3
votes
2answers
65 views

A bijective hash function

Suppose: $H: \{0,1\}^{n} \rightarrow \{0,1\}^{n}$. $H$ is bijective. It is difficult to derive $x$ from $H(x)$. Is this type of function possible? What would the strength of it be? I realize that ...
0
votes
1answer
76 views

Reducing the probability that a password will be guessed

In a file users' password hashes are stored. There are 2 approaches to reduce the probability that the password is guessed. Increase the size of the salt from 12 bits to 24 bits. Increase the length ...
1
vote
1answer
32 views

Encryption strength when using a PBKDF

Suppose: $m$ is a positive integer. $n=2 \times m$. $H: \{0,1\}^{*} \rightarrow \{0,1\}^n$. $p$ is a password. $s$ is a salt. $K=\{0,1\}^n$ is the derived key. $PBKDF(p,s)$ derives a key from $p$ ...
2
votes
1answer
62 views

Does XChaCha20 exist with similar properties to XSalsa20?

Does XChaCha20 (built from ChaCha20 just like XSalsa20 is built from Salsa20) exist and have XSalsa20's desirable properties?
3
votes
1answer
83 views

How to prove hardness of approximate-GCD problem?

I am trying to prove the security of my system using the hardness assumption of the approximate-GCD problem using contradiction, i.e. If the attacker is able to break in our scheme, then attacker ...
0
votes
2answers
59 views

Stripping off message authentication or signature

If attackers can strip off RSA / EC / -DSA digital signature and conduct CCA on AES-CTR or CBC payload, why can't they do the same for AES-GCM?
1
vote
0answers
70 views

Format Preserving Encryption FF1 Samples

NIST published methods for Format-Preserving Encryption and there is a link in the document giving examples for FF1 methods. I am confused by the value of R given in the examples. $R$ is the output ...
0
votes
2answers
173 views

Is there a way to create an encrypted email with no third party application

I would like to send and receive emails between group of people. Is there a way to create it with no need to use some other 3rd party application. Is there a way just to transfer files between 2 ...
1
vote
1answer
76 views

How to use AES-CBC to password-encrypt a small block of stored data

I have a small block of data (16 bytes) that I am required to encrypt, store (in an unsecure, visible location). Later I have to retrieve and decrypt it. I must use AES-CBC and a user-entered ...
12
votes
1answer
1k views

Easy explanation of “IND-” security notions?

There are many schemes that can advertise themselves with certain security notions, usually IND-CPA or IND-CCA2, for example plain ElGamal has IND-CPA security but doesn't provide IND-CCA security. ...
6
votes
1answer
324 views

Security of a simple AEAD construction

Introduction I want securely communicate with a small 8-bit device which has a very limited program program memory (Arduino Uno - 32K). My goal is to minimize code size and RAM usage. There are ...
5
votes
1answer
286 views

How does Microsoft's BitLocker Recovery Code work?

I don't trust Microsoft BitLocker because it is closed source, which makes it so people can't audit it. But now I'm really interested in how its recovery code works. In my own knowledge, encrypted ...
3
votes
1answer
128 views

Time taken for a brute force attack on a key size of 64-bits

You can do $2^{30}$ encryptions per second and the key size is 64 bits. $2^{64}/2^{30}$ should give me the time taken for a brute force attack? What if you double the key size? How can I calculate ...
4
votes
1answer
94 views

Beavers Triple Vs BGW Multiplication on MPC

Typically MPC protocols that are secure against semi-honest adversaries recommend the use of the revised GMW multiplication protocol by Gennaro et al. This is not the case against Active adversaries ...
2
votes
0answers
58 views

AES encryption time difference for same data [duplicate]

When encrypting same data with AES, encryption time for the first encryption is longer than the second one. In the same program, I have written encrypt() function ...
5
votes
1answer
71 views

Can RSA-PSS signing be made deterministic without loss of security?

Is it possible to make RSA-PSS signing deterministic without loss of security? Specifically, I would use Blake2b of the key and message to generate the salt.
2
votes
1answer
115 views

Top notation as exponent

In cryptography, what does the Top notation mean? It resembles a capital T: T In particular, I saw it in the follwing context: $c = (c_1,c_2)^\top$ where $c_1$ and $c_2$ are homomorphic ciphertexts
0
votes
1answer
80 views

What is necessary for generating an elliptic curve?

Let's say I want to generate my own elliptic curve with an order whose bit length is $n$ (specifically 2048, 4096, and/or 8192)? How would I do this? What needs to be done? What software can do this? ...
0
votes
2answers
44 views

Is ssl_sign safe as it is using OPENSSL_PKCS1_PADDING

I've been hearing a lot of scare talk about PKCS1 padding attacks. Is PKCS1 v1.5 safe in signatures as it is practically transporting only message digest? I'm using it in one of my applications so i'...
0
votes
0answers
12 views

Do eCryptfs encrypt same source files into same retulsing encrypted files?

Can I assume that for given eCryptfs mount, files with byte-by-byte same content will map into byte-by-byte same encrypted files in underlying filesystem (i.e. after umount) ? (i.e. if resulting ...
-2
votes
3answers
546 views

How to reverse this hash function?

I have a function that takes an $m$ byte inputs $x_i$ and maps it a 32 byte outputs $y_j$. The hash function is defined as: $$y_j = \sum_{i=1}^{m} (x_i)^{i-1} \pmod {127}$$ The input is restricted ...
1
vote
0answers
29 views

WPA-PSK authentication without password [closed]

Suppose I have a service in which I allow users to share their WiFi connection. When sharing their connection, users provide their WPA PSK password to my service. Now if somebody wants to use this ...
6
votes
2answers
101 views

Confusion about definition of homomorphic encryption

I am trying to better understand homomorphic encryption, but I feel like I keep getting inconsistent information in the papers that I am reading. One of the papers I am reading says the following: ...
0
votes
1answer
57 views

How does Hash with random cryptography work?

I am new in this world of cryptography and while reading some articles I got stuck into something that I did not understand correctly: the use of hash based on random cryptography. On my knowledge, ...
13
votes
5answers
735 views

Could one construct a cipher that is secure for friendly parties to use but insecure for hostile parties?

Consider the situation of a nation state (Blue) at war with another nation state (Red). Blue wants to deploy a secure cipher that blue currently can not break, but they are considered that Red could ...
3
votes
0answers
64 views

Majority encryption algorithm?

Assume that I want to leave an encrypted message to a group of $n$ people in a way that they can only decrypt it if they work together in the following sense: For some fixed $k < n$ every sub-...
1
vote
1answer
82 views

Does OpenSSL apply ASN1 encoding to the hash before signing using ECDSA?

I read on stack overflow that OpenSSL performs ASN1 encoding to the hash before signing it for, for ECDSA. In other words, OpenSSL performs the following steps when for an Elliptic curve key ...
1
vote
2answers
120 views

Understanding Hash function notations

I am implementing a Certificateless Cryptography Algorithm by referencing the paper An Efficient Certificateless Encryption for Secure Data Sharing in Public Clouds. In this paper on page no. 4 during ...
5
votes
0answers
60 views

Software timing attack using Kocher method

What's the minimum number of random sample points needed in Kocher's timing attack, so that we can determine enough valid measurements of $A_{i,r}$ and $D_{i,r}$? I'm working from this paper: Volker ...
0
votes
2answers
106 views

What is the strength and attack vectors of this simple scheme?

Let's say that Alice wants to communicate some secret value $S$ to Bob through a public channel in a way that other people don't know what the value was. To do this, Alice publishes that secret value ...
3
votes
1answer
87 views

Why is 128-bit considered “medium term” security?

Why is 128-bit encryption considered good enough for medium term security only? How is expected to be eventually broken? Quantum computing or brute force attack?
5
votes
1answer
367 views

Does salt size affect password hash security?

I use PBKDF2-SHA512 with an iteration count of 128,000 to hash my passwords. I use a CSPRNG to generate a salt per password. However, I am unsure about the ideal size of the salt. I have read a lot of ...
0
votes
0answers
27 views

Weaknesses of RSA for $e_A\neq e_B: \gcd(e_A,e_B)=1$ [duplicate]

If two people, A and B, are using a common RSA modulus N, and they select distinct exponents $e_A$ and $e_B$, such that they are coprime, why does this mean that someone can decrypt (in polynomial ...
1
vote
0answers
32 views

Increase in security with increase in size of salt [duplicate]

Does increasing or doubling the size of salt result in improved security? It will require some extra time to brute force the passwords, but any added advantage?
3
votes
1answer
100 views

ElGamal - What if the Decisional Diffie-Hellmann problem could be solved?

I have read that if the DDH problem could be efficiently solved, the IND-CPA would not hold for ElGamal. I don't see why it makes ElGamal less secure if you have $g^a$, $g^b$ and $g^c$
0
votes
1answer
72 views

Inverting RSA using an oracle

Say you are given an efficient deterministic algorithm 'I' that can invert the RSA function on 1% of the points in $Z^*_{N}$. That is to say that if y $ \in $ $Z^*_{N}$ is a "good" point for 'I', then ...
1
vote
1answer
43 views

SHA-1: number of possible inputs, number of possible outputs, how many inputs have the same output,

From Wikipedia I read SHA-1 function produces an output of 160 bits and expects a max message size of $2^{64}-1$ bits. Is that right? If I order all possible inputs I would produce $\approx 2^{2^{64}}...
3
votes
0answers
41 views

Committing to a secret shared value

Suppose I run a multiparty protocol. At the output stage, I have the function output in a secret shared manner among all parties. Now I want all the parties to commit to their value of the secret ...
8
votes
1answer
2k views

Sending KCV (key check value) with cipher text

I was wondering why it is not more common to send the KCV of a secret key together with the cipher text. I see many systems that send cipher text and properly prepend the IV to e.g. a CBC mode ...
3
votes
3answers
74 views

Are there any hash functions that use integers of arbitrary bit size (e.g., int63)?

More precisely, I am looking for hash functions where the internal state can be decomposed as a vector of integers of size 63. That is, machine integers where one of the bits is fixed and unusable.
1
vote
1answer
52 views

Huffman coding is a prefix code, proof [closed]

How would you prove that Huffman coding $C$ is a prefix code, i.e $$\forall x,y \in C,\ \forall z \in \lbrace 0,1\rbrace^*,\ x\not=y||z$$ Using induction according to the number of items to code? ...
14
votes
3answers
2k views

Is there any more information on this RSA backdoor?

Someone posted this on reddit a few days ago, and I haven't found much about it except for links back to the same page. Here's the article http://programmingpraxis.com/2016/01/19/rsa-encryption-...
4
votes
1answer
68 views

How does order-preserving encryption work on string?

I have read “How does order-preserving encryption work?”. After that, I completed order-preserving encryption on integer data. Now, I have four questions in this subject: Is it possible to apply ...
1
vote
0answers
133 views

Vigenère cipher frequency analysis not working

I'm writing a C++ program to use the Kasiski method to crack the Vigenère cipher. I'm testing it with a bit of Shakespeare encrypted with the key ABCDEFGHIJKL. It ...
18
votes
0answers
50 views

Is there a key for AES-128, 256 etc, where cleartext equals ciphertext? [duplicate]

Is there a key for AES-128, 256 etc, where cleartext equals ciphertext? (regardless of modes like chaining or ctr-cbc)
4
votes
1answer
78 views

Password-based encryption with small typos allowed

I'm trying to write some software to encrypt and decrypt some messages with a password. I am trying to implement it using this post: How can I securely convert a "string" password to a key used in ...
3
votes
1answer
87 views

Schnorr signature security level as compared to AES/RSA

As per the Schnorr's original paper (1991), The Security Complexity $2^t$: We wish to choose the parameters $p$, $q$ so that forging a signature or an authentication requires about $2^t$ steps by ...
0
votes
1answer
42 views

Calculating characters from the sum of ASCII pairs

I have a series of messages (limited to only a-z and A-Z) that are encoded such that the sum is taken of every two ASCII values and concatenated. For instance, the message ...
1
vote
2answers
137 views

Best non-digital cipher?

Is there an undisputed cipher that was considered the best before the computer age? (This is not suppose to be a discussion, it's either a yes or no.) Please give a brief description why it is yes ...
0
votes
2answers
96 views

Brute force knowing a part of the password

So here is the problem, my brother forgot about the eight digit numeric part of his password, he is using Ubuntu 15.04 default whole disk encryption To make it clear, this was his password (example) ...

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