NIP-76
======

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

`draft` `optional`

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

Events tagged with `rp` or `prp` require AUTH for download.

## Read Permission

The `rp` tag takes a pubkey in lowercase hex as value. 

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

When responding to `REQ`s, if an event contains `rp` tags, relays MUST verify that the authenticated user is either the event's author or one of the keys in the `rp` set before delivering it to the client.

## Probabilistic Read Permissions

Probabilistic permissions are implemented using Bloom filters that represent a set of authorized pubkeys. These permissions are expressed as a colon-separated value comprising:
1. the number of bits in the bit array,
2. the number of hash rounds applied, and
3. the bit array encoded in Base64.
4. the salt encoded in Base64.

```json
["prp", "<bits>:<rounds>:<base64>:<salt>"]
```

Bloom filters MUST use `SHA256` functions applied to the concatenation of the key, salt, and index, as demonstrated in the pseudocode below:

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

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

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

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

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

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

        return true
    }

    fun encode() {
        return size + ":" + rounds + ":" + base64Encode(bits.toByteArray()) + ":" + base64Encode(salt) 
    }
    
    fun decode(str: String): BloomFilter {
        val [sizeStr, roundsStr, bufferB64, saltB64] = str.split(":")
        return BloomFilter(sizeStr.toInt(), roundsStr.toInt(), base64Decode(bufferB64), base64Decode(saltB64))
    }
}
```

When responding to `REQ`s, if an event contains `prp` tags, relays MUST verify that the authenticated user is either the event's author or matches any of the filters before delivering it to the client.

If both `rp` and `prp` tags are present, the authenticated user MUST either be in the `rp` set or match any `prp` filter.

### Test cases

The filter below has 100 bits and uses 10 rounds of hashing, which should be capable of handling up to 10,000,000 keys without producing any false positives.

```json
["prp", "100:10:AAAkAQANcYQFCQoB:hZkZYqqdxcE="]
```

It includes keys `ca29c211f1c72d5b6622268ff43d2288ea2b2cb5b9aa196ff9f1704fc914b71b` and `460c25e682fda7832b52d1f22d3d22b3176d972f60dcdc3212ed8c92ef85065c`