Now we're starting to see the first companies deploy BIP70, we're encountering a need for identity delegation. This need was long foreseen by the way: it's not in BIP70 because, well, we had to draw the line for v1 somewhere, and this is an issue that mostly affects payment processors. But I figured I'd start a thread anyway because people keep asking me about it :)


Identity delegation means that a payment request can be signed by someone who is not holding the certified private key. The most obvious use case for this is payment processors like BitPay and Coinbase who currently have to sign payment requests as themselves. Other use cases might involve untrusted sales agents who want to be able to accept payment as their employer, but cannot be trusted with a long-term valuable secret, e.g. because they take their phone into areas with high crime rates. 

The lack of this is ok for v1 but not great, because:

1) It requires the name of the *actual* recipient to be put in the memo field, otherwise you don't have the nice receipt-like properties. The memo field is just plain text though, it doesn't have any exploitable structure.

2) It gives a confusing UI, the user thinks they're paying e.g. Overstock but their wallet UI tells them they're paying Coinbase

3) Whilst these payment processors currently verify merchants so the security risk is low, in future a lighter-weight model or competing sites that allow open signups would give a weak security situation:  a hacker who compromised your computer could sign up for some popular payment processor under a false identity (or no identity), and wait until you use your hacked computer to make a payment to someone else using the same payment processor. They could then do an identity swap of the real payment request for one of their own, and your Trezor would still look the same. Avoiding this is a major motivation for the entire system!

Also it just looks more professional if the name you see in the wallet UI is correct.

Proposed implementation

We can fix this with a simple extension:

enum KeyType {
  SECP256K1 = 1

message ExtensionCert {
  required bytes signature = 1;
  required bytes public_key = 2;
  required KeyType key_type = 3;
  required uint32 expiry_time = 4;
  optional string memo = 5;

// modification
message X509Certificates {
  repeated bytes certificate = 1;
  repeated ExtensionCert extended_certs = 2;

message PaymentRequest {
  // new field
  optional bytes extended_signature = 6;

This allow us to define a so-called extended certificate, which is conceptually the same as an X.509 certificate except simpler and Bitcoin specific. To create one, you just format a ExtensionCert message with an ECDSA public key from the payment processor (PP), set signature to an empty array and then sign it using your SSL private key. Obviously the resulting (most likely RSA) signature then goes into the signature field of the ExtensionCert. The memo field could optionally indicate the purpose of this cert, like "Delegation to BitPay" but I don't think it'd ever appear in the UI, rather, it should be there for debugging purposes.

The new ExtensionCert can then be provided back to the PP who adds it to the X509Certificates message. In the PaymentRequest, there are now two signature fields (this is for backwards compatibility). Because of how the mechanism is designed they should not interfere with each other - old implementations that don't understand the new extended_signature field will drop it during reserialization to set signature to the empty array, and thus signature should not cover that field. On the other hand, extended_signature would cover signature. Thus, for full backwards compatibility, you would:

1) Sign the payment request using the PP's SSL cert, i.e. sign as

2) Then sign again using the PP's delegated ECDSA key, i.e. sign as the merchant

The finished protobuf would show up in old clients as signed by and by new clients as signed by even though Overstock did not provide their SSL key to coinbase.

If you have only an ExtensionCert and not any X.509 cert of your own, then you cannot of course make backwards compatible signatures in this way, and in that case you would miss out the signature field and set the pki_type to a new value:  "x509+sha256+excert". Old wallets would see that they don't understand this pki_type and treat the request as unverified.

For maximum security the merchant may choose to set very short expiry times (like, a day) and then have a cron job that uploads a new ExtensionCert at the end of each expiry period. This means in the case of PP compromise, the system reseals very fast.

Alternatives considered

We could always use a new pki_type and not bother with the two signature fields. However, this means old wallets will show payment requests as untrusted during the transition period. Some signing is still better than none, security-wise.

We could attempt to fix the above by introducing a use of User-Agent field to the case where a payment request is fetched via HTTP, so the server can customise the PaymentRequest according to the capabilities of the client. However, sometimes payment requests are not fetched via HTTP, for example, they may be attached to an email, sent via an IM network or sent over a Bluetooth socket. Nonetheless this may be a useful thing to consider for future cases where the protocol may not be extended in a backwards compatible manner.

We could create the extension cert as an X.509 cert, rather than a custom type. However most CA's set path length constraints on their intermediate certs that forbid this kind of extension (I forgot why, possibly some kind of anti-DoS mitigation). Also re-using X.509 for the extension cert would open up the risk of it being accepted by a bogus SSL stack that didn't check the key usage constraints extension, and that would allow for SSL delegation as well. It seems safer to just use a different format that definitely won't be accepted.

Feedback welcome.