nips/13.md

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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 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.
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", "20"]]}
```
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 note 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;
}
```
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