NIP-50: Keywords Filter

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+NIP-50
+======
+
+Keywords Filter
+---------------
+
+`draft` `optional` `author:brugeman`
+
+## Abstract
+
+Many Nostr use cases require some form of search by `content` field. The meaning and format of `content` differs 
+with event kind and use case. Thus, this NIP only establishes a general framework for search over `content`,
+and specifies the details for kind `0` and kind `1` events.
+
+## `keywords` filter field 
+
+A new `keywords` field is introduced for `REQ` messages from clients:
+```json
+{
+  "kinds": <an array of kind numbers>
+  "keywords": <an array of strings>
+}
+```
+A filter with `keywords` field MUST also include `kinds` field. This NIP specifies the behavior for kinds `0` and `1`, 
+if none of these kinds are provided along with `keywords`, relay SHOULD ignore the filter, and MAY send
+a `NOTICE` to inform the client. Future NIPs will specify behavior for other event kinds.
+
+A filter matches if one of the provided `keywords` matches. Each keyword string may contain several conditions - `words`, 
+all the words must match for a keyword to match. The meaning of `words` depends on event kind.
+
+Clients SHOULD query several relays supporting this NIP and specific kind to compensate for potentially different 
+implementation details between relays (like string sanitization, etc).
+
+Clients SHOULD verify that events returned by a relay match the specified `keywords` in the way that suits the
+client's use case, and MAY stop querying relays that have low precision.
+
+## Keyword search for kind `0` and kind `1` 
+
+For kind `0` and kind `1`, each keyword is a lowercase string consisting of words separated by spaces (in most languages), punctuation/control 
+characters are ignored. 
+
+For kind `1`, an event must include all words in it's `content` field for it to match the keyword.
+
+For kind `0`, an event must include all mentioned words in the json object stored at the `content` field. Recommended 
+json fields for matching are `about`, `display_name`, `name`, `nip05`, `lud06` and `lud18`.
+
+A keyword may contain a word starting with prefix `option:` (like `option:no_spam`), such words specify additional 
+constraints for event matching, and are reserved for the future extensions. Relay SHOULD ignore `option:` words that it does not support.
+
+Relay MAY support these options for kind `0` and kind `1` events:
+- `option:no_spam` - only include events deemed 'high-quality' by whatever internal metric the relay adopts
+