# All Questions

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### A lower bound on the insecurity of CipherCloud?

CipherCloud claims to support , among other things, searchable encryption. A bunch of speculation seems to suggest they did this via some breathtakingly incompetent means( unfortunately such ...
2k views

### Is javascript RSA signing safe?

I'm currently working on a secure open source messaging system (https://github.com/DSMNET/DSMNET). Currently I'm using the cryptico.js (https://github.com/wwwtyro/cryptico) library to encrypt all the ...
490 views

### RSA leak bits to factor N

Suppose you randomly generate large primes p and q as in RSA, and then tell me N=pq but not p or q. Then, you would like to actually let me factor N, except you should tell me as few bits of ...
4k views

### How can we reason about the cryptographic capabilities of code-breaking agencies like the NSA or GCHQ?

I have read in Applied Cryptography that the NSA is the largest hardware buyer and the largest mathematician employer in the world. How can we reason about the symmetric ciphers cryptanalysis ...
140 views

### Can stream ciphers (usually) be “run backwards”?

Is it possible to reconstruct the previous output bits of a stream cipher, e.g. RC4, when only the current state is known, or is that computationally hard, or even impossible (due to ambiguous ...
266 views

### What are some of the major differences between PyNaCl and PyCrypto? [closed]

It seems like one can generate keys and encrypt data with both libraries. Is one better than the other? Do they attempt to solve different problems? Is one more secure than the other?
570 views

### Is there an encryption/decryption algorithm that can give two different outputs?

The recent revelations that airport staff can detain travellers in an interrogation room and force them to give over passwords to encrypted-HDD laptops under threat of jail time has made me think ...
185 views

### Can GCM be used in incremental mode?

I want to sync blobs over the network and the most frequent change will be appending to blob. To minimize data amount sent I would prefer to encrypt appended data, send it over the network, append to ...
601 views

### What is an active S-Box?

I don't understand what an active s-box is (in reference of design criteria 8 of the DES). Because according to DES, all 8 s-boxes are used every round. But this is not what active means. So my ...
586 views

### Will rehashing an SHA256 hash continually, eventually produce every possible value?

So let's say you had infinite time and energy. You have a hashed string of some sort. Because you have infinite time and energy, you can produce a collision(or the original value) easily enough. But, ...
137 views

### One-way function and $EXP$

All examples of one-way functions I have see till now are closely related to the assumption that $NP\neq P$ (or even weaker ones, such as $UP\neq P$), but why not considering the theorem $P\neq EXP$? ...
965 views

### RSA PCKS1 v2.1 RSAES-OAEP algorithm

Using RSA public key (n,e) the encryption of a message is c= (m^e) mod n and the corresponding message is decrypted as ...
4k views

### Are there hash algorithms with variable length output?

I understand that for example MD5 produces a 128 bit hash value from a given text of variable size. My question is if there is a hash-like algorithm that will produce a hash value where one can ...
252 views

### Encryption schemes and pseudorandom permutations

Can an encryption scheme (deterministic or not) be viewed as a pseudorandom permutation taking as input a message and returning a value of the same bit-encoding length ? Thank you.
188 views

### Key exchange and man in the middle issue. Secure solution

I am trying to design MiTM protected key exchange algorithm. For instance I am using RSA asymmetric keys. Alice and Bob respectively have each their own pair of keys. And they should be able to ...
253 views

### PBKDF2 uses HMAC-SHA1 to generate keys, but what is the key for the HMAC?

I was reading up on PBKDF2 and noted the following: DK = PBKDF2(PRF, Password, Salt, c, dkLen) PRF is a keyed HMAC, but I can't seem to find out what they use ...
1k views

### Using one-way hash functions as the encryption method

Suppose two parties want to communicate securely with each other (Bob and Alice) using a simple messaging system in English. There are approximately 180,000 currently used words in the English ...
753 views

### Self-expiring symmetric keys, or: cryptography in absence of secure deletion

I can encrypt some data D using a random symmetric key K, obtaining a ciphertext C, and then encrypt K with my public key Pub and obtain H. So far so good: I can only decrypt C if I have H and my ...
391 views

### Why the symmetric key layer in PGP?

I've been familiarizing myself with the basics of PGP. If I understand correctly, PGP symmetrically encrypts the data of interest using a random single-use key, then encrypts the encrypted data and ...
164 views

### Any problems with this secure time synchronization scheme?

I have a time authority and I want to securely set a client's time to this authority's time/date within a precision of $\delta$ seconds. The authority's public key is known to the client. This was my ...
1k views

### What is the progress on the MIT LCS35 Time Capsule Crypto-Puzzle?

Ron Rivest posed a puzzle in 1999. MIT LCS35 Time Capsule Crypto-Puzzle. The problem is to compute $2^{2^t} \pmod n$ for specified values of $t$ and $n$. Here $n$ is the product of two large ...
371 views

### How terribly flawed is this design for key storage?

Dave here, with a plan for storing secrets. Threat model: Database and keyfile may (will?) be stored on un-trusted servers. Database contains many valuable secrets (passwords), encrypted by key ...
147 views

### Feedback requested on a method of posting a message without revealing the author

So I was thinking about variations on the Dining Cryptographers problem - In some cases, it's useful to be able to post a message without revealing the source, but with the additional constraint of ...
137 views

### Is it possible to translate a piece of language into your own without knowing the language? [closed]

I understand the the texts within the Voynich Manuscript are of an unknown language, and yet cryptographers are still working on it and still trying to decipher it. Does that mean that it is possible ...
360 views

### AES CTR with similar IVs and same key

Lets say there is a piece of software that uses AES CTR to encrypt different messages using the same key but with slightly different IVs So for example, a 16 byte IV, the 2nd 8 bytes are always the ...
2k views

### Initialization vector in symmetric-key encryption

Can we use symmetric-key algorithms without an initialization vector? I am making an app where both the sender and receiver share a key and there is no way to create an initialization vector for each ...
86 views

### ASN.1 OID of bcrypt

What is the ASN.1 OID associated to bcrypt (the key derivation scheme)? For instance, PBKDF2 has 1.2.840.113549.1.5.12 and it is therefore possible to store a ...
96 views

### Where can I double check my elliptic curve results?

I am trying to do some elliptic curve calculations by hand, just to refresh myself on how the system works. I calculated some points and did some operations by hand. I am trying to double check my ...
212 views

### How to renew keys for a statically-encrypted database

Preamble Before I ask this question, please accept that I fully understand that 'rolling your own' is never a good idea, and that I am a relative n00b. The only reason I am doing this the way I am is ...
416 views

### Storing Credentials for a Third Party

(Originally posted to SOF, but I got no answers) I am designing a small application for use inside my company. My application needs to connect to a third party website on behalf of my application ...
117 views

### Create a field in PBC

Edited (I removed the emphasize on Integers): My question is partly cryptography and partly programming, I would appreciate any help on any aspect of it :) I want to use PBC library to do the ...
316 views

### What is the relation between hash chaining and chosen prefix attack

I read that hash chaining can be used to prevent hash collisions, while the chosen prefix attack allows hash collisions for algorithms like MD5. How are they related? Does hash chaining prevent the ...
732 views

### CKM_RSA_PKCS vs CKM_RSA_X_509 mechanisms in PKCS#11

What is the exact difference between the CKM_RSA_PKCS and CKM_RSA_X_509 mechanisms in the PKCS#11 v2.20 specification (Please ...
435 views

### RSA smaller number work-out-by-hand not working - I think I made a mistake

I tried out the paper/pencil explanation @ http://sergematovic.tripod.com/rsa1.html, and it seemed to make sense just fine until I came to decryption. Here is what I worked out: Key Creation: Choose ...
585 views

### Why restricting SHA3 to have only two possible capacities?

I just read the presentation slides of John M. Kesley (from NIST) for his invited talk at CHES 2013 about SHA-3 and learned that NIST is going to standardize Keccak with a possibly modified padding ...
245 views

### How can I take advantage of repeated patterns in non random RSA prime factors?

I am researching vulnerable RSA moduli which are composed of primes generated with poor entropy. Having a list of these primes I searched for variable sized repeated patterns among them and I noticed ...
1k views

### Why is the complexity of RSA-1024 80 bit and not 86 bit?

Why is the complexity of RSA-1024 80 bit and not 86.76611925028119 bit? Here is the complexity for the GNFS (pulled from the linked Wikipedia article): \exp\left( ...
139 views

### PKCS#11: Can C_GetMechanismInfo() return CKR_OK on a disabled mechanism?

According to PKCS#11 v2.20, "C_GetMechanismInfo obtains information about a particular mechanism possibly supported by a token." Does that mean ...
247 views

### Proof of correctness of a homomorphic ElGamal sum

Let's suppose we are using the exponential ElGamal as a public-key encryption scheme, so that we encrypt $g^m$ instead of $m$, for some generator $g$. Let $x$ be the private key, and $h=g^x$ be the ...
370 views

### RSA Attack with some known values

There is an RSA implementation which I need to attack to get the encrypted messages. Some of the values are known. Plain text value for input. So if I enter a message, say it "A", the plain text is ...
163 views

### RC4 system pitfalls

A system fell into my neck, where lots of files are splitted into several segments and encoded via RC4. RC4, because client side - slow computer, slow flash arithmetic ops - can only decode big blobs ...
436 views

### How to hash a list of multiple items?

In some protocols, a key is derived from a hash of many parameters (K=H(param1,param2,param3)) Is there a hash function that takes many parameters as input ? Or we can simply use a function like ...
371 views

### Do ciphertexts leak information about their algorithmic creators?

If an adversary holds thousands of encrypted files that were encrypted with the same cipher, can he determine which cipher method had been used on the files? Assume that the adversary has no prior ...
186 views

### Proof of work for determining whether a number is prime?

I have an idea for a system that would be outsourcing some brute force calculations to many users in hopes of finding divisors of a number. However, there is a possibility that a given number would be ...
308 views

### Hamming Distance vs. Index of Coincidence

When analyzing a repeating-key xor cipher to find the key length, I've read about two key methods (assuming there aren't just repeating chunks of ciphertext for Kasiski's method), for some assumed key ...
227 views

### Multiple iterations of AES for key derivation?

The keepass password manager uses multiple rounds of AES for its key derivation, as described here. I was somewhat surprised that it wasn't using pbkdf2, bcrypt, or something more common. ...
899 views

### HMAC-SHA1 input size

I know that the HMAC is a message authentication code that uses a cryptographic key in conjunction with a hash function (SHA1 , MD5, etc.). The HMAC output is 160 bits for HMAC-SHA160 and 256 bits for ...
342 views

### Finite fields and ECC

I understand modular arithmetic(or at least I think I do!) and I've tried to read and learn about how the Math in RSA works(and I think it went pretty well). I've been reading up on ECC and it looks ...