[bitcoin-dev] Proposal: extend bip70 with OpenAlias

[Thomas Voegtlin <thomasv@electrum.org>]

Dear Bitcoin developers,

I would like to propose an extension of the signature scheme used in
the Payment Protocol (BIP70), in order to authorize payment requests
signed by user@domain aliases, where the alias is verified using
DNSSEC (OpenAlias).

Note that the Payment Protocol already includes the possibility to
sign requests with user@domain aliases, using so-called "SSL email
certificates". Email certificates do not require ownership of a domain
name. They are usually delivered by a trusted CA, to the owner of an
email address.

So, why extend BIP70? Well, I believe that SSL email certificates, as
they exist today, are not well suited for payment requests. The core
issue is that email certificates are not delivered by the entity that
owns the same domain. This has the following implications:

1. No cross-verification. Two different CAs may deliver certificates
   for the same email address. Thus, if a user's mailbox is
   compromised, the hacker can obtain a new certificate for the
   compromised email address, from another CA, and sign payment
   requests with it.  OTOH, if the certificate was delivered by the
   same entity, they could require revokation of the existing
   certificate before issuing a new one. Revocation of a certificate
   would require signing a challenge with the corresponding private
   key.

2. Dilution of responsibilities. Three parties are involved in the
   security of an email certificate: the owner of the email address,
   the CA who signs the certificate, and the owner of the domain
   hosting the email service. If something goes wrong and a user
   claims that a payment request was not signed by them, it is not
   possible to determine who is to blame: the user, the domain owner
   or the CA? Any of these parties could have obtained or issued a new
   certificate.  OTOH, if the alias "user@domain" was issued by
   "domain", we would have clear semantics and clear
   responsibilities. Instead of involving three parties, as in "User X
   hosted at domain Y was verified by trusted authority Z who is not
   shown in the alias", the alias only involves two parties: "user X
   was verified by domain Y". If domain Y misbehaves and issues a
   second certificate for user X, while the first certificate is still
   valid, then the first certificate can serve as a public proof that
   they misbehaved.

3. Lowest common denominator: email is only a communication channel,
   used for authentication by some CAs. Other CAs may decide to use
   other, possibly better, identity verification procedures. However,
   because of the absence of cross verification, the security of the
   whole scheme will always be the security of an email address,
   because it remains the method used by less regarding CAs.

In fact, these issues are so bad that I believe BIP70 should be
amended to reject email certificates.

These issues would be solved, if we could enforce that the user@domain
certificate was delivered by the same entity that controls the domain.
How can we do that? Clearly, we need to change the certificate chain
validation procedure. I see two methods to achieve this:

  1. Keep using TLS and change the certificate chain validation.
  2. Use DNSSEC and Openalias.


Method 1: Modified chain validation.
------------------------------------

This introduces a new type of user certificate, where:

 - The commonName is a user@domain alias.
 - The certificate for user@domain must be issued by a domain
   certificate for the same domain (with some rules to allow
   wildcards).
 - Validation of the user@domain certificate does not require the
   issuer certificate to have a CA bit.

This solution would probably be the easiest to deploy, because it uses
TLS certificate chain validation, which is already available in BIP70
compatible wallets. However, it will break compatibility with the
existing certificate validation procedures.


Method 2: DNSSEC and OpenAlias.
-------------------------------

OpenAlias (http://openalias.org) is a standard for storing Bitcoin
addresses and public keys in DNS TXT records. DNSSEC chain validation
imposes that a record is signed by its parent.

In order to use DNSSEC with BIP70, we may add a new pki_type to BIP70
payment requests (let me call it 'dnssec+btc'), that indicates that
the request has been signed with a Bitcoin public key, and that the
chain validation uses DNSSEC. The chain of signatures may be included
in the payment request.

This solution has my preference. It has been implemented in Electrum
and will be available in version 2.4.



Please let me know what you think. Standardizing that proposal will
probably require a new BIP number, because BIP70 is already final. I
am willing to help doing that. OpenAlias developers have also expressed
their support, and are willing to provide assistance.