_p$ order preserving for the points $(x,y) \in \mathbb{Z}_p$?

Let $E\colon y^2=x^3+ax+b$ be an elliptic curve, and consider its realisation over the finite field of prime order $p$: $E(\mathbb{F}_{p})$. Then if $0<a,b$ is the following true? \forall ...
96 views

Why doesn't this dummy mutual authentication protocol provide mutual authentication?

I am a student taking a cryptography course so forgive me if this comes off as a silly question. This is an assignment question: ...
62 views

If H(m) = 0 for some m, how can a DSA signature be forged?

If we know for some message $m$ that $H(m) = 0$, how can we forge a DSA signature with only the public key? I got that $g^s*r = (g^x)^r$ where $x$ is the private key, but that's one equation with 2 ...
66 views

How widely is Perfect Forward Secrecy in HTTPS clients? [migrated]

We are hosting an SaaS website and API with the https protocol, both of which are accessed by numerous users with a lot of different browsers and client-side apps. What would happen if I made PFS ...
29 views

Why doesn't this authentication protocol work? [duplicate]

I am a student taking a cryptography course so forgive me if this comes off as a silly question. This is an assignment question: Alice ------R--------> Bob Alice <-----[R]B------ Bob Alice ...
65 views

Differences between working of Tor and onion routing [migrated]

I am learning about Tor and Onion routing and I'm trying to compare both services. From the wiki link, I can see that Diffie Hellman (DH) handshake is used and when look into the “tor-design.pdf” for ...
72 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 ...
94 views

Is a simple stream cipher “partially homomorphic” if no integrity check is applied?

My understanding is that, simply put, a stream cipher is just a CSPRNG such that $R(i,k)$ will produce a deterministic but statistically random sequence, where $i$ is an IV, and $k$ is the session ...
99 views

Weak Stream Cipher cryptanalysis

Hi I have a stream cipher which uses a keystream of: F(r) || F(r+1) || F(r+2) || ... Suppose F(r) is the RSA encryption algorithm: ...
153 views

What is the difference between a 'cipher' and a 'mode of operation'?

What is the difference between the term cipher (a name like RIJNDAEL) and mode of operation (like ECB)? Aren't these both terms for the encryption/decryption technique?
192 views

Are there any bijective one-way functions not based on number-theoretic hardness assumptions?

I'm trying to find a bijective function $y=F(x)$ which should be easy to compute in one direction but hard to compute in the other, where the one-way property is not based on a number theoretic ...
8k views

Who uses Dual_EC_DRBG?

Recent news articles have suggested that the NSA may be involved in trying to influence the cryptography in public standards or commercially deployed software, to enable the NSA to decrypt the ...
45 views

The real-life meaning of proving over a group that doesn't support the oracle?

If I proved a scheme's security under GDH assumption, in real-life, if this DDH oracle does exist, then it's good, but what about other side ? In real-life, if this DDH oracle doesn't exist, then ...
7k views

How can I use SSL/TLS with Perfect Forward Secrecy?

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 ...
104 views

What is the chance of a collision when using SHA-3?

I currently am writing a web app and am using two random hashes that are before and after the string and are both 50 char long (alphabet, numbers, and special chars). ...
205 views

Deciphering a Message [closed]

I have this message to decipher: UYDVKUYEHJNRFRLYZIPVSUQYGAFFBXKAJLVRIETJIUOTSVKXHQZUWYLIOQHQEJR I have tried using methods such as: Ceaser cipher, Atbash ...
122 views

Please explain Pairing based Cryptography with numeric example [closed]

I read concept of Pairing based Cryptography. But while trying understand the concept by doing mathematical calculation, I am facing much problems. So, could you please explain Pairing based ...
68 views

VPN Authentication over WiFi — Is it secure? [migrated]

Count this question as part of my ignorance on how authentication scheme is done on your typical VPN session. But here goes: We often advise the security-conscious user that if one must use an ...
67 views

Is the composition of collision resistant hash with non collision resistant hash a collision resistant hash?

Suppose $h_1$ is collision resistant, and $h_2$ isn't. Will $h(x)=h_2(h_1(x))$ always be collision resistant? [Edited:] the output of h2 must be uniformly distributed
127 views

Blum-Blum-Shub Cryptosystem

I am given $N$, which is Bob's encryption key. I was able to successfully factor $N$ into $N=p*q$. Alice encrypts a messages for bob and sends $a = [1,1,1,0,0,0,1,0,1,1,0,...]$ $b = 1545164156...$ ...
90 views

Graphically representing points on Elliptic Curve over finite field

I have taken elliptic curve $E\colon y^2=x^3-4x+20$, defined over $\mathbb{F}_{29}$. The number of points on the curve, $\left|E(\mathbb{F}_{29})\right|=37$. I took base point $P=(1,5)$, and got ...
66 views

$\phi$ function in Dual_EC_DRBG

I am trying to understand the operation of the Dual_EC_DRBG. I'm reading the formal specification (SP 800-90) and can't seem to find a definition of the $\phi$ function used throughout the definition ...
78 views

Why use two affine transformations in Multivariate Cryptography?

I'm reading Multivariate Cryptography and would like to know why we use two affine transformations to scramble the central map? Why not use only one?
319 views

How to prove the security of the PRNG?

Are there any realties tests or criterias that prove the security of the PRNG? What kind of tests or criteria?
201 views

Testing the Fortuna random number generator?

I have implemented the Fortuna random number generator as described in chapter 10 of "Practical Cryptography" (Ferguson and Schneier, Wiley, 2003), the result can be found at ...
77 views

What is the function of the secret key “r” in Poly1305?

Poly1305-AES uses two per-connection keys $k$ (for AES) and $r$ (for Poly1305's compression function) and a per-message nonce n to execute. I've read the original paper on the implementation, and I ...
62 views

Layered email encryption with multiple private keys on separate media [migrated]

I'd like to encrypt e-mail messages so that multiple physically separated keys are needed to decrypt them. For example, I'd like to have three private keys stored on different media, as in: key A ...
110 views

Splitting a password for dual roles [duplicate]

I would like to prompt users for a single passphrase to establish trust with separate, normally (but not always) complementary systems from one password input. I'm essentially looking for a box where ...
57 views

What is the difference between, and security of $Z_p$ and $F_p$?

While reading some papers, I found that sometimes $Z_p$ was used but sometimes $F_p$ was used ($p$ is prime). Usually, the author will only use their subgroups like this: choose a big prime factor of ...
87 views

What does this step in this protocol for the Yao's Millionaire Problem mean?

I was reading “An Efficient Protocol for Yao’s Millionaires’ Problem” (Ioannidis and Grama 2003). In the proposed protocol in section three, it is written: (Step 4) For every $i$, $1 \le i \le d$, ...
208 views

Is this encryption scheme perfectly secure?

Let $m = 6$, and let $\mathbb{Z}_m$ denote the set $\{0,…,m-1\}$. Let $X \mod m$ denote the remainder obtained when dividing $X$ by $m$. (a) Consider the symmetric encryption scheme in ...
162 views

Does a playlist of songs or movies mixed together contain enough random enough for OTP key material?

Ok hypothetical scenario. Lets say I want to generate a lot of key material quickly for a one-time pad. This key material will be divided up and used for many different messages in the future. I ...
490 views

Would this simple encrypted chat program be feasible using One Time Pads?

Lets say I want to build a simple chat program that has encryption that is impossible to crack for anyone, even a theoretical government with a massive super computer. Ok here's my idea. You'll need ...
99 views

According to this document the padded message has the following structure: $EM \;= \; 0x00 \; || \; 0x02 \; || \; PS \; || \; 0x00 \; || \; M$ What is the purpose of this null byte at the beginning ...
148 views

XOR cipher Pseudo-random key strength

When implementing a XOR cipher with a pseudo-random key equal in length to the plaintext, Is there a significant difference in potential key strength between keys drawn from a pool of either 1000 or ...
378 views

What stops the Multiply-With-Carry Random Number Generator from being a Cryptographically Secure Pseudo-Random Number Generator?

Despite the fact that Marsaglia's MWC PRNG (multiply-with-carry random number generator) is considered to be "the mother of all RNGs", it does not seem to be considered to be a CSPRNG ...
147 views

Can you help me understand hash-preserving encryption?

I have rather simplistic understanding of cryptography, so I have to beg a pardon in advance if the question has internal contradictions. Imagine data $A$ with a hash $hashA$. This data is encrypted ...
119 views

Can AES decrypt with a wrong key?

The title is asking the question in layman terms, but what I really mean to ask is this: Let $x \in \lbrace 0,1 \rbrace^{128}$ (an arbitrary input block) and let $K \in \lbrace 0, 1 \rbrace^{128}$ ...
317 views

Is there a practical zero-knowledge proof for this special discrete log equation?

We have a multiplicative cyclic group $G$ with generators $g$ and $h$, as in El Gamal. Assume $G$ is a subgroup of $(\mathbb{Z}/n\mathbb{Z})^*$. There are two parties, Alice and Bob: Alice knows: ...
114 views

Publicly exposed hash of private key

Would exposing a cryptographic hash function's digest (e.g. SHA-3) of RSA private key data compromise the key? If so, what are the possible (cryptanalysis-) vectors for attacking the key if an ...
21 views

User's Privacy in certificateless cryptography

In practically, while doing registration with Key generation center in certificateless Signature,How can be a user do registration to the Key generation center without providing (Global ...
28 views

Why have AH and ESP security protocol when we could just encrypt packet with a symmetrical key?

Why have AH and ESP security protocol when we could just encrypt / decrypt packets with a symmetrical key and with some block cipher using that symmetrical key?
11k views

What is safer: ZipCrypto or AES-256?

Like in title: which one of these encryption methods (ZipCrypto, AES-256) is more secure and why? I am asking about it because I'd like to know which should be preferred when compressing files with ...
34 views

Can you clarify the proof that secure LR-two party protocols do not exist?

In the paper “Multiparty Computation Secure Against Continual Memory Leakage” on pg. 1237, the footnote #1 discuss why it is not possible to construct a leakage resilient two-party protocol. But I'm ...
53 views

Can minimum encrypted message length be used to identify public key algorithm or derived some of its features?

Encrypting a very small message using public key cryptography always result in larger encrypted messages. Is it possible to deduce the algorithm used, key length, etc just by looking at the minimum ...
52 views

What are the key lengths used in (EC)DHE cipher suites in SSL?

What are the key lengths used in (EC)DHE cipher suites in SSL? Browsers only display key lengths for public key encryption…