# All Questions

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### “Fine grain Cross-VM Attacks on Xen and VMware are possible” and OpenSSL 0.9.7

Here's a paper that just made its way into Cryptology ePrint Archive: Fine grain Cross-VM Attacks on Xen and VMware are possible!. Its a good survey of past attacks and presents a couple of new ...
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### Is less security required for a short stream cipher than for the AES enciphering of very long messages? [closed]

Criticisms of a cipher system such as 'the ciphertext from one message must be indistinguishable from the ciphertext of a second message" surely only apply when there are very large amounts of ...
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### Key construction in the Full Cramer-Shoup cryptosystem

The Full Cramer-Shoup Encryption scheme needs to choose six random numbers from $\mathbb{Z}_q$, which we denote them by $x_1,x_2,y_1,y_2,z_1,z_2$. Then the scheme hides these random numbers in the ...
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### Why does computing g^a * g^{-a} with the PBC library result in zero?

My example code is as follows: /* * Example 1 * 1) Calculate g^a * 2) calculate g^{-a} * 3) multiply g^a * g^{-a} * */ Note: here ...
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### Definition of one-way functions: randomly-chosen point vs every point

Notation: $\Sigma^k$ is the set of $k$-strings on the alphabet $\Sigma$; $\Sigma^\ast$ is the set of all finite dimensional strings on the alphabet $\Sigma$. In the context of computer-science a ...
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### Leaving authentication data blank less secure for AES GCM?

The AES GCM example for Ruby / OpenSSL shown at: http://ruby-doc.org/stdlib-2.0/libdoc/openssl/rdoc/OpenSSL/Cipher.html has a GCM example of the following: ...
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### Stateless hash based public key cryptography?

Merkle-Winternitz signatures based on fractal hash trees are an attractive alternative to other post-quantum cryptographic schemes, in particular since they are conceptually simple, the security ...
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### Is it possible to track down copies of WW2-era codebooks?

I've got an academic curiosity regarding some of the old Japanese naval codes used during World War II specifically. I'm wondering if any of the codebooks from this era have been made available, or ...
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### Usage of different size chunks during AES encryption/decryption

I want to encrypt user's files on our server. For this purpose I decided to use the PHP Mcrypt library. I chose to use the Rijandel-128 algorithm and CBC mode for encryption. Since files are quite ...
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### Cryptography Implementation in software

I am trying to implement a password manager in C and I had a question about the proper steps in implementing the crypto. I looked at some implementations, google talks on crypto and what the standards ...
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### Is a tweakable block cipher still considered deterministic in nature?

"a block cipher is a deterministic algorithm operating on fixed-length groups of bits, called blocks, with an unvarying transformation that is specified by a symmetric key. - Wikipedia " But with a ...
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### Diffusion in Shamir's secret sharing scheme

The popular command line utility ssss implements the classic Shamir's secret sharing scheme over the generic field $GF(2^q)$ with $8 \le q \le 1024$. When $q>=64$, the constant coefficient ($c_0$) ...
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### XORing a header with SHA512-hash - secure?

I am writing a application that encrypts text several times and stores the result. The result looks like complete random bytes, therefore I add some random bytes at the end. To let the program know ...
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### MAC security and adversaries with memory

The usual definition of MAC security is (roughly) that there is no "efficient" adversary that has non-negligible advantage in the "MAC game" where the MAC game is as follows: the challenger chooses a ...
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### Is there a reason that the N value in scrypt is limited to powers of two?

I have been looking at the scrypt hashing algorithm and am confused as to why the N value, which determines the overall cost, is limited to powers of two. There are ...
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### Is the strength of RSA over quadratic or other cyclotomic fields as strong as over the integers?

If we assume the strength of RSA is based on the difficulty of factoring (which I know we can't guarantee) and we compose the modulus of some other quadratic ring that is a unique factorization domain ...
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Referring to both Wikipedia page and ECDSA-cert paper I can understand that, given $\mathcal{E} = \mathcal{E}(a,\,b,\,\mathbb{F}_{2^m})$ as our elliptic curve on $\mathbb{F}_{2^m}$ group $G \in ... 1answer 76 views ### Composing hashes and/or MACs Suppose I have n hash/MAC functions, of which n-1 are extremely insecure, leaving only one that is in fact cryptographically secure. If I use all of them on a message and the XOR them together the ... 0answers 46 views ### Is anyone aware of Du atallah multiplicative secret sharing scheme for dot products for > 2 party scenario? I am working on Du Atallah's multiplicative secret sharing scheme for more than 2 party scenario. Is anyone aware of its multiparty version (more than 2 parties). The paper for 2 party can be found ... 1answer 98 views ### Existence of a map$\phi:\mathbb{Z}_{N^2}^* \mapsto \mathbb{F} $Is there a map between the group of$\mathbb{Z}_{N^2}^*$where$N$is a composite number , a product of two equal size secure prime numbers$p$and$q$and a finite field$\mathbb{F}$, such that for ... 1answer 303 views ### SSL-like protocol with public-key hard-coded in the client Imagine a SSL-like protocol that instead of using certificates signed by a trusted CA, has the server's public key hard-coded in the client. My question is: what happens if the server's private key is ... 1answer 292 views ### How can I convert numbers into prime numbers? I'm working with one-way accumulators, but I'm not knowledgable in cryptography. Is there an easy peasy way to hash numbers (or whatever) into prime numbers? Obviously I'd like it to be collision ... 1answer 121 views ### Has there ever been more then a theoretical difference between preimage resistance and second preimage resistance? In other words, has there ever been a point in time in which having the content of a message has actually helped break a hash function? 1answer 346 views ### Block cipher and parity of permutation Can anyone explain the following text passage to me? Most real-world block ciphers build even permutations, because it's hard to build odd ones using small operations (32 bit) on larger (128 ... 1answer 65 views ### Positioning of keys in encrypted text I'm just looking at very simple encryption. I've noticed that the key is typically either at the front or at the back of the encrypted text. This is quite easy to break if you know the key. It can ... 1answer 554 views ### What is a 'secret key factory'? What precisely is it doing? [closed] I have found a Java implementation of AES CBC mode that runs in Netbeans. The lines below appear to create the key from password and salt: ... 1answer 1k views ### Does IKEv2 protocol have two modes like IKE IKE Protocol have 2 modes, that modes have different structure. Does IKEv2 protocol have two modes like IKE. 2answers 784 views ### Are there any elliptic curve asymmetric encryption algorithms? RSA offers the functionality of encrypting (short messages, or symmetric keys) with a public key, and decrypting with a private key. However, RSA key generation is extremely expensive, especially for ... 1answer 84 views ### Exchanging Keyspace and Message space in PRF Suppose there is a secure PRF$F: K \times X \to Y$. Then is$F': X \times K \to Y$, defined by$F'(x,k):=F(k,x)$, a secure PRF? In my opinion it should be because exchanging the keyspace with ... 2answers 231 views ### does re-encrypting the same value with multiple keys reduce security I found myself wondering today, how much security is lost if you take a plaintext - assume that its content, including any metadata is unknown to an attacker, for example it may be random data - and ... 2answers 576 views ### Does AES-128 have the same strength as AES-256 with a padded key? When I use the same 128-bit key for AES-128 and AES-256 with a known/public padding for the latter, is there some weakness in AES-256 that is not present in AES-128 with essentially the same key? ... 2answers 46 views ### How to make sure the pre-agreed information safe for DH-Key Exchange The definition of DH key exchange was given as the method let two authenticate each other & exchange the crypto key over an insecure channel. DH-key-exchange was innovated to defence ... 1answer 98 views ### In Pedersen Key Distribution, can the public key be persistent? I am implementing a key distribution protocol described by Torben Pedersen in A Threshold Cryptosystem without a Trusted Party (EUROCRYPT'91). In the protocol, the$n$parties distribute a public key ... 2answers 963 views ### Are AES-256's related-key weaknesses exploitable if it is used to build a hash? Assume it is made a hash based on AES-256 encryption (perhaps because this is hardware-accelerated, but no standard hash is); and it is used the Merkle–Damgård structure, that is padding of the ... 1answer 568 views ### How to select$g\$ in Paillier Cryptosystem

For my cryptography class project in university I have selected Paillier Cryptosystem as a course project ...