# All Questions

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### Is there a string that's hash is equal to itself?

I was wondering if there's any string that has a hash equal to itself, so that – when using any (none specific) hash function – the hash would be equal to that string? so that: ...
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### Verifying signature owner (without verifying the actual document)

Programming in Java. I have an RSA key-pair, a document, and a signature created with the Java Signature class, using SHA512WithRSA. In order to verify the signature, of course, I need to provide the ...
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### What does “G2” mean when used with X509 certficates and certificate authorities?

For example "Google Internet Authority G2"?. I thought it was another way of specifying Class 2 (for organizations, for which proof of identity is required) but then see certificates such as "VeriSign ...
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### Can we use numbers as a pad in the Vernam cipher - why or why not?

I was playing with the Vernam cipher on some online converter. But when I tried to encrypt my message string with numbers, it remained unchanged. Moreover, it was ignoring numbers and was encrypting ...
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### Regarding Key Strength with DES and Blowfish

I had a small discussion with my friend on combining two encryption algorithms based on 64-bit keys. Say I have a plain text of 64 bits which I want to encrypt. My Key size is 128 bits. I will use ...
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### How can AES be considered secure when encrypting large files?

Why is AES considered to be secure when encrypting large files since the algorithm is a block cipher? I mean, if the file is larger than the block size, the file will be broken down to fit the ...
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### AES Affine Transformation Polynom Representation

I have been reading on the polynomial representation of the AES Sbox in the PDF “Essential Algebraic Structure Within the AES” by Murphy and Robshaw (www.isg.rhul.ac.uk/~sean/crypto.pdf) on page 7, ...
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### stream cipher computation cost [closed]

With AES-CTR the computation time to encrypt a message of lengh $n$ blocks is $n$ computations of the block cipher. What is the running time to encrypt a message of $n$ bits using a stream cipher, ...
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### How can one generate pairs/triplets/…/n-ary MD5 collisions

Is there a way to generate n strings with the same MD5 hash?
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### How can I know when a file was signed?

I have been thinking about digitally signed documents (Word files and PDF files) and can not get over the fact about - how can I securely know when the file was signed? Scenario: If the date of the ...
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### Feistel cipher vs Substitution-permutation network - number of rounds

So I have been doing for the last couple of days comparison between Feistel cipher and Substitution-permutation network. Now I can say I know a lot about them and their similarities and differences ...
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### Data I/O operations for encrypted files

I need to implement a simple approach on a Linux system to encrypt/decrypt data and I would appreciate any feedback from you. Basically, I need to change a bit the behavior of the functions ...
I was reading upon Biham and Shamir's paper and a fact has been presented over there: if $P_1 = \bar P_2$ and I choose a key $K_1 = \bar K_2$ then in that case $$T_1 = DES(P_1, K_1)$$ $$T_2 = ... 2answers 231 views ### Are there cryptographic hash functions with homomorphic properties? [duplicate] Are there cryptographic hash functions that have homomorphism-like properties? E.g. satisfying following relation h(a || b) = h(a) · h(b), where h(x) is hash function itself, x || y is ... 1answer 106 views ### Trapdoors for lattices I refer to an article https://eprint.iacr.org/2011/501. I focus on (a bit modified) Algorithm 1 which runs as follows (in my understanding): For given n, m\in \mathbb N, q=2^k and a distribution ... 1answer 187 views ### Equal length of primes in paillier cryptosystem In the key generation step of paillier cryptosystem , In order to satisfy \gcd(pq,(p-1)(q-1))=1 , we can take equal length primes. Instead of taking(length as parameter to generate p,q) equal ... 1answer 131 views ### How often do I need to rotate AES 256 keys I'm trying to design a protocol for a resource constrained environment. The device has to communicate with a server through an insecure node. My plan is to use AES 256 end to end for all ... 2answers 281 views ### Which block cipher mode(s) is most appropriate in these applications? I have a few questions I'm stuck on for a cryptography course I'm doing, any help would be much appreciated. The first application is for decryption on a multicore processor. I assume any of ECB, ... 2answers 184 views ### Quantum key exchange skepticism/confusion I was hoping somebody could explain some issues I have with quantum key exchange that I don't quite understand. I've read bits and pieces about BB84 but I'm sure my questions probably apply to other ... 2answers 256 views ### How can I map arbitrary group elements to unique integers without using Hash functions? Let's say, I have a group G of large prime order p. A set S consists of n random elements chosen from G. Without using a collision resistant hash function H, how can I map elements of G ... 1answer 442 views ### Isn't a simple Vernam cipher as secure as known symmetric key algorithms? I have just read little about the Vernam cipher and the problems of implementing it. But, in practice, would not the key, if generated by a commonly used programming language's ... 1answer 1k views ### Can I remove new lines in a public key? Can I remove new lines from the rsa public key file to get a one line string? So the question is if the key looks like this AAA BBB CCC or ... 2answers 490 views ### SHA512withRSA - Looking for details about the Signature Algorithm I am trying to find information about the Signature Algorithm SHA512withRSA and have been unsuccessful so far. In the current state, the signature is too long, so I would like to check the code for ... 2answers 871 views ### breaking fully homomorphic encryption schemes Fully homomorphic encryption schemes allow one to evaluate any arbitrary computation over encrypted data. Intuitively this seems to be too weak, irrespective of how we achieve this. An adversary who ... 0answers 97 views ### Privacy-Preserving Protocols and Proofs of Security While dabbling in privacy-preserving protocols (mainly using Semi-Homomorphic Encryption) and coming up with miscellaneous ideas for comparison tests or other similar primitives, based on obfuscation ... 0answers 47 views ### Is there a practical succinct interactive argument for fortress draws in chess endgames? (In theory, it is known that collision-resistant hashing suffices for the type of protocol that I'm asking about: page 3.) Is there a known practical candidate protocol for succinctly (and ... 0answers 43 views ### Selecting a Bouncy Castle named curve for ECC [duplicate] I have been tasked with implementing ECDSA and ECDH in Java using the Bouncy Castle library. However, I am unsure of what curve to use. I am aware of the named curves listed in BC, and I've done a ... 0answers 73 views ### Does the initial exchange of random tokens in SKID3 authentication assure against replay attacks? As the question suggests, I wanted to get a sanity-check on this issue... is it the case that the use of a random token exchange during SKID3 authentication itself constitute a prevention of a replay ... 2answers 111 views ### Why verify padding at all? Whenever a ciphertext is decrypted using a block-cypher, we need to remove the padding. There are different ways to add padding, but they usually set the last byte of the last block to the number of ... 2answers 220 views ### Are there valid attacks on full SHA-1? Google today announced they are sunsetting SHA-1. Its fine by me. But that made me realize I am not keeping up with the research on SHA-1. The Wikipedia page only says Stevens' attack is the most ... 2answers 89 views ### How do you create a large PRP from a small one? How can you efficiently create a pseudorandom permutation with a large domain given a PRP with a small domain? Meaning: given an n-bit PRP, how can I create an n k-bit PRP? I'm only interested in ... 1answer 107 views ### Generating S boxes that satisfy Coppersmith's criteria? I'd like to generate all possible 6-bit to 4-bit S-Boxes that satisfy the criteria for S-Box design given by Coppersmith, but I have a few doubts: How many such S-Boxes are possible? Is there any ... 1answer 198 views ### “Practical” operations supported by functional encryption? I'm curious about what operations have been developed into functional encryption schemes. What I mean by that is: what operations can be performed over encrypted ciphertexts? Obviously homomorphic ... 1answer 85 views ### What does \to mean in the following definition of a block cipher? In the following picture \kappa is the key length and n is the block size. I understand that \{0,1\}^\kappa means all possible combination of keys and \{0,1\}^n means all possible combination ... 0answers 274 views ### DES S-Box properties Two of the properties that lead to the design of DES S-Boxes are: 1)For any non-zero 6-bit difference between inputs, no more than 8 of the 32 pairs of inputs exhibiting that difference may result ... 1answer 115 views ### Bit padding instead of PKCS#5 padding Padding oracle attacks are a huge nuisance when using CBC mode encryption without authentication. Wouldn't all those padding oracle attacks be avoided if we'd just use bit padding instead? Or is does ... 1answer 224 views ### How to prove Security of Onion Layers of encryption? CryptDB has Onion layers of Encryption to provide wider functionality from weaker forms of encryption. How do we prove such things are indeed secure ? Intuitively It seems ok. Are there any parallels ... 1answer 449 views ### Why does FIPS 186-4 require specific sizes for keys? In FIPS 186-4, page 32, about FFC crypto it is required that the length of p will be exactly 1024 bit and the length of q will be exactly 160 bit. Why is the requirement not stated in terms of ... 1answer 111 views ### What's causing the poor randomness in this program: the LCG, or the program logic itself? (Crypto Gods, I should begin by stressing that I haven't lost my mind: I'm not doing this in real life, I'm just trying to understand the theory behind what's happening. With your help, hopefully I ... 3answers 1k views ### Shannon confusion and diffusion concept I read the document(not the whole document) from Shannon where he speaks about the concepts of confusion and diffusion. I read in many places(not in the document but around the internet) that ... 1answer 94 views ### Base64 for a hash algorithm [closed] May be a silly question, but I am really curious. If a hash algorithm uses Base64 in the process of hashing a string for example, it is still considered a hash algorithm, even though it uses an ... 1answer 121 views ### What is the most secure key expansion routine? I am looking at G now, but is there a better, (in terms of security and speed), means by which a byte array can be expanded that doesn't involve another cipher, just a simple algorithm? I need to ... 0answers 68 views ### Cracking an appliance's network protocol I'm trying to crack my thermostat's network protocol. I've captured several rounds of network traffic and here is what I've got to work with. Communicating via HTTP POST The POST data is JSON ... 1answer 303 views ### Why is Rijndael key length restricted? Why is Rijndael restricted to key length in {128, 160, 192, 224, 256} bits (and not larger)? The algorithm looks to me like it would support an arbitrary-sized key (multiple of 32). The rounds ... 1answer 222 views ### Is it possible to generate a secure permutation F over 32-bit integers even if F(0) … F(n) is public knowledge? I'm currently interested in the problem of generating random-looking URLs from sequential database IDs, like how they do it in link shorteners. One way to do this is to encrypt the sequential database ... 1answer 66 views ### Is using SHA256 to extract entropy from a p-rand nonce a good way to create keying material? Using SHA256(N), where N is 64 bytes of output from the Microsoft Windows CryptGenRandom function. Are there any known vulnerabilities with using this method to ... 0answers 348 views ### Elliptic curves in java I am writing you because I want to implement the DPS signature with elliptic curves in Java. I found this library http://docs.oracle.com/javase/7/docs/api/java/security/spec/package-summary.html which ... 0answers 40 views ### ECIES: Purpose of optional shared information? According to Wikipedia the ECIES algorithm has two optional shared information S_1 and S_2. They are used as follows: Generate a random shared secret Z according to ECIES, which will never be ... 2answers 225 views ### Would xoring 2 independant AES CTRs to produce p-rand introduce vulnerabilities? Assuming$$AES_{k_1}(C_1) \oplus AES_{k_2}(C_2) where AES is used in CTR mode, $C$ is a 128 bit incrementing counter, keys and counters for both transforms are random as well as unique, and the ...
Assumptions: $A$ wants to communicate with $B$ $A$ knows a public key $P_B$ which is trusted by a third party and belongs to $B$ $A$ knows the address of someone who pretends to be $B$ $A$ wants ...