# All Questions

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### ElGamal signature - exploiting fallacious implementation

This question is related to ElGamal signature scheme as defined here ElGamal signature without calculating the inverse Show how one could exploit an implementation ElGamal signature scheme in which ...
2k views

When a user on facebook grants an app access to their account, an API key is issued to the app. This key is app and user-specific. This process is described in Facebook's developer documentation. ...
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### ANSI X9.31: the purpose of the date/time vector in the PRNG?

What is the exact purpose of the date/time vector $dt$ in the ANSI X9.31 PRNG? $$I := E_K(dt)$$ $$R := E_K(I \oplus V_{old})$$ $$V_{new} := E_K(R \oplus I)$$ Specifically, the document seems ...
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### ElGamal signature without calculating the inverse

I stumbled upon this question in some textbook. Propose a variant of ElGamal signature scheme such that there is no need to calculate the inverse $k^{-1}$ as it is usually done using the EEA. ...
1k views

### What is the most practical fully homomorphic cryptosystem?

Craig Gentry recently gave the first fully homomorphic cryptosystem. Quite a bit of work has been done since extending his work. It seems, however, that no system is practical for real world use. ...
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My application can authenticate via openid and oauth (facebook, twitter, etc) and also with its own authentication system. I previously switched hashing from MD5 to SHA1 and during migration I had to ...
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### Does NTRU decrypt correctly now?

The NTRU public-key cryptosystem has a lot of interesting properties (being resistant to quantum computer attacks, being standardized by several important bodies), but it also has a pretty unique ...
350 views

### What algorithm does PGP use to encrypt email?

I know it uses RSA/DSA to create keys, but does it use that same algorithm for the actual cipher?
518 views

### KeeLoq showing that decryption is indeed the inverse of encryption

In some text I am reading, there is an exercise asking to show that KeeLoq decryption function is the inverse of the encryption function. Details about KeeLoq are given in the Wikipedia article. As I ...
421 views

### Would RSA-encrypting a private key for itself constitute a vulnerability?

I'm planning to encrypt some individual files for storage, using the GnuPG implementation of RSA. If I happened to encrypt the private key corresponding to the public key used for encrypting -- either ...
5k views

### Is modern encryption needlessly complicated?

RSA, DES, AES, etc. all use (relatively) complicated mathematics to encrypt some message with some key. For each of these methods, there have been several documented vulnerabilities found over the ...
6k views

### Why is public-key encryption so much less efficient than secret-key encryption?

I'm currently reading Cryptography Engineering. After giving a high level explanation of the difference between secret-key encryption and public-key encryption, the book says: So why do we bother ...
237 views

### What is the sign bit for in Feige-Fiat-Shamir?

The Feige-Fiat-Shamir identity scheme is based on a ZKP assuming that square roots are "hard" modulo an integer of unknown factorization. The "parallel version" of this protocol includes a "sign bit" ...
3k views

### How can we reason about the cryptographic capabilities of code-breaking agencies like the NSA or GCHQ?

I have read in Applied Cryptography that the NSA is the largest hardware buyer and the largest mathematician employer in the world. How can we reason about the symmetric ciphers cryptanalysis ...
586 views

### Alphabetic Substitution with Symbols

I was reading on a site about the Zodiac Killer and how he used a basic substitution cipher, but instead of substituting english letters and characters he substituted symbols. I was wondering, if you ...
4k views

### Types of Cryptography for a 4-8 bit microcontroller

This is more of a research question. I was wondering what types of crypto algorithms would work best on a small 4-8 bit micro controller. I recently read a paper called Security Considerations for ...
1k views

### Calculating the amount of zero bits to be appended to the message

From FIPS 180-3 Suppose that the length of the message, M, is l bits. Append the bit “1” to the end of the message, followed ...
1k views

### GPG symmetric encryption and javascript

I have a use case where I need to encrypt some text in the browser using javascript. Only the encrypted text will be stored on the server. The user should then be able to take the encrypted result and ...
386 views

### Pairing-friendly curves in small characteristic fields

There are several well-known techniques to generate pairing-friendly curves of degrees 1 to 36 on prime fields GF(p): Cocks-Pinch, MNT, Brezing-Weng, and several others. In extension fields GF(p^n), ...
883 views

### What is the signature scheme with the fastest batch verification protocol for multiple signers?

I'm looking for a scheme where signing may be costly (slow) but batch verification with different signers is the fastest possible. Modified ECDSA allows batch verification for different signers with ...
630 views

### What is the smallest plaintext/ciphertext size for an algorithm like?

When I read about a choosen plaintext attack, for example on AES the block size is always 128 bits, does it mean the attacker will only supply 128 bits data words as "plain text" ?
191 views

### Is there a cumulative commitment scheme?

For a certain application I need a commitment scheme where each user could make a commitment, and a single verification operation could verify all the commitments simultaneously, faster than single ...
1k views

### How to practically find solutions to a discrete logarithm?

Are there any ongoing or current practical attempts to solve instances of the discrete logarithm problem of the order of magnitude used in cryptographic applications, for example with a 256 bit ...
135 views

### How to collect, process, and transmit data securely?

In my question "Authenticating data generated by a particular build of an open source program", Dave Cary requested that I post a question stating my real problem on a high level rather than the ...
222 views

### Are there reference implementations of ECQV implicit certificates?

I am interested in exploring ECC implicit certificates, specifically using the ECQV protocol. While the actual implementation would not difficult to perform using building blocks provided by most ECC ...
2k views

### What place do prime numbers have in cryptography?

My understanding of hashing and encryption is rather limited. I certainly do not understand the mathematical formulas at play in these algorithms. With that said, what part do prime numbers play in ...
179 views

### Authenticating data generated by a particular build of an open source program

[I was torn between posting here or security.stackexchange.com. In the end, I felt that this was more of a design question, rather than an implementation question and so chose this forum.] My ...
386 views

### Does MD5 generate 128 independent bits?

I heard that there are 128 stochastically independent bits in an MD5 output. Is that true? If so, are there any citations or proofs for that?
268 views

### If a cryptanalytic breakthrough is made, what process should be followed?

If a researcher manages to make a cryptanalytic breakthrough on a cryptographic algorithm or protocol that is in use, what should they do? Has this ever happened before? What are the implications for ...
190 views

### How are constructs with data-dependent swaps and rotations cryptanalyzed?

Linear and differential cryptanalysis seem well suited for constructs with a (relatively) simple fixed structure of boolean expressions. But some ciphers incorporate swaps of array elements where the ...
2k views

### Could one prevent double spending in decentralized digital currencies (like Bitcoin) without all transactions being public?

A recent approach to creating a decentralized online currency, called Bitcoin, has been generating some interest. The goal is to have a way to transfer currency without a central authority and without ...
1k views

### Can you create a strong blockcipher with small blocksize, given a strong blockcipher of conventional blocksize?

Suppose I want a strong 20-bit blockcipher. In other words, I want a function that takes a key (suppose the key is 128 bits), and implements a permutation from 20 bits to 20 bits. The set of ...
264 views

### Is it safe to encrypt a public key alongside a message with AES?

If I have a message M and a public key P, and I encrypt them alongside one another: CIPHER = AES(M + P, aes_key) Is CIPHER now weakened if someone knows what P ...
2k views

### What is the post-quantum cryptography alternative to Diffie-Hellman?

Post-quantum cryptography concentrates on cryptographic algorithms that remain secure in the face of large scale quantum computers. In general, the main focus seems to be on public-key encryption ...
326 views

### Looking for cryptographic secure hash algorithm(s) that produces identical root hash for differently sliced hash list

I have a scenario similar to the one described in Wikipedia: hash list, but with a twist. I'm looking for a cryptographically secure hash function that would create the same root hash for the same ...
474 views

### Is digest=HASH(HASH(a)+HASH(b)) equivalent to publishing two digests?

Is combining digests (created using a hash function) using arithmetic addition, and then hashing and publishing the result, less secure than publishing the set of digests? Does the answer change if ...
1k views

### Dictionary attack on pass-phrases on common algorithms

I don't yet perfectly understand the difference between brute-force and dictionary attack since this differentiates one attacking the key and another attacking password: apparently attacking passwords ...
412 views

### Are there public slow-but-strong algorithms out there that resist brute-force attacks better?

I'm reading that AES uses 4x4 bytes (4*4*8 = 256 bits key) matrix for performances matters (since it's a requirement for common standard encryption algorithms), but are there implementations with ...
4k views

### Why does nobody use (or break) the Camellia Cipher?

If Camellia is of equivalent security and speed to AES, concerns arise. First of all, assuming the above, why is Camellia so rarely used in practice? Why aren't there any breaks in Camellia? Does ...
1k views

### Any good file format alternative to PGP for encrypting data at rest?

I'd like to create an encrypted file that: Can be decrypted on a variety of platforms (mainly Windows, OS X, and Linux) with knowledge of the key. Can be decrypted with existing tools (perhaps ...
677 views

### Using an RSA private key simultanously as an AES encryption key to generate random numbers?

Currently I'm implementing a PRNG for an embedded system. Using the RFC 4086 I've decided to use the X9.17 - to be more specific the successor X9.31- standard to implement my PRNG. X9.17 uses DES, but ...
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### How long would the 100 Year Cryptography Project have secured its data had it been started 100 years ago?

The goal of the Tahoe-LAFS 100 Year Cryptography project is to "enhance Tahoe-LAFS's cryptographic system so that Tahoe shipped today/next year might remain safe from cryptographic attacks for a 100 ...
137 views

### What is sent on the initial communication with Kerberos?

When a user wants to communicate with another user (let's say Alice and Bob), what is sent over the network during the initial communication? Alice uses her Kerberos client and enters her password ...
600 views

### Is public-key cryptography the only option in this scenario?

Two parties: a client and a server are to a agree on a symmetric key. Both the client and the server are aware of a master password. The way this is currently done ...
137 views

### Proof that Alternating Step Generator and modifed ASG' have equivalent security?

The Alternating Step Generator (ASG) is a PRNG combining 3 LFSRs. Output of the ASG is the XOR of the output of two clock-gated LFSRs. At each step, a single one of these LFSRs is clocked, according ...
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### Is there an algorithm to find the number of intersections of two sets?

Suppose both I and my friend have a set of integer numbers. We want to know the number of common elements in our two sets but without knowing elements of the sets of each other. So I don't want my ...
459 views

### Are these emerging threats against AES affecting your designs?

Recentally, an attack on AES was discovered which reduces its computationally complexity, by a very slight amount. The first key recovery attack on the full AES-128 with computational complexity ...
5k views

### Now that quantum computers have been out for a while, has RSA been cracked?

D-wave systems has released a commercially viable quantum computer. This means in theory, that all asymmetric encryption algorithms — such as RSA — are now useless due to the speed at which quantum ...
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 ...