3
votes
2answers
280 views

Does encrypting a hash of the message yield a secure MAC?

Is $mac = enc(hash(x))$ a secure MAC for hash = SHA256 and enc AES in CBC mode or ECB mode. Normally AES-CBC is malleable. But in this case I use a hash function for the one-way property. For ...
3
votes
1answer
449 views

Why does NaCl's crypto_secretbox for xsalsa20-poly1305 require the first 32 bytes to be zero?

The NaCl web site says this is a requirement of the API, but its seems more than just an API issue. It means the first 32 bytes of the first salsa20 block are effectively ignored. I didn't see ...
3
votes
2answers
525 views

Encoding information in packet lengths to actively sidestep encryption

If you've only got an encrypted data channel can you actively sidestep the encryption and communicate with an outside party who can see the data but cannot decrypt it? Yes - if you can find some ...
3
votes
2answers
310 views

bcrypt and pbkdf2 double hashing

If I want to protect myself from implementation of algorithm issues and inherent design flaws of algorithms in password hashing, what would be most secure way to combine two hashes? ...
3
votes
1answer
854 views

Meet-in-the-middle with checking complexity

In regards to meet in the middle type attacks, I have been considering the amount of operations in order to successfully find a key given two sets of plaintext / ciphertext pairs. All of the sources I ...
3
votes
1answer
146 views

Could using sequential plaintext uncover AES key?

I am encrypting a string in a format like this: ...
3
votes
2answers
286 views

Is the polynomial reconstruction problem really “intractable”?

In the family of the believed (experts haven't come yet with a solution) intractable problems in cryptography belongs the Polynomial Reconstruction problem in which an attacker is given $n$ points in ...
3
votes
1answer
290 views

Using same AES-GCM setup for multiple packets?

Most cryptographic libraries I've encountered uses some variant of processPlaintext(...) and then doFinal() to produce the ...
3
votes
3answers
259 views

Construct a random permutation from a random function?

How can we construct a (pseudo) random permutation from a (pseudo) random function ? I've seen a method from Luby-Rackoff which allows to construct a PRP of length $2N$ from a PRF of input/output ...
3
votes
4answers
571 views

RSA by hand - did I do something wrong? (c = m on encryption)

to understand RSA better I am doing a little calculation by hand, this is what I got: Choosing: $p = 3$ $q = 5$ $n = 15$ $\phi(pq) = 2 \cdot 4 = 8$ $e > \phi(n) => e = 13$ $e \cdot d = 1 ...
3
votes
2answers
341 views

What are the standard procedures in cryptanalysis to analyze unknown ciphertext?

What are the "standard procedures in cryptanalysis" to analyze unknown ciphertext? In other words: Are there any protocols, officially acknowledged checklists or something like that which represent a ...
3
votes
2answers
170 views

Is this exponentation or bitwise XOR

I'm reading about SRP from this page and came accross the line that says a party computes v = g^x I am unfamiliar with reading cryptography texts. Does this mean gx, or g ⊕ x?
3
votes
1answer
479 views

How much data can I encrypt with AES before I need to change the key in CBC mode?

In my cryptography class, the instructor suggested that in order to give the attacker a minimal advantage of $1/2^{32}$, we have to change the key after $2^{48}$ blocks are encrypted. It seems that ...
3
votes
2answers
415 views

Is there a way to do fair exchange between two parties who don't trust each other?

Let's suppose we have an Alice who knows a secret key A, and Bob who knows key B. Using their own keys, they each encrypt a message (Alice encrypts $m_A$, Bob encrypts $m_B$) with their own key, and ...
3
votes
3answers
252 views

Ciphers in CBC mode reveal place of change in plaintext

Theoretically, when using a symmetric block cipher in CBC mode, the current block is dependent on the previous block. Suppose one plaintext is encrypted using CBC, and then one bit of it is changed, ...
3
votes
2answers
552 views

Is this hand cipher any more secure than the Vigenère cipher?

I know that inventing one's one crypto always sucks, but the problem is that hand ciphers are usually very insecure very slow. This is an attempt to make a relatively secure, keyable, and ...
3
votes
1answer
297 views

When confusion is applied during encryption?

I know confusion is used in cryptography to make it more difficult to identify any relationship between the ciphertext and the symmetric key. I want to know when the confusion is applied during ...
3
votes
1answer
605 views

XOR cipher for encrypting compiled C code

I'm exploring ways of encrypting Intel hex files we send to customers for flashing onto an embedded device. The embedded processor itself has a built-in mechanism that prevents anyone from reading the ...
3
votes
2answers
1k views

Validating successful decryption in AES

I have a program which uses AES-256 in CBC mode to encrypt and decrypt files. As I have quickly realized, AES will even use an incorrect passphrase to decrypt data, which leaves me with no way to ...
3
votes
1answer
201 views

Non-cryptographic hash function as MAC for stream ciphers

I understand that for a stream cipher to be useful, there must be a way to verify that the message was not tampered with (bits were flipped by an attacker). So, instead of using some cryptographic ...
3
votes
2answers
1k views

Which categories of cipher are semantically secure under a chosen-plaintext attack?

I am trying to get my head around the circumstances under which a cipher is (or is not) semantically secure under a chosen-plaintext attack. I can't seem to find a good reference explaining this. I ...
3
votes
2answers
342 views

Crypto USB Devices - where is the PIN/Password stored?

I am currently looking at the security of cryto USB drives for storing x509 certificates. I have one in possession currently. It can be read/written to using the Microsoft Crypto APIs. Anytime one ...
3
votes
1answer
246 views

A question regarding relevance of vulnerability of MD5 when linking multiple records together

I have been studying a Supreme Court case "IMS Health vs Sorrel". In this case a Friend of Court brief filled by "Electronic Privacy Information Center" states that use of MD5 hash function to link ...
3
votes
1answer
246 views

S-box with differential uniformity = 2

I read that we do not know if there exists an 8x8 sbox with differential uniformity = 2. I suppose we cannot compute every possible sbox because there are $64!$ possible s-boxes. Am I right?
3
votes
2answers
271 views

Information leakage in real-time voice encryption

I saw some time ago a post discussing the issues surrounding encryption of speech/voice in VoIP and how it can leak information (can't find it now). Can someone provide an insight into some of the ...
3
votes
2answers
261 views

Proof that a function constructed from a PRP is, or is not a PRF?

Let $E$ be a family of Pseudo-Random Permutations of $b$ bits, with $E_K$ indistinguishable from random permutation with effort less than $O(2^b)$, easily computable as well as the inverse ...
3
votes
2answers
11k views

FIPS 140-2 Compliant Algorithms

Is there any reference to check the list of encryption & signing algorithms which are compliant to FIPS 140-2. After an exhaustive search I could find only "AES". Any suggestions would be much ...
3
votes
2answers
631 views

Additive ElGamal cryptosystem using a finite field

I'm trying to implement a modified version of the ElGamal cryptosystem as specified by Cramer et al. in "A secure and optimally efficient multi-authority election scheme", which possesses additive ...
3
votes
2answers
360 views

Untraceable communication protocol

I am doing a research about secure communication protocols. I would be interested to know whether a protocol exists such that it grants that the two end-points taking part to the communication cannot ...
3
votes
1answer
616 views

Advanced Encryption Standard Algorithm Secret Key

just a follow up question with this question.. How does the key schedule of Rijndael looks for keysizes other than 128 bit? A 256-bit key can still encrypt a block of plaintext in 128 bit? but in ...
3
votes
3answers
2k views

Does encrypting twice using the same block cipher produce a security weakness?

If I use the output of a cipher, for example a block cipher such as AES and encrypt it again with the same algorithm, I read that this introduces weaknesses into the overall security of the system. ...
3
votes
1answer
208 views

Is a subset of a random set still random?

Suppose you have an infinite list of random bytes, and you select the first N bytes that correspond to printable ASCII characters. Is this list of N bytes still as secure (e.g. for the purpose of a ...
3
votes
1answer
99 views

Is the following scheme secure for different cipher

Let's say I have two keys K1 and K2. two messages M1 and M2 of the same length. Cipher (E,D) 3 ciphertexts: C11, C12,C22 where Cij = E(Ki, Mj) In situation ...
3
votes
1answer
415 views

What is the computational cost of a public key certificate signature verification?

What is the computational cost of a certificate signature verification in terms of exponentiation, multiplication and other computation operations?
3
votes
2answers
201 views

Helper data authentication in PUFs

As I understand, PUFs work by using two procedures: generation and reproduction. Generation reads a value $w$ from a fuzzy source and generates a key $R$ and helper data $P$. Then, in the reproduction ...
3
votes
1answer
298 views

How safe is it to derive MAC key from a hashed password?

Imagine I have a blob that I want to encrypt-then-MAC. Now, what I can realistically ask my users for (out of UX considerations) is just an encryption password. Naturally, I bcrypt original password ...
3
votes
1answer
148 views

Is a using salt important when creating a hash data validator?

I am creating a service that will return an set of objects, which will be used by multiple systems. At the end of the process, one (or more) of the objects will be sent back to our system for ...
3
votes
2answers
150 views

What is sent on the initial communication with Kerberos?

When a user wants to communicate with another user (let's say Alice and Bob), what is sent over the network during the initial communication? Alice uses her Kerberos client and enters her password ...
3
votes
1answer
79 views

what is the public information in Elliptic curve cryptosystems [closed]

Currently my knowledge about Elliptic curve is quite limited to the textbook and I don't know how a practical Elliptic curve cryptosystem works. I read an example about key exchange using Elliptic ...
3
votes
2answers
239 views

Brute-force attack given small search-space and hash prefix

Suppose we have a value $secret which is a random 56-bit (7-byte) string. Suppose an adversary wishes to discover $secret. The ...
3
votes
1answer
78 views

Non adjacent form of an integer is unique

I have tried to look up the proof for NAF (Non-adjacent form) being unique for every integer, but as far as I have seen, textbooks only mention it as a property of NAF, but no proof is given. Also I ...
3
votes
1answer
61 views

dh parameters someone else generated

Am am auditing some software that includes compiled in DH parameters (2048 bits). I can't be sure who generated the DH parameters or how they were generated. Question: Can DH parameters be weak ...
3
votes
2answers
263 views

Does RSA operate over a Finite Field (Galois Field)?

Is it correct to say that RSA operates over a Finite Field (Galois Field)? In this case GF(p)? I do understant that the modulo in RSA is not itself a prime number, but all the operations ...
3
votes
3answers
475 views

SHA1 Collisions - what about practical attacks?

I understand the theorical problem with hash collision but when it comes to practice, I get very confused. Suppose a attacker would like to forge a certificate (or any kind of strucutred piece of ...
3
votes
2answers
186 views

Ways to make a “doomsday” cryptocurrency which becomes untradable

As a social experiment (not a money-making scheme), I'm interested in developing a crazy cryptocurrency which, by its very design, will become worthless and untradable after a certain point. Ideally, ...
3
votes
2answers
93 views

Why verify padding at all?

Whenever a ciphertext is decrypted using a block-cypher, we need to remove the padding. There are different ways to add padding, but they usually set the last byte of the last block to the number of ...
3
votes
2answers
193 views

Attacking unuauthenticated RSA

In symmetric cryptography, a scheme is not considered secure if the data transmitted is only encrypted. The adversary will be able to modify messages to generate encrypted messages of his choice ...
3
votes
2answers
123 views

CBC-MAC key can be added to ciphertext?

I use AES both CBC and CBC-MAC to encrypt some stuff. I generate one key for CBC and one different key for CBC-MAC. Does the second key (for CBC-MAC) need to be secret? How to join such key with the ...
3
votes
1answer
517 views

Why does AES-GCM need MAC? (TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256)

"TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256" is just one example for a cipher suite, as far as I'm concerned it means this: it uses the TLS protocol, exchanges keys with Elliptic Curve Diffie Hellman ...
3
votes
2answers
147 views

Reductionist proofs of decisional problems to computational

Are they any reductionist proofs where an attacker $\mathcal{I}$ for a well established computationally "hard" problem $\mathsf{Π}$ is employing an attacker $\mathcal{A}$ who we assume is able to ...

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