3
votes
2answers
234 views

Efficiency of finding sub group order vs factorization

Suppose you got a prime $p = 2\mathbb\Pi_{i=0}^{n-1}q_i+1$, where $2^{k-1} \lt q_i \lt 2^k$ for some $k$ and all $0 \le i \lt n$, and that you also got a generator $g$ of one of the prime order sub ...
3
votes
2answers
123 views

Secure Sketch Construction

I would like a user to choose a point on an image which is represented by (x,y). This is meant to be a security token. When a user uses this location to login they are unlikely to to choose the exact ...
3
votes
2answers
272 views

Quadratic residuosity problem reduction to integer factorization

How can one show how to reduce the quadratic residuosity problem to an integer factorization?
3
votes
2answers
716 views

Encrypted counter as IV in CBC mode encryption using same key

I'm currently working with a secure transport protocol that defines the IV to be a counter (incremental nonce) to be encrypted with the same key. This is a followup to a protocol that did not provide ...
3
votes
3answers
174 views

Hash Family Example

I am reading my lecture and I found this definition: Let be $F$ be a family of hash functions, that is, a parameterized set $$F=\{g_k:\{0,1\}^{*}\rightarrow\{0,1\}^{n}|k\in K\},$$ where $n\in ...
3
votes
2answers
362 views

How to calculate leaked information from a check digit

Given a check digit calculated using mod 11, of an encrypted numeric sequence of 8 digits, how much information can be known about the plaintext? More specifically, a single 3DES encrypted CBC block ...
3
votes
2answers
220 views

Have these compositions of block ciphers the same security?

I'm interested by the compositions of the block cipher DES, instantiated with independent keys $k_1$, $k_2$ and $k_3$. Are these 3 compositions equivalent in terms of security? ...
3
votes
1answer
1k views

Encrypt a single file, chunk-by-chunk, each chunk using different key (AES)

Encrypt a single file, chunk-by-chunk, each chunk using different key. I am a security newbie (only took 2 security courses before) But currently I am using this encryption method for my Android ...
3
votes
1answer
134 views

Potential vulnerability in DH key selection - am I understanding this right?

I'm reading through a DH implementation, and I think I found a potential hole. Public $p$ and $g$ values are properly selected. A candidate secret value $\bar a$ is pseudo-randomly selected such ...
3
votes
1answer
263 views

How to use HMAC for large ciphertexts over TLS

HMAC used to secure chosen cipher-text attacks. if we are encrypting a large file (video) and sending it over TLS for decryption, how server can check MAC for ciphertext when we didn't sent whole of ...
3
votes
1answer
576 views

Cracking an RSA with no padding and very small e

I have a project wherein I have to crack a given cipher text encrypted using RSA and have been given N and e. Can someone suggest an RSA attack using a very small exponent e(here e=3) and no padding?
3
votes
2answers
577 views

Timing attack on modular exponentiation

It is known that computing $a^x \bmod N$ takes $O(|x| + \mathrm{pop}(x))$ multiplications modulo $N$, where $|x|$ is the number of bits of $x$ and $\mathrm{pop}(x)$ is the number of $1$ bits (Hamming ...
3
votes
1answer
127 views

How hard are discrete logarithms problems in $\mathbb Z^{*}_{n}$ and $\mathbb Z^{*}_{n^2}$, where $n$ is the RSA $n=pq$

Use the notations form the Wikipedia article Paillier Cryptosystem , assume that the chipertext $c$ and $c^{\lambda} \mod n^2$ are both given, is it possible to compute $\lambda$ easily?
3
votes
1answer
184 views

Why does Rijndael with a 256-bit block require the bottom two rows to be shifted one more space than usual?

From Wikipedia: For a 256-bit block, the first row is unchanged and the shifting for the second, third and fourth row is 1 byte, 3 bytes and 4 bytes respectively. Why the change? Is there any ...
3
votes
1answer
128 views

Hashing a password with the password?

I was wondering if hashing a password with the password would be a good way of encrypting the password. So, the user must know his/her password to get the same result as the one in the database. Also, ...
3
votes
2answers
559 views

How does using salt reduce rainbow table attack?

I could create a rainbow table for a particular salt value and still create a successful attack similar to one without salt value.
3
votes
2answers
370 views

How do single use passwords work for an encrypted message

LastPass encrypts its data before sending it to their servers. The key for the data is generated using a combination of the master password and username. How then can a one-time password be used to ...
3
votes
2answers
359 views

Are hash trees an alternative, quantum-resistant signature scheme which can replace RSA?

Can hash trees provide quantum resistant signatures to replace RSA for signing securely? What is the key size and how many times can we use same key?
3
votes
1answer
856 views

Why can't I break ElGamal encryption by brute-forcing the secret exponent?

I am doing a course on cryptography on coursera and one of the topics covered was the ElGamal Encryption system. I am using the terms as defined in Wikipedia. Alice publishes $g$ and $g^x$. ...
3
votes
1answer
995 views

How to generate a public key from a private ECDSA key?

Having some specific ECDSA curve and a private key, how does one calculate the public key? I am having a hard time finding the algorithm and equations for it.
3
votes
1answer
272 views

RSA Without Padding?

I've been looking at the weakness with plain/textbook RSA, where the same message is encrypted and sent to multiple destinations. In this case, it is possible to recover the message. Given that an ...
3
votes
2answers
167 views

Is there an advantage to storing keys split between several hashes?

I have a question about the way to store a key or password that was used for encryption, so that the application can check if the user put in the right key for decryption. If I make a mistake, please ...
3
votes
2answers
2k views

Cracking the Beaufort cipher

Is there any easy way to crack a Beaufort cipher? We have a Vigenère table, and are trying to guess the keyword. Any easier way?
3
votes
1answer
204 views

Impact of algorithms for factoring using elliptic curves over $\mathbb{Q}$

Recently a few papers have appeared that describe a new approach to factoring, using elliptic curves over $\mathbb{Q}$. See, e.g., Factoring integers and computing elliptic curve rational points, ...
3
votes
1answer
406 views

Is frequency analysis a useful tool against encryption by multiplication?

If I transform natural plaintext by: making each letter two decimal digits, considering the whole as a decimal number; multiplying by the key (some integer constant), giving the ciphertext; would ...
3
votes
1answer
154 views

Attack by replaying messages

Assume Alice and Bob communicate over an insecure channel using one-time perfectly-secret encryption together with one-time secure message authentication code. Say Eve has the ability to eavesdrop and ...
3
votes
1answer
156 views

Correct method to encrypt data so that it can be decrypted only by Alice and Bob

I need to build a system that stores encrypted transactions. Transaction involves always two parties. Payer and payee. Both must be able to download encrypted transaction from server and decrypt it ...
3
votes
1answer
280 views

Why not encrypt salt?

Assuming I had to distribute salt+ciphertext together over an insecure channel, isn't it better to store the salt encrypted? By encrypted I mean with a block cipher and with key and IV derived from ...
3
votes
1answer
1k views

AES CCM vs CCMP

Are the terms AES CCM and AES CCMP are equivalent, or is there any technical differences between the two?
3
votes
1answer
193 views

What are the dangers of predictable (repeated) plaintext structures?

When using a "good", modern cipher (specifically one that provides ciphertext indistinguishability), is it a problem at all if there is some well-known structure in all plaintexts? For example, ...
3
votes
1answer
316 views

Conforming Randomness To An Alphabet

Imagine that we're trying to create a function to generate a random string conforming to a user-supplied alphabet. That way, users can generate random strings with given characters. Something like: ...
3
votes
1answer
471 views

Is symmetric encryption vulnerable to plain-text-attacks?

Imagine that Bob sends a message to Alice for symmetric encryption to send to Charlie. (Only Alice and Charlie know the key.) Alice sends the encrypted message back to Bob to send to Charlie. Can Bob ...
3
votes
1answer
2k views

“Signing” with public key

For this question, the following caveats and assumptions hold: There exists a 2048-bit RSA key pair used exclusively for signing/verification The private key is kept completely private There exists ...
3
votes
2answers
513 views

Is it possible to figure out the public key from encrypted text?

Suppose Alice sends messages to Bob by encrypting the messages with Bob's public key. Eve knows that the data is encrypted using RSA, but does not know the public key. Can Eve figure out the public ...
3
votes
1answer
4k views

RSA cracking: The same message is sent to two different people problem

Suppose we have two people: Smith and Jones. Smith public key is e=9, n=179 and Jones public key is e=13, n=179. Bob sends to ...
3
votes
2answers
352 views

How is de-synchronisation of HOTP solved?

From RFC 4226 I understand how HOTP generates one-time passwords by incrementing a counter and uses the 'look-ahead' window to try to resynchronise (from this counter), if the user tries a few wrong ...
3
votes
1answer
220 views

A set of key pairs and one hash to secure them

I have a simple problem: I have a set of users' ECDSA key pairs, and say I want to encrypt them with a simple algorithm. I have access to one variable that uniquely identifies the user, so I hash it ...
3
votes
1answer
20 views

Deriving a key for HMAC, using PBKDF2

What is the recommended way to derive a key for an HMAC-SHA256, from a passphrase and salt? I'd like to compute an HMAC-SHA256. I have a passphrase, and a message. I note that RFC-7518 (JWA) ...
3
votes
1answer
46 views

Multi-target attacks on AES-CTR with a random nonce

128-bit block ciphers are vulnerable to multi-target attacks where the attacker seeks to attack a collection of keys instead of a single key. A simple example: Generate keys $k_1, k_2, ...
3
votes
1answer
37 views

What information should I send along with an AES ciphertext and is there any standard to do that?

While using some crypto implementations I got to see that many times a comment is added to the ciphertext. For example: ...
3
votes
1answer
363 views

Is XOR adequate for encrypting truly random data? [duplicate]

It is well-known that the two-time pad is very insecure. However, it seems unexploitable in the case where the plaintext is indistinguishable from random (ex. symmetric keys). Is this property ...
3
votes
1answer
109 views

Are there any successful preimage attacks?

Are there any successful preimage attacks on any serious cryptographic hash algorithm or at least on serious-looking non-cryptographic hashes (like MurmurHash)?
3
votes
1answer
48 views

Faster alternative to HKDF

HKDF is used in one of my projects to derive separate keys from a common master key. But profiling has shown it to be the performance bottleneck under certain circumstances. Is there any faster ...
3
votes
1answer
183 views

Homemade Randomized RSA

Considering the following encryption scheme where RSA is used to encrypt a plaintext m and then we choose a random r and compute: $$A = r + m^e \bmod N$$ and $$B = r^e \bmod N$$ So the ciphertext is ...
3
votes
1answer
93 views

Would SHA-1 be safe for certificates with short validity?

There are articles floating around the topic of SHA-1 deprecation. The gist of it is that in (mostly) developing countries, people are still using mobile/computing devices that are 5 years old (or ...
3
votes
1answer
48 views

Truncating ciphertexts on ring-LWE schemes

On the section 5.4 of the paper Improved Security for a Ring-Based Fully Homomorphic Encryption Scheme, the authors explain how to discard some bits of the ciphertexts to get smaller ciphertexts and ...
3
votes
2answers
112 views

secure PRF or not

I am new in Cryptography and I saw this question in a note I solved it but I'm not sure about my answers. Let $F : \{0, 1\}^n × \{0, 1\}^n→ \{0, 1\}^n$ be a secure PRF (i.e. a PRF where the key ...
3
votes
1answer
83 views

Why can you reverse a modulo function when knowing its primes

We are dealing with cryptography in school right now and superficially went over the Rabin cryptosystem with the (apparently usual) example of p=7 and q=11 etc (we didn't do RSA). I understand that ...
3
votes
2answers
74 views

Securely generating auth tokens

(Disclaimer: I'm an experienced server-side Java programmer, but with little crypto experience.) Objective: I've got a login server which, given a login+password pair, should issue a unique auth ...
3
votes
2answers
111 views

What is the use case for XOF functions (i.e. SHAKE128/256)?

FIPS 202 defines 2 functions: SHAKE128 SHAKE256 As extendable-output functions (XOFs) that can have variable output length. But in Appendix A.2 marks: it is possible to use an XOF as a hash ...

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