# Tagged Questions

Cryptography is the practice and study of logical means used to achieve information confidentiality, integrity and authenticity. It covers, among other things, encryption (making some data unreadable except for those who know a given secret element, called a key), data hashing (in particular for ...

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### Principle of asymmetric algorithm in plain english

I was giving a presentation to my colleagues about cryptography basics in which I explained about asymmetric algorithm and its use. One of the common question from the audience about asymmetric ...
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### What are the differences between an encryption seed and salt?

Today I had a hard time explaining the difference to a friend. I know seeds are used when generating "random" strings. And salts are used when providing different outcomes to a hash. What is a ...
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### Rainbow table concept for Prime numbers

Is there a concept where pre-computed tables can be used for prime number factorization ? Is it possible that a computer can generate millions of prime numbers, store it and then effectively determine ...
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### How can I explain “zero knowledge proof” to an end user?

A ZKP allows proof of knowing the answer to a secret, without actually disclosing what that answer is. Is there any analogy that can help people put this concept into everyday practice? A "lie to ...
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The password hash used for MySQL passwords prior to version 4.1 (now called OLD_PASSWORD()) seems like a very simple ad-hoc hash, without salts or iteration counts. See e.g an implementation in ...
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### Is Schneier's “Applied Cryptography” current?

I've been recommended Bruce Schneier's 1994 book Applied Cryptography as an introduction to the use and the inner workings of cryptography. Is this book still current and a good introduction?
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### Generate cryptographically strong pseudorandom numbers in Javascript?

Is there any good way to generate cryptographically strong pseudorandom (or true random) numbers in Javascript? The crucial requirement: if a.com's Javascript generates some random numbers, no one ...
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### Collision rate for different hash algorithms

Is there any collision rate measure for popular hashing algorithms (md5, crc32, sha-*)? If that depends only from output size, it's quite trivial to measure, but I suppose that depends also of ...
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### How quickly can these password schemes really be beaten?

If you use a quick hashing algorithm like MD5 or SHA-1 to hash passwords and you don't use any salt at all, how quickly could one expect a hacker to find my password out? If I use a truly random salt ...
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### Is it possible to securely store passwords using reversible encryption?

Everyone says that you need to use a non-reversible hash when you store passwords so that even if your database is leaked, the passwords themselves are still safe. I'm wondering if there is anyway to ...
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### Why are there few (none?) easy to use encryption libraries?

If I search stack overflow for how to safely encrypt data, one of the first hits is someone's custom encryption scheme. I've seen several similar questions on this site, and in general they are all ...
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### GnuPG/OpenPGP Key Practices

I'm looking for input on GnuPG (gpg) best practices. It's been discussed some on the gnupg-users mailing list, but I wanted to get as diverse a view as possible, so I thought to bring the topic to ...
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### How secure is Keychain on OS X?

Keychain is a built-in app on OS X that stores all of the user credentials, and allows for one to keep encrypted notes in an easily accessible way. Does anyone know how secure Keychain is? I know ...
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### Does my PGP key really need to match the e-mail address for which I'll be using it?

In considering whether or not I want to join the big web o' trust and put my keys on a key server, I got to thinking about how it would affect my e-mail address' exposure. I generally try to keep my ...
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### Would an encryption scheme that generates an extra key to be securely stored offsite be a safe backdoor?

Let me be clear first and foremost: I do not think installing a backdoor in security algorithms is a good idea. They undermine the trust in the software and in the company that provides the service. ...
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### Is it dangerous to publicly share ciphertext?

Alice wants to share a message with Bob, but Alice and Bob can never be in the same place at the same time. We can assume they both know each others public keys (or agreed on a shared key, if that ...
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### What are the realistic, and most secure crypto for Symmetric, Asymmetric, Hash, Message Authentication Code ciphers?

I'm interested in updating this two pronged question for 2011: What cryptology is most appropriate for low-powered devices (such as a cellphone), and yet still effective? What cryptology is most ...
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### Multi-tenancy, SSL Certificates, and Subject Alternative Name

The X509 specification allows a CA to issue a single certificate for multiple hostnames, by using the "Subject Alternative Name" extension. From the spec: The subject alternative name ...
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### What happens when an Intermediate CA is revoked?

Currently I'm working on a certificate manager that allows our product to securely connect to remote webservices (over TLS/SSL). For security, we use Certificate Revocation List checking (or CRL-...
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### In what ways does Full or Partial Homomorphic Encryption benefit the cloud?

Can someone explain, in plain English, the practical ways FHE and PHE can be leveraged in the cloud? Some interesting (and confusing) links include this Microsoft Research PDF and this wiki entry. ...
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### The AES Crypt implementation hashes a password 8192 times to generate the key. Is this necessary?

I'm considering encrypting some small files (hundreds of kb apiece) using the AES Crypt reference implementation. Looking at the source, it seems that the encryption key is derived from the IV and the ...
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### How should passwords be stored if they must be recoverable?

By "encrypting" passwords you are violating CWE-257: Storing Passwords in a Recoverable Format. But this is exactly what the French government wants. They want to be able to obtain the passwords of ...
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### SHA, RSA and the relation between them

SHA is the hashing mechanism. However, RSA is the encryption algorithm. So does RSA algorithm use SHA hashing mechanism to generate hashing keys which in turn is used to encrypt the message?? ...
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### Storing passwords in reversible form - a genuine use-case

What's wrong with you, you crazy fool, you're not supposed to be able to retrieve a password in plain text! I know. Hear me out. Assume I've got a service that's similar to Mint.com. For those that ...
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### Encryption and the “security time decay” of prior encrypted data

This question is on the assumption that any data once encrypted, may (eventually) be decrypted through Brute force (compute power/time) Exploits in the cryptography used Theft of private keys ...
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### Forced into using a static IV (AES)

We've had to extend our website to communicate user credentials to a suppliers website (in the query string) using AES with a 256-bit key, however they are using a static IV when decrypting the ...
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### What is gained by hashing the last block on-device?

Recently I encountered the notion of "hashing the last block on-device". It means that when computing a digital signature, the hash that serves as input for the actual signature computation is not to ...
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### What's the mathematical model behind the security claims of symmetric ciphers and digest algorithms?

Why can SHA-1 be considered a secure hash function? That's something I still wonder about. I understand the concepts of why modern asymmetric algorithms are deemed to be secure. They are founded on ...
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### Recommendations for a Certificate Management tool for Linux [closed]

A client is looking to roll out OpenVPN to all its mobile employees and will use certificates on both the server and all clients. This creates a new challenge to manage all these certificates and ...
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### Does Windows really still use unsalted MD4 for password storage?

I find it quite difficult to believe that the passwords in Active Directory for Windows 2008 R2 are still stored using the unsalted MD4 (aka "NT Hash") algorithm. Can it really be true? I've been ...
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### Why should I sign data that's already encrypted? [duplicate]

My question is about the difference between signing and encrypting. Why would you still sign something if you are already going to PKI encrypt it? Doesn't the PKI encryption inherently provide ...
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### Do salts have to be random, or just unique and unknown?

First of all, my motive is to avoid storing the salt in the database as plain text. As far as this question is concerned, the salt is not stored in the database. After discussion in comments and in ...
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### CryptoWall 3 - how to prevent and how to decrypt?

My father's computer is now infected with CryptoWall 3, according to the link below. http://www.bleepingcomputer.com/virus-removal/cryptowall-ransomware-information#CryptoWall Is there a way to ...
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### What does “key with length of x bits” mean?

I'd like to know what it means to say "the cryptosystem C uses keys with a length of x bits". I do not understand what the bits length means... doesn't it depend on the encoding? The same word encodes ...
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### What is the potential impact of the alleged OpenBSD IPSEC attack?

Recently there is a bit of concern over encryption back doors in IPsec and while the status of this has not been confirmed, I don't know what impact something like this might have. For instance, does ...
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### Doubling up or cycling encryption algorithms

I've occasionally read the suggestion to enhance security by either doubling up on encryption algorithms (encrypt a message once with one algorithm, then encrypt the ciphertext again with a different ...
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### Why aren't digital signatures the standard way to authenticate web clients?

The web is a mess of incompatible cookie schemes, asinine requirements ("your password must contain letters, numbers, and at least one symbol from !()-_., and must begin with a letter, and once you ...
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### XKCD #936: under what assumptions is 1000 guesses/second over a network plausible?

In XKCD #936, a rate of 1000 password guesses/second is proposed as a "plausible attack on a weak remote web service". Under what assumptions is this rate plausible? It seems much too high to me. I ...
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### If I have two identical linux virtual machine images, will they generate identical random numbers?

A really noob question, I know, but something I was wondering about this morning. If this were the case, would they encrypt things identically? Or maybe someone could direct me to a helpful research ...
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### What should I know before configuring Perfect Forward Secrecy?

PFS has gained attention in our audit department because of its innate ability to limit our exposure if someone steals our private key. What pitfalls or common mistakes should I be aware of before ...
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### What did Blackberry do wrong?

The NIST defines a vulnerability in RIM Blackberry encryption discovered last October. Apparently, Blackberry's flavour of PBKDF2 was weak. They say: The offline backup mechanism in Research In ...
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### Salting password with the first 8 bit of the password itself

Do you guys think salting password with the first 8 bit of the password itself will have same result as using stored salt to hash password? [Clarification] I am not storing salt but creating from the ...
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### Is PBKDF2-based System.Cryptology.RFC2898DeriveBytes() “better” for Unicode Password hashing than traditional methods?

When is it appropriate to use RFC2898DeriveBytes versus a typical hash? Update I now understand that a KDF is typically used to create a symmetric key for possible use in encrypting a stream. I ...
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### Is advanced mathematics relevant to security beyond cryptography?

Short question. The title says it all... is advanced mathematics (i.e. beyond the classic college level calculus sequence) relevant to any field of computer security besides cryptography? By relevant, ...
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### Looking for example of well-known app using unsalted hashes

Does anyone know of a well-known application that still uses unsalted hashes for password storage? I'm looking for an example for an upcoming lecture on Rainbow Tables, and I think it would add ...
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### License key / Serial number generator and checker

I need a serial number generator and accompanying checker. I would like to be able to set a salt (and maybe a length). The generator should only produce serial numbers that pass the test of the ...
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### Can I use a private key as a public key and vice versa?

I have code to encrypt data using a public key and decrypt it using a private key. This is useful when a client wants to send data to a server and know that only the server can decrypt it. But say I ...
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### At what point does adding more iterations to PBKDF2 provide no extra security?

If my true passphrase is used only to generate a hash which is used as the cipher's actual key, doesn't that mean it's possible to try and brute force the cipher itself? I know it would take an ...