# All Questions

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### Example of FHEI scheme?

I am reading this paper Toward Secure Multikeyword Top-k Retrieval over Encrypted Cloud Data. I am trying to work out an example of the FHEI (Fully homomorphic Encryption over Integers) scheme ...
87 views

### Proof of non-membership on a merkle tree

Assume a user U and a server S. User uploads its data and wants later to perform an authenticity check. It also sends a merklee tree to the server. Lets say we would like the user to ask for a ...
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### infinite one-time-pad based on reference compression. can this be made more practical?

first of all I would like to start with this does not modify one-time-pad at all. nor does it apply any excessive limitations to the people using this said algorithm. a one-time pad can't be ...
225 views

### Affine Cipher over an Affine cipher

I would like to know your view about the title, encrypting a plain text with an affine cipher then encrypting that ciphertext once more using the same cipher, but of course different keys. Would it be ...
188 views

### Should I use RSA encryption since RSA is said to be broken by NSA? [closed]

I am developing a simple web app that uses asymmetric encryption, but I am not sure whether to chose RSA for the reasons mentioned above. What are your suggestions? Also if I decided to go with RSA ...
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### Are low security curves for ECDSA not supported?

I am working in a very bandwidth restricted environment, and we are looking to implement ECDSA digital signatures. Although they are no longer recommended, we might be looking to use curves that are ...
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### Containers for secret key

What 'container' is used normally to store/transport secret keys (for symmetric algorithms)? I have come across PKCS12 format where a public key certificate as well as a private key can be bundled ...
407 views

### What is difference between meet in the middle attack and man in the middle attack?

From my understanding man-in-the middle attack works as follow: Alice and Bob agreed to use Diffie-Hellman using $a$ and $q$. Alice sends $Y_A$. Before it reaches Bob, Charlie ...
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### Difference between AES CMAC and AES HMAC?

Can someone elaborate on how 'signing' is done using AES- CMAC and AES-HMAC? HMAC Signing as I understand: Compute the HMAC( Hash the key and the input concatenated in a special way) Verification: ...
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### Generating a key to use with AES

I'm going paperless, and I want to encrypt the documents that I scan. I'd like to use CommonCrypto to perform AES-XTS encryption on each file. I'm a software engineer, and I like the idea of being ...
120 views

### Can a book cipher be cracked without key?

Book cipher works this way : Choose a reference book. For each word find its position in the book. replace each word by its coordinates (ie: 2,3,4 = page 2, 3rd line, 4th word). How would ...
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### Can we break ECDLP with this machine?

Let $P$ and $Q$ are two points of NIST elliptic curve $E$ (defined over $F_{2^m}$ with prime $m$) and $k$ is a private key such that $k.P=Q$. Also we have a machine that is able to leak some ...
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### Simplied DES why 10-bit key?

In Simplified DES algorithm, the plain text is 8-bit & the key is 10-bit, why is that, why can't both be of 8-bits each?
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### Faster alternative to HKDF

HKDF is used in one of my projects to derive separate keys from a common master key. But profiling has shown it to be the performance bottleneck under certain circumstances. Is there any faster ...
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I have found many sources against cascading encryption, however I am having trouble finding arguments FOR using cascading. AES-256 + BlowFish + RC5 etc... each with a different key. This is for ...
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### Are the ECDSA public keys of a stable/fixed length relative to curve size? [duplicate]

I am working in an environment with strict bandwidth requirements, and we need predictable bit lengths of the stuff we send. We need to broadcast ECDSA public keys, and we are using OpenSSL to ...
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### Can I shorten the large ECDSA public key output file from OpenSSL?

I am working in a very bandwidth restricted environment where I from time to time have the need to distribute public keys. The distribution happens in bit format, so there is no need for encoding, at ...
126 views

### Understanding the length b used in MD5

I am trying to understand MD5 hash algorithm from the link http://www.ietf.org/rfc/rfc1321.txt At first I was not able to understand padding of MD5. Then I asked a question in this link: To ...
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### FDE Protection against firmware backdoors [migrated]

In early 2015 news broke that the NSA was installing firmware backdoors in many hard drives. I was wondering, if there was a BIOS and Hard Drive backdoor how well would full disk encryption protect ...
20 views

### What is the canonical way of creating merkle tree branches?

I'm currently looking into creating some compact merkle tree branches to prove that a given hash was included in a given merkle root. My initial thought was to list the leaf, the merkle root and all ...
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### 1 bit shannon entropy CPU instructions?

It has been proved, that the xor operator is suitable for entropy accumulation, due to its theoretical shannon entropy of 1 bit. There are six truth tables with ...
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### Does a “commutative” asymmetric system exist?

Completely crypto noob here. I'm looking for a cryptosystem made of a func f + key1 + key2 such that they satisfy: g(f(msg, key1),key2)=msg g(f(msg, key2),key1)=msg key1 != key2 Does such a ...
38 views

### Security parameter p =O(n)

In many homomorphic encryption scheme, a security parameter is calculated as p =O(n). How to use the complexity order as values? Is there any specific method with an appropriate example?
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### Security parameter p=O(n) [duplicate]

In many homomorphic encryption scheme, a security parameter is given as p =O(n). How to use the big oh values for calculation? Is there any method to do that.
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### Is finding collisions in a part-hash not often enough a bad problem?

My situation: I've been working now for a couple of months on my own unique hash function, I've changed it many times and had two main versions but I won't bore anyone with the details of my work; at ...
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### Question on data patterns within a Mask [closed]

I have already asked a similar question to this today and was told to go elsewhere, went to data analysis and they told me to come back here.... I am trying to write a Data Mask pattern and I would ...
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### Homomorphic encryption diagram and trouble about a paper

I'm reading a paper based on Homomorphic Encryption: “Fully Homomorphic Encryption over the Integers: From Theory to Practice” (PDF). Now, I need a diagram of this algorithm if possible with all the ...
292 views

### Periodic One Way Function

Is there any notion of a periodic OWF? I.e. an OWF $f$ that on every input $x$ in its domain it holds that $f(f(f\ldots f(x))\ldots )=x$ when $f$ applied many times, each time on the previous ...
181 views

### Scrypt not “old enough” to be safe?

I just check all the questions with scrypt tag about that and usually it's said that scrypt is nice in theory and theoretically better than bcrypt and PKDF2, but it's too young to be "completely ...
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### Performance of ECDSA, ECKCDSA and ECGDSA

It is proven that ECDSA algorithms are faster in key and signature generation compared to RSA. In addition, the signatures are much shorter. However, I would like to know the performance difference ...
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### PBC library: group of composite order for pairing operation

How to generate composite order group using PBC library? With PBC library, element_init_G1(g,pairing) statement creates element $g$ for group of prime order. I want ...
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### Is the length limit of AES-GCM per key or per (key, nonce) pair?

AES-GCM has a limit on the length of the plaintext it encrypts: $2^{39} - 256$ bits. But is this limit per key or per (key, nonce)? That is, if I encrypt a message with the maximum length, and encrypt ...
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### Why is CBC-MAC with length appended not secure?

I've just read about CBC-MAC for the authentication of arbitrary length messages. In particular I cannot figure out why appending the message length before computing the MAC yields a non-secure MAC. ...
394 views

### To understand a fact related to padding in MD5

I'm trying to understand to padding process of MD5 from the link: http://www.ietf.org/rfc/rfc1321.txt In this link I have found the following description of padding: The message is "padded" ...
127 views

### Collision free one way function

I was playing with a function that I think is collision free and uninvertible assuming the hardness of integer factorization. I am unfortunately not as skilled at math as I would like to be, and do ...
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### Hard problem and public key

We Knowing that make a public key cryptography system want a Hard Problem. Can we use from "Ramsey Number" as a Hard Problem to make a public key cryptography system?
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### Can homomorphic operations on many different ciphertexts be multiplied by a same constant?

Assume Alice encrypts messages $m_1$,...,$m_n$ using secret keys $k_1$,...,$k_n$ on a homomorphic encryption scheme (BHHO), so she would get $c_1=Enc_{k_1}(m_1)$,...,$c_n=Enc_{k_n}(m_n)$. Then Alice ...
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### compact rsa private key format

As shown in Why does my SSH private key still work after changing some bytes in the file? a typical RSA private key file contains. The public modulus $n$ and exponent $e$; The private ...
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### Encryption used in a commercial software product [closed]

I found (through some reverse-engineering) that the following encryption scheme is used in a commercial software that is quite widely used in Germany (Security by obscurity at its best). I have the ...
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### Why is AES-SIV not used, but AESKW, AKW1?

I'm trying to investigate different key wrapping algorithms for my implementation. I've noticed that AES-SIV is very rarely implemented by most of open source libraries. Most of them implements key ...
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### Elliptic ElGamal Public Key Cryptosystem doubt

I need an example of Elliptic ElGamal Public Key Cryptosystem. I have been trying with some values but I don't get the right solution. I have $p=13$, the elliptic curve $E:y^2=x^3+11x+7$ and a point ...
341 views

### Need for salt with IV

I have mainly referred to this question http://stackoverflow.com/questions/1949640/does-iv-work-like-salt . Definitely in case of using a salt, IV has a use of adding randomisation to each and every ...
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### Proven secure scheme under random oracle

Currently I am trying to understand random oracle model in order to make a small presentation about it but I seem to be very confused about it. Since it's an hypothetical model without a real life ...
38 views

### How to test an implementation of a homomorphic scheme?

I want to implement the following homomorphic encryption scheme from On-the-Fly Multiparty Computation on the Cloud via Multikey Fully Homomorphic Encryption by Lopez-Alt et al. I use C++ and I want ...
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### This question about combining cryptography and steganography

In combining $RSA$ system into $LSB$ steganography system in bmp image, Since every pixel of image is beteen $[0,255]$ integer numbers, if we choose $p$ and $q$ that $N=p*q>255$, How embedding ...
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### Recover secret $x$ when $c\equiv m^x \pmod p$ with public $p$ (modified)

Given an encryption system where $c\equiv m^x \pmod p$, $p$ is a known prime, 1. Is it possible to recover $x$ with a known plaintext attack? Given $(p,\text{factorization of }\varphi(p),m,c)$ 2. Is ...
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### (Non-) Perfect Secrecy of Vernam Cipher Using $E(m) = m \oplus k \oplus \operatorname{rev}(k)$

Given the cipher $$E(k, m) = m \oplus k \oplus \operatorname{rev}(k)$$ where $\operatorname{rev}(k)$ is the reversed binary of $k$, how would one prove that the cipher is not perfectly secret. I ...
3k views

### Why does my SSH private key still work after changing some bytes in the file?

I (for a test) just randomly altered a private RSA key by opening it up in Vim and changing a few bytes. It is the private part of an SSH key pair used for logging in on a remote system. Puzzlingly, ...
I know that – within the elgamal cryptosystem – the values of $a$ and $b$ are public. But which values are used to create public keys?
I am looking at why certain public keys in RSA can't be used, such as: 2 and all other even numbers (since $\gcd(\phi,e)$ must equal 1) 1 (since the cipher text would equal the message) 0 (since the ...