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Jun
23
comment What algorithm would give the shortest ciphertext for very short plaintexts?
Do not do this. Especially if you have a very small plaintext space, ECB performs poorly. Imagine using AES in ECB mode on messages with length 1 bit. Every message would be an encryption of $0$ or $1$. Finding out what is $E(0)$ and $E(1)$ is not that much guesswork any more. Especially with short messages, it is very important to have some form of randomization to hide the fact that the same message has been encrypted.
Jun
23
answered Choose a random number that is different from a bunch of other secret numbers
Jun
23
comment Functions that are only second-preimage resistant?
The definition of collision resistance means "find two values, which result in the same hash". If such a value is already baked in the definition of the function, collision resistance is broken. Additionally, the (small) difference between first and second preimage resistance becomes clear when you look for the $a\neq b$ statement, although in real functions are considered pretty much the same. But for example the identity function is collision resistant and second preimage resistant but not first preimage resistant.
Jun
23
comment Functions that are only second-preimage resistant?
Dingo, you might want to state what your actual goal is. Avoiding to use a "normal" cryptographic hash function? Or do you actually want to have something that is not collision resistant? As practical example of such a function, you can use MD5, which is broken for collision resistance but preimage resistance is still considered okay-ish.
Jun
23
comment In public key cryptography, how many keys are required for secure communication between n parties?
You're welcome. But tbh, most of this discussion still should be deleted. This isn't a discussion forum. Next time you have a question try asking the real question instead of providing the answer and asking "Am I right?". If you focus on what is actually unclear to you, you will get more helpful answers. And I suggest keeping an open mind instead of insisting on your own thought process.
Jun
23
comment In public key cryptography, how many keys are required for secure communication between n parties?
I'm voting for closing this thread now, because it started to become a discussion. Besides being pretty much "below homework level", this question isn't beneficial for anyone else, because the basic understanding of the question is just wrong.
Jun
23
comment In public key cryptography, how many keys are required for secure communication between n parties?
I have absolutely no clue what that diagram shows, but your thought process is totally wrong, yes. The assumption in your question is "there are public keys". and that means "they are secure". Questioning the assumption of the question is plain wrong.
Jun
23
revised In public key cryptography, how many keys are required for secure communication between n parties?
removed "cryptanalysis" tag
Jun
23
comment In public key cryptography, how many keys are required for secure communication between n parties?
As mikeazo said, the answer is $2n$: each user has one private key and one public key. That's it. But because public keys are public, everyone has all $n$ public keys (including his own). Since people don't need their own public key, it's enough to know the own private key and all $n-1$ public keys of the other users.
Jun
23
comment Is email an example of public-key cryptography?
The lockbox analogy is usually used for symmetric encryption or commitments, and it is a little off for public key encryption (everynoe can create the lock, but only the owner has the key to unlock it). However, I think email is an even worse example, since it provides a false sense of confidentiality or is based on wrong assumptions about email. There is no confidentiality for transmission of emails. There is no proof that an email actually came from a certain email address or that the email wasn't tempered with. Email doesn't fulfill any security definition on its own.
Jun
23
reviewed Reviewed In public key cryptography, how many keys are required for secure communication between n parties?
Jun
23
comment In public key cryptography, how many keys are required for secure communication between n parties?
Why would every party have one public key and $n$ private keys? That would mean there are $n$ public keys and $n^2-n$ private keys in the total system. Usually there is one public key for every private key, so the answer you proposed in the question is wrong. It is correct, that each user will have a total of $n$ keys, but the distribution of public and private keys will be different, and there are duplicates over all users (the questions states how many keys are required, not how many keys are in the system).
Jun
23
answered How to prove NIZK proof of knowledge?
Jun
13
reviewed Close Algorithm accepting every passphrase to fool unlegit user
Jun
13
comment Algorithm accepting every passphrase to fool unlegit user
There is no question here, and reference requests are offtopic. Beside it is pretty close to Ricky's question; although I wouldn't tag it as a duplicate
Jun
13
comment RSA decrypting of a huge file by parts
There is just not enough to go for in this question. Is some knd of hybrid encryption used? What's the keysize? What length is this "line" you're speaking of? Are you talking about usual ASCII chars there? Any other unit than bits and bytes is just not specific enough. And nto your last question: No, unless this "line" has exactly as many bits as your modulus in RSA, it will not be the same length (assuming you use RSA for that operation).
Jun
12
reviewed No Action Needed Elliptic Curves of different forms
Jun
12
comment Python KDF - Master Key and Derivatives
Welcome to crypto-exchange. It seems to me, that you don't need a KDF function to achieve this but a "2-out-of-3" secret sharing scheme. Btw, the combination $DK_1+DK_2+DK_3$ can be ommited, since it's already covered by all 3 previous ones (someone who knows 3 keys can also unlock the file by only using two of them already)
Jun
12
comment BigInteger in Java and Randomness
BigInteger does not contain PRNG code at all. It uses RNGs only. Btw, the statement to use SecureRandom is even in the JavaDoc of the Random class. Last sentence of the first paragraph
Jun
12
revised Given $n$ bits, how many “truly random” sequences/numbers can be constructed?
added 887 characters in body