# All Questions

579 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?
286 views

### Is there a problem with constructing a CSPRNG using MD5 or SHA1?

Is there any security concerns with building a CSPRNG using a broken hash function like MD5 or SHA1? The design is such that a CRC-like function is used for mixing entropy and MD5 is used as the ...
490 views

### CSPRNG in JavaScript using Audio and Video

I ported GnuPG to Javascript using Emscripten (not released yet). Now I need a good replacement for /dev/random//dev/urandom. ...
353 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 ...
252 views

### synchronization of counters in HOTP

How is synchronization of counter values achieved in the HOTP protocol? As I understand it, the server increments its counter value only if a match (of the OTP value) is found. What happens at ...
349 views

### Is it considered insecure in an HOTP implementation to publicly provide the next counter?

I am working on a project that needs to securely authenticating one or more smartphone clients with a server running on a microcontroller so it has very limited resources. I have found plenty of ...
502 views

### Why does HOTP use such a complex truncate function?

In the HOTP protocol after calculating a 20 byte hash it is truncated to 4 bytes. For this first an offset is calculated (low-order 4 bits of the last byte) which determines the four bytes to be ...
2k views

### How does HOTP keep in sync?

My understanding of HOTP is that each password is unique and based on a counter. $$PASSWORD = HOTP_1(K,C)$$ Where $C$ is an incremental counter. What I wish to know, is how you keep the client ...
2k 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 ...
412 views

### Is it generally possible to employ brute force methods when the encryption scheme is not known? Why or why not?

Lets say you are presented with an encrypted string of bits or text, and no other information. What would be necessary before you could apply brute force methods to decrypting the string? How would a ...
14k views

### Why use an Initialization Vector (IV)?

Why use an Initialization Vector (IV)? How are IV's used? What are the advantages/disadvantages of using an IV? Why use an IV instead of a longer key in which some section of the key is pubic? What ...
198 views

### GGM PRG construction: Why do we need to change keys all the time with underlying PRG

When constructing a PRF that has n bit input using the GGM PRG, why do we always have to recursively run the PRG using its previous output as a seed key n times. Instead, why don't we run the PRG n ...
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410 views

### Amateur question: two-way RSA?

Is this a sensible scheme for 2-way client/server communication? A client connects to a server. The server and client both generate RSA keys, and send over the public ones to each other. If the client ...
260 views

### Preimage resistance hash in digital signature

I'm studying about preimage resistance property of the hash functions. In particularly I'm reading as the missing of this property can be fatal in digital signatures that use RSA. Further details: ...
73 views

### Why is it impractical to generate a semiprime dictionary? [duplicate]

This might be a very simple question. However, I am just learning the concept, so just excuse me. I am wondering why there is not any attempt to generate all semiprime numbers? (as an dict. attack to ...
582 views

### length extension attack on an MD5 hash of a text file of about 10K Bytes?

How much computing time on a typical desktop computer would it take to find a new length and new data to extend a text file of about 10K Bytes with a given MD5 hash?
333 views

### Can Poly1305 be used with block ciphers running in CTR mode?

Is the use of Poly1305 limited to stream ciphers? (note, I'm not talking about Poly1305-AES )? Can it be used with block ciphers running in CTR mode? If so, what other considerations/limitations are ...
580 views

### Given a private RSA key, how do we get the public key?

Is it possible to pre-choose a private RSA key, then obtain a public key from it?
899 views

### Difference entity authentication and implicit key authentication

From the Handbook of Applied Cryptography, in discussions of key sharing algorithms, I see definitions: Implicit key authentication is when one party is assured that no other aside from a ...
1k views

### Finding CRC collisions for specific divisor

My current textbook (Information Security: Principles and Practice by Mark Stamp) discusses how to determine the CRC of data via long-division, using XOR instead of subtraction to determine the ...
128 views

### Differential privacy definition

Differential privacy defines "privacy" of a mechanism $A$ as the "closeness" of the two distribution $Pr[A(D) \in S]$ and $Pr[A(D') \in S]$ where $D,D'$ differ in one element. And the distance between ...
115 views

### Can substrings of a long string be efficiently authenticated?

Suppose one has a (strong) shared secret key and wishes to authenticate a very long string in a way that will allow the verifier to verify small substrings without reading much of the stored data and ...
134 views

### What kind of cryptography should i use?

I have a trusted third part A that issues an access token (xml file) to an untrusted client C that uses this token to log into an untrusted server S and access to the authorized files. I want only ...
514 views

### What are some good references for the implementation weaknesses in RSA

I'm pretty sure I understand textbook RSA. Choose p and q, large primes, and compute n=pq. Choose e such that $gcd(\phi(n),e)=1.$ Publish n and e. Compute d such that $de=1$ (mod $\phi(n)$). To ...
299 views

### What's efficient MPC protocol for determining if sum's bigger than y?

My secure multi-party computation (MPC) in need is simply to determine if a sum of two private variable is bigger than a given value $y$, as $f(x_0, x_1) = [(x_0 + x_1) > y]$ in which the value ...
570 views

### Can you identify the public key used to encrypt something?

If I encrypt a string with a public key, does the encrypted ciphertext reveal the public key I used to encrypt it? Basically, I don't want anyone to know who the ciphertext is addressed to. I'm ...
3k views

### How long to bruteforce a RSA key [duplicate]

Suppose I have a 2048 bit RSA public key, and want to brute force the corresponding private key. I guess there are 2048^16 possible combinations? How long would this take me to brute force with an ...
611 views

### When using HMAC, does key encoding matter?

Given: H is a good hash function with block size L. K is a key of length >= L (recommended by RFC 2104). Khex and Kbase64 are ASCII encodings of K. In the HMAC algorithm, is there a good reason to ...
409 views

### Is triple des similiar to RSA in that they message size is limited to the key size?

Is triple des similiar to rsa in that the message size you can encrypt is limited (unlike AES)? Yes you can break the message size into parts and apply it, but I'm not interested in doing that so I ...
83 views

### “proof of access” schemes

What is the state of the art way when implementing a scheme challenging a party to prove they have access to certain data? What I'm looking for something along the lines of Give them ...
171 views

am sorry, an a beginner, so please forgive me if i make mistakes, so here is what I understood: PBKDF2: uses HMAC, so, the message is the password, and the key that crypts the message is the salt? or ...
170 views

### How to compare two datasets „anonymously”?

Ok, I hope this question makes some sense because I am not so sure how to word it any differently… Imagine the following situation: There are 10 defined colors (blue, orange, yellow etc.) There are ...
200 views

### Is this OTP scheme safe?

I have a message $m_1$ and I want to encrypt it by XORing it with two keys $k_1$ and $k_2$: $$c_1 = m_1 \oplus k_1 \oplus k_2$$ So far so good. Now I was wondering if I could create a "fake" result ...
83 views

1k views

### Why are bitwise rotations used in cryptography?

Any understanding I have of cryptography stops right around the cipher level. As such, I'm just curious as to why bit shifts and moreover circular bit shift are so prevalent in cryptography.
130 views

### How can I split a message in parts of similar size or smaller?

I have a 130-160 characters message that I need to split in say, 3 parts, and be able to reconstruct it by recovering all 3 parts. I also need that these parts are type-able, meaning that they can't ...
374 views

### IV Security Clarification

After doing lots of reading on SO and other websites relating to AES cryptography, I am trying to understand the security issues surrounding IV's. There seems to be a lot of confusion and ...