# All Questions

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### Is it possible to match encrypted documents using user-defined search terms?

Suppose I am storing a number of encrypted documents in a database. I would like to make it possible to identify the subset of documents whose contents match user-specified search terms without a) ...
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### Why can't one implement bcrypt in Cuda?

I had heard that although it's easy to implement message digest functions like MD5, SHA-1, SHA-256 etc. in CUDA (or any other GPU platform), it is impossible to implement bcrypt there. bcrypt is ...
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### Blum Blum Shub vs. AES-CTR or other CSPRNGs

Following on from D.W.'s comments on a previous question, what properties does Blum Blum Shub have that make it better / worse than other PRNGs? Are there significant implementation difficulties or ...
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### Calculating RSA private exponent when given public exponent and the modulus factors using extended euclid

When given $p = 5, q = 11, N = 55$ and $e = 17$, I'm trying to compute the RSA private key $d$. I can calculate $\varphi(N) = 40$, but my lecturer then says to use the extended Euclidean algorithm to ...
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### From hash to Cryptographic hash

After reading some excellent papers on SipHash, I understood that good non-cryptographic hashes such as MurmurHash and CityHash are not secure for MAC usage, due to a certain type of DDos attack ...
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### Why is triple-DES using three different keys vulnerable to a meet-in-the-middle-attack?

Among other sources, this wikipedia entry states that triple DES using three seperate keys (k1, k2, k3) is vulnerable to meet-in-the-middle-attacks, while triple DES using only two keys (k1, k2, k1) ...
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I was looking at the algorithm for Twofish, and I noticed that in some places a XOR is used, but in others, they use "addition modulo-32." What makes modulo-32 special? Why not always use XOR? Why not ...
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### How does the key schedule of Rijndael looks for keysizes other than 128 bit?

It said in Wikipedia that: [....] Rijndael can be specified with block and key sizes in any multiple of 32 bits, with a minimum of 128 bits. The blocksize has a maximum of 256 bits, but the ...
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### Using a Non-Random IV with modes other than CBC

The weakness CWE-329 is an interesting problem with CBC mode. However, does this same weakness affect the other modes of operation that rely upon an IV such as: PCBC, CFB and OFB? My gut feeling is, ...
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### Is a known plaintext, ciphertext, and public-key a viable attack on RSA?

Assume Alice and Bob are using RSA to create a common session key and Cindy is listening, attempting to obtain the session key. Alice and Bob each have their public- and private-key pairs ...
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### Besides key and ciphertext sizes what are other advantages of elliptic curve versions of various protocols?

There are elliptic curve variants of Diffie-Hellman, ElGamal, DSA and possibly other protocols/algorithms. I know that these elliptic curve variants have smaller key and ciphertext sizes which will ...
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### What is a smart card?

In many cryptographic protocols, some information is transmitted within smart cards. So, what is a smart card? Is it a physical card? What are they used for in cryptographic protocols?
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### Does encrypting twice using the same block cipher produce a security weakness?

If I use the output of a cipher, for example a block cipher such as AES and encrypt it again with the same algorithm, I read that this introduces weaknesses into the overall security of the system. ...
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### Can you make a hash out of a stream cipher?

A comment on another question made me wonder about something: Assume you're on a rather constrained platform — say, a low-end embedded device — with no built-in crypto capabilities, ...
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### How random are commercial TRNGS

I'm thinking about buying a USB TRNG. How do I evaluate its randomness? I'm sure some are better than others but which is which? Are thermal-noise better than radio-noise TRNGs?
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### Can AES decryption be used as encryption?

Definition E: AES encryption D: AES decryption x: plain text y: encrypted text k: key In original AES cipher, encryption: y = E(x, k) decryption: x = D(y, k) Then I define the "reverse AES ...
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### AES CBC mode or AES CTR mode recommended?

what's benefits and disadvantages of CBC vs CTR ? which one is more secure ?
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### Why is padding used for RSA encryption given that it is not a block cipher?

In AES we use some padded bytes at end of message to fit 128/256 byte blocks. But as RSA is not a block cipher why is padding used? Can the message size be any byte length (is the encrypting agent ...
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### Why do we need asymmetric algorithms for key exchange?

In SSL protocols, both symmetric and asymmetric algorithms are used. Why is it so? The symmetric algorithms are more secure and easier to implement. Why are asymmetric algorithms usually preferred in ...
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### Does AES-CTR require an IV for any purpose other than distinguishing identical inputs?

I'd like to encrypt files deterministically, such that any users encrypting the same plaintext will use the same key and end up with the same ciphertext. The ciphertext should be private as long as ...
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### Why doesn't preimage resistance imply the second preimage resistance?

Let the preimage resistance be defined as »given a hash value $h$, it is hard to find any message $m$ such that $\operatorname{hash}(m)=h$«, and let the second preimage resistance be defined as »given ...
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### Low Public Exponent Attack for RSA

I'm having trouble understanding the algorithm for finding the original message $m$, when there is a small public exponent. Here is the example I'm trying to follow (you can also read it in the 'Low ...
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### How to salt PBKDF2, when generating both an AES key and a HMAC key for Encrypt then MAC?

When using Encrypt-then-MAC with AES and HMAC by password, and given 128 bits of payload with the ciphertext to store a random salt, which would be more secure: Using PBKDF2 with then entire 128 bit ...
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### What are the requirements of a nonce?

Sometimes I read that a nonce has to be a random number but I disagree. A nonce just can't repeat itself. You could increase in by 1 every time if you are sure it would never repeat.
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### Sending KCV (key check value) with cipher text

I was wondering why it is not more common to send the KCV of a secret key together with the cipher text. I see many systems that send cipher text and properly prepend the IV to e.g. a CBC mode ...
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### Why, or when, to use an Initialization Vector?

i'm trying to figure out when an Intialization Vector (IV) should be used. There are anecdotal reports that WEP was broken because of weak IV's. It's also claimed that if two pieces of plaintext are ...
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### Hill Cipher known plaintext attack

I know a plaintext - ciphertext couple of length 6 for a hill cipher where its key is a [3x3] matrix. Based on what I've read and learned, to attack and crack keys of [n x n], if we know a plaintext ...
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### What is “Implicit Authentication”?

What is “Implicit Authentication” in the context of authentication methods? I searched the Web but could not find any article that describes this. If anyone can describe it, that would be a great ...
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### 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?
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### Webapp password storage: Salting a hash vs multiple hashes?

For security's sake, of course it's blasphemous to store passwords in plain-text; using a hash function and then doing a re-hash and comparison is considered much better. But, if bad guys steal your ...
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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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### Relation between attack and attack model for signatures

I would like to know: What is the relationship between an attack and an attack model. For example, let $\Pi$ be the Lamport signature scheme. This signature has it's security based on the one-way ...
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### How much data can I encrypt with AES before I need to change the key in CBC mode?

In my cryptography class, the instructor suggested that in order to give the attacker a minimal advantage of $1/2^{32}$, we have to change the key after $2^{48}$ blocks are encrypted. It seems that ...
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### 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 ...