# All Questions

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### How can we get CA's public key?

To get a public key of some organization or someone we want to send an encrypted message to, we need to make a request to CA asking that organization's public key. CA then returns X509 certificate. It ...
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### Why routers don't just use Diffie-Hellman protocol?

Routers could just use Diffie-Hellman protocol with any computer who tries to connect to that router. Then it would be impossible to eavesdrop for anyone. Only router and computer would know the ...
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### Attack XOR encryption of binary data compressed by zlib with known key length (very short key)

I am trying to break a packet format. The packet format simply packs several files into one big file. The file contents are plain. But the index data which contain offsets, file sizes and filenames ...
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### What complexity class is Bitcoin's proof-of-work (hashcash) in?

It's clear that, as a first pass, Hashcash is in FNP. For instance, say that R(x,y) is a binary relation, where x is a "maximum" target hash, and y is an input string (working block + nonce), R(x,y) ...
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### SHA-1 Keyed Hash Function

I know that SHA-1 is an unkeyed cryptographic hash function when used in practice. But, in the theory, all hash function are defined with keys. My question is: How I will be able to formalize the ...
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### How should we interpret the cryptanalysis results of SIMON and SPECK?

The NSA recently released SIMON and SPECK light weight block ciphers. Although initial spec release did not have much of cryptanalysis details, two works later appeared providing the cryptanalysis for ...
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### Is there a proof for showing any cryptogram is crackable?

I commonly hear statements along the lines of "all cryptograms are crackable - it's only a matter of time". Is there a proof to show that any cryptogram is "crackable"? The proof may be of a more ...
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### modfied man in the middle attack diffie hellman

I have been given a problem in preparation for my cryptography final that I'm not sure how to solve. It asks me to suppose a scenario where instead of where an attacker would intercept some message ...
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### How do I decrypt an Adobe Triple Des ECB mode encrypted password? [migrated]

So as you may have heard. The adobe user database of 130 million was leaked recently. I want to find out what password I used for the adobe account I created to make sure that I'm not using it ...
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### OpenPGP/X.509 bridge: how to verify public key?

I'd like to use OpenPGP authentication over TLS, but lack of implementations made me use a temporary solution: an OpenPGP/X.509 bridge certificate. The approach is very similar to the approach used ...
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### Achieving End to End encryption with TLS [migrated]

The scenario is that a client browser needs to be able to view certain sensitive information that resides in a mainframe sitting in the Internal network via a web/application server that resides in ...
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### Recent attacks on RSA

At Blackhat 2013 this week, there was a talk saying RSA is (essentially) doomed in the near future. networkworld.com ~ “Black Hat: Elliptic curve cryptography coming as smarter algorithms threaten ...
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### Learning to encrypt/encryption algorithms [closed]

I am a beginning programmer interested in coding an application of my own to encrypt sensitive files of mine. For example: lists of passwords, bank accounts, Credit Card numbers etc. I was wondering ...
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### What (precisely) is a block cipher?

If I follow the wikipedia or crypto.stackexchange definition, any simple XOR encryption where the key is as long as the plain text should qualify as a secure block cipher. Now I thought what would ...
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### Help me describe/identify this challenge-response protocol/algorithm?

My area of expertise is reverse engineering, specifically embedded systems. I do attack cryptographic systems, but this largely involves key recovery or exploiting the implementation. I was asked to ...
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### Is compressing data prior to encryption necessary to reduce plaintext redundancy?

As explained in William Stallings' Book, in PGP encryption is done after compression, since it reduces redundancy. I couldn't relate encryption strength with redundancy. Could anyone explain more on ...
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### Is a private RSA key vulnerable to a chosen plaintext encrypted with that private key?

I grasp that having known plaintext encrypted with the public key doesn't provide a means of discovering the private key (that being the whole point...). But if one were in a situation where they ...
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### Is there a cryptographic hash function that can be performed with pencil and paper?

Imagine I'm signing up for the 99th new web site this month. I somehow take my secret key (which I have written down on a card in my wallet) and the domain name of the site and feed them both into ...
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### What is wrong with AES-CTR-HMAC-SHA256 - or why is it not in TLS?

It seems the only specified CTR mode ciphers in TLS are all GCM based. GCM ciphers run AES-CTR and do authenticated encryption with a MAC based on Galois-field ...
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### Can you explain partial private key extract algorithm in certificateless Public key cryptography?

Related to the algorithms of certificate-less Signature approach of Al-Riyami and Paterson… Why is there a need for a "partial private key extract algorithm"? Why should it be needed when ...
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### Ideal system for an encryption scheme

What is the ideal system for an encryption scheme? For a pseudorandom permutation the ideal one is a random permutation, for a pseudorandom function the ideal one is a random function. For an ...
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### Explicit Key SHA-1

I'm reading “Collision-Resistant Hashing? Towards Making UOWHFs Practical” and in the Section 2 it says: Hash functions like MD5 or SHA-1 have no explicit key. But no notion of ...
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### How to store keys for a cascading encryption?

What is the best way to implement a cascade encryption? I'm trying to figure out how to cipher a string (or message) using Serpent-Twofish-AES and then store the keys. I'll provide an explanation, ...
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### MD5 theoretical question

Ok, so this is a very possible stupid question, but one of those things that has sat at the back of my mind for a while. We all know that MD5 has issues, and these have been known for about a decade. ...
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### How does secret sharing solve the partial exposure problem?

I have been trying to understand how secret sharing methods like Shamir's secret sharing solve the problem of a share revealing information about the secret. I guess there are some random numbers ...
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### Why is appending the key to a mesage and then hashing that insecure if the hash isn't weakly collision resistant? [duplicate]

Suppose I have H(M|K) and that H is not weakly collision resistant. If I have a message mac pair (M,MAC), how is it possible to find another message mac pair (M2,MAC2)? My thinking for this problem is ...
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### Is this password migration strategy secure?

I want to upgrade the security of some existing databases of users' authentication tokens strictly for the purpose of making sure that if the database is stolen, attackers will not be able to guess ...
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### What is the difference between various modes in FPE?

Format preserving encryption has three modes FE1( FFX) , FE2(VAES) , FE3(BPS) . All of them are based on Feistel Networks .Can somebody explain how they differ from each other ?
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### Can anyone give an example where (asymmetric) crypto can go wrong due to selection of wrong groups?

Basically the title says it all. It would be great if someone could tell give an example using provable security. More information about groups can be found at: ...
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### What security differences are there between E(p',“well-known-plaintext”) vs. hash(p') where p'=KDF(p)

I am using a KDF to derive a key, p' from a password p. I would like to know if the password is correct before using the derived ...
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### What is the name of this kind of protocol

There is a communication protocol that I believe creates the equivalent of a one time pad, with the downside that the secret message must be transferred multiple times. The protocol is so simple that ...
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### Can two people with different one time pads securely exchange a message like this?

Alice has a message, generates a one time pad, encrypts her message and sends it to Bob. Bob generates his own one time pad, encrypts the message again, and sends it back to Alice. Alice then decrypts ...
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### Are there any differences in operation between existing Diffie-Hellman specifications?

Are there any differences in operation between existing Diffie-Hellman specifications, especially those within the RSA PKCS#3, ANSI X9.42 and RFC 2631 standards?
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### Are there public $p$ and $q$ numbers for use in DSA?

There are many RFC documents giving large primes to use in Diffie-Hellman. However, I couldn't find standards on the $p$ and $q$ large primes used in the DSA signature scheme. This is proving to be a ...
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### Is it true for Java that the transformation mode and padding is ignored when using RSA?

Consider GregS's comment to this question (more specific to bouncycastle's provider implementation): http://stackoverflow.com/questions/11717646/bouncycastle-rsa-key-size-and-padding The mode, ...
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### Why don't we use MACs to store passwords?

These days, the best practice for storing passwords is to use a scheme like scrypt or bcrypt. The goal of these schemes is to make dictionary attack inefficient for an attacker but it also slows down ...
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### How can two (or more) parties share and agree upon a common random seed?

I really don't know how to call this simple problem: Two (or more) parties need to establish a common (non-secret) value to be used as a seed for a deterministic RNG. The only requirement is that each ...
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### Can Diffie-Hellman generate values in a specified range?

Is there any way to generate values only between $0$ and $255$ with the Diffie-Hellman method? I need to create keys for communication algorithms with $32$ and $16$ numbers between that range. The ...
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### Dependence on Keyed Hash Function

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” which states: With an ACR hash function $F$ the key $K$ is announced and the adversary wins if she manages to ...
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### No Birthday Attack to TCR

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” , which compared TCR (Target Collision Resistant) and ACR (Any collision Resistant). It says we wish to stress ...
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### Weaker Notion of Target Collision Resistance

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” which states: While it might be easy to find a collision $M,M'$ in $F_K$ by making both $M,M'$ depend on $K$ ...
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### Why does WPA-PSK not use Diffie-Hellman key exchange?

Is there a reason, why in pre-shared key mode, Wi-Fi session key is not secured with something like Diffie-Hellman but instead is derived from PSK key and some information exchanged in clear during ...
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### Is there a relation between security of rounds of DES and Luby Rackoff rounds?

DES uses 16 rounds of feistel networks but not considered secure . Where Luby Rackoff constructions which are inspired by DES its proven by Patarin that 7 rounds is secure . Is there a relation ...
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### Public-key cryptosystems without poly-time quantum attacks

It's well-known that Shor's algorithm can solve integer factorization, discrete logarithm and discrete log over elliptic curves in cubic time. This implies that cryptosystems like RSA, ElGamal, and ...