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:


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:


- 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.


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