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", "20"]
  ],
  "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
int zero_bits(unsigned char b)
{
        int n = 0;

        if (b == 0)
                return 8;

        while (b >>= 1)
                n++;

        return 7-n;
}

/* find the number of leading zero bits in a hash */
int count_leading_zero_bits(unsigned char *hash)
{
        int bits, total, i;
        for (i = 0, total = 0; i < 32; i++) {
                bits = zero_bits(hash[i]);
                total += bits;
                if (bits != 8)
                        break;
        }
        return total;
}
```

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;
}
```

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.