1. Is it true to say both SDC and XMR have built their own cryptonote systems from scratch?
As far as I can tell that is true. The SDC implementation was apparently built from scratch (I haven't personally reviewed it) and from the whitepaper is effectively cryptonote with some relatively minor tweaks.
The XMR cryptonote implementation was apparently built (presumably also from scratch) by cryptonote itself, a shadowy (no pun intended) group that has no direct participation in the cryptocurrency community as far as anyone knows. They also created something called cryptonotecoin, but only as a proof-of-concept that is designed to have no value (the blockchain resets itself after a month or something).
2. Is it true to say Shadow>Shadow transactions offer the same security as Monero? Perhaps better?
Essentially the same, if widely used, but see below. The reduced number of denominations provides a small improvement in anonymity, but also an increase in transaction size, so it isn't really clear this ends up being a net gain in anonymity (compared to say an increase in mix factor with comparable increase in size).
3. Is it true to say providing a working GUI wallet for cryptonote transactions that SDC made great strides forward for user-adoption of a working cryptonote implementation?
I haven't seen the SDC wallet but from the screen shots in the OP it certainly looks nice. I'd give it high marks based on that alone.
1. smooth said: "If you mint and redeem frequently, you will have issues that reduce your anonymity". Is this a problem? Is so solutions? Discuss
At that point you are effectively using a traceable coin with a short trip through an untraceable coin, which makes the untraceable coin something of mixer, and that in turn only helps if there is a high level of activity, but still reduces anonymity given time-based attacks.
Consider that you take 53,17 SDC, convert it to shadow, send it to someone (for example to pay for a private purchase), and soon thereafter he converts it back to 53.17 SDC. During which time there hasn't been a whole lot of activity (certainly no other transactions in that same amount or close to it). Is this really fooling anyone?
(I have pointed out similar issues with the suggestion that Bitcoin add anonymity using a cryptonote-based sidechain.)
2. smooth mentioned advantages ("This is indeed something new, and there are some advantages to it…"). However these were not mentioned. Any suggestions?
The advantages would be easier integration with services (exchanges, etc.) that expect everything to operate just like a Bitcoin clone, not and deal with stealth addresses, mix factors, etc.
Also lower cost for the non-anon transactions, since they are smaller and less resource intensive, but the trade off is moving activity to the (lower cost) non-anonymous coin reduces activity in the anonymous coin, which in turn reduces effective anonymity.
3. My understanding is that XMR does not use a blockchain db. Working out your db structure I bleieve is why you're still developing the gui wallet. In any case what are the implications (pros/cons) re the differing db structures
XMR stores the bloockchain in memory, which means you currently need at least 4 GB to run a node with reasonable performance, and this is growing, albeit slowly. DB development is progressing, also slowly. By contrast, SDC I understand uses the Bitcoin DB code so probably leveldb for storage? That isn't blazingly fast, but generally fast enough (especially for a coin with much less activity than BTC), and eliminates the issue of high memory requirement.
4. Do we still need SDC>SDC? Useful for non-anon transactions? Does it compromise our anon to keep SDC>SDC an option?
That is a tradeoff you will have to make. See above.
Hope this helps.
