NIP-13
======

Proof of Work
-------------

`draft` `optional`

This NIP defines a way to generate and interpret Proof of Work for nostr notes. Proof of Work (PoW) is a way to add a proof of computational work to a note. This is a bearer proof that all relays and clients can universally validate with a small amount of code. This proof can be used as a means of spam deterrence.

`difficulty` is defined to be the number of leading zero bits in the `NIP-01` id. For example, an id of `000000000e9d97a1ab09fc381030b346cdd7a142ad57e6df0b46dc9bef6c7e2d` has a difficulty of `36` with `36` leading 0 bits.

`002f...` is `0000 0000 0010 1111...` in binary, which has 10 leading zeroes. Do not forget to count leading zeroes for hex digits <= `7`.

Mining
------

To generate PoW for a `NIP-01` note, a `nonce` tag is used:

```json
{"content": "It's just me mining my own business", "tags": [["nonce", "1", "21"]]}
```

When mining, the second entry to the nonce tag is updated, and then the id is recalculated (see [NIP-01](./01.md)). If the id has the desired number of leading zero bits, the note has been mined. It is recommended to update the `created_at` as well during this process.

The third entry to the nonce tag `SHOULD` contain the target difficulty. This allows clients to protect against situations where bulk spammers targeting a lower difficulty get lucky and match a higher difficulty. For example, if you require 40 bits to reply to your thread and see a committed target of 30, you can safely reject it even if the note has 40 bits difficulty. Without a committed target difficulty you could not reject it. Committing to a target difficulty is something all honest miners should be ok with, and clients `MAY` reject a note matching a target difficulty if it is missing a difficulty commitment.

Example mined note
------------------

```json
{
  "id": "000006d8c378af1779d2feebc7603a125d99eca0ccf1085959b307f64e5dd358",
  "pubkey": "a48380f4cfcc1ad5378294fcac36439770f9c878dd880ffa94bb74ea54a6f243",
  "created_at": 1651794653,
  "kind": 1,
  "tags": [
    ["nonce", "776797", "21"]
  ],
  "content": "It's just me mining my own business",
  "sig": "284622fc0a3f4f1303455d5175f7ba962a3300d136085b9566801bc2e0699de0c7e31e44c81fb40ad9049173742e904713c3594a1da0fc5d2382a25c11aba977"
}
```

Validating
----------

Here is some reference C code for calculating the difficulty (aka number of leading zero bits) in a nostr event id:

```c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int countLeadingZeroes(const char *hex) {
    int count = 0;

    for (int i = 0; i < strlen(hex); i++) {
        int nibble = (int)strtol((char[]){hex[i], '\0'}, NULL, 16);
        if (nibble == 0) {
            count += 4;
        } else {
            count += __builtin_clz(nibble) - 28;
            break;
        }
    }

    return count;
}

int main(int argc, char *argv[]) {
    if (argc != 2) {
        fprintf(stderr, "Usage: %s <hex_string>\n", argv[0]);
        return 1;
    }

    const char *hex_string = argv[1];
    int result = countLeadingZeroes(hex_string);
    printf("Leading zeroes in hex string %s: %d\n", hex_string, result);

    return 0;
}
```

Here is some JavaScript code for doing the same thing:

```javascript
// hex should be a hexadecimal string (with no 0x prefix)
function countLeadingZeroes(hex) {
  let count = 0;

  for (let i = 0; i < hex.length; i++) {
    const nibble = parseInt(hex[i], 16);
    if (nibble === 0) {
      count += 4;
    } else {
      count += Math.clz32(nibble) - 28;
      break;
    }
  }

  return count;
}
```

Querying relays for PoW notes
-----------------------------

If relays allow searching on prefixes, you can use this as a way to filter notes of a certain difficulty:

```
$ echo '["REQ", "subid", {"ids": ["000000000"]}]'  | websocat wss://some-relay.com | jq -c '.[2]'
{"id":"000000000121637feeb68a06c8fa7abd25774bdedfa9b6ef648386fb3b70c387", ...}
```

Delegated Proof of Work
-----------------------

Since the `NIP-01` note id does not commit to any signature, PoW can be outsourced to PoW providers, perhaps for a fee. This provides a way for clients to get their messages out to PoW-restricted relays without having to do any work themselves, which is useful for energy-constrained devices like mobile phones.