64
votes
Accepted
Why do 5G, 4G, etc., use non-conventional algorithms?
These decisions are driven by silicon. Most specifications for hardware are built around a minimally viable CMOS implementation (ex: MPEG-1, lightweight cryptography via NIST 8114). This is ...
- 4,524
57
votes
Accepted
Signal vs Telegram in terms of protocols?
Alright, I'll bite.
First, let me propose bounding the discussion to just the core of the protocol.
In particular, let's not get hung up on:
Social engineering attacks
How broadly the end-to-end ...
- 786
55
votes
Accepted
One Encryption, Many Decryption Keys
This is known in the cryptographic literature as "traitor tracing". See, e.g., the following seminal paper:
An efficient public key traitor tracing scheme. Dan Boneh and Matt Franklin. CRYPTO 1999....
- 36k
49
votes
Should we MAC-then-encrypt or encrypt-then-MAC?
Although there are already many answers here, I wanted to strongly advocate AGAINST MAC-then-encrypt. I fully agree with Thomas' first half of the answer, but completely disagree with the second half. ...
- 27.4k
35
votes
Is it easy to crack a hashed phone number?
No, it is not a good idea to hash phone numbers. There are only a limited number of phone numbers, so it is pretty easy for an adversary to try and hash all of them. Then you can simply compare the ...
- 90.2k
24
votes
Why is SSL on top of TCP?
why the SSL is not under the TCP ( Transport layer ) ?
Because SSL can use TCP [1] to transport SSL records, and so SSL relies on TCP as a service.
That is, SSL takes the user data stream, and ...
- 140k
18
votes
Accepted
Is it possible for Alice and Bob to both sign a message "simultaneously"?
This is one of the earliest questions that was asked in modern cryptography. There is a proof that you cannot achieve completely fair contract signing. However, there are some reasonable alternatives. ...
- 27.4k
17
votes
Accepted
Salary Negotiation Problem
Solutions to Yao's Millionaire's Problem should suffice for this computation. In that setup, there are two parties each with an input. The output reveals whose input is larger, and nothing else.
So ...
- 38.3k
17
votes
Accepted
DRM simple clone
I heard of DRM but could not get a reliable implementation of DRM
There is a good reason for this: DRM is a hard problem, and a solution to it could be leveraged to obtain incredible amounts of money....
- 19.4k
14
votes
Accepted
Why do we implement a protocol?
I assume the question is related to academic work: why do we implement a protocol if we already know how efficient it is by a complexity analysis? The answer depends very much on the type of protocol. ...
- 27.4k
13
votes
How does a rolling code work?
It is worth pointing out that Samy Kamkar realized and implemented (in 2015) what is now forehead-slappingly obvious in retrospect - it's perfectly practical to have a radio TX+RX unit that snoops a ...
- 231
13
votes
Accepted
How to use proof of lack of knowledge?
In general, you cannot prove lack of knowledge, because even if you did know something you shouldn't, you can always pretend that you don't know it and carry out the proof as if you didn't know it.
...
- 45.7k
13
votes
Is it easy to crack a hashed phone number?
It is always a bad idea to hash data that has a limited set of length or characters.
A phone number in Germany for example has normally no more than 12 digits.
The first digit is always a ...
- 378
12
votes
Accepted
Why does TLS do Authenticate-then-Encrypt instead of Encrypt-then-Authenticate?
SSL was designed long ago when encrypt-then-MAC wasn't that popular yet. Even TLS 1.2, published in 2008, is pretty old by now, and while encrypt-then-MAC was preferred by then, the practical risks ...
- 24.6k
12
votes
Accepted
Is there a formal language to define a cryptographic protocol?
Disclaimer: I use Coq on daily basis...
About the tools
As you are looking for a formal verification, I would advise you to take a look at Coq. Even though mainly used by Academics, it provides a ...
- 9,909
12
votes
Why is SSL on top of TCP?
The Secure Socket Layer is as the name suggests build on sockets, which provide error free, ordered stream of data. TLS is the differently named successor to SSL; TLS 1.0 succeeds SSL 3. TLS likewise ...
- 90.2k
12
votes
Is it easy to crack a hashed phone number?
In the general sense, The problem is known as the small input space on the hash functions, and in short simple hashing won't be secure.
If you hash data ( here a phone number) and an attacker tries to ...
- 46k
11
votes
Equal sign with a "c"
To quote the paper
Two probability ensembles... are said to be computationally indistinguishable, denoted $X\stackrel{c}{\equiv}Y$, if...
This is found in section 2.
- 38.3k
11
votes
Accepted
Obfuscating functions that are mostly zero
I provide a summary below of what is currently known (to my knowledge) regarding obfuscation of various class of "mostly-zero" functions.
From Indistinguishability Obfuscation
What we can obfuscate: ...
- 18.6k
11
votes
Accepted
Examples of protocols that are insecure when run concurrently
Consider the function $f : \{L,R\} \times \{ U,D \} \to \{0,1,2\}$ defined by the following table:
$$
\begin{array}{c|cc}
f & L & R \\
\hline
U & 0 & 0 \\
D ...
- 11.2k
10
votes
Why do we implement a protocol?
Do we implement it for proof of concept?
Absolutely. It's very easy to miss vital points if no implementation exists. W3C for instance doesn't even allow protocols to be standardized without ...
- 90.2k
10
votes
Why is SSL on top of TCP?
While the other answers are good, I think they're missing the core point: TCP is what's there. You can't just define arbitrary new layer-4 protocols on top of IP and expect existing operating systems ...
10
votes
Accepted
Why does the SSH protocol generate two keys: an encryption key for client to server communication and server to client communication?
There's a line of thought in crypto when it comes to keys: one usage, one key.
That gives a first level of rationale towards using separates keys in each direction: each direction is a different usage ...
- 134k
9
votes
Accepted
Is it usual for a signed message to contain the public key of the signer?
I'd say that most of the time the signature is accompanied by the certificate of the signer. This certificate contains the public key. Most container formats such as CMS (used in S/MIME, also known as ...
- 90.2k
9
votes
Signal vs Telegram in terms of protocols?
Besides Joshua Warner's excellent answer, I do also want to point out that someone has to "roll their own crypto" at some point for there to be any designs and implementations at all.
On that front, ...
- 10.9k
9
votes
Exchanging encrypted messages, and decrypting without a public key?
You're describing a form of three-pass protocol, which is a communication mechanism where neither party needs to know each other's secret key. Wikipedia describes a helpful metaphor using a box that ...
- 190
9
votes
One Encryption, Many Decryption Keys
I can't really prevent "bad" recipients from the leaking the key (or the plaintext, for that matter), but I would like to be able to distribute unique keys to each recipient so that if a key is leaked,...
- 14.3k
9
votes
Accepted
Can I use a HMAC for Replay Attack protection?
An old but excellent paper on this topic is Tuomas Aura's Strategies against Replay Attacks.
The simplest version of the "Hashed Full Information" method would be to include the MAC of the ...
- 345
9
votes
Is there any protocol for proving that a message was written at a certain time?
In general, no.
You can prove a message was written no later than time T by hashing the message with a collision-resistant hash, and communicating the hash to some trusted third party that records it ...
- 191
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