There was some doubt about whether this post is purely theoretical, or whether it in fact allows one to de-anonymize the sdc chain in practice. In fact, I originally thought it would be too much effort to install these other coins clients, which I am really familiar with, and then muck about in their code enough to get it working. However, after some prodding, I have created a simple replacement for their “ringsig.cpp” (see https://github.com/ShenNoether/Deanon) in the shadowcoin code, which, when run, after resyncing the chain, you will be able to determine who the signer of any ring sig is (read the debug.log in .shadowcoin directory). For example, here is the output according to the first ring signature sent on their blockchain:

ProcessBlock: ACCEPTED a801e125053dcc556b94
verifying ring sig asdf

index i = 0 / 4

index i = 1 / 4

index i = 2 / 4

index i = 3 / 4
signer is index 3