# All Questions

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### Supersingular Isogeny Key Exchange broken?

Found this report detailing a quantum algorithm for computing isogenies between supersingular elliptic curves. http://cacr.uwaterloo.ca/techreports/2014/cacr2014-24.pdf with the quote ...
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I would love to have a system where the password is never send in plaintext (even with TLS) to authenticate a user. I've come up with this protocol, but I'd rather use something proven. Registration: ...
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### Envelope Encryption with asymmetric keys vs KMS

In Envelope encryption a symmetric secret key is used to sign the data and then another key is used to encrypt the secret to produce an encrypted secret key. The encrypted messages is packaged with ...
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I have an OLD work database that seems to have a simple encryption on the PW. I have one users whose pw is 7777 the cipher text is ...
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### Knowing pre-encryption data, can we find the private key CryptoWall used?

I'm just repurposing a question already asked about Cryptolocker for CryptoWall: "If we know exact contents of some of the files prior to them being encrypted, would it be feasible to use those ...
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### What is the difference between a random oracle and a probabilistic algorithm? [on hold]

What is the difference between a random oracle and a probabilistic algorithm?
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### How can I handle the situation if key size is less than 64 bits in DES?

If the data is not a multiple of 64 bits, then I can use padding. I know that it is not secure. But if the key is less than or more than 64 bits, should I ask the user to enter the key again, or ...
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### Ensuring integrity and confidentiality together with symmetric encryption

I've been required to answer a question: As usually, we have Alice and Bob. Suppose Alice wants to send a file F to Bob, ensuring integrity and confidentiality. They share a symmetryc key ...
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### How to encrypt/decrypt data in an SPI BIOS FLASH

I need some info/suggestions about the cryptographic algorithm that make me sure that the executed bios by the CPU is an authenticated bios. The check has to be performed by a FPGA. I have SPI Flash ...
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### How to build a secure cryptographic algorithm [closed]

I need some info about the building of a cryptographic system. I would like to know the flow to follow and if there is some books or paper that covers this topic, because I know to concatenate ...
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### Encryption of 8 bit of block data

I'm looking for a cryptographic algorithm that allows encrypting only 8 bits of data in a block. Is there any such algorithm that is considered secure? The reason I'm asking is because I have to ...
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### OTP same message encrypted twice (with different keys)

Lets say we have a message $m$ that we encrypt with two keys $k_1, k_2$ such that $k_1 \ne k_2$. This will produce two ciphertexts $c_1, c_2$ such that $c_1 = m \oplus k_1, c_2 = m \oplus k_2$. The ...
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### Why an upside down path on the evaluation of branching program on encrypted data?

Suggested by Ricky Demer in this post, I am reading the paper "Evaluating Branching Programs on Encrypted Data"(TCC 2007), which uses one-round strong OT protocol to implement homomorphic evaluation ...
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### Are there any encryption schemes that enable to permute homomorphically?

According to the Barrington's theorem, any circuit in NC1 can be converted to a branching program, whose main operation is the composition of permutations (along with the choosing of permutations ...
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### Recommended password complexity for SSH key encryption using AES-256-CBC

When protecting SSH private keys with password-based encryption, what would be a good minimum password complexity+length standard to make cracking the password too difficult to be worthwhile anytime ...
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### how to do AES decryption knowing the key (matasano) [closed]

I'm doing the challenge in matasano and I'm doing something wrong cause I don't understand what to do. All I want to do for now is to decrypt the first block (128 bits) and then I will code the rest ...
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### What is the recommended key length for digital signature? [closed]

What is the recommended key length considered secure in order to sign a flash memory using PKCS #1.5? What is the main difference between PKCS#1 1.5 to 2.2?
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### How feasible would it be to generate 300 million public key pairs in 8 hours?

Assume there is a hypothetical situation where one would need to generate hundreds of millions of public key pairs of reasonable strength in a limited time. What would be required in terms of ...
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### Is double encryption using AES using different key lengths (128 bit and then 256 bit) vulnerable? [duplicate]

I know that AES 256 bit is far more than enough to secure a file. I just want to know if there are any vulnerabilities or known attacks which can make such this combination less secure "double ...
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### Is there a partially homomorphic quantum secure public key cryptosystem with IND-CCA1 security?

I recentely asked "IND-CCA1 RSA padding?" about whether there is a IND-CCA1 secure variant of RSA. The original version of the question also allowed usage of ECC which would allow usage of ElGamal, ...
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### Modular reduction for NIST prime P256— understanding the data

I am working on a project where I need to implement elliptic curve cryptography, I am struggling from a long time in order to understand the working and the process. Modular finite field arithmetic, ...
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### Digital signature algorithms within the European Union

I've just noticed that US NIST has accepted DSA as a digital signature standard. Does the European Union have a similar standard, specifying an algorithm to use when digitally signing messages? I ...
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### Why the $IV$ used in Merkle-Damgard has to be fixed to a specific value?

I just can't figure out why on earth the $IV$ in Merkle-Damgard has to be fixed (that's what the Katz-Lindell book says)? Because even if you choose it randomly from say $\{{0,1}\}^n$ then the ...
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### OPE ON SYNONYM BASED RANKED KEYWORD SEARCH

I had already studied, order preserving encryption(ope) is suitable for ranked keyword search. But i want to use order preserving encryption(ope) for synonym based ranked keyword search. it this ...
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### How can you produce visual cryptography?

I would like to be able to "code" images such as the one seen here on Wikipedia. Is this generally produced with software, or does this require complex programming? Ideally, I would love to be able ...
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### Why is there no 'ECDSA' version of 'DHE-RSA-CHACHA20-POLY1305'?

So I was just checking my TLS cipherlist and noticed that there was a 'DSS' / DSA / ECDSA version of every ...
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### Possible to determine equivalence of hash codes from different hashing functions?

Let $H_{1}(x)$, $H_{2}(x)$, ..., $H_{n}(x)$ be a list of $n$ secure one-way hash functions such that for a given input $x$ each $H_{i}(x) \neq H_{j}(x)$ when $i \neq j$. Give one hash function to ...
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### Enhanced Security Services / CAdES for PKCS7 - multiple signers for SignerInfo's?

In RFC5652, Cryptographic Message Syntax (CMS), section 5.3. SignerInfo Type, there's a sid field that maps to this: ...
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### For each of the following modifications to the Merkle-Damgard transform, determine whether the result is collision resistant or not

$(a)$ Modify the construction so that the input length is not included at all (i.e., output $z_B$ and not $z_{B+1}=h^s(z_B||L)$). $(b)$ Modify the construction so that instead of outputting ...
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### Request for review: known plaintext mitigation by rotation and xor encoding

When using structured data inside a plaintext (such as gzip headers, xml, etc.) they generate known plaintext which may be useful in cracking the ciphertext. So I had these two ideas on how to make ...
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### Safety of AES ECB when used with openssl_seal function

I'm trying to use PHP to encrypt files with a public key. I'm using the function openssl_seal that "encrypts data by using RC4 with a randomly generated secret key." RC4 is considered unsafe so the ...
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### How can i work on coding theory? [closed]

I interested to work and create a Creativity on coding theory(modern codes). Please introduce me the best book and a matter in this case,i have read coding theory.
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### How to switch your computers random number generator? [closed]

I am interested in changing my computers default PRNG within bsafe from the broken dual_ec_drbg to one of the other included options. I have been unable to find any documentation on how to do this. ...
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### Probability of $D_k(a*k+b) =p$ for a PRP

I'm wondering what the probability of a PRP having numerous keys (say 3 or more) with the same simple idempotent, or near identity, function. For example, does there exist values of $p, a, b, x_1,$ ...
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### Question about OR operation in fully homomorphic encryption

This page (which won't let me post a comment, sadly!) describes how the original FHE paper by Craig Gentry describes FHE. (Other references to this stuff can be found on this question.) It mentions ...
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### Combine two sha512 hashes to a single hash

Assume I have two data fragments: $Frag_1$ and $Frag_2$. I can build sha512 hashes the usual way with $SHA512(Frag_1)$ and $SHA512(Frag_2)$ and then hash the two fragments appended to each other: ...
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### Is there a standard for public key encrypting and signing short messages?

Some systems, that operate over flaky bearers, need to send signed, encrypted messages that are entirely self contained - i.e. send a single 300 byte UDP rather than have a 10Kb, two-way, certificate ...
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### How to formally prove that a given hash construction is collision resistant or not?

Is there a precise procedure or strategy to follow in order to prove in a formal way the (not) collision-resistance of a certain given hash construction? I.e. let $H_1$ and $H_2$ be two hash ...
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### Is it possible to generate backdoored DH parameters?

I know it has been already asked and answered whether it's possible to generate weak DH parameters. But "recentely" we experienced the Logjam attack, which makes use of the pre-computation ...
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### RC4 via nonlinear mixer — sound?

We implemented a rc4 (ARC4) variant for creating a cryptographic stream - question is: Is this sound? (I'll give an example for 32 bit variant, though 64 bit/larger is possible) Algorithm is as ...
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### Diffie-Hellman in the wild

Say we have a public network and lots of peers. Each may need to communicate with another peer. Would there be any huge security issue to each peer making the the public part of their key openly ...
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### Digital Signature Attack

In a typical digital signature scheme in which in order to sign we compute: hash of the message: $h(m)$ resulting hash signed with the private key $[h(m)](private key)$ If there were a collision ...
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### Is a 256 bit HMAC-SHA256 key compromised if the tail is used for identification?

Using a truncated hash of a key seems to be a safe way to identify the key (i.e. key fingerprint), but what about truncating a key itself in the case of HMAC-SHA256? Example: ...
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### SHA512 vs HKDF key expansion

I'm working on a secure file storage server. This server allows users to upload and download files based on a "seed". This "seed" is a 128-bit value. From this 128-bit seed, we run SHA512 to derive a ...
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### How does the birthday paradox apply to random keys?

I'm developing a web app where I wish to use pseudorandom keys for items, perhaps akin to Imgur. I was thinking about how many combinations I need to reduce the chance of someone guessing any random ...
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### Using scrypt to generate key + IV/nonce together

Is there any security issue in using scrypt for Key + IV/nonce generation? In other words, can I safely use only a salt for scrypt and derive the nonce along with the key? For example: I have data I ...
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### Real-world example of using ring signature [closed]

Code also will be great. There is one in wikipedia, but it's awful. Have you got something more usable?
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### Is entropy being lost when the hash of a secret has a specific format?

Bitcoin's mini private key format works by brute forcing random data to produce a secret so that when appending ? and then hashing with sha256 it will produce a ...