All Questions

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Encryption of a number with RSA bigger than $n$

I'm having trouble understanding why encrypting a number $m$, which is larger than $n=p\cdot q$. I've looked into the following threads: Why RSA can't handle numbers above 76? Why does plain RSA ...
574 views

Proof for exponentiation in modular arithemtic

If $e$ is a natural number, then this is true: $$m^e \bmod\ n = (m\bmod\ n)^e\bmod\ n$$ This is often used when encrypting, especially with RSA, since one can avoid directly calculating $m^e$, ...
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What security do Cryptographic Sponges offer against generic quantum attacks?

In the face of non-quantum attacker, Keccak[r=1088,c=512] with 512 bits of output provides: Collision resistance up to $2^{256}$ operations Preimage resistance up to $2^{256}$ operations Second ...
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Going Blind, Group or Ring?

I have several problems approaching the following problem: Let's assume that we have Alice, Bob and Cathy. Alice wants to present a message to Cathy, showing her that someone else has signed it (Bob), ...
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Cracking Stream Cipher

I need to crack a stream cipher. I have a set of plain text and encrypted text pairs. The hint to the question asks me to use entropy analysis and look at the hex dump of the encrypted text. However, ...
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State of the art in zero knowledge proof compilers?

What is the current state of the art zero knowledge proof compiler ? I need one that can minimally handle double exponentiation by a known value E.g. $$Pok\{(\alpha):h=g^{\alpha^b}\}$$ where b, ...
352 views

Why is CBC based on AES malleable in blocks other than the first block?

Perhaps I don't understand the answers to Malleability attacks against encryption without authentication, so trying to be more concrete... please be patient. If I want to change the first block and I ...
132 views

DES-encrypted cipher

Suppose that you knew that a plaintext was encrypted with DES and consisted only of 16-bit Unicode letters, numerals, and punctuation marks available on a standard English-language QWERTY keyboard. ...
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Is knowing the distribution of input domain only way to do cipher text only attacks?

Imagine there is a $n$ bit block cipher. The attacker has harvested some how all possible $2^n$ cipher texts for some given key. Without the knowledge of the distribution of input domain as in ...
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Why can ECC key sizes be smaller than RSA keys for similar security?

I understand how ECC is based on the discrete log problem and RSA on integer factorization. I've read several references that show how a solution to either of these problems can typically be adapted ...
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Is there a metric (term) for work required to decrypt a public key?

Any public key decryption can be decrypted given enough time and computing power. Is there a metric or term for this? Something like it would require on average 2^43 1024 bit hashes to find private ...
736 views

How is HKDF-Expand better than a simple hash?

Why do we need HKDF-Expand if we can simply hash the pseudorandom key to make it longer? The docs (in the link) say: The second stage "expands" the pseudorandom key to the desired length... ...
104 views

Using homomorphic encryption, I would like to be able to take an encrypted integer and either add 1 or -1 for a new encrypted value. I do not want the encrypted value to be recoverable - just the ...
533 views

Shamir's ID based signatures

I am currently implementing Shamir's ID based signature algorithm as proposed in Adi Shamir, Identity-Based Cryptosystems and Signature Schemes. Advances in Cryptology: Proceedings of CRYPTO 84, ...
174 views

implication of tweak on bruteforcing a block cipher

Consider a KPA attack, where the attacker gets known plain text and the corresponding cipher text. Since the encryption algorithm is known, he can brute force all possibilities of key bits. What is ...
1k views

Which blind signature schemes exist, and how do they compare?

I'm looking into blind signature schemes for use as digital cash. I have come across blinded RSA, and Lucre(DH based). Are there other schemes available, and how do they compare? I suspect there ...
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Using cryptographic hash to match AES key size

I am making a tool for myself where I intend to store cipher text. I wish to use AES together with arbitrary sized keys to create these cipher text strings. The idea is to use a cryptographic hash ...
414 views

Best way to get 32 bytes from PBKDF2

Use the PBKDF2 output as an HMAC key, hashing two different inputs - once for 20 bytes and once for 12 : ...
163 views

Security based on PRF

I've seen that there is several kinds of PRF, and sometimes people speak about strong PRFs. When proofs of a protocol or algorithm are given based on the assumption of the use of a PRF, are they ...
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Merkle signature + PRNG?

In the section 3 (page 44) of the book "Post Quantum Cryptography", it says: Then each one-time signature key must be generated twice, once for the MSS public key generation and once during the ...
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How to specify last $t$ bits are only sent when a signature is sent?

I have read that in cases where there is a limit on the amount of data sent over the network, one idea is to sent only $t$ bits of data rather than sending all the bits. For example, in the Schnorr ...
138 views

Can you explain the counter in the Key derivation function of PACE?

I try to understand the PACE-Protocol, especially the Key Derivation Function in ECDH on page 33. Can someone explain me, what the Counter is good for? Often there are iteration counters in KDF - but ...
151 views

Combating traffic analysis over request-response protocols

Suppose I am to design a request-response protocol (similar HTTP). For the sake of simplicity let us assume that this is a "chat" protocol where the client can only perform two actions: Contribute a ...
268 views

Usage of Fermat primes in RSA

Why are Fermat primes ($2^{2^n}+1$) useful as $e$ (the public key) in RSA encryption? I understand why $2^n+1$ primes are useful, because they would simply be ...
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Decrypting files with an unknown method but a known result

I am assuming there is a very simple way of working this out by brute force, but I am not sure if there is a better way. I have a file of data that I wish to get (my data, generated by a machine). ...