I just received these half-bitcoin and one-bitcoin physical pieces from NolaCoin in Louisiana, and they are beautiful.
They’re brass at heart, plated into different colors with a striking mirror finish. The half-bitcoin piece is a delicate rose gold/coppery color, and the one-bitcoin piece is nickel-plated to a lustrous silver. But beyond their beauty, they have several innovations over other physical bitcoin designs.
While most other designs hide the private key on an insert which is held behind a hologram sticker on the back of the coin, these have both the public address and private key etched into the metal of the coin with a 10 watt laser (according to the creator’s post here). The public address is readily visible on the front, and the private key is hidden on the back.
This is great not only because it makes the public address readily available for verifying the balance, but also because of the relative indelibility of the etching; if a more traditional key-printed-on-insert physical bitcoin gets caught in a fire or other calamity, it’s only as tough as whatever material the key is printed on, but with these the key is as tough as the metal of the coin.
It’s even greater that the hidden private key is not just presented as an alphanumeric string, but also as a QR code which is likewise etched into the metal and entirely scannable. Typing in letters and numbers is for suckers!
Because someone had to do it, I opened one up to see how it worked. There are essentially three parts to the security over the key: a plastic film, a scratch-off hologram, and a gold ring sticker around the edge.
The gold ring overlaps the scratch-off hologram, making it more difficult to tamper with the hologram in the center. Once you’ve scratched off the hologram, you can make out the characters of the key, but there is an additional layer of plastic film still over the metal. Upon removing that, there was a fair amount of sticky residue which kept the QR code from scanning in, but a little rubbing with soap and warm water got the surface cleared away down to the mirror finish, and at that point I scanned the key into Mycelium easily.
This is a great design, and I look forward to seeing what he comes up with next.
Update: Arash Dini, the coins’ creator, explained to me that contrast on the QR-code etching on the nickel-plated coins is sharper and more likely to scan first-time, and though I haven’t opened one of those up yet, that certainly makes sense, as you can see the difference in the lettering on the front. He also said that if you leave the flash on your smartphone on, the reflection will help the scanning. In other words, it should be possible to scan the QR code just by scratching off the hologram, but without the (very minor) hassle of removing the final layer of plastic film and cleaning residue off the coin.
Standard disclaimer: It’s always, to some degree, a bad idea to let anyone else have access to a private key which controls any of your bitcoin wallets; in a sense, it goes counter to the bitcoin system itself. Once someone else has handled the key which controls your funds, you have to trust both that they exercised adequate security procedures while handling your key, and that they have not save copies of your key to exploit at some point in the unforeseen future. While some (such as Mike Caldwell of Casascius) have established trusted reputations, newer operators can only prove their trustworthiness over time.