# All Questions

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### Is it theoretically possible to construct a string that contain its own hash value?

After saw the xkcd comic self-description, I wonder is it theoretically possible to construct a self-descriptive string that contains its own hash value? Let's say the string's md5 value is ...
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### Hill cipher, unknown letter value

I've been struggling on this problem for a while now : the Hill cipher is well-known to be vulnerable to known-plaintext attack due to its linearity. Given a key matrix $K$ of size $n\times n$, one ...
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### How to construct encrypted functions (with either public or private data)?

Homomorphic encryption is often touted for its ability to Compute on encrypted data with public functions Compute an encrypted function on public (or private) data I feel I have a good grasp of #1 ...
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### Software implementation of a commutative cipher?

I've got an application (detailed below) that calls for the use of a cipher that is commutative. I've been doing some googling & reading, and there are two algorithms that seem to get mentioned ...
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### Why is AES not a Feistel cipher?

I am studying for an exam right now. And I wanted to make sure I got this point correct. AES is not a Feistel cipher because the operations in AES are not invertible. Is the above statement ...
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### Why can't the IV be predictable when its said it doesn't need to be a secret?

I heard multiple times not to reuse the same IV and IV should be random but doesn't need to be secret. I also heard if the IV is something like sequential numbers or something predictable I should ...
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Given a CBC ciphertext and IV, how can I find the encryption key? We are limited with an 8 chars key, each char in the range of [a..h], so I can generate every possible key (these are only $8^8 = ... 1answer 286 views ### What does the expression$1^n$mean as a function argument? In a paper about predicate encryption or attribute based encryption, the setup function is mentioned with the$setup(1^n)$or$setup(1^l)$. I want to know what is meant here. Is it multiples of ones ... 2answers 587 views ### AES Key Length vs Block Length This answer points out that certain key and block lengths were a requirement for the AES submissions: The candidate algorithm shall be capable of supporting key-block combinations with sizes of ... 0answers 194 views ### How do I encrypt with the private key? [duplicate] Possible Duplicate: RSA encryption with private key and decryption with a public key This wording is creeping everywhere (e.g. there): "I encrypt with the private key" and even sometimes, ... 1answer 638 views ### Can Elgamal be made additively homomorphic and how could it be used for E-voting? Elgamal is a cryptosystem that is homomorphic over multiplication. How can I convert it to an additive homomorphic cryptosystem? How can I use this additive homomorphic Elgamal cryptosystem for ... 5answers 570 views ### Approach towards anonymous e-voting I want to implement an internet-based e-voting system. Voters shall be able to cast their vote for one out of n possible candidates. Each candidate has his own ballot-box kept by and at a trustworthy ... 2answers 484 views ### Can we use elliptic curve cryptography in wireless sensors? Can we use elliptic curve cryptography in wireless sensors? If so, how do you map points to message characters? 4answers 12k views ### How can I generate large prime numbers for RSA? What is the currently industry-standard algorithm used to generate large prime numbers to be used in RSA encryption? I'm aware that I can find any number of articles on the Internet that explain how ... 6answers 8k views ### How does asymmetric encryption work? I've always been interested in encryption but I have never found a good explanation (beginners explanation) of how encryption with public key and decryption with private key works. How does it ... 3answers 2k views ### How are primes generated for RSA? As I understand it, the RSA algorithm is based on finding two large primes (p and q) and multiplying them. The security aspect is based on the fact that it's difficult to factor it back into p and q. ... 3answers 995 views ### Is this password migration strategy secure? I want to upgrade the security of some existing databases of users' authentication tokens strictly for the purpose of making sure that if the database is stolen, attackers will not be able to guess ... 6answers 11k views ### Who uses Dual_EC_DRBG? Recent news articles have suggested that the NSA may be involved in trying to influence the cryptography in public standards or commercially deployed software, to enable the NSA to decrypt the ... 3answers 3k views ### What is the relation between RSA & Fermat's little theorem? I came across this while refreshing my cryptography brain cells. From the RSA algorithm I understand that it somehow depends on the fact that, given a large number (A) it is computationally ... 2answers 6k views ### What's the fundamental difference between Diffie-Hellman and RSA? What is the difference in the purpose of DH and RSA? Aren't they both public-key encryption? 5answers 1k views ### Galois fields in cryptography I don't really understand Galois fields, but I've noticed they're used a lot in crypto. I tried to read into them, but quickly got lost in the mess of heiroglyphs and alien terms. I understand they're ... 1answer 2k views ### Does the generator size matter in Diffie-Hellman? For the Diffie-Hellman protocol I've heard that the generator 3 is as safe as any other generator. Yet, 32-bit or 256-bit exponents are sometimes used as generators. What is the benefit of using ... 2answers 1k views ### How do I apply differential cryptanalysis to a block cipher? I've read a lot of summaries of block ciphers particularly with regards to the NIST competitions stating that reduced-round block ciphers are, for example, vulnerable to differential cryptanalysis. I ... 3answers 1k views ### How robust is discrete logarithm in$GF(2^n)$? "Normal" discrete logarithm based cryptosystems (DSA, Diffie-Hellman, ElGamal) work in the finite field of integers modulo a big prime p. However, there exist other finite fields out there, in ... 3answers 414 views ### Purpose of outer key in HMAC From what I know, the HMAC constructions has two strength: It's resistant to length extensions Since the key is consumed before the message, the attacker does not know the initial state, preventing ... 4answers 992 views ### Why is it important that phi(n) is kept a secret, in RSA? Why is it important that$\phi(n)$is kept a secret, in RSA? 3answers 2k views ### Information leakage from the ecryptfs filesystem I'm wondering what information might be leaked from the ecryptfs filesystem. This is what Ubuntu uses if you check the box for "encrypted home directory" when using the desktop installer, so is ... 2answers 3k views ### “SHA-256” vs “any 256 bits of SHA-512”, which is more secure? In terms of security strength, Is there any difference in using the SHA-256 algorithm vs using any random 256 bits of the output of the SHA-512 algorithm? Similarly, what is the security difference ... 3answers 4k views ### Why is public-key encryption so much less efficient than secret-key encryption? I'm currently reading Cryptography Engineering. After giving a high level explanation of the difference between secret-key encryption and public-key encryption, the book says: So why do we bother ... 4answers 782 views ### Is Wiener's attack on RSA extendable to larger keys with low hamming weight? Using small private exponents with RSA improves performance. However, it has been shown (Wiener, 1990) that if$\log d \leq \frac14 \log N$, the private exponent$d$can be reconstructed from the ... 3answers 1k views ### How well does scrypt perform on different architectures / OSes? The scrypt algorithm seems to be a prominent feature in the "CPU friendly" Bitcoin clones for the proof-of-labor part. I've heard claims that it's relatively slow on Windows and/or Intel compared to ... 1answer 2k views ### What is a white-box implementation of a cryptographic algorithm? What is a white-box implementation? Does a white-box implementation have specific properties? 5answers 364 views ### How exactly is “true randomness” defined in the realms of cryptography? Especially in relation to stream ciphers, I frequently read about (sometimes theoretical, sometimes practical) attacks that are able to "distinguish a ciphertext from a truly random stream". What's ... 1answer 1k views ### Is Truecrypt's multiple/cascading encryption safe? Is Truecrypt use of cascading encryption safe? Is it useful? Truecrypt is arguably one of the most popular and widely used encryption applications in use today, yet it seems to use a very ... 2answers 620 views ### Webcam random number generator I have a question about random number generators. I have read from a real random number generator, based on a webcam ("randcam"). My problem is, that I do not really understand how the generation of ... 0answers 188 views ### Is there a “brainwallet” for GPG keys? [closed] Brainwallet is a program that takes a passphrase (hopefully a good one) and deterministically creates a Bitcoin wallet. This wallet can be used for transactions, then deleted from the computer. To get ... 5answers 810 views ### Any efficient text-based steganographic schemes? While there are sophisticated and efficient steganographic schemes with images as cover available, I am yet ignorant of the existence of any fairly efficient and secure schemes with texts as cover. ... 3answers 4k views ### Is it possible to derive the encryption method from encrypted text? Is it possible to identify the encryption method, or at least rule out some of them, by looking at the encrypted text? For example, if you have 3 encrypted strings where the first 10 characters are ... 1answer 3k views ### Why do we use encrypt-decrypt-encrypt (EDE) in 3DES, rather than encrypting three times? I'm wondering why we use encrypt-decrypt-encrypt sequence in 3DES with three keys instead of three times encryption with three different keys? 5answers 2k views ### What is the post-quantum cryptography alternative to Diffie-Hellman? Post-quantum cryptography concentrates on cryptographic algorithms that remain secure in the face of large scale quantum computers. In general, the main focus seems to be on public-key encryption ... 5answers 747 views ### Why does PBKDF2 xor the iterations of the hash function together? The definition of PBKDF2 states that I obtain a derived key* by calling a pseudorandom function a bunch of times recursively: ... 2answers 1k views ### What's is the main difference between a key, an IV and a nonce? What are the main differences between a nonce, a key and an IV. Without any doubt the key should be kept secret. But what about the nonce and the IV. What's the main difference between them and their ... 2answers 272 views ### Why should I use Authenticated Encryption instead of just encryption? There are various different modes of operation for block cipher use, some of which provide "encryption" and some of which provide authenticated encryption. Why should I use an authenticated ... 1answer 251 views ### Alice trusts Bob only when Bob trusts Alice some story first: Alice and Bob both have public/private key pairs. Now Bob wants Alice to sign his public key id. Alice agrees but only when Bob signs the public key id of her. Is this something ... 2answers 1k views ### How does order-preserving encryption work? Order-preserving encryption (OPE) is, apparently, a method of encrypting data so that it's possible to make efficient inequality comparisons on the encrypted items without decrypting them. I've been ... 1answer 300 views ### Correct way to map random number to defined range? Say that we have a secure random number generation that outputs 32 bit random numbers, so it's output is a true random number between 0 and a MAX. What is the best way to map this random number to a ... 2answers 294 views ### Which risks are associated with deriving multiple keys from the same DH secret Z? NIST recommends Krawczyk's HMAC-based key derivation function (HKDF) in SP-800-56C (PDF). HKDF shall e.g. be used to create keys from shared secrets after Diffie Hellman key establishment. NIST ... 2answers 3k views ### Impacts of not using RSA exponent of 65537 This RFC says the RSA Exponent should be 65537. Why is that number recommended and what are the theoretical and practical impacts & risks of making that number higher or lower? What are the ... 2answers 3k views ### Rijndael vs. Serpent vs. Twofish: General comparison Can anyone explain (or give a link to document about) why Rijndaal won the AES, especially comparing it to other finalists (Serpent and Twofish)? What criteria were used to make decision? Or is there ... 4answers 3k views ### How can one securely generate an asymmetric key pair from a short passphrase? Background info: I am planning on making a filehost with which one can encrypt and upload files. To protect the data against any form of hacking, I'd like not to know the encryption key ($K\$) used for ...

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