4
votes
1answer
138 views

Should different key pairs be used for signing and encryption?

In the recent iOS Security white paper from Apple (February 2014), the section on iMessage discusses using two different asymmetric key types as part of its standard operation: When a user turns ...
4
votes
2answers
220 views

Authenticated encryption without padding

I want to encrypt data and protect its integrity and confidentiality. However, I cannot increase the length of the data. Are there any cipher modes of operation which provide confidentiality and ...
4
votes
1answer
196 views

Homomorphic Encryption and Semantic Security using Lattices?

I've been reading Brakerski and Vaikuntanathan's "Efficient Fully Homomorphic Encryption from (Standard) LWE" and I'm still digesting pieces at a time. Under section 1.1, "Re-Linearization: Somewhat ...
4
votes
2answers
227 views

Is the following symmetric design secure?

Assume: $O$ be a reversible random permutation oracle on a finite set and $O^{-1}$ the inverse permutation (pretty much equivalent to a random permutation: What is the difference between a bijective ...
4
votes
2answers
825 views

Why is sharing the seed and using SecureRandom “deterministically” so bad?

I've recently been reading an article about "using cryptography to store credentials", and I would love it someone explained this in more detail… Up until Android 4.1, Android used a SecureRandom ...
4
votes
2answers
2k views

Which stream cipher can we replace the RC4 in the SSL?

In cryptography, the Fluhrer, Mantin and Shamir attack is a particular stream cipher attack, a dedicated form of cryptanalysis for attacking the widely-used stream cipher RC4. The attack allows an ...
4
votes
1answer
2k views

Can an RSA private key have several public keys?

Ok, so my crpto lecturer in Uni posed this question at the end of the RSA key generation lecture as a brain teaser. I have been thinking about this and I think I have come up with a way ( I am aware ...
4
votes
2answers
216 views

how to iteratively calculate a^emod n with modulus n sized 4096 bits

In most sites the exponent of the RSA public key is 24 bits. But the modulus can get to 4096 bits size. I have an accelerator that can get max. 2112 bit size modulus. It calculates ...
4
votes
2answers
179 views

Can S/MIME be still considered secure?

Previoulsy I had asked this question at http://stackoverflow.com/questions/18235983/can-s-mime-be-still-considered-secure but I feel this forum is topic-wise the right place. Recently there has been ...
4
votes
2answers
4k views

AES encryption using a Diffie-Hellman key exchange

I have two systems which need to encrypt their communication by AES-128. The key for the AES encryption should be exchanged with the Diffie-Hellman (DH) algorithm. Now, I've read about DH and got a ...
4
votes
2answers
277 views

Does SRP reduce to DH key exchange when shared password is not secret?

I can find JavaScript implementations of SRP (Secure Remote Password protocol), but nothing that inspires confidence for Diffie-Hellmen key exchange. I also have a separate need for SRP later. I ...
4
votes
2answers
402 views

How do I unpack the x and y values from the BITSTRING in a DER ECDSA public key?

In ASN.1, the X and Y values for a 256-bit elliptic curve key are stored as a single 66-byte ASN.1 BITSTRING. Are the values just the first and second half of this bitstring? The private key is an ...
4
votes
1answer
829 views

What happens if an RSA key pair has identical public and private exponents?

Rather, is it possible for big prime numbers? Classroom examples usually involve smaller primes, so for example if you are given a prime number pair $p = 3$, $q = 13$ you would get $n = 39$ and $e = ...
4
votes
4answers
639 views

Which block cipher modes of operation allow a predictable IV?

Recently I found out that in the modes CBC and PCBC the IV may be passed in cleartext but never must be predictable. However for this part of my app I rather have the IV be predictable and unique ...
4
votes
2answers
570 views

Adaptive Hash Functions: How to tell how many iterations were used?

If an adaptive hash function like bcrypt or PBKDF2 is used in hashing passwords, the number of iterations used in the hashing process can be configured. For a penetration tester or a malicious ...
4
votes
2answers
566 views

How did LinkedIn “salt” all their passwords?

First, just to make sure I understand "salting" correctly: You randomly generate a string to append to the password before hashing it, so as to increase its length and make precomputed tables much ...
4
votes
1answer
397 views

How secure is my OTP program?

I'm writing an One-Time Pad encryption program, because I got really interested in the idea of " encryption which has been proven to be impossible to crack if used correctly". I'm writing the program ...
4
votes
2answers
3k views

Why DSA cannot be used for encryption?

Here it is mentioned that DSA cannot be used for encrypt. But Both RSA and DSA can be used to generate public and private keys, right? (Or am I wrong?). Then why can't I use the DSA public key to ...
4
votes
1answer
198 views

Do public/private pairs work both ways?

Public/private key algorithms such as RSA encrypt a message with a private key but only decrypted with another (mathematically related) key. Can the public key decrypt the messages encrypted with ...
4
votes
1answer
199 views

Question about hash collisions

If we have a hash function $h(x)$ and then a hash function $H(X) = h(h(X_0) || h(X_1))$ where $X_0$ is the first half of $X$, $X_1$ is the second half of $X$ and $||$ is concatenation. Then assuming ...
4
votes
1answer
776 views

If you had to implement the BGN Cryptosystem, how would you do it?

If you had to implement BGN, how would you do it? I'm looking for an implementation of the public-key cryptosystem due to Boneh, Goh, and Nissim (aka BGN), or at least some suggestions on ...
4
votes
2answers
376 views

Provable Encryption

Is it possible to encrypt data in a way that it can be proven that the data is encrypted, without revealing the key? Alice chooses some plaintext, then she encrypts it with a certain scheme. She ...
4
votes
1answer
1k views

Why is ElGamal considered non-deterministic?

One difference between RSA and ElGamal is that ElGamal isn't necessarily deterministic (while RSA is). What makes it non-deterministic? Is this advantageous to security? How else does this property ...
4
votes
2answers
419 views

What is the appropriate public key encryption for secure coin flipping?

I quote Bobby's question here since I encountered the same one... Random Coin Flip using ElGamal and a Trusted Party Consider the following protocol for two parties to flip a fair coin. Trusted ...
4
votes
3answers
509 views

Does the XML Encryption flaw affect SSL/TLS?

A "practical attack against XML's cipher block chaining (CBC) mode" has been demonstrated: XML Encryption Flaw Leaves Web Services Vulnerable. Does this weakness of CBC-mode which is used here also ...
4
votes
1answer
179 views

When is an asymmetric scheme considered broken?

Does the following quote imply that valid encrypted data can be created and decrypted by someone other than the owner of a private key: An asymmetric encryption scheme is considered to be broken ...
4
votes
2answers
2k views

Brute forcing CRC-32

I'm working on a cryptosystem which uses IDEA. The designer made the mistake of including a CRC-32B hash of the password unencrypted in the header, so that the system can quickly reject bad ...
4
votes
1answer
79 views

Authenticating Very Short (Sub-Block) Data

If I'm encrypting very short/small data - like an int or a long (32 or 64 bits), does it make any sense to authenticate the ciphertext, when using an "online" mode of operation (like EAX)? ...
4
votes
1answer
203 views

Encryption-Decryption-Encryption

I have learned from Applied Cryptography that triple encryption with three independent keys in encryption-decryption-encryption (EDE) process gives security not higher then $2^{2n}$ when it comes to ...
4
votes
2answers
116 views

Are variable-length crypto hash functions still susceptible to collisions?

I just recently learned that the SHA-3 finalist Keccak allows for variable length output. As the only answer to this question states, "[it] need to have an output length at least equal to the input ...
4
votes
3answers
148 views

Do I need to use AE/AEAD in my particular scenario? Or is it redundant?

This is my scenario: A small structured text message (1-2kB) is to be sent over an insecure channel. Everything except for the header portion (which contains a title, timestamp, category and tags) ...
4
votes
3answers
256 views

Is it secure to use Diffie-Hellman key agreement to generate a nonce?

I have a system, using AES, in one of the modes that uses a nonce and authentication. We have a pre-shared key, and to agree about initial nonce we could use Diffie-Hellman, using the resulting ...
4
votes
3answers
321 views

Can I create a fixed length output from a fixed length input?

I use AES256 for various purposes in my app. However, the purpose I am now trying to achieve is the following: Input is for example AU00000001 and I would like a ...
4
votes
1answer
405 views

how to use common modulus attack?

I am struck with the following problem: Let Alice, Bob, Chris and Eve communicate over a public network. They encrypt all messages they send using RSA system. Bob and Chris have the RSA modulus ...
4
votes
1answer
311 views

Is knowing the private key of RSA equivalent to the factorization of $N$?

Given the RSA modulus $N$ the fastest method to factor it is of sub-exponent order. But, now if I know the private key $d$ of RSA, does that mean I can factor $N$ efficiently?. It intuitively seems ...
4
votes
2answers
288 views

Is Chaocipher a secure cipher under ciphertext-only attack?

Chaocipher was invented by John F. Byrne in 1919. The algorithm was recently revealed – see Moshe Rubin's Chaocipher Revealed, the Algorithm (PDF). While a known plaintext attack successfully finds ...
4
votes
2answers
318 views

Is it possible to create a “proof-of-upload” system for BitTorrent ratio tracking?

One issue that private BitTorrent trackers that track users' share ratios often run into is how to keep track of people who are screwing with their upload statistics, something known as ...
4
votes
2answers
1k views

Is PBKDF2-HMAC-SHA1 really broken?

I just read through this article which demonstrates practical (and seemingly trivial) collisions in PBKDF2-HMAC-SHA1, and provides a few examples of collisions. Am I missing something here? Is ...
4
votes
1answer
127 views

Why has the sponge construction's generic collision finding attack a complexity of O(min(2^(-n/2) , 2^(-c/2)))?

I'm trying to learn about sponge function for creating hash functions and generic attacks on it. I'm looking for the collision finding attack scenario which leads to $O(\min(2^{-n/2} , 2^{-c/2}))$ ...
4
votes
1answer
289 views

EC ElGamal versus static+ephemeral ECDH

A client application needs to encrypt a UDP datagram for a server with known EC public key $P$. Performing a full ECDH key exchange would defeat the benefit of using UDP as a connectionless protocol. ...
4
votes
1answer
200 views

Solve a Modular Exponentiation

It might be common, but if we had to solve an equation like this $m=s^{e}$ mod $n$ where $m,e,n$ are known. How can we find $s$. What optimisations could be applied? And what would the complexity of ...
4
votes
1answer
2k views

Is AES still secure considering all this NSA/Snowden scandal? [duplicate]

Im trying to understand if nowadays using AES is secure or not, after reading some arguments on Schneier’s blog (to be more exact, a comment to “The NSA Is Breaking Most Encryption on the Internet”) ...
4
votes
1answer
158 views

How can I tell how many bits of security a secure hash function has?

How can I tell how many bits of security a secure hash function has? For example, how would I calculate or tell how many bits of security a secure hash function with 160 bits output would have? ...
4
votes
3answers
235 views

Can insecure algorithms be combined to form a secure algorithm?

This is hypothetical as I can't think of any reason to do this, but out of curiosity... Could I, for example, take the MD5 digest of a message and concatenate it with the SHA-1 digest (not quite ...
4
votes
3answers
188 views

Why can an RSA signature be authenticated ONLY with the signer's public key?

I'm an undergraduate Math student currently taking a cryptology course. I have a question about the RSA signature authentication mechanism. An RSA signature can be authenticated with the public key ...
4
votes
2answers
361 views

How can I implement the elliptic curve MOV attack myself?

I understand and have implemented elliptic curve signatures in Python without the use of libraries like Sage, and would like to implement the MOV attack against certain weak types of elliptic curves. ...
4
votes
1answer
394 views

Why does only length prepending improve the security of CBC-MAC

I know that length prepending improves security of CBC-MAC. However, wouldn't inserting the length elsewhere (middle, end or any other part of message) be equally good? After all, even the length is ...
4
votes
2answers
531 views

Increasing number of rounds in AES

AES-128 requires 10 rounds ( AES Description, Wikipedia ) However Bruce Schneier has recommended AES-128 use 16 rounds ( Schneier on Security: New Attack on AES ) Could that be implemented as two ...
4
votes
1answer
6k views

How are the AES S-Boxes calculated?

I'm trying to understand how the AES S-Boxes are calculated. I understand how the multiplicative inverse is calculated over GF(2^8), but I'm confused by the description of the affine transformation. I ...
4
votes
2answers
679 views

With a true random number generator at hand, how to implement one-time pad?

I have a Raspberry Pi single-board computer that happened to have an hardware true random number generator, based on some quantum effects on the processor silicon, baked into its BCM2835 chip and I ...

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