John Deters

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	1,035 reputation	bio	website		visits	member for	9 months
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May 2	comment	Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash? I'm still guessing at your intent, but it seems like you want to create parallel systems that can yield different digests given the same inputs. Some implementations use unique salt as the differentiator, by taking the hash of message M+"system1", M+"system2", etc. Otherwise you could use different hash algorithms, but that would be complex, confining, and inflexible.
May 1	revised	Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash? correction
May 1	answered	Book Didactic Security Notions
May 1	revised	Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash? clarified the purpose of salt
May 1	answered	Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash?
May 1	comment	chaining rsa with ecies Now I understand your point. Sorry if the description was confusing, I was trying to state exactly that: to store the TDEA key alongside with the private key. Yes, if you encrypted the TDEA key and kept it with the data, that would be utterly useless.
Apr 30	comment	How to compare two datasets „anonymously”? @RickyDemer, post that as the answer, because it's the answer.
Apr 30	comment	chaining rsa with ecies It's not bogus, but it's another layer of protecting the TDEA key. Sure, he could superencrypt the TDEA message with AES, too. There's no particular rule saying keys must be encrypted only with asymmetric algorithms, and data must only be encrypted with symmetric algorithms. That's the common use of public key cryptography, but this application may not even have that need.
Apr 30	answered	chaining rsa with ecies
Apr 29	comment	toy hash algorithm At this point the idea probably isn't to get students to cryptographically break a hash function, it's more likely intended to teach them "why it's important that it's hard to discover a collision" and "why it's important that you can't recover the original message from the digest".
Apr 29	comment	Custom crypto library in C There was a weakness in OpenSSL that brought timing attacks to the attention of the IT industry at large a few years ago, and yes, they've been fixed. I don't specifically recall, but the code submitted for AES consideration was likely a reference implementation. I'm still uncomfortable with your "non-standard" reference above. Algorithms and protocols don't become standards until they've been studied and attacked mercilessly. Threefish, no matter how cool Bruce is, is not a standard, and has not been studied as well. Bruce would be the first to admit it.
Apr 29	comment	Custom crypto library in C "Need"? That sounds like a severe case of Not Invented Here syndrome, and is the bane of secure systems. What you "need" is a valid business case. What is your business reason for writing your own? Money? Consider the potential for loss and lawsuits if someone breaks your home-written system. Pride? Pride does not deliver value to your customers. Fame? You'll certainly get notoriety if your system is broken, but that's not the same as fame. Learning? That's a valid reason for you, but don't inflict it on your customers.
Apr 29	answered	toy hash algorithm
Apr 29	answered	risk of attacker decrypting RSA ciphertext without public or private key
Apr 29	answered	Custom crypto library in C
Apr 28	answered	RSA - Ecrypting the same data with the same public key = same ciphertext?
Apr 27	comment	Requiring a “supervisor” key pair and a “user” key pair to decrypt multiple-recipient messages So this is still the solution you are looking for. No one person has enough shares to decrypt it. SSS is used to reveal only the one secret key, which is for a symmetric algorithm like AES, used to encrypt the sensitive data. AES has no known clear text attack, so it's safe. SSS leaks no info about the other keys, so it's also safe.
Apr 26	comment	Attacking historical ciphers methodology I'd do them in a slightly different order, just based on ease of the tasks: Caesar first, then Vigenere, then looking at bigrams for Playfair. I hope he has a lot of ciphertext if it's Playfair.
Apr 26	comment	Tamper-proofing log files Sorry, I was referring only to the direction the comments have taken, not to the solutions.
Apr 26	comment	Algorithm digipass-like Alice just needs to sign the time stamp before sending the challenge X, and validate both the time stamp's signature and the 30 second rule on Bob's reply before decrypting his Y. It's pretty straight forward. Or are you thinking there's a homomorphic algorithm that is externally verifiable?