i just used standard 75 g/m printing paper, but the folding part wasn't easy, and actually left the paper a little creased, it's easier to get about half off the qr at at a time and maybe photographing it with a good enough camera to joint it.
but as you point paper stock should have a huge effect on this technique, it's hard enough with standard paper, so a thicker or stiffer one should make it much harder.

i just generated a huge random qr an started randomly using gimp's clone tool (no face intended)

maybe oversimplifying things, but since you want just a moderate lvl of security, but ease off use just make a small app that generates the key on a trusted server, then encrypts it with symmetrical encryption, send the bitcoin address, secure delete it and then distribute the encrypted key along with a portion off the key so than whenever n out of m of the recipients join their parts there's at least one full copy off the symmetrical key.
and another app that receives such parts and outputs the bitcoin key.

that way you just have to trust the server in which the apps reside and generating the key is only a matter off imputing parameters of n, m, and delivery method for each member's parts and data.

and receiving the key will be a matter of imputing one copy of anyone encrypted data, the minimum amount off decryption fragments and where to deliver the key

and that way as long as the same code is running you don't even need to use the same server for getting the key back (although both will have to be trusted)

edit: after goggling i realized my solution was already posted somewhere else, along with another great one:
http://stackoverflow.com/questions/1719376/how-to-encrypt-something-so-that-can-be-decrypted-using-any-two-of-three-keys
the checksum one is great for your intention in my opinion, along with the best rated one (incredibly similar to mine)

sorry for the edit, but i rushed to post

also remember the web of trust is there for one reason, get some people with recognized signatures to sign your key, i will help a lot in telling it apart from a fake one.

http://www.gnupg.org/gph/en/manual.html#AEN335

and signing your gpg signature where you publish it via your most recognized bitcoin address gives users another way to determine it's really you signing

on a side note, its way to easy to fake a creation date on a key (just mess with the clock), so being "the first one" means nothing.

--edit: and a lot of end users will be confused giving them just a console command, instead you can instruct them to use a frontend or gui implementation of pgp like kgpg (linux), cryptophane (windows) or apg (android).

my suggestion would be to change the back pattern which now i think its the most vulnerable, and besides its the one less likely to be confused with a qr for something like this:



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now candle it from the front, so the dispersion off the front obscuring pattern's light works in your favor, and if you did it correctly any part off the qr that touches the back obscuring pattern should be visible, but any part that is'n touching it should still scatter.

before photographing make sure to press gently on the white surface on the back so all the qr touches the pattern and therefore becomes clear.

in this step i can confirm that a cellphone light in a dark room is enough to make it perfectly clear to the human eye, so a good camera should be able to clearly pick it up.

since i was already using duck tape, i though about using it to obscure qr, so i did this:


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[IMG=http://img5.imageshack.us/img5/3336/38ki.jpg]http://[/img]

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but still wasn't enoug, because upon retriing one off my previos attemps at candling i used this old beast:


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note: this thing actually can lift again a small piece off tinfoil falling towards it when it fires

and actually i got the best picture i have been able to get from this particular flash on my phones camera:


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if you notice the black shadow at right it's because this flash is faster than my cameras frame rate (60fps) and i couldn't light the whole frame, so under this conditions it's imposible to get my cellphone to focus or at least get any detail at all

proposed solution at next post....

actually i have a theory about the reddit flash way:
i have noticed that in most cases laser toner is much less translucent that inkjet ink, so if the wallet was printed with a cheap color inkjet printer, and then the codes added with a laser one (seen it recommended for durability purposes) it's possible that such difference is enough to render useless the obscuring pattern when a bright enough light is used

update:
i managed to reproduce the effect on a wallet printed on inkjet in one go, but sadly my phones camera is unable to photograph any detail at such light conditions, but ill explain in detail:
first thing is slightly pressing on the sides to separate the layers, so the light of the front obscuring pattern gets scattered before reaching the qr, so you will need to make the 3rd fold separate from the first two, which is actually quite harder than separating the first one, so you need to make a fold like this:

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make sure you don't make it like this:

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and it should end up looking like this:

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notice the small creases that show that it seems to be "inflated".

front:

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note:i used duck tape because i was doing several tries and it was easier and cheaper just using duck tape.
in case there is the slightest gap between the edge off the tape and the edge off the fold you can insert a needle and it will make it much easier, so never make any wallet like this:

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but alternatively you could just pierce a small and discrete hole on any crease and do the same

continue in next post...

actually it has some sort of special covering on one side which is the printable one (has to be single face, but it folds nicely) because the ink doesn't stick well on the other side.
and it can tolerate some water when printed with a regular canon printer

have you tried alumajet (http://www.horizonsisg.com/alumajet.html) i have printed in it and it looks great, and it is prety durable. Besides it would help making the wallets look valuable

i would be wary of 3pass printing on a paranoid level, because a laser (like on the previos tests) with the correct frecuency coud candle the qr from 1 ink while the obscuring pattern having a different pigment could be transparent to that particular frecuency

thanks for the service for newbs btw addres in my sig

mirense esto: https://en.bitcoin.it/wiki/Weaknesses para una idea de vulnerabilidades mas exacta, pero como la pag dice para transacciones pequeñas no es problema aceptar una 0 confirmaciones, porque el gasto de hacer esto seria mayor que la ganancia en la mayoria de casos

lo que veo algo dificil es que los de avalon dicen ser opensource, pero ahun no he visto ningun .eagle o algun equivalente que sea un open source hardware tambien, entonces en cuanto al ensamblaje ondemand o diy la cosa se puede complicar

si tienes logs de tus transacciones deberias enviarselos a ellos para que puedan replicar el exploit y corregirlo antes de que a ellos o a otro trader puedan terminar costandoles mas que 4 btc

hola, soy alejandro y vivo en colombia, llevo solo un par de semanas en el bitcoin, pero ya ando ensayando la venta en bitmit y comenzando con el mining