# All Questions

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### How to perfrom modular division while numerator is lesser than the denominator?

I'm implementing point addition and point doubling of elliptic curve cryptography. The formula that I'm using for slope is Point addition: $S = \frac{(P_y-Q_y)}{(P_x-Q_x)}$ where $P$ and $Q$ are the ...
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### How secure is Shamir's Secret Sharing for password sharing when attacker has t-1 shares?

I am designing protocol to share a random generated n long password between k parties using Shamir's Secret Sharing. I know that share alone does not reveal much information about the original ...
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### Will entropy be lost by using a DRBG?

Lets assume we have a DRGB (deterministic random bit generator) which is seeded by a good true RBG (random bit generator). Before any bit has been read from the DRBG, the entropy is clearly the number ...
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### Are all encryption tools made equal?

I'd like to encrypt work-related data backups stored on an external drive. The backups contain proprietary company information and trade secrets. My current backup program, Comodo Backup, allows me ...
96 views

### Cracking a hash which descends to a single value after repeated hashes

Assume I have a hash function (such as the one below) and a cipher, and I know that if I repeatedly hash any string over and over again, it will eventually descend to one unique value. Is there any ...
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### gpg --gen-random quality level: is higher “better”?

The GNU Privacy Guard manual pages have this to say about using the gpg --gen-random 0|1|2 count command: Emit count random bytes of the given quality level 0, ...
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### How many bits of an exponent are leaked when doing a powmod?

How many bits of the exponent $x$ are leaked when you calculate and reveal $g^x \pmod p$ for some generator $g$ of $\mathbb Z^∗_p$? The low bit of $x$ is obviously leaked: the low bit equals ...
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### What about using only XOR gates in homomorphic encryption?

What if we were substitute the ANDs with XORs in some homomorphic encryption scheme like BGV(https://eprint.iacr.org/2011/277.pdf), LTV ( https://eprint.iacr.org/2013/094.pdf) ? It may be more ...
101 views

### Proving that a node is the only child

Say I have a graph of messages like this: A | B / \ C D By including the parent hash in the message to be hashed I can show that B is a child of A, ...
83 views

### AES/DES question about determine a key

I have a question. If you receive a DES encrypted message for which you would NOT recognize the plaintext if it were decrypted, is there any way to potentially determine the key if all keys are ...
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### Can a billion elliptic curve keys be generated on a laptop in less than an hour?

I want my application to generate an EC key pair. The first four bytes of the sha256 hash of the public key should contain a known IP address. As hashes are one-way functions, I need to brute force ...
85 views

### Can a file encrypted with one tool be decrypted with different tool? [closed]

I'm new to cryptography and I'm trying implement a file encryption feature into a Python program but I'm running into a big issue...what is needed for someone else to decrypt the file?! Obviously, I ...
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### Is there any benefit of changing the AES-CCM encryption key periodically even if the Nonce space is not exhausted yet?

I would like an answer that could be sustained by a mathematical proof. Let's say I'm using AES-CCM to encrypt a lot of messages, and I always use a different nonce. I know that when the nonce space ...
111 views

### Can homomorphic decryption of DES be practical?

I was reading this paper Homomorphic evaluation of the AES Circuit by Gentry et al. when I thought if something similar can be done with DES or 3DES, e.g. it is plausible to decrypt DES ...
66 views

### What is the complexity of finding SHA-1 collisions?

Given the recent news about SHA-1 collisions, what is the complexity of finding a SHA-1 collision? Marc Steven's HashClash is still detailing about 260. Does the previous complexity still hold?
71 views

### Are HTTPS web sessions non-repudiable?

(This is probably a basic question, and may be a duplicate; if so, just let me know.) Suppose there are two clients A and B, ...
82 views

### Are there full cycle cryptographic/one-way hash primitives?

I'm looking for behavior similiar to that of LCGs, (i.e. input and output sizes are same). Full cycle of $2^{32}$ different inputs generates full cycle of $2^{32}$ different outputs, distribution of ...
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### X509 certificates: Two Algorithm Identifiers?

I'm wondering why there are two Algorithm Identifiers in a X509 certificate. One is in the "to be signed" part and the other one is at the end right before the actual signature. Are they used for the ...
129 views

### Why not use OAEP for signatures?

There seems to be a distinct lack of implementations of RSA signatures using OAEP to pad the digest. Other than the old, funky, padding schemes, it's PSS all the way. PSS seems to have the whiff of ...
71 views

### How do I tack on “additional data” to libsodium's public-key authenticated encryption?

I note that libsodium provides AEAD when using symmetric cryptography. And yet, under section 8 (Public-key cryptography) there are no public-key methods described which allow for additional data ...
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### How to understand RSA Proofs of correctness at Wikipedia

From Wikipedia(https://en.wikipedia.org/wiki/RSA_%28cryptosystem%29#Proofs_of_correctness): $m^{ed}$ ≡ $m$ $\pmod{q}$ $m^{ed}$ ≡ $m$ $\pmod{p}$ then $m^{ed}$ ≡ $m$ $\pmod{pq}$ Question: if $m^{ed}$ ...
143 views

### Is simple XOR with secret key using DHE secure?

Client and server have given generator 3 and modulus which is 256 bit prime same all the time for all clients and server (simply hardcoded). They both generate a private exponent and send result to ...
121 views

### Is it safe to exchange cryptographic salt and initialization vector in messages?

I have a client which does the following: ...
102 views

### What is tiger192,4 in PHP?

PHP supports hashing of the following Tiger algorithms: tiger128,3 tiger160,3 tiger192,3 tiger128,4 tiger160,4 tiger192,4 Wikipedia's entry on Tiger says there's Tiger and Tiger2 and provides ...
141 views

### Whats the point of an IV if public? [duplicate]

What is the point of the initialization vector in cryptographic modes like AES-CBC when it is sent in the public? Cant any attacker just listen and reverse the IV? and as such defeating its purpose? ...
63 views

### Fixed-points in block ciphers; why don't they indicate a vulnerability?

I've seen a few questions here on fixed-points in ciphers, most asking about the possibility and existence. Most of the answers however pointed out that fixed points are not exactly a security threat. ...
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### Use of the IV for the CBC Block Cipher in TLS1.2

I'm struggling to understand at what point a message is encrypted when looking at RFC 5246. The spec gives the generic block cipher in section 6.2.3.2 as: ...
87 views

### What is matrix branching program?

Recently I am reading something about order-revealing encryption (by Boneh at al. in EuroCrypt 2015) and encountered "matrix branching programming". It seems like it took me forever to understand ...
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### Supersingular Isogeny Key Exchange broken?

Found this report detailing a quantum algorithm for computing isogenies between supersingular elliptic curves. http://cacr.uwaterloo.ca/techreports/2014/cacr2014-24.pdf with the quote ...
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### SHA512 vs HKDF key expansion

I'm working on a secure file storage server. This server allows users to upload and download files based on a "seed". This "seed" is a 128-bit value. From this 128-bit seed, we run SHA512 to derive a ...
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### Prove that H(s) = G(!s) is a CPRNG if G(s) is one

Given a cryptographic pseudo-random number generator $G$, prove that $H$ with $H(s) = G(!s)$ is also a cryptographic pseudo-random number generator. Here $s$ is a binary string; $!s$ is its ...
104 views

### Base point in Ed25519?

The paper "High-speed high-security signatures" by Bernstein et al. introduces the Edwards curve Ed25519. Concerning the base point $B$, it says that $B$ is the unique point $(x, 4/5)\in E$ for ...
101 views

### UKS attack on KEA protocol

I'm trying to understand the UKS attack on KEA protocol. This is described in this paper: Security Analysis of KEA Authenticated Key Exchange Protocol So to summarize: KEA protocol: UKS ...
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### What does “simulator” and “interactive” mean in a cryptographic credential system's definition of security?

Definition 1 on Page 7 of “An Efficient System for Non-transferable Anonymous Credentials with Optional Anonymity Revocation” by Camenisch and Lysyanskaya, uses the term “simulator” in defining the ...
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### Can CFB mode of block cipher be used as a commutative encryption?

Can CFB mode of block cipher be used as a commutative encryption? Is it possible to use CFB mode of AES block cipher to create commutative encryption? With CTR it is possible but not secure.
131 views

### Hash functions to generate one time pad

Newbie question... may be... Stream cypher, will XOR message bits with one time pad bits Suppose I have a pseudo random number generator, like a Mersenne twister or sort of. I could use the ...
153 views

### Why don't use random padding in RSA?

Consider the following scenario: Sender S sends a message to receiver R. He uses a hybrid encryption scheme with RSA as key-encapsulation algorithm and AES-256-CTR (4byte-counter) and SHA256-HMAC. ...
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### Is there any good reason to chain HMACs in this manner?

I am looking at some decompiled Java code from an Android app. As a security measure, a signature is passed as a parameter to a number of JSON requests. The method that generates the signature is as ...
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### Block Ciphers - Standardize plain text

I need to use AES-128 to encrypt a plain text about 720 bits. Is it correct to say that – in this case – the plain text will be divided in 5 blocks of 128, the result will be equal 640 bits, and the ...
85 views

### Logjam-style attack on Factoring?

We're all aware of the Logjam attack, which is known as "FREAK on discrete logarithms". The attack works by doing a large pre-computation step, which needs only to be done once per field and then ...
109 views

### NTRU less secure than previously thought?

A new paper (https://eprint.iacr.org/2015/552.pdf) says: This makes it possible to asymptotically and heuristically break the NTRU cryptosystem in subexponential time (without contradicting its ...
130 views

### Is secure communication without public-key crytography feasible?

We all know about public-key cryptosystems, and we know some of these are computationally secure (for the time being, of course). Are there any algorithms out there which allow safe communication ...
192 views

### RSA public key recovery from signatures

Is it possible (how) to recover public (512 bit long) RSA key from multiple signatures having corresponding plain texts. Padding is not randomized. I need it to verify any future message comming from ...
266 views

### How many bits of entropy can I expect from /dev/urandom in iOS?

Apple's latest security documentation indicates that it now uses CTR_DRBG for generating pseudo-random numbers (previously Yarrow instead of CTR_DRBG), using "timing variations during boot" and ...
136 views

### Does the One Time Pad rely on confusion or diffusion?

From what I could research, XORing a random key adds confusion. But I do not undertand the rationale for that classification. Shannon's confusion is supposed to obscure the relationship between the ...
323 views

### How to prove that someone encrypted a specific (large) chunk of data

Alice encrypts her data with HER secret key. Then she sends the encrypted data to Bob who should encrypt it with HIS secret key. Caroline should then eventually get this double encrypted data. ...
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### Inversion Free Direct Conversion between Twisted Edwards (X,Y,Z) and Montgomery (X,Z)

The Wikipedia page for Montgomery curves shows how to convert points on a twisted Edwards curve to and from points on an equivalent Montgomery curve. However, their description and the original ...
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### Does the Paillier system remain secure if it is used to encrypt only binary values, i.e. {0, 1}?

Is there any security compromises if the Paillier system was used to encrypt only binary message in {0, 1}? i.e., plaintexts are either 0 or 1.
108 views

### Yubico's take on U2F key wrapping

I've been reading up on Yubico's implementation of U2F. What I understand so far is that two things are calculated. So upon registration first you calculate a private key by \$PrivateKey = ...
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### Why must the sender and receiver be synchronised in synchronous stream ciphers?

I would like to know why the sender and receiver must be synchronized in synchronous stream ciphers as said in synchronous stream cipher. Why is there no synchronization between the sender and ...

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