NIP-76
======

Relay Read Permissions
----------------------

`draft` `optional`

Tag names `rp` (read permission) and `prp` (probabilistic read permission) define which keys are authorized to download an event from the relay. 

Events that include an `rp` or `prp` require AUTH to be downloaded.

## Read Permission

The `rp` tag accepts a list of pubkeys 

```json
["rp", "<pubkey1>", "<pubkey2>", "<pubkey3>"]
```

Relays MUST check if the authed user is one of the keys in the `rp` before sending the event to the client.

## Probabilistic Read Permissions

Bloom filters are bit arrays that encode keys `n` times. They are represented by a base64 encoded tag value with the `n` as the third element. 

```json
["prp", "<bit size>:<rounds>:<base64>"]
```

Bloom filters MUST use `SHA-256` functions of the key + iterating index as the psedocode below: 

```js
class BloomFilter(size: Int, n: Int, buffer: ByteArray) {
    val bits = BitArray(buffer)

    fun bitIndex(value: ByteArray, index: Byte) {
        return BigInt(sha256(value || index)) % size
    }

    fun add(pubkey: HexKey) {
        val value = pubkey.hexToByteArray()

        for (index in 0..n) {
            bits[bitIndex(value, index)] = true 
        }
    }

    fun mightContains(pubkey: HexKey): Boolean {
        val value = pubkey.hexToByteArray()

        for (index in 0..n) {
            if (!bits[bitIndex(value, index)]) {
                return false
            }
        }

        return true
    }

    fun encode() = size + ":" + rounds + ":" + base64Enc(bits.toByteArray()) // base64 might include extra 0 bits to fill the last byte
    
    fun decode(str: String): BloomFilter {
        val parts = str.split(":")
        return BloomFilter(parts[0].toInt(), parts[1].toInt(), base64Decode(bits.toByteArray()))
    }
}
```

Relays MUST check if the authed user is in the filter before returning the event.

### Test cases

The filter below has 100 bits, with 10 rounds or hashes that should be able to match 10,000,000 keys without a single false positive.

```json
["prp", "100:10:QGKCgBEBAAhIAApO"]
```

It includes keys `ca29c211f1c72d5b6622268ff43d2288ea2b2cb5b9aa196ff9f1704fc914b71b` and `460c25e682fda7832b52d1f22d3d22b3176d972f60dcdc3212ed8c92ef85065c`