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Mar
24
comment Are there any practical implementation of a homomorphic hashing or signature scheme?
Appears these system frequently depend upon the hardness of discrete log. I'd be interested in seeing a post-quantum homomorphic hash.
Feb
24
comment Universal reencryption for signatures
An interactive signing process is fine since actually blind signatures are preferable. It's not quite as bad to violate the cross origin policy between the signing authority and the verifiers as it would be to violate it between multiple verifiers. An interactive verification process sounds more problematic.
Feb
24
comment Universal reencryption for signatures
Oops. I've clarified the question. now
Feb
15
comment How does the wider cryptographic community view non-abelian group based cryptography?
crypto.stackexchange.com/questions/18680/…
Oct
23
comment Symmetric-like cyphers with several steps
I'm asking for a cypher with an algebraic property of the key material that allows the keys to be split into multiple parts to be used separately. There are several ways to do with with elliptic curves, but one needs something with symmetric-like speed and security, ideally including immunity to quantum computers.
Oct
23
comment Symmetric-like cyphers with several steps
Any cypher I know has the property that identical key material must be used both for encryption and decryption, meaning $k_i$ is a substring of $k_j$ for some $i \ne j$. At a consequence, any system using onion encryption has at most one "cut-out" node : en.wikipedia.org/wiki/Cut-out_%28espionage%29
Oct
13
comment Using a product of a series of curve25519 scalars as a private key
Alright, I suppose that answers everything then. In particular, one should not subtract curve25519 scalars because doing so would wipe out the most significant bit. And multiplication might get tricky too. Although one could protect against this by checking that the implementation does not fuck up and start with the most significant bit.
Oct
12
comment Why are the lower 3 bits of curve25519/ed25519 secret keys cleared during creation?
Ahh, thanks for the explanation. I just asked a slightly expanded version of this question here : crypto.stackexchange.com/questions/29791/…
Jan
17
comment Information leakage from the ecryptfs filesystem
Is the only big difference between eCryptFS and EncFS that EncFS is userspace with all that entails, and requires users reserve another directory elsewhere for the encrypted files?
Dec
24
comment How does one design a traffic analysis resistant protocol?
Agreed, making the server useless was the whole point talking about "mailboxes". I think what I'd be most interested in is any rigorous analysis of what makes a server useless for traffic analysis while still making ordinary usage reasonable.
Dec
23
comment How does one design a traffic analysis resistant protocol?
I'd consider that "traffic analysis" refers to any attempt to discern the traffic pattern, but especially contacts/friends lists. I suppose one should view this on-server traffic analysis as a completely separate problem form internet traffic analysis.
Dec
23
comment How does one design a traffic analysis resistant protocol?
I'd implicitly assumed that the IM server cannot be trusted, maybe I should edit the question, but everything you say still applies of course. I suppose you could achieve this extra traffic merely by using Tor hidden services with all nodes configured as Tor relays.
Dec
21
comment How close is homomorphic encryption to handling regular expressions?
Is homomorphic encryption even the correct term when we aren't talking about the ring structure? It'll work, I suppose.
Dec
3
comment How do we find the last two correspondences in an otherwise known even permutation?
You could achieve sublinear space by modifying this algorithm to implement the set as a Bloom filters for the done array, but you must watch that the false positive rate doesn't grow too much, i.e. cycles cannot be too long. en.wikipedia.org/wiki/Bloom_filter
Dec
3
comment Prevent double-spending with decentralized digital currencies without all transactions being public?
Agreed, digital currency theory should remain inside crypto.SE.
Dec
2
comment How do we find the last two correspondences in an otherwise known even permutation?
Yes, you must choose the queries, obviously. Just fyi, online algorithm is the technical term for an algorithm that acts on data arriving in a fixed sequence without any chance for going backwards.
Dec
2
comment How do we find the last two correspondences in an otherwise known even permutation?
Create an object Pmod with two variables a and b such that Pmod(a) = n-1, Pmod(b) = n-2, and Pmod(j) = P(j) otherwise. For i=0..n-3, if done[i]==0 then : Initialize j=i. Loop setting done[j]=1 and j=Pmod(j)$ until done[j]==1. If j==n-1, set a=i. If j==n-2, set b=i. You compute the answer using the final values of a and b.
Dec
2
comment How do we find the last two correspondences in an otherwise known even permutation?
In that case, my second linear time algorithm requires an $n$ bit array plus a few $O(log(n))$ bit counters and integers.
Dec
2
comment How do we find the last two correspondences in an otherwise known even permutation?
If your going for n bits plus O(1), then : Iterate thorough the entries exhausting each elements cycle as far as possible to gradually build out the cycle(s) that end in $n-2$ and $n-1$. You use the n bit array to prevent reprocessing cycles.
Dec
2
comment How do we find the last two correspondences in an otherwise known even permutation?
Inverse : Initialize $inverse[i] = -1$ for $i=1..n$. Set $inverse[P(i)] = i$ for $i=1..n$. If $inverse[n-2] = -1$ or $inverse[n-1] = -1$, then use parity to decide whether it's a fixed point or maps to $n-1$ or $n-2$, respectively.