# Tagged Questions

Questions about the meaning and proper use of specific technical terms or notation within cryptography.

127 views

### What do they mean when saying that a certain value should be “super-logarithmic”?

What do they mean when saying that a certain value should be “super-logarithmic”? I've found the Wikipedia definition of a “super-logarithm”, but I'm having trouble understanding how a given value ...
480 views

### Difference between a circuit and a normal function

I've seen the word circuit used in many crypto contexts (for example, regarding FHE in this pdf). I've always thought of a circuit as another word for a program or function. But is there a difference -...
34 views

### What is many:1 and collision in MD5?

This is in reference to this answer on Stackoverflow. If you read the first paragraph of the answer, you'll find this line: It's always a many:1 mapping (meaning there will always be collisions) ...
484 views

### Are hash functions chaotic?

I'm looking into "bitcoin like" mining processes. Say a security process uses a hash of data as proof that the data was known at some point in time. From what I understand, hash functions are ...
49 views

### What does this advantage statement signify?

In many Attribute Based Encryption papers I see this line... The advantage of an adversary A in this game is defined as Pr[b' = b] − 1/2. Can someone explain what it means to me? I assume it means ...
780 views

### What is adversary's advantage in cryptography and why we use it?

The definition of adversary's advantage seems a bit odd for me and I am wondering why do we use it to measure the power of an adversary rather than just use the probability of a PPT adversary ...
106 views

### Can a hash have more than one input?

From a definition stand point, can a hashing algorithm have more than one input, ex: $H(m_1, m_2, m_3)$? If not why are algorithms like bcrypt, scrypt and pbkdf2 referred to as password hashing ...
68 views

### How to meter and limit number of files encrypted?

First my apologies if this isn't the right forum for this but here's my problem : I have a black box that takes an input file and produces an output file. I want to sell this black box to a customer ...
79 views

### What is the non-programmable random oracle model?

I would like to know the difference between the random oracle model and the non-programmable random oracle model. ​ What is the difference?
45 views

### How would you name and/or describe this algorithm?

First up: this is not a decoding request or anything. This is about terminology. Twice per year, we email another department to ask for a new licence file for their software. They are always slow to ...
56 views

### Term for the ratio of ciphertext to plaintext size?

Is there a term that determines the ratio of size of ciphertext $|c|$ and plaintext $|n|$? I vaguely recollect reading it as scale out factor. Is there a better term? Or a popular term for this?
44 views

### Is there a difference between $\mathbb{Z}_2^b$, $\text{GF}(2^b)$ and $\text{GF}(2)^b$?

I have read a lot about Galois Fields (GF). They are also presented in The Design of Rinjdael on page 13. In computer memory, the polynomials in $F[x]|_l$ with $F$ a finite field can be stored ...
4k views

### What is a “freestart collision”?

In their work on SHA-1 collisions (cf. the EUROCRYPT-2016 paper “Freestart collision on full SHA-1” by Stevens, Karpman, and Peyrin) Stevens et al show that they are able to generate "freestart ...
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### How distinct are the meanings of the terms “CSPRNG,” “DRBG” and “stream cipher”?

Are there universal, consensus definitions for the following terms? Cryptographically secure pseudo-random number generator ("CSPRNG") Deterministic random bit generator ("DRBG") Stream cipher ...
3k views

### Why should I make my cipher public?

As I understand it, the less people know about the internals of my protocol or cipher, the more secure the protocol is. However Kerckhoffs's principle states that A cryptosystem should be secure ...
3k views

### Terminology: differences between the terms “pre-master secret”, “master secret”, “private key”, and “shared secret”?

Both crypto.SE and security.SE have excellent Q&As about how TLS generates session keys (I have linked some at the bottom). In reading these threads I'm having troubles with terminology since the ...
132 views

### What is a branch number in the PAEQ algorithm?

I'm reading an article about authenticated encryption algorithm - PAEQ. It uses a permutation function AESQ, which has transormations from AES. And there is written about MegaSBoxes, which contain 4 ...
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### Is the 'nonce' in bitcoin really a nonce?

Is the 'nonce' in bitcoin really a nonce? Reading definitions of nonce and salt it seems the number is used as a salt but the same value can occur in multiple blocks; there's no requirement or indeed ...
131 views

### For what does “Rcon” stand for in Rijndael/AES?

How do we expand the abbreviation “Rcon” used in the Rijndael key schedule? I know what it is, I know how it works; I just do not know for what it is a shortcut for. I mean: if "Sbox" is "...
1k views

### Easy explanation of “IND-” security notions?

There are many schemes that can advertise themselves with certain security notions, usually IND-CPA or IND-CCA2, for example plain ElGamal has IND-CPA security but doesn't provide IND-CCA security. ...
242 views

### What is the difference between PBE and symmetric key encryption?

What is the difference between PBE (password based encryption) and symmetric-key algorithms? My understanding is that symmetric-key algorithms like AES and Twofish are used in PBE. Hence PBE is just ...
14k views

### What is the main difference between a key, an IV and a nonce?

What are the main differences between a nonce, a key and an IV? Without any doubt the key should be kept secret. But what about the nonce and the IV? What's the main difference between them and their ...
494 views

### What is cryptographic agility?

I keep seeing the term "Cryptographic Agility" referenced. What does it mean? For instance: https://msdn.microsoft.com/en-us/library/windows/desktop/bb204775(v=vs.85).aspx One of the key value ...
54 views

### Is rubberhose cryptanalysis a sidechannel - formally?

I recently was engaged in a discussion in Sec.SE where somebody used the term "sidechannel" to actually mean "circumvent the encryption". By what he said it was clear, that he wanted to not bother ...
8k views

### Simply put, what does “perfect secrecy” mean?

I would like to ask for a clear (but maybe not so deep) explanation of what the term "perfect secrecy" means. As far as I have researched and understood, it has to do with probabilities of assuming ...
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### What do associated data and message numbers mean?

I'm reading an article about authenticated encryption and don't understand some inputs to an encryption function. They are associated data, public and secret message numbers. Can you explain me their ...
3k views

### Are checksums essentially non-secure versions of cryptographic hashes?

Are checksums basically toned-down versions of cryptographic hashes? As in: they are supposed to detect errors that occur naturally/randomly as opposed to being designed to prevent a knowledgeable ...
2k views

### Simulation-based security?

I've been reading Introduction to Modern Cryptography by Katz and Lindell as an introduction to cryptography. The book seems to use the term 'simulator' when it talks about a game like, for instance, ...
113 views

### Classification of a stream cipher

How would one classify a stream cipher that is neither synchronous, nor self synchronizing? I would have thought asynchronous, but from all the sources I can find, asynchronous is synonymous with ...
90 views

### What is the correct name for bytes which are to be signed?

I have some bytes, and I sign them with my private key using ECDSA. The resultant (71) bytes are the signature. But what is the name of the bytes I have signed? (If this were encryption, they would ...
110 views

### A general definition for cryptocurrencies

I am trying to find the definitions to several terms related to digital currencies. Unfortunately, I am always ending up on Bitcoin-oriented pages and websites which only take Bitcoin itself into ...
229 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 safe"....
129 views

### Name and security of these non-invertible functions?

I would like to learn more about a specific set of non invertible functions: The function should accept 2 inputs: one hidden and the other visible. The output and the function should also be visible....
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3k views

### What are SNARKs?

What does it mean and what is it used for, I have been hearing this term a lot lately. From the context I've heard it talked about it seems to be connected with zero knowledge?
2k views

### What is it meant by a “hybrid argument”?

Can anyone explain (or point to a reference for) what a hybrid argument is in a security proof, and when it's convenient or preferable to use it? Among some of the places where I've seen it mentioned,...
335 views

### Difference between actual attacks and theoretical attacks on SHA cryptographic series

Could anyone tell me what the difference is between a theoretical attack (Like the one done on SHA-1) and an actual, practical attack (Like the one done on SHA-0)? Is a theoretical attack a proposed ...
112 views

### Hash and salt or salt and hash?

People say "hash and salt", but aren't you supposed to salt password the first, and then hash it? So that means its called "salt and hash" right?
88 views

### What is the difference between “securely realizes” and “securely implements”?

In some security proofs it is stated that "a protocol securely realizes an ideal functionality" while in some others "a protocol securely implements an ideal functionality". Is there a meaningful ...
129 views

### Explanation of part of a visual cryptography algorithm

I have been working on a project involving visual cryptography and I am stuck with the following problem. My question is related to this paper, AN IMPROVED VISUAL CRYPTOGRAPHY SCHEME FOR SECRET ...
125 views

### Why are there so many stream ciphers out there and even ongoing research?

What make them better than OTP using a CSPRNG initialized with a secret key?
445 views

### What is 'key agility' in relation to symmetric-key encryption?

I sometimes see, in discussions of symmetric ciphers, reference to the 'key agility' of a particular algorithm. It seems to be related to the difficulty of switching encryption keys, but I don't ...
180 views

### MD5 Collision Attack Explained

In one of my assignments, I came across this line Due to MD5’s length-extension behavior, we can append any suffix to both messages and know that the longer messages will also collide. This lets ...
342 views

### SHA-256 “almost unique”?

I have seen numerous references on the internet of people describing SHA-256 as generating an "almost unique" hash. Exhibit A. there are more. Is there some mathematical basis to the almost ...