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Some brief thoughts: Shared secret Generation: $$s=E_a(B)=E_b(A)$$ The shared secret is generated by encrypting the other users public key with your private key. This is effectively an ECDH step, which is very reasonable, and one of the key aims of C25519$^{[1]}$. Key Generation: $$s_0=\mathrm{SHA256}(s); s_i=\mathrm{SHA256}(s_{i-1})$$ First, using the ...


4

The current specification says that tracker GET requests specify the following variables: uploaded=... (bytes) downloaded=... (bytes) left=... (bytes) This is great for public trackers but is poorly designed for private trackers. The problem is that the numbers don't always add up as they should and this can be for several reasons. For example, you might ...


2

Below is one possibility, but for a large set of values not a really efficient one as the work and the size is linear in the number of values. Say we are working in a cyclic multiplicative group $G$ of prime order $p$ with generators $g$ and $h$ (such that the discrete logarithm between $g$ and $h$ is unknown to the users) and I assume that the values on ...


2

It's actually somewhat simpler than I'd thought, since I don't think it would help much to usually be able to avoid contacting the tracker. Each time a (registered) client joins the Torrent, they establish a secure channel for their communication with the tracker in that session. The tracker informs the downloader and uploader of the other's IP address and ...


2

Unless you are absolutely sure that you don't need to and that the cost is going to be significant then I would absolutely say you should use authenticated encryption. One reason is bit-flipping attacks - flipping a few bits at the 'right' point in your encrypted message might lead well to a message that is legal (the classic example is if someone learns ...


1

Your approach makes getting information other than count of cards in possession of each player at least as hard as breaking the PRFness of HMAC. To make it information-theoretically impossible "for all the ... each player", $\;\;$ if different card_values have different lengths then use $\;\;$ SHA256(commonly_agreed_public_salt:card_value) $\;\;$ ...


1

Moxie Marlinspike calls it in his article http://www.thoughtcrime.org/blog/the-cryptographic-doom-principle/ the doom principle: if you have to perform any cryptographic operation before verifying the MAC on a message you’ve received, it will somehow inevitably lead to doom. He also demonstrates two attacks which are possible because of trying to ...



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