0
votes
1answer
72 views

SSL / TLS digital signature exchange

If server is sending its digital signature, in which message / where does the digitally signature is provided to the client. Is it after the server_hello message because there the public key of the ...
4
votes
3answers
87 views

Is there a way to use bcrypt with passwords longer than 72 bytes securely?

From my understanding BCrypt truncates the password to 72 bytes. If a password is longer than 72 bytes, what is a way to store that password using bcrypt securely without compromising it? Or is this ...
0
votes
1answer
30 views

Why perfect secrecy can be ensured when a plain message and a cipher-text based on one-time pad are correlated?

First, some well-known results are as follows. For random variables $X$ and $Y$, we have $$H(X,Y) \leq H(X) + H(Y).$$ The equality is achieved when $X$ and $Y$ are independent. Second, in one-time ...
22
votes
2answers
3k views
0
votes
0answers
15 views

Website Certificate with different public key [migrated]

I've installed a ssl certificate from https://www.startssl.com/ in a server running with plesk. The website is the following: https://refood.sal-pimenta.pt/ As you can see the certificate is fine but ...
0
votes
1answer
28 views

what is legend of h(r) in this protocol?

can you help me what is legend of h(r) in below protocol? ‎‎$A‎\leftarrow‎{B}\quad:{h(r)‎‎}{,B}{,P{_A(r,B)}}‎\\A‎\rightarrow‎{B}\quad:r$
2
votes
1answer
133 views

Modes of encryption for hard drives?

What are the currently recommended modes of encryption for large disks? Basic requirements would seem to be that you can update an encrypted sector independently of others, which rules out most ...
5
votes
0answers
52 views

Factoring two RSA moduli $N_i=p_i\cdot q_i$ knowing that $p_2=p_1+2$?

It is given two RSA moduli $N_1$ and $N_2$, known to be of the form $N_i=p_i\cdot q_i$, with $p_i$ and $q_i$ unknown primes, and such that $p_2=p_1+2$. Can we make use of that relation to factor the ...
1
vote
0answers
21 views

Difference between IPSEC SA and CHILD SA [migrated]

Let us consider two network entities. ...
0
votes
0answers
44 views

GPG: What public key do I have to publish? [migrated]

I just created a master key and two subkeys. The first subkey is used for encryption, the second one for signing. Now i am wondering what public key do I have to publish on my website. Does is ...
-1
votes
0answers
31 views

nonce is a number that use once.so why in many protocol use nonce several times?

the Nonce is "a number that use once".so why in many protocol use nonce several times? how use nonce in protocols? if problem is save of nonce in database,how saved time stamp and signature in ...
0
votes
2answers
112 views

Asymmetric encryption that is secure for (almost) any foreseeable future

Recently, I've been reading a couple of papers about building cryptosystems that are secure for up to 30 years. But for some applications, this seems a bit low to me. So I am wondering if considering ...
4
votes
2answers
152 views

Use cases for “online” authenticated encryption?

What is the the point of an "online" mode for an authenticated cipher? I understand what "online" means in this context. However, I have trouble coming up with applications that would benefit from ...
4
votes
1answer
180 views

Factoring two RSA modulus with known $|p_1 - p_2| < \ell $

I have two 1024bit RSA modulus $N_1 = p_1q_1$ and $N_2 = p_2q_2$ such that $0 <|p_1 - p_2| < \ell$, and $\ell$ is at most 64bit integer. Can I factorise $N_1$ and $N_2$? What's the answer when ...
0
votes
0answers
25 views

Security for multiple functions for a functional encryption scheme

Assuming a functional encryption scheme $F$ in which an untrusted server is able to learn the sum, the multiplication and the sum of the inverses of an $n$ dimensional vector $x \in \mathbb{F}_N^*$ ...
2
votes
1answer
51 views

Secure ElGamal with OAEP

Is it possible to make ElGamal IND-CCA2 using OAEP or OAEP+? The reason I ask is that I recently answered this question and it came to my mind that OAEP or OAEP+ might be possible solutions. Note ...
0
votes
1answer
63 views

Inserting a backdoor into a cryptographic hash function

I'm curious as to how a backdoor can be built into a hash function. What might it take the form of? What's probability of getting one that can be backdoored in a way that can actually undermine its ...
2
votes
1answer
74 views

RFID Protocol Cryptanalysis

Assume we have the following scheme for RFID: TAG & READER both have initially k keys. Every session the TAG computes $k_i$=F($k_{i-1})$ where F is a function which computes XOR of previous key ...
6
votes
1answer
229 views

The effect of truncated hash on entropy

Suppose i have a 128 bits random binary string (128 bits of entropy), then i hash it using SHA-256, then i take the first 128 bits of the output hash. Does the taken bit string still have (almost) 128 ...
2
votes
0answers
30 views

Encrypt-Mix-Encrypt: Full Diffusion?

I've read this paper about the encrypt-mix-encrypt mode for ciphers and was asking myself if this mode provides "full" diffusion. So if an attacker alters one bit of the ciphertext, how many bits ...
0
votes
0answers
40 views

why are both ipad and opad required for HMAC?

I was reading about HMAC and why it is crucial to avoid security issues with normal prepending / appending the secret k to a message (issues due to merkel damagard construction). I am not familiar ...
1
vote
2answers
82 views

What do these notations mean in the definition of Perfect Secrecy, if we read those in English?

If m: message, M: message space, k: key, K: keyspace, c: cipher, C: cipher space and $E_k$: encryption function, such that $E_k(m) = c,\ m,m^* \in M,\ k\in K,\ c\in C.$ Then, what do the following ...
1
vote
1answer
38 views

Are there equivalent IVs in DES/CBC?

I'm playing around with the CryptoPP library with DES/CBC and I've noticed that for some sets of data, if I decrypt the ciphertext with Key1, IV1, I get a good cleartext result -- but if I decrypt the ...
3
votes
0answers
60 views

How to recover $e$ from $f_A(e) = Ae \mod q$ when knowing trapdoor

I have a silly question, but I don't know a solution, so I need to question. Assume, with a algorithm $TrapGen(1^\lambda)$, it generates $A\in\mathbb{Z}_q^{n\times m}$ with a basis $B ...
0
votes
1answer
54 views

Choosing finite field size in Shamir's Secret Sharing Scheme

The Wikipedia article on Shamir's Secret Sharing says to that to have information theoretical security the splitting algorithm should be evaluated using finite field arithmetic on the field ...
1
vote
1answer
40 views

Errata for NIST SP 800-63

I'm looking through the latest version of NIST SP 800-63, and there is a table (Table A.1) in the Appendix that supposed give the "entropy" of a password for a 94 character alphabet for various ...
3
votes
1answer
253 views

Weaker alternative to Ed25519?

For me, the main advantages of Ed25519 are that it avoids patents (by computing in one dimension) and that it is fast. However, the 128bit security of Ed25519 is sometimes too strong to comply with ...
1
vote
1answer
72 views

Benefit of using random key in Shamir's Secret Sharing

I am implementing Shamir's Secret Sharing, and I find that in (t,n)-threshold scheme, the shares are just using 1,2,3...,n as the key to form the shares ...
0
votes
1answer
36 views

Value of S and r in Fiat-Shamir Protocol

In the book Cryptography and Network Security by Forouzan,in chapter 14, Fiat-Shamir protocol, its mentioned that two large prime numbers p and q are chosen and kept secret. However n=p*q is made ...
1
vote
0answers
53 views

Standards/RFCs for non-Diffie-Hellman perfect forward secrecy?

Some time has passed since PFS introduction and we now have secure&proven asymmetric ciphers that can generate strong keys very efficiently (elliptic RSA variants and/or QD-Mceliece can generate ...
5
votes
2answers
190 views

Interpretation of the results of NIST (p)NRG suite

I have problems interpreting the NIST (sts-2.1.2) suite results. After running the statistics with 100 samples (each sample of 1000000 byte length) a new prng i got this result: ...
1
vote
1answer
36 views

Meaning of Signature hash algorithm field in certificate

I'd like to understand what is the meaning of this field, especially considering that when calling a crypto library to sign data the digest method is specified by the caller. For example, this simple ...
0
votes
2answers
112 views

General questions about cryptography for password security

I'm pretty new to this and find it quite interesting, but my brain won't process how it works so I have to ask some questions :) Why is cryptography used? Is it for extra security once the password ...
-1
votes
0answers
62 views

What are the difference between cryptographic primitives and encryption primitives?

For example, from reading I found that the cryptographic primitives of both AES and DES are Confusion and Diffusion. However the encryption primitive for DES was Substitution and Permutation while ...
1
vote
1answer
79 views

How does the Blowfish algorithm key initialization work

I'm having some trouble understanding the Blowfish encryption algorithm. From the Wikipedia article... The secret key is then, byte by byte, cycling the key if necessary, XORed with all the ...
0
votes
1answer
41 views

Rationale of “r” AES key use in OTR version 3 AKE protocol?

I just tried to review & understand AKE (Authenticated Key Exchange) protocol as defined in OTR secure messaging protocol version 3 here , and aiming to achieve Perfect Forward Secrecy I am a ...
-3
votes
0answers
83 views

Decryption using Key and vector. [closed]

Decrypt the enclosed text using not so advanced encryption technique. Key is “abcdefgh” and vector is “abcdefgh”. ÷ó*ÁÛýÁ n9†“Ü,YÓ©?†]žÀ
-4
votes
0answers
117 views

How to decrypt the text using the key and vector? [closed]

Decrypt the enclosed text using not so advanced encryption technique. Key is “abcdefgh” and vector is “abcdefgh”. ÷ó*ÁÛýÁ n9†“Ü,YÓ©?†]žÀ
-3
votes
0answers
55 views

Question about feistel network [on hold]

What’s the output of two round Feistel network, 1) when input is 1^32||0^32 (L0 is 32 bit 1, R0 is 32 bit 0)? 2) when input is input is ^064(L0 is 32 bit 0, R0 is 32 bit 0)? This is all I have...I ...
-4
votes
0answers
42 views

Why is plain ElGamal not secure?

Why is plain ElGamal not secure? What is the proof behind it?
-4
votes
0answers
37 views

What’s the output of two round Feistel network [closed]

What’s the output of two round Feistel network, 1) when input is 132||032 (L0 is 32 bit 1, R0 is 32 bit 0)? 2) when input is input is 064(L0 is 32 bit 0, R0 is 32 bit 0)?
2
votes
1answer
87 views

Encryption in which ciphertext is bigger than plaintext?

Is there an encryption which would produce a ciphertext longer than the plaintext? (please ignore block-chaining IV stuff, which would result in a data being generated from the plaintext). The point ...
0
votes
1answer
88 views

How does chosen ciphertext attack on Elgamal work?

Can it be proven that attacker can obtain the full message if he knows some plain-ciphertext pairs?
2
votes
2answers
52 views

Zero knowledge-proof for discrete log that is not honest-verifier

Take a cyclic group of prime order. The Schnorr-protocol for proving knowledge of the discrete logarithm of some group element is honest-verifier zero-knowledge, meaning that if the verifier chooses ...
0
votes
1answer
41 views

if using cbc+hmac dealing with small size of data(less than 128bit) , will it cause any problem?

if use AES-cbc,HMAC-SHA256, and there is some txt needed to be enc, and I divide this txt into small packages with variable length, then enc them. some of these package can be less than 128bit. Is ...
1
vote
3answers
68 views

Why do we get the following equation in case of a One Time Pad?

If m: message, M: message space, k: key, K: keyspace, c: cipher, C: cipher space and $E_k$: encryption function, such that $E_k(m) = c,\ m,m^* \in M,\ k\in K,\ c\in C.$ My first question is, is the ...
1
vote
1answer
38 views

Using similar passwords and salts for PBKDF2

I'm trying to generate a bunch of pseudo-random keys for AES using PBKDF2, where the AES ciphers will be used in CTR mode as pseudo-random number generators. My goal is to create a 2-dimensional ...
0
votes
1answer
227 views

Timelock puzzle improvment

I came across this question with this answer about a cryptographic timelock-puzzle that needs approximately 30 years to be solved. There is also an explanation with source code for that puzzle ...
1
vote
1answer
34 views

CTR_DRBG versus OFB_DRBG

As far as I know, in the development of ANSI X9.82, there was consideration of OFB_DRBG, a pseudorandom number generator based on block ciphers like CTR_DRBG, but it appears to have been rejected of ...
0
votes
0answers
25 views

AES-XTS: find key from ciphertext and plaintext [duplicate]

Let's say I have a plaintext, the corresponding ciphertext encrypted using AES-XTS, and the initialization vectors. Is there any way - not brute force, of course - to compute the key? Even if there's ...

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