# All Questions

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### Are there any secure commutative ciphers?

This answer lists two commutative cipher algorithms - Pohlig-Hellman and SRA. However, they don't appear to be too secure. My question is, here there any commutative ciphers out there that are secure ...
329 views

### How do we compute IV+1 in CTR mode?

This may be a very simple question, but I have not found info about this in any place. My question is about CTR mode: When it says $IV+1$… How do we make this operation? For example, if I have the IV ...
595 views

### How does SafeNet MobilePASS generate passwords?

We use MobilePASS at work but the latest version of the android client seems to be buggy so I wanted to have a go at implementing the algorithm myself. You can download the client to play with here: ...
28 views

### Unknown compression algorithm [on hold]

I got data which is compressed but I fail to find the compression algorithm. The data is part of a larger file from which I know the layout, So I managed to find out few things. What I know: I ...
442 views

### Difference between a nonce and IV

I know the generic difference between a nonce and an IV. I am specifically looking for a clarification on these terms as used in the "Evaluation of Some Blockcipher Modes of Operation" by Phil ...
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### How HOTP values are validated according to RFC 4226

In Section 7.2 "Validation of HOTP Values" of the HOTP spec (RFC 4226) it says, in part, The HOTP client (hardware or software token) increments its counter and then calculates the next HOTP ...
1k views

### Does SHA-1024 hash exist?

Does SHA-1024 cryptographic hash function exist similarly to SHA-512? If not, what's the reason for that? Links: SHA-2 at Wikipedia
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### SRP-6 vulnerabilities when N is small

I'm one of the developers of an application which uses SRP-6 as the authentication mechanism. The authentication part of the code is very old and uses N with only 256 bits (all arithmetic is done in ...
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### How much would it cost in U.S. dollars to brute force a 256 bit key in a year?

I am often told that any key can be broken and that it is only a matter of time and resources for any key to be broken. I know that this it technically true. However, I think that there is probably a ...
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### GPG & Mcrypt - difference?

in short I'm learning GUI programming in python and thought it could be interesting to make a small GUI for symmetric encryption. Not being mad/mathematical/experienced (take your pick) enough to try ...
206 views

### Is my id obfuscation algorithm secure?

I'm designing an id obfuscation system. My system includes: Embedded chips, each chip has a unique 32-bit address (id). Server Insecure low-bandwidth unstable one-way channels from the chips to the ...
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### Rainbow tables, where have I got it wrong?

There seems to be some error in my understanding of the concept of rainbow tables, despite going through quite a bit of articles on it. Let me show what I don't seem to agree with. My plain text is ...
110 views

### Where do I truncate the bits in SHA 256 hash algorithm?

I'm implementing SHA-256 in PHP using strings. Where do you truncate in the algorithm? There are a lot of places where there's binary addition happening so the number of bits in $W(i)$ keep ...
552 views

### Is Curve25519-java secure?

I have only about 2 weeks of cryptography experience mostly in the form of questions on bitcoin.se. Is Curve25519-java up to date with current Curve25519 standards? Is Curve25519 itself secure? ...
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### Can I construct a feasible stream cipher out of HMAC and a secure hash algorithm?

I have constructed a stream cipher from a secure hash algorithm and a HMAC. Here is a brief description of the algorithm: Let: (Actually Objective-C styled pseudo code) ...
907 views

### Is there a hash function with 2048bit output?

Is there a publicly available cryptographic hashing algorithm with 2048 bit output? The standard ones are "only" up to 512 bit (SHA-512, WHIRLPOOL). (2048 bits are 256 bytes, so it would be useful ...
123 views

### Simple proof that shows AES is not a uniform permutation on any n-bit string?

Is there a simple proof that shows AES is not a uniform permutation on any $n$-bit string? Since I'm just starting with crypto, I'd like to see a simple yet elegant proof for the said property. ...
193 views

### Attacking a hardware AES implementation if it leaks the intermediate round states

Let's say that we have a hardware AES implementation that, on request, will encrypt or decrypt a 16-byte block of data in ECB mode using a fixed key, but refuses to reveal its fixed key. In other ...
3k views

### What algorithm could/should be used to encrypt files for storage on untrusted servers?

I need an encryption algorithm that works fast on large files and doesn't increase the file size. It should use a key to encrypt/decrypt data. The files will be sent using REST and over HTTPS, but a ...
173 views

### The security of an encrypt-and-MAC

I would like to know what is the security of an encrypt-and-MAC with different keys for each transaction For example: the one-time-pad with $k_1$ and the HMAC with $k_2$ C = M \oplus k_1\\ MAC = ...
586 views

### Where can I get information on how to implement AES?

I want to write AES from scratch without using the built in libraries of java. I know there are hundreds of AES programs (and websites) out there but I can't seem to find a site that gives me the ...
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### Number of keys when using symmetric and asymmetric encryption?

Quoting part of my homework/assignment: How many keys are required for secure communication among 1000 person if: Symmetric key encryption algorithm is used? Asymmetric key encryption ...
420 views

### Is there a symmetric-key cryptography based on key establishment techniques?

Most of the current key exchange techniques are based on public-key cryptography. Are there any key exchange/establishment techniques based on symmetric-key cryptography too? In my setup primary ...
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### Session-specific Symmetric Key Derivation Using SHA256

I was wondering if there were any security implications of deriving session keys from a symmetric key using SHA256 in the following manner: $Ks = SHA256(Nonce||K)$, where $Nonce$ is the session ...
118 views

### ML/NN Cryptanalysis

Is it computationally plausible to use NN/ML to do any of the following tasks? "Real" cryptanalysis. Cyclic neural networks for extended computation, but break the hash all the way through. ...
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### How feasible is word-level frequency analysis over English (or any language)?

Say I have some black box which, given any English word, deterministically outputs a token for that word. Assume our black box is implemented using strong cryptography, i.e. the hardness of reversing ...
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### My SSH server public key is 2048 bits, but my account's private key is 4096. What is my effective security?

My SSH server has a 2048-bit public key. But my private key, for clients to login, is 4096-bit. In this case, do I have the brute force protection of 2048 or 4096 bits? My worry is that someone ...
154 views

### Advantages of 64-bit CPU smartphone

Advantages of having a 64-bit CPU vs 32-Bit CPU smartphone … 3. Complex Encryption. Improved Security: 64-Bit can process incredibly large numbers allowing you to better encrypt data against ...
247 views

### Can I use HMAC-SHA1 in counter mode to make a stream cipher?

Say I have an embedded device which is only capable of doing HMAC-SHA1 transformations (that device is, in fact, a YubiKey). Would it be secure to feed it like a block cipher in counter mode to get a ...
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### How compared encryption algorithm in terms of efficiency

I want to compare two cryptographic algorithms. The first algorithm is RSA, and second algorithm is ElGamal elliptic curve cryptography. Now, I’m looking for a way to compare the speed of the two ...
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### Construct block cipher from a smaller one with mixing function

I read about the AEZ encryption scheme as presented at the CAESAR competition. To me it seems like a construction of an arbitrary length block cipher from a smaller one. The key component is the ...
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### How dead is braid based cryptography

Braid groups has drawn the attention of cryptographers for a few years, as a promising platform for post-quantum cryptographic protocols. The security of the proposed schemes mostly relied on ...
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### How can I fix a corrupted PGP file? [migrated]

I've encrypted a file with PGP but for "security reasons" I thought it would be good to damage the header and add some extra bytes so that even if someone finds it it won't know for sure it's a PGP ...
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### Strengthening security of an encrypted file against forensic detection [on hold]

I have created a Truecrypt container for backup in the cloud, and I want to obfuscate to the greatest extend possible which software and algorithm was used to encrypt the data and to slow down any ...
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### Chosen plaintext attack on textbook RSA decryption

Let $(n,d)$ be an RSA private key and $(n,e)$ the corresponding private key. Generate a secret random $r$ from $(\mathbb{Z}/n\mathbb{Z})^*$ Compute $C’ = C (r^e) \bmod n$ Submit $C’$ as a chosen ...
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### Big crunch, the Data way? [on hold]

before exposing my problem I beg you all to excuse my bad english :).. let's take the bible book as an exemple with its 3,116,480 letters. (a file could also be considered as a book with its ...
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### Hash or compression function in Merkle tree nodes?

Wikipedia says about a Merkle tree: Nodes further up in the tree are the hashes of their respective children. Usually, a cryptographic hash function such as SHA-2 or SHA-3 is used for the hashing. ...
398 views

### How to perform authentication without central server in P2P?

How can one be sure that the man who you're talking with is the one who you think he is? i.e. How can one perform authentication in P2P network without a central trust server or Certificate Authority? ...
107 views

### Luby-Rackoff theorem confusion

The Luby-Rackoff theorem states that if a round function is a secure pseudorandom function (PRF) then 3 rounds are sufficient to make the block cipher a pseudorandom permutation (PRP). PRPs are ...
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### How do the following new (2013) ECC curves compare in security or efficiency? [closed]

I read about the following "safe" ECC curves and notably, secp256 and all the NIST curves are marked as "unsafe" when compared to more modern curves. I need a curve for signing or encryption, (or ...
45 views

### Why can't you decrypt an encrypted message with just the public key? [duplicate]

How does asymmetric crypto work? For example, if you use PGP in emails, you generate a private key that is known only to you and a public key, which is available to everyone. Is there a simple way ...
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### Block-cipher based vs Hash based MAC

When it comes to MAC algorithms one can have many choices. CBC-MAC, CMAC, PMAC etc are some MAC algorithms that rely on block ciphers (ex AES) to generate a MAC. On the other hand HMAC is very popular ...