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Date: Wed, 24 Apr 2013 16:01:51 -0700
Message-ID: <CA+CODZFNo6KRW-wKvVbnesQMEm9k5MEoTPEPXepCKKnzt+1E6g@mail.gmail.com>
From: Jeremy Spilman <jeremy.spilman@gmail.com>
To: bitcoin-development@lists.sourceforge.net
Content-Type: multipart/alternative; boundary=047d7b677b3e94e32d04db234944
Subject: [Bitcoin-development] Cold Signing Payment Requests
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Payment Protocol uses x509 certs to sign a Payment Request. This
allows wallets to display meta-data from the cert to the payer instead
of the address, which should make it easier to verify where money is
being sent, and make it harder for an attacker to change the address
displayed to a user so that coins are not sent to the wrong place.

The difficulty is that Payment Requests must be generated live, and
therefore the key used to sign those requests must also be live,
exposing the key to theft similar to a hot wallet. Steal the key,
forge payment requests, and the payer sees a 'green box' but the coins
go to the attacker. The question... is there a way to sign something
once, with a key kept offline, which verifies the address in the
Payment Request belongs to the payee?

1) Given a 'parent' cert which is kept offline, and a child
certificate of 'parent' which is kept hot on the payment server.

2) Given a public key and chain code { pubKey, code } under BIP32 we
generate child keys as I = HMAC(code, Kpar || i), Ki = I[0:32] * Kpar.

3) If we sign Kpar with the parent cert's key offline, we can sign the
remaining less critical data (address, I[0:32], amount, description,
etc.) with the child cert's key.

4) The payer verifies Kpar, and verifies the address by calculating
Hash160(Kpar * I[0:32])

In fact, there's no requirement to use BIP32 to calculate I[0:32], it
could also just be randomly generated.

Any I[0:32] included in the Payment Request, even if it is tampered
with, will correspond to an address for which the payee can calculate
the corresponding private key.

So the idea is your 'most trusted' cert would be used offline only to
sign a Kpar once, and a 'less trusted' cert would be used to sign the
other stuff, like 'amount', 'description', 'merchant-data', and the
'I[0:32]' as well.

I'm not an expert on x509, but I imagine the trouble is, how does the
payer know which cert is which? I was originally thinking the parent
cert would be an intermediate CA cert used to sign the child cert, but
I guess good look getting one of those, even with a name constraint,
from a Root CA. I'm not sure if you can do better than just a
'convention' such as one is an EV cert and one is not. Perhaps the
less trusted cert is actually self-signed using the EV cert, but that
requires special validation, since its no longer a standard
certificate chain. I would love to hear a better idea.

Any comments if this is something worth pursuing? I think there are
definitely benefits if merchants can keep the key signing the address
offline.

Thanks,--Jeremy

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<div dir=3D"ltr"><pre style=3D"color:rgb(0,0,0);word-wrap:break-word;white-=
space:pre-wrap"><font face=3D"arial, helvetica, sans-serif">Payment Protoco=
l uses x509 certs to sign a Payment Request. This allows wallets to display=
 meta-data from the cert to the payer instead of the address, which should =
make it easier to verify where money is being sent, and make it harder for =
an attacker to change the address displayed to a user so that coins are not=
 sent to the wrong place.

The difficulty is that Payment Requests must be generated live, and therefo=
re the key used to sign those requests must also be live, exposing the key =
to theft similar to a hot wallet. Steal the key, forge payment requests, an=
d the payer sees a &#39;green box&#39; but the coins go to the attacker. Th=
e question... is there a way to sign something once, with a key kept offlin=
e, which verifies the address in the Payment Request belongs to the payee?

1) Given a &#39;parent&#39; cert which is kept offline, and a child certifi=
cate of &#39;parent&#39; which is kept hot on the payment server.

2) Given a public key and chain code { pubKey, code } under BIP32 we genera=
te child keys as I =3D HMAC(code, Kpar || i), Ki =3D I[0:32] * Kpar.

3) If we sign Kpar with the parent cert&#39;s key offline, we can sign the =
remaining less critical data (address, I[0:32], amount, description, etc.) =
with the child cert&#39;s key.

4) The payer verifies Kpar, and verifies the address by calculating Hash160=
(Kpar * I[0:32])

In fact, there&#39;s no requirement to use BIP32 to calculate I[0:32], it c=
ould also just be randomly generated.

Any I[0:32] included in the Payment Request, even if it is tampered with, w=
ill correspond to an address for which the payee can calculate the correspo=
nding private key.</font></pre><pre style=3D"word-wrap:break-word"><span st=
yle=3D"white-space:pre-wrap;color:rgb(0,0,0)"><font face=3D"arial, helvetic=
a, sans-serif">So the idea is your &#39;most trusted&#39; cert would be use=
d offline only to sign a Kpar once, and a &#39;less trusted&#39; cert would=
 be used to sign the other stuff, like &#39;amount&#39;, &#39;description&#=
39;, &#39;merchant-data&#39;, and the &#39;I[0:32]&#39; as well.</font></sp=
an></pre>
<pre style=3D"word-wrap:break-word"><span style=3D"color:rgb(0,0,0);font-fa=
mily:arial,helvetica,sans-serif;white-space:pre-wrap">I&#39;m not an expert=
 on x509, but I imagine the trouble is, how does the payer know which cert =
is which?</span><span style=3D"font-family:arial;white-space:pre-wrap;color=
:rgb(0,0,0)"><font face=3D"arial, helvetica, sans-serif"> I was originally =
thinking the parent cert would be an intermediate CA cert used to sign the =
child cert, but I guess good look getting one of those, even with a name co=
nstraint, from a Root CA.</font></span><span style=3D"color:rgb(0,0,0);font=
-family:arial,helvetica,sans-serif;white-space:pre-wrap"> I&#39;m not sure =
if you can do better than just a &#39;convention&#39; such as one is an EV =
cert and one is not. Perhaps the less trusted cert is actually self-signed =
using the EV cert, but that requires special validation, since its no longe=
r a standard certificate chain. I would love to hear a better idea.</span><=
br>
</pre><pre style=3D"word-wrap:break-word"><font color=3D"#000000" face=3D"a=
rial, helvetica, sans-serif"><span style=3D"white-space:pre-wrap">Any comme=
nts if this is something worth pursuing? I think there are definitely benef=
its if merchants can keep the key signing the address offline.</span></font=
></pre>
<pre style=3D"word-wrap:break-word"><font color=3D"#000000" face=3D"arial, =
helvetica, sans-serif"><span style=3D"white-space:pre-wrap">Thanks,</span><=
/font>
<span style=3D"white-space:pre-wrap;color:rgb(0,0,0);font-family:arial,helv=
etica,sans-serif">--Jeremy</span></pre></div>

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