8
votes
2answers
2k views

On-the-fly computation of AES Round Keys for decryption?

The usual implementation of AES first computes all the Round Keys sequentially starting from the key, and stores them in RAM for later uses. However, when enciphering a single block with a key that ...
4
votes
2answers
79 views

Does a key derivation function always generate an even distribution of bits in the resulting key?

I am a student of crypto. During our study course, we went into the AES-128 algorithm. All the examples given used an initial key k, 128 bits and we only briefly touched on the key derivation of ...
0
votes
3answers
113 views

How can complexity be increased or decreased in AES?

I have been studying data compression for a while. For educational purposes, after a lot of reading, I managed to create a software that performs encryption and authentication using AES256-GCM. ...
0
votes
1answer
58 views

Security when using same salt for different passwords, compared to using different salts?

I have this question I am struggling about: For example, someone suggested improving UNIX authentication by defining 3 different passwords for a user that are stored in the shadow file. All three ...
0
votes
0answers
30 views

How are the coefficients of the polynomial used in Mix Column stage of AES chosen?

In the book (Cryptography, Theory and Practice by William Stallings), it states: the coefficients of the matrix are based on a linear code with maximal distance between the code words, which ...
1
vote
0answers
78 views

Encrypted comms between devices on LAN

I'm trying to encrypt a protocol that shares data between devices on a local area network (i.e.no connection to the internet). I don't want to use predefined keys because they're too easy to read from ...
1
vote
1answer
35 views

How does BB84 fare against an attacker who can entangle qubits?

Consider the following attack on the BB84 quantum key exchange protocol (as far as I can tell, it should work similarly for E91): After Alice sends her key bits encoded in qubits with randomly chosen ...
0
votes
2answers
63 views

Diffie Hellman Exchange Confusion [closed]

I am working on this question and I am wondering I have figured out the secret key, but my problem is I don't know how to use the secret key to decrypt the ciphertext. Thanks for the help!
3
votes
0answers
62 views

How to select corresponding ciphertext in anonymous Identity-based broadcast encryption

In identity-based broadcast encryption, suppose the broadcast ciphertext $(r_1,r_2,\dots,r_i,U,W,V)$ is sent to the receiver. It does not describe which user corresponds to which ciphertext part. User ...
1
vote
1answer
40 views

Is the “Hash Tree” described in this paper the same as a Merkle Hash Tree?

I'm implementing an algorithm from a paper, and part of it calls for using a "hash tree". Since I'd never heard of that data structure I looked it up on Wikipedia. It turns out that the name hash tree ...
4
votes
1answer
107 views

What does the probability subscript mean in Shannon's secrecy definition?

Shannon's secrecy can be defined as: $$P_M (M=m) = P_{SK,M}(M=m|E(SK,m)=c)$$ What does $P_M$ means? (same question for $P_{SK,M}$) I know that is the probability space M, M being the messages; I do ...
-1
votes
1answer
24 views

Which one has better Performance?

Sending secret value in identity-based encryption Using xor : $K \oplus e(g,h)^{\gamma}$ Using symmetric key: Assuming $Key=e(g,h)^{\gamma}$ and encrypt $K$ using $Key$. Which one is better in ...
-1
votes
1answer
54 views

How efficient is OpenSSL? [closed]

It is apparent that OpenSSL is widely deployed and used, but how efficient is OpenSSL? What's some efficiency concerns of it? What kind of methods to they use to make OpenSSL efficient? Do they also ...
3
votes
0answers
164 views

Yaos Millionaire Problem: Why distance >= 2?

I'm currently reading about Yao' Millionaire Problem: http://research.cs.wisc.edu/areas/sec/yao1982-ocr.pdf Alice and Bob want to know which of them is richer. Let $j \in \{1, \cdots 10\}$ be Bobs ...
3
votes
1answer
118 views

TLS RSA Certificate with Elliptic Curve Negotiation

When I look at Google's Public Key in Firefox I get PKCS #1 SHA-256 With RSA Encryption Yet according to SSL Labs Google has EC 256 bits / SHA256withRSA and supports ...
-3
votes
2answers
60 views

If we know a sha256 hashed value and its message length can we find the message?

Is it possible to find the original message if we know a sha256 hashed value and its message length?
5
votes
1answer
667 views

Encrypting user data with their password?

I had an idea once that all of the user data (including the username) could be encrypted with the user's password. Obviously, most sites would store different data, so using the same password and ...
2
votes
3answers
203 views

Hypothetical encryption technique, is it secure?

This to me, sounds like a great idea, but with all rosy-tinted ideas it needs some grounding in reality. Here are the steps of the hypothetical system (with the hypothetical part highlighted): User ...
3
votes
0answers
58 views

Simultaneous 2 way exchange without trusted third party

Stakeholders: Alice - has a message, M containing a secret, SA that Bob wants, where H(SA) is a hash which may be used by Bob to redeem some Bitcoin time-locked transaction sent previously by Alice. ...
1
vote
0answers
36 views

How to set attributes to private key on PKCS#12 (key usage)

I make a certificate X509 with library Bouncy Castle on Java. I need set a Key Attribute to private key. ...
1
vote
1answer
44 views

Prove that a function is not PRF

Let $G$ $PRG$ and $G'(s)$ equal to the first $n$-bits of $G(s)$, where $|s| = n$. Show that $F_k(x) = G'(k) ⊕ x$ is not a $PRF$. My attempt: Lets assume a $k$ and 2 messages $x_1$ and $x_2$. So, for $...
1
vote
0answers
36 views

Additive homomorphic encryption over small fields

Are there encryption schemes that are additively homomorphic with respect to small fields such as $\mathbb{F}_{2^4}$ or $\mathbb{F}_{2^8}$?
4
votes
0answers
41 views

Do we still need HMAC now that SHA3 is here? [duplicate]

In Another look at HMAC from Koblitz and Menezes, they state that HMAC came along because doing h(secret || message) was vulnerable to length extension attacks. But as far as I know, these kind of ...
7
votes
2answers
540 views

Is HMAC needed for a SHA-3 based MAC?

HMAC does nested hashing in order to prevent Length Extension Attacks. Given that you use the SHA-3 hash (which is resistant against length extension attacks), would you still need to go through that ...
1
vote
1answer
37 views

Prove that a function is PRF

Let $F : K×K \rightarrow \{0,1\}^{128}$ be $PRF$ function. Is $F'$ $PRF$ ? $ F'(x) = \begin{cases} 0^{128} & \quad \text{if } x = 0\\ F_k(x) & \quad otherwise \end{cases} ...
1
vote
1answer
65 views

RC6 Key Size Understanding

I need help to understand how RC6 works. Many sites show that the maximum key size of RC6 is 2040 bits (255 Bytes). I can also have 0..255 rounds and 4 times 32bits words in the 128bit block variant ...
1
vote
0answers
18 views

How to calculate how many trials I need to guess the plain text if a simple P-Box is used to make the cipher text

If I have 64 bit as input for a straight box that use a transposition process to encipher the plain text and the given 64 bits contains 60 bits as (1s), then how many trial I should do?
4
votes
1answer
85 views

How to prove a function is pseudorandom function?

I am currently enrolled in a cryptography course, which uses the book by Katz and Lindell. I am using a function having $F_{k}$ = $A_{k}(B_{k}(r,m),r)$, the random number is generated using the ...
1
vote
1answer
106 views

Ciphertext with uneven letter distribution

I have a ciphertext encrypted with an unknown method. It is part of a challenge (personal quest), so I can not post it. The letter statistics is the following: ...
-2
votes
1answer
81 views

How many hashes one needs in order to reverse engineer hash function?

If I know the pattern of a hash string, particularly dllldlll or llldllld (d=digit, l=low case letter), is it possible to reverse engineer the hash function? If so, how many hashes ones needs to do so,...
-2
votes
1answer
57 views

Carmichael's function in Cryptography

The Carmichael's function says that for $a\in \mathbb{Z}_n^*$, if $gcd(a,n)=1$, then \begin{equation} a^{\lambda(n)} \equiv 1 \;(mod\; n). \end{equation} My aim is to find $a$ if factorization of $n$ ...
0
votes
1answer
40 views

What is a simple information authentication protocol for constrained devices?

Background: I have an application where devices will poll information from some source on the network. These devices are very basic and have modest processing and memory capabilities. Hashing (and ...
0
votes
0answers
33 views

Security or not in Key distribution

For example, There are 3 clients. I want to deliver specific value $x$ to all clients. $x\leftarrow {0,1,2,...,q-1}$ where q is a large prime number. By using function $f(x)=x*y_{i}*z_{i}$ where i=...
1
vote
4answers
208 views

Can you use a hash as a password?

There are several methods of making a strong password, making it longer is one of them. now we also have hashing algorithms which have outputs of 512, so i wonder, would it be safe to basiclly use a ...
1
vote
0answers
59 views

Software Security Impossible in Non-Trusted Hardware?

Fundamentally it is impossible to protect software if you can't trust the execution platform (e.g. using a TPM), so what is the point of software security then? With vendors claim that their white-...
5
votes
1answer
105 views

Encrypted firmware

I have seen many papers on "encrypted firmware" (for example, firmware on Apple Watch like devices) . Can you tell me how it works? Is there a stub in the firmware which is not encrypted and can ...
4
votes
2answers
97 views

Recover elliptic curve order from ECDSA signatures

I need the elliptic curve order to recover the private key from two signed messages with ECDSA. What I have: two signatures signed by the private-key I want to get the messages that have been ...
13
votes
2answers
1k views

Do key collisions for symmetric ciphers exist?

I'm not sure if I'm calling the thing (key collisions) correctly, and that's probably why I couldn't find any information about it on Google. Still, my question is as follows. Note that I don't work ...
3
votes
0answers
92 views

What is the lightweight key exchange protocol of choice?

Assume I have to implement a secure channel protocol by myself. Assume further that for some obscure reasons (library restrictions?) I can't use any of the pre-made libraries for SSH, TLS and such. ...
1
vote
0answers
47 views

Safe MAC proof check

Let $(Mac, Vrfy)$ be a safe defined $MAC$ over $(K,M,T)$ where $ M = \{0,1\}^n$ and $T = \{0,1\}^{128}$. Is the following $MAC$ safe? Show your proof. $Mac'(k,m) = Mac(k, m)$ $Vrfy'(k,m,t) = \{Vrfy(...
1
vote
1answer
35 views

Affine decryption

A message has been converted to ASCII and then encrypted with the formula: $$ax+b \equiv n \pmod {215475}$$ The encrypted message is: ...
1
vote
0answers
22 views

What do Quantum computers mean for security? [closed]

I know that Quantum computers can speed up brute forcing a key by a ton, but how much? And what does that mean for privacy? Does this just mean that the FBI's side of that court case has won until the ...
1
vote
1answer
91 views

Encrypting a string as smaller pieces vs. encrypting it as a whole

Is there any difference between these two ways to encrypt a string using FPE? Say the string has the format is specified by this rule First part is in the set $S = \{\text{AA,AB,AC,BA,BB,BC,CA,CB,CC}...
1
vote
1answer
75 views

Security of a MAC

Let $(Mac, Vrfy)$ be a safe defined $MAC$ over $(K,M,T)$ where $ M = \{0,1\}^n$ and $T = \{0,1\}^{128}$. Is the following $MAC$ safe? Show your proof. $Mac'(k,m) = Mac(k, m ⊕ m)$ $Vrfy'(k,m,t) = ...
3
votes
1answer
76 views

Highest prime factor that is Safe for a particular scheme

My question is how many bits of prime number is secure so that it cannot be factored from very large number? Until today how large prime factor is found in large number? Quantum computing find ...
0
votes
0answers
15 views

Self-signed certificate becomes invalid after some time [migrated]

I have a web site hosted on IIS webserver. I've created a self-signed certificate for the web site and installed on a client machine. When I install the certificate to a client machine, I can ...
-2
votes
0answers
36 views

Secret Key Encryption in identity based encryption

If we assume $x$ be secret value $x\leftarrow{0,1,2,....,q-1}$ where q is a prime number Instead of using public key encryption schemes, using a function such as $f(x)=x*y/z$ to conceal $x$ value ...
9
votes
2answers
242 views

In the Quadratic Sieve, why restrict the factor base?

In the Quadratic Sieve, when factoring a number $N$, many descriptions and most implementations select as the factor base the set of small primes $p_j$ less than some bound $B$ restricted to having ...
4
votes
1answer
67 views

Number of Sbox in a Cipher

Camellia has 4 SBOX and so does the TwoFish(key dependent 4 Sboxes). Where as AES uses only one. Would it have made AES more secure if AES had 4 or more SBOX too? Does using more Sbox with similar ...
6
votes
1answer
82 views

How to calculate active s-boxes from branch number?

If MDS in AES has branch number 5 (so 5 active s-boxes in 2 rounds), wouldn't that mean 4 rounds of AES has $5*2=10$ active s-boxes? AES paper says it has 25 ($5^2$?) active s-boxes in 4 rounds. How ...

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