nips/13.md

NIP-13

Proof of Work

draft optional author:jb55 author:cameri

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 which 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.

Mining

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

{"content": "It's just me mining my own business", "tags": [["nonce", "1", "20"]]}

When mining, the second entry to the nonce tag is updated, and then the id is recalculated (see NIP-01). 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

{
  "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 note id:

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;
}

Querying relays for PoW notes

Since 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 on mobile