[Bitcoin-development] Blockchain as root CA for payment protocol

[Bitcoin-development] Blockchain as root CA for payment protocol

[Timo Hanke]

There have been proposals to use the blockchain to establish
"identities". firstbits is a simple example. I would like to announce a
project that extends this idea to turn the blockchain into a "root CA"
that can sign arbitrary certificates. The purpose is to use these
certificates in the payment protocol, where some might consider
traditional centralized root CAs unsatisfactory. 

Code is here: https://github.com/bcpki

Technical specification and full-length examples are found in the wiki.
I therefore spare myself from repeating the details here, even though,
of course, discussion about those details is welcome on this list.

Excerpt from README.md follows:

First, we have drafted a quite general specification for bitcoin certificates (protobuf messages) that allow for a variety of payment protocols (e.g. static as well as customer-side-generated payment addresses).
This part has surely been done elsewhere as well and is orthogonal to the goal of this project.
What is new here is the signatures _under_ the certificates.

We have patched the bitcoind to handle certificates, submit signatures to the blockchain, verify certificates against the blockchain, pay directly to certificates (with various payment methods), revoke certificates.
Signatures in the blockchain are stored entirely in the UTXO set (i.e. the unspend, unprunable outputs). 
This seems to make signature lookup and verification reasonably fast: 
it took us 10s in the mainnet test we performed (lookup is instant on the testnet, of course).

Payment methods include: static bitcoin addresses, client-side derived
payment addresses (pay-to-contract), pay-to-contract with multisig destinations (P2SH)

Full-length real-world examples for all payment methods are provided in the tutorial pages.
These examples have actually been carried out on testnet3.

For further details and specifications see the wiki.

timo hanke

Re: [Bitcoin-development] Blockchain as root CA for payment protocol

[Peter Todd]

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On Fri, Feb 08, 2013 at 11:03:54AM +0100, Timo Hanke wrote:
> First, we have drafted a quite general specification for bitcoin certificates (protobuf messages) that allow for a variety of payment protocols (e.g. static as well as customer-side-generated payment addresses).
> This part has surely been done elsewhere as well and is orthogonal to the goal of this project.
> What is new here is the signatures _under_ the certificates.
> 
> We have patched the bitcoind to handle certificates, submit signatures to the blockchain, verify certificates against the blockchain, pay directly to certificates (with various payment methods), revoke certificates.
> Signatures in the blockchain are stored entirely in the UTXO set (i.e. the unspend, unprunable outputs). 
> This seems to make signature lookup and verification reasonably fast: 
> it took us 10s in the mainnet test we performed (lookup is instant on the testnet, of course).

Why don't you use namecoin or another alt-chain for this?

The UTXO set is the most expensive part of the blockchain because it
must be stored in memory with fast access times. It's good that you have
designed the system so that the addresses can be revoked, removing them
from the UTXO set, but it still will encourage the exact same type of
ugly squatting behavior we've already seen with first-bits, and again
it'll have a significant cost to the network going forward for purposes
that do not need to be done on the block chain.

In https://github.com/bcpki/bitcoin/wiki/Technical you say that you have
a minimum amount required for an outpoint to be valid, set at 0.05BTC.
That's a nice touch, and sort of works because this is a consensus
protocol, but if the exchange rate climbs significantly there will be a
lot of pressure to reduce that value. (setting minimum value by chain
height) What will happen then is there will be a mad rush to squat on
previously unaffordable domains, further disrupting Bitcoin's purpose as
a financial network.

In addition you'll also have a second problem: squatting of subsequent
transactions, particularly for valuable bcvalues. Basically if someone
already has "microsoft" insert bcvalues after their tx in case they
accidentally spend it. Of course, this will be done by people buying
bcvalues as well. Again, all this further clogs up the UTXO set.

I also can't figure out why you say signature lookup and verification
takes 10s - this should be an O(1) operation if you maintain a mapping
of candidate pubkeys to linked-lists of sorted applicable transactions.

Finally, why is this implemented within the reference client? Use the
raw transaction API and make up your own database. If you want, create a
RPC command that allows you to query the UTXO set directly; this would
be a useful feature to have. This patch will never be accepted to the
reference client, so you'll just wind up having to maintain a fork. Even
for a prototype this approach is ill-advised - prototypes have a bad way
of turning into production code.


In short, please don't do this.

-- 
'peter'[:-1]@petertodd.org

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Re: [Bitcoin-development] Blockchain as root CA for payment protocol

[Timo Hanke]

On Fri, Feb 08, 2013 at 06:01:08AM -0500, Peter Todd wrote:
> On Fri, Feb 08, 2013 at 11:03:54AM +0100, Timo Hanke wrote:
> > First, we have drafted a quite general specification for bitcoin certificates (protobuf messages) that allow for a variety of payment protocols (e.g. static as well as customer-side-generated payment addresses).
> > This part has surely been done elsewhere as well and is orthogonal to the goal of this project.
> > What is new here is the signatures _under_ the certificates.
> > 
> > We have patched the bitcoind to handle certificates, submit signatures to the blockchain, verify certificates against the blockchain, pay directly to certificates (with various payment methods), revoke certificates.
> > Signatures in the blockchain are stored entirely in the UTXO set (i.e. the unspend, unprunable outputs). 
> > This seems to make signature lookup and verification reasonably fast: 
> > it took us 10s in the mainnet test we performed (lookup is instant on the testnet, of course).
> 
> Why don't you use namecoin or another alt-chain for this?

Because namcoin tries to solve a different problem, DNS, whereas I want
to establish an identity for a payment protocol. Your incoming payments
will land on addresses that are derived (regardless which way) from this
idenity. This makes your identity as important (securitywise) as
anything else involved in the bitcoin protocol. Therefore I would not
want to have payment-ids rely on anything _less_ than bitcoin's own
blockchain. In particular not on PKI with centralized root CAs. But also
not on namecoin or any other (weaker) alt-chains.

You can argue that alt-chains _can_ be as strong as bitcoin, but they
don't _have to_ be. There is no guarantee how many people will
cross-mine. The alt-chain could even disappear at some point. If at some
point your alt-chain is no longer being worked on, then how do you prove
that some old bitcoin transaction went to an address for which there was
a valid id/certificate at the time of sending? If the certificate is
based inside bitcoin's blockchain then you will have a proof for the
correct destinations of all your old transactions as long as bitcoin
exists.

Besides all this, as you mentioned namecoin specifically, that is
overkill if you just want to link two hashes together. A single 2-of-2
multisig output would suffice for that. 

> The UTXO set is the most expensive part of the blockchain because it
> must be stored in memory with fast access times. It's good that you have
> designed the system so that the addresses can be revoked, removing them
> from the UTXO set, but it still will encourage the exact same type of
> ugly squatting behavior we've already seen with first-bits, and again
> it'll have a significant cost to the network going forward for purposes
> that do not need to be done on the block chain.

You are probably right that storing this in the _spent outputs_ would be
better. There doesn't seem to be any type of client out there that would
benefit from having to search UTXO only. 

Timo

Re: [Bitcoin-development] Blockchain as root CA for payment protocol

[Luke-Jr]

On Saturday, February 09, 2013 2:33:25 PM Timo Hanke wrote:
> > Why don't you use namecoin or another alt-chain for this?
> 
> Because namcoin tries to solve a different problem, DNS, whereas I want
> to establish an identity for a payment protocol.

What is the technical difference here? Namecoin ties names to data; DNS is a 
specific namespace in it. There is no reason I know of that this identity 
stuff cannot be a new namespace.

> You can argue that alt-chains _can_ be as strong as bitcoin, but they
> don't _have to_ be. There is no guarantee how many people will
> cross-mine.

This is true of namecoin, but it does not have to be true of new merged-mined 
data. You could very well require the Bitcoin proof-of-work to be valid and 
the master header to be in the Bitcoin blockchain.

> The alt-chain could even disappear at some point. If at some point your alt-
> chain is no longer being worked on, then how do you prove that some old
> bitcoin transaction went to an address for which there was a valid
> id/certificate at the time of sending? If the certificate is based inside
> bitcoin's blockchain then you will have a proof for the correct destinations
> of all your old transactions as long as bitcoin exists.

Yes, if people stop using your system, it won't work. Consider that a "this 
idea failed" scenario, where it doesn't matter.

Luke

Re: [Bitcoin-development] Blockchain as root CA for payment protocol

[Peter Todd]

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On Sat, Feb 09, 2013 at 03:33:25PM +0100, Timo Hanke wrote:
> > Why don't you use namecoin or another alt-chain for this?
> 
> Because namcoin tries to solve a different problem, DNS, whereas I want
> to establish an identity for a payment protocol. Your incoming payments
> will land on addresses that are derived (regardless which way) from this
> idenity. This makes your identity as important (securitywise) as
> anything else involved in the bitcoin protocol. Therefore I would not
> want to have payment-ids rely on anything _less_ than bitcoin's own
> blockchain. In particular not on PKI with centralized root CAs. But also
> not on namecoin or any other (weaker) alt-chains.

In what way are you not solving the same problem as DNS? I don't mean
the Luke-Jr's (quite correct) technical point about key-value maps, I
mean the human problem that I have these unique numbers that I can't
memorize, and I have some non-unique names that I can.

By creating Yet Another Totally Different System you are just creating
another way that users can be confused into thinking some name snatched
up by some scammers in some little-used PKI system is who they are
supposed to be communicating with. Fortunately your PKI system isn't
actually used and probably never will be, so it's not a big deal yet,
but ultimately you are adding to the problem.

Go work on namecoin and make it more usable. Then add some PKI to it
using the *same* domain names so when I see a PKI certificate for "foo"
I know it must be the same "foo" website I just visited and the same
"foo@foo" I just emailed.

> You can argue that alt-chains _can_ be as strong as bitcoin, but they
> don't _have to_ be. There is no guarantee how many people will
> cross-mine. The alt-chain could even disappear at some point. If at some
> point your alt-chain is no longer being worked on, then how do you prove
> that some old bitcoin transaction went to an address for which there was
> a valid id/certificate at the time of sending? If the certificate is
> based inside bitcoin's blockchain then you will have a proof for the
> correct destinations of all your old transactions as long as bitcoin
> exists.

Alt-chains don't have to be based on mining you know. Your proof-of-work
can be replaced by proof-of-sacrifice, specifically Bitcoins. A
particularly nice scheme is to use transaction fees and Bitcoin block
height. Specifically every block in your alt-chain will have a merkle
path to a transaction in a Bitcoin block. Of course there can be more
than one such block, so you introduce a tie-breaker rule: the
transaction with the highest mining fee wins.

The reason why this is nice is because it becomes really easy to be sure
that a better chain won't turn up after the fact - make sure the
transaction linking the alt-chain to the Bitcoin block has the highest
fee in the block. Thus if you want to, say, register a domain name, do
so in the alt-chain, then "mine" the block by creating a suitable
transaction. Make sure it's the biggest fee, wait a few confirmations,
and you're good to go with the same level of security as Bitcoin proper.

Because the rule is that a merkle *path* exists, multiple alt-chains can
use this mechanism at the same time, with the exact same security
guarantee re: max fees. (note that you're chain needs to store copies of
the txin's for the tx sacrificing the fee, transactions by themselves do
not prove fees) Multiple parties can also colaborate to create the
transaction, each providing, and signing for, an input for their portion
of the total fee.


There is the problem that miners get to keep the fee, thus they can
create these special proof-of-sacrifice transactions at low cost, and
potentially make it difficult to get a block mined, or to be sure a
block won't be undone later. This problem can be solved with my
"two-step sacrifice" protocol.(1) Essentially you create a transaction
that is invalid until some time in the future and sacrifices Bitcoins to
mining fees, then create a second transaction that includes the first
one as data. You publish the second in the block chain, proving the
whole world had an opportunity to mine it. Eventually the first is in
fact mined, thus sacrificing Bitcoins to a miner you have no control
over. For a alt-chain you would consider the sacrifice to be a "balance"
and then spend that balance as required in later blocks in a way that is
guaranteed to be public so you can still check the security guarantee of
knowing your tx had the max fee. For instance with the contract protocol
I describe in (1), shave off what ever percentage of the original
sacrifice, linking the merkle-root of the merkel tree of alt-chains at
the same time. Anyone can still monitor the set of all two-step
sacrifices and associated contract movements and check that their one in
a block was the largest possible. Finally if you want to be nice, modify
the contract value rules so only the successful max contract value tx
has it's balance decreased.

1) https://github.com/petertodd/trustbits/blob/master/fidelitybond.md

Actually, you know gmaxwell, the above would be a great way to run the
alt-chain I'm probably going to need for the fraud proofs in Trustbits.
Although it does have the minor problem of being ludicrously complex...

> > The UTXO set is the most expensive part of the blockchain because it
> > must be stored in memory with fast access times. It's good that you have
> > designed the system so that the addresses can be revoked, removing them
> > from the UTXO set, but it still will encourage the exact same type of
> > ugly squatting behavior we've already seen with first-bits, and again
> > it'll have a significant cost to the network going forward for purposes
> > that do not need to be done on the block chain.
> 
> You are probably right that storing this in the _spent outputs_ would be
> better. There doesn't seem to be any type of client out there that would
> benefit from having to search UTXO only. 

The blockchain grows at a maximum rate of 55GiB/year. Do you think your
users will all want to have that available just to validate some PKI
certificates?

-- 
'peter'[:-1]@petertodd.org

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Re: [Bitcoin-development] Blockchain as root CA for payment protocol

[Timo Hanke]

On Sat, Feb 09, 2013 at 07:01:48PM +0000, Luke-Jr wrote:
> On Saturday, February 09, 2013 2:33:25 PM Timo Hanke wrote:
> > namcoin tries to solve a different problem, DNS, whereas I want
> > to establish an identity for a payment protocol.
> 
> What is the technical difference here? Namecoin ties names to data; DNS is a 
> specific namespace in it. There is no reason I know of that this identity 
> stuff cannot be a new namespace.

It's not about technical differences, but about the different use or
purpose, which can result in different security demands. I argue that
DNS has a lower demand in this respect than payment ids have. So DNS
data can be in a chain with a hashrate lower than bitcoin's hashrate but
payment ids _for_ bitcoin have to be in a chain with equal hashrate.

> > You can argue that alt-chains _can_ be as strong as bitcoin, but they
> > don't _have to_ be. There is no guarantee how many people will
> > cross-mine.
> 
> This is true of namecoin, but it does not have to be true of new merged-mined 
> data. You could very well require the Bitcoin proof-of-work to be valid and 
> the master header to be in the Bitcoin blockchain.

Ok, true. This does the trick. If few miners merge-mine then the new
chain just becomes slower. But is this still an alt-chain? It is not
independently verifiable anymore, like the alt-chains described in the
wiki are. Instead, you need to refer to the bitcoin's chain to see if
the target is correct. Not sure if I got you right on this. But it seems
to be essentially a more efficient version of what I proposed, rather
than a true alt-chain.

I suppose you suggest to place the master header hash into the coinbase.
A drawback may be that it puts miners at a great advantage over regular
users. This could (but doesn't have to) become relevant depending on
your counter-measures against excessive alias registration. I think
Peter addressed this (below). 

On Fri, Feb 08, 2013 at 06:01:08AM -0500, Peter Todd wrote:
> Finally, why is this implemented within the reference client? Use the
> raw transaction API and make up your own database. If you want, create a
> RPC command that allows you to query the UTXO set directly; this would
> be a useful feature to have. This patch will never be accepted to the
> reference client, so you'll just wind up having to maintain a fork. Even
> for a prototype this approach is ill-advised - prototypes have a bad way
> of turning into production code.

This was not intended to be a prototype and will certainly not be
maintained. It is a demo to be run on the testnet to get a feel of how
the user interface (RPC) and the "work flow" could look like, starting
from the creation of a certificate all the way to paying to a
customer-derived payment addresses (pay-to-contract) when the merchant's
base address is defined in the certificate. There's an appeal to be able
to issue
./bitcoind sendtoalias foo deadbeef 10
and being sure that 10 BTC go, e.g., to a unique P2SH multisig address
that is derived for order number 'deadbeef' from the two pubkeys that
foo defined in his certificate. And having the certificate verification
happen automatically in the background. The demo is in the reference
client a) to simulate this feel, b) because it was the fastest way to
code it. Apart from that, it could have just as well been separate, and
an UTXO query-RPC would certainly be nice.  

Another reason for this demo was the fun of devising a certificate that
can handle all this. 

BTW, I'm sure that some form of certificate handling will find its way
directly into the reference client. The user will want to trust only one
piece of software running on one piece of dedicated hardware. 

On Mon, Feb 11, 2013 at 06:21:03AM -0500, Peter Todd wrote:
> In what way are you not solving the same problem as DNS? I don't mean
> the Luke-Jr's (quite correct) technical point about key-value maps, I
> mean the human problem that I have these unique numbers that I can't
> memorize, and I have some non-unique names that I can.
> 
> By creating Yet Another Totally Different System you are just creating
> another way that users can be confused into thinking some name snatched
> up by some scammers in some little-used PKI system is who they are
> supposed to be communicating with. Fortunately your PKI system isn't
> actually used and probably never will be, so it's not a big deal yet,
> but ultimately you are adding to the problem.

What exactly is the problem, the "little-used"? Otherwise it's the same
as it is now, you pick up an interesting domain name on the street, type
it in, and start communicating with who you think it is, and maybe even
pay them. The EV that centralized PKI offers prevents only some attacks,
and may even create a false sense of security.

> Go work on namecoin and make it more usable. Then add some PKI to it
> using the *same* domain names so when I see a PKI certificate for "foo"
> I know it must be the same "foo" website I just visited and the same
> "foo@foo" I just emailed.

No. I outlined above why I think namecoin is unsuitable (but Luke's
suggestion for a new alt-chain may be). BTW, if you want to link aliases
of some "bitcoin PKI" and "domain names", they should not correspond
bijectively to each other. The "bitcoin alias" corresponds to a payment
base address, which I see as a more universal identifier than a domain
name. First, bitcoin alias foo can have several domains foo.com,
foo.co.uk, etc. but still only one "bitcoin certificate". This can be
achieved by either writing the domain names directly into the
certificate, or better, by having the "bitcoin certificate" sign an SSL
sub-certificate and tell your browser to ask bitcoind to verify it.
Second, a bitcoin alias can be meaningful without any domain names, e.g.
it can be a certificate for all vending machines of a certain seller,
with who you interact directly, say NFC. To summarize, I would either
not link bitcoin aliases with domain names, or put SSL certificates
"below" bitcoin certificates in the verification chain.

Also BTW, it is the beauty of the pay-to-contract principle that
authenticated communication is not even required. This means that if you
know you have the correct "bitcoin certificate" stored with your
bitcoind and you pay to it, then it doesn't matter if you ordered on a
wrong or fake website. The worst-case scenario is that you have to call
in via another channel and re-submit your order, but funds are never
lost.

> Alt-chains don't have to be based on mining you know. Your proof-of-work
> can be replaced by proof-of-sacrifice, specifically Bitcoins. 
> [...]
> 1) https://github.com/petertodd/trustbits/blob/master/fidelitybond.md

Yes, I already read your discussing of sacrifice-methods after your
first reply and found it interesting.. The problem I see is to
dynamically (and automatically) adjust the minimum sacrifice amount. A
strict limit on the number of registrations like namecoin has is not
desirable. A constant sacrifice, as you mentioned earlier, is also
undesirable. A good measure could be the average transaction fee taken
over several blocks. Maybe the minimum sacrifice should be a constant
times that. 

> > You are probably right that storing this in the _spent outputs_
> > would be better [than UTXO]. There doesn't seem to be any type of
> > client out there that would benefit from having to search UTXO only. 
> 
> The blockchain grows at a maximum rate of 55GiB/year. Do you think your
> users will all want to have that available just to validate some PKI
> certificates?

Actually yes. They are also willing to do that to verify their payments,
so why not for the certificate? But true, this was the reason why I
thought of putting it in the UTXO. To enable some future dedicated
"hardware wallet" to keep only the UTXO and to verify against it. Where
would you store you alt-chain block header hashes? UTXO? 

BTW, suppose you avoid any squatting (there would certainly be ways to
do that). Then with my original proposal of one transaction per
registration you would grow the UTXO by O(n), n is the number of users.
Each user would create a small constant number of aliases. This number
will certainly be dwarfed by the number of unspend outputs that each
user keeps anyway as a result of their regular transactions. So it would
not present a problem for the UTXO. Actually, anything above the dust
threshold should not be a problem, or the system is misdesigned.  But of
course the alt-chain header hashes would only take O(1), much better.

Timo

Re: [Bitcoin-development] Blockchain as root CA for payment protocol

[Gregory Maxwell]

On Mon, Feb 11, 2013 at 11:12 AM, Timo Hanke <timo.hanke@web.de> wrote:
> It's not about technical differences, but about the different use or
> purpose, which can result in different security demands. I argue that
> DNS has a lower demand in this respect than payment ids have. So DNS
> data can be in a chain with a hashrate lower than bitcoin's hashrate but
> payment ids _for_ bitcoin have to be in a chain with equal hashrate.

It seems you're not very well informed about what namecoin does— it's
a multiple namespace key-value store. And, as Peter pointed out— a
non-parasitic system can have exactly the same POW hashpower. Namecoin
chose a model which made it so that namecoin could survive even if
Bitcoin failed, but you don't have to.

I strongly recommend you listen to Peter and Luke— externalizing the
costs of your services onto people who do not care about them is not
going to produce good results for anyone. It's possible to accomplish
what you want to accomplish without taking resources from the users of
the Bitcoin currency without their consent— and you have people here
who are willing to help you figure out how.

Re: [Bitcoin-development] Blockchain as root CA for payment protocol

[Rick Wesson]

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I prefer to leverage the signing of the (.) root in the DNS tree. The
amount of effort in signing the root holds more weight than building a CA
off the bitcoin blockchain.

If you want to associate identifiers for payment addresses I suggest
putting those in DNSSEC signed records in the DNS.

For routing around x.509 CAs I suggest participating in the DANE working
group in the IETF.

-rick


On Fri, Feb 8, 2013 at 2:03 AM, Timo Hanke <timo.hanke@web.de> wrote:

> There have been proposals to use the blockchain to establish
> "identities". firstbits is a simple example. I would like to announce a
> project that extends this idea to turn the blockchain into a "root CA"
> that can sign arbitrary certificates. The purpose is to use these
> certificates in the payment protocol, where some might consider
> traditional centralized root CAs unsatisfactory.
>
> Code is here: https://github.com/bcpki
>
> Technical specification and full-length examples are found in the wiki.
> I therefore spare myself from repeating the details here, even though,
> of course, discussion about those details is welcome on this list.
>
> Excerpt from README.md follows:
>
> First, we have drafted a quite general specification for bitcoin
> certificates (protobuf messages) that allow for a variety of payment
> protocols (e.g. static as well as customer-side-generated payment
> addresses).
> This part has surely been done elsewhere as well and is orthogonal to the
> goal of this project.
> What is new here is the signatures _under_ the certificates.
>
> We have patched the bitcoind to handle certificates, submit signatures to
> the blockchain, verify certificates against the blockchain, pay directly to
> certificates (with various payment methods), revoke certificates.
> Signatures in the blockchain are stored entirely in the UTXO set (i.e. the
> unspend, unprunable outputs).
> This seems to make signature lookup and verification reasonably fast:
> it took us 10s in the mainnet test we performed (lookup is instant on the
> testnet, of course).
>
> Payment methods include: static bitcoin addresses, client-side derived
> payment addresses (pay-to-contract), pay-to-contract with multisig
> destinations (P2SH)
>
> Full-length real-world examples for all payment methods are provided in
> the tutorial pages.
> These examples have actually been carried out on testnet3.
>
> For further details and specifications see the wiki.
>
> timo hanke
>
>
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