Hi Thomas,

I am working on an experimental implementation [1] of a new payment request format in Trezor T. In some respects it's similar to BIP-70. The main differences are:

1. There is no reliance on X.509, since that seems to have been the main reason for BIP-70's downfall. The signature is mandatory, since for us the main feature is protection against a man-in-the-middle attack. So in this sense it's more similar to BOLT11.

2. It can be used to solve a similar problem with coin exchange. When you are sending BTC to a trusted exchange service and expecting another cryptocurrency in return, say LTC, you want to be sure that you not only have the correct BTC address, but also that the exchange service has your correct LTC address.

3. It uses an optional nonce for replay protection.

The two interesting parts in [1] are probably the `TxAckPaymentRequest` protobuf message [2] and the signature verification [3]. The protobuf message is only for communication between Trezor and the host software running on the user's computer. It's not intended for interchange between wallets. We haven't defined the interchange format yet. I intend to create a SLIP documenting all this.

Andrew

[1] https://github.com/trezor/trezor-firmware/compare/andrewkozlik/payreq2
[2] https://github.com/trezor/trezor-firmware/blob/andrewkozlik/payreq2/common/protob/messages-bitcoin.proto#L403-L427
[3] https://github.com/trezor/trezor-firmware/blob/andrewkozlik/payreq2/core/src/apps/bitcoin/sign_tx/payment_request.py

On Fri, Feb 19, 2021 at 1:43 PM Charles Hill via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

Hi, Thomas,

I developed a URL signing scheme for use with LNURL as a method for
authorizing payments on behalf of offline devices /applications. It's
not specifically off-chain or on-chain related, but could be repurposed.
The gist of the scheme is as follows:

Before any signing is done:

0) Generate an API key (ID/reference, secret, encoding) to be shared
between a server and an offline device or application.

To generate a signature:

1) Generate a random nonce (unique per API key)

2) Build a query string with the `id`, `nonce`, `tag`, "Server
parameters" (see [Subprotocols](#subprotocols) above), and any custom
parameters. The `id` parameter should be equal to the API key's ID.
Example:
`id=b6cb8e81e3&nonce=d585674cf991dbbab42b&tag=withdrawRequest&minWithdrawable=5000&maxWithdrawable=7000&defaultDescription=example&custom1=CUSTOM1_PARAM_VALUE&custom2=CUSTOM2_PARAM_VALUE`.
Note that both the keys and values for query parameters should be URL
encoded. The following characters should be __unescaped__: `A-Z a-z 0-9
- _ . ! ~ * ' ( )`. See
[encodeURIComponent](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/encodeURIComponent#description)
for more details.

3) Sort the query parameters by key (alphabetically). This is referred
to as the "payload". Example:
`custom1=CUSTOM1_PARAM_VALUE&custom2=CUSTOM2_PARAM_VALUE&defaultDescription=example&id=b6cb8e81e3&maxWithdrawable=7000&minWithdrawable=5000&nonce=d585674cf991dbbab42b&tag=withdrawRequest`

4) Sign the payload (the sorted query string) using the API key secret.
Signatures are generated using HMAC-SHA256, where the API key secret is
the key.

5) Append the signature to the payload as follows:
`custom1=CUSTOM1_PARAM_VALUE&custom2=CUSTOM2_PARAM_VALUE&defaultDescription=example&id=b6cb8e81e3&maxWithdrawable=7000&minWithdrawable=5000&nonce=d585674cf991dbbab42b&tag=withdrawRequest&signature=HMAC_SHA256_SIGNATURE`.

You can find more details here:

https://github.com/chill117/lnurl-node#how-to-implement-url-signing-scheme


I would change a few things with this scheme to fit better with the
use-case you describe. For example:

* Remove the "tag" and LNURL-specific parameters

* Instead of HMAC-SHA256 with a shared secret, it could use pub/priv key
signing instead. The lnurl-auth subprotocol has an interesting approach
to protecting user privacy while allowing verification of signatures.
See for more details on that:

https://github.com/fiatjaf/lnurl-rfc/blob/master/lnurl-auth.md


- chill


On 2/19/21 10:14 AM, Thomas Voegtlin via bitcoin-dev wrote:
> I never liked BIP70. It was too complex, had too many features, and when
> people discuss it, they do not even agree on what the main feature was.
>
> Nevertheless, there is ONE feature of BIP70 that I find useful: the fact
> that payment requests were signed. I am making this post to discuss this.
>
> When I send bitcoins to an exchange, I would like to receive a signed
> request. I want to have a proof that the exchange asked me to send coins
> to that address, in case it has been hijacked by some intern working
> there. If that feature was implemented by an exchange, it would guide my
> decision to use that exchange over its competitors.
>
> I do not think that a single exchange ever implemented that, but I guess
> this is because BIP70 is a terrible standard. LN payment requests are
> signed, do not require SSL, do not require interactivity, and therefore
> exchanges use them. Can't we achieve the same for on-chain payments? Is
> anyone working on that?
>
> I would be more than happy to remove BIP70 support from Electrum, if
> there was another standard for signed requests.
>
> Thomas
>
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