diff --git a/README.md b/README.md
index 24106e1e..68ed0a30 100644
--- a/README.md
+++ b/README.md
@@ -35,6 +35,7 @@ NIPs stand for **Nostr Implementation Possibilities**. They exist to document wh
 - [NIP-56: Reporting](56.md)
 - [NIP-57: Lightning Zaps](57.md)
 - [NIP-65: Relay List Metadata](65.md)
+- [NIP-XX: Proof of Freshness](XX.md)
 
 ## Event Kinds
 | kind          | description                      | NIP                     |
@@ -104,6 +105,7 @@ When experimenting with kinds, keep in mind the classification introduced by [NI
 | subject    | subject                 |                   | [14](14.md)              |
 | d          | identifier              |                   | [33](33.md)              |
 | expiration | unix timestamp (string) |                   | [40](40.md)              |
+| f          | block hash (hex)        |                   | [XX](XX.md)              |
 
 ## Criteria for acceptance of NIPs
 
diff --git a/XX.md b/XX.md
new file mode 100644
index 00000000..4c657531
--- /dev/null
+++ b/XX.md
@@ -0,0 +1,78 @@
+NIP-XX
+======
+
+Proof of Freshness
+------------------
+
+`draft` `optional` `author:shafemtol`
+
+Proof of freshness proves that the id of an event, and thus its signature and
+associated proof of work, was generated after some specific point in time.
+
+The proof of freshness is provided in an `f` tag, with the tag value being the
+hexadecimal representation of the hash of a recent Bitcoin block header.
+
+
+Rationale
+---------
+
+Proof of work, described in `NIP-13`, can be used to deter spam by adding
+computational cost to the creation of events. However, it does not prevent a
+spammer from generating large amounts of events over time offline in order to
+later flood the network by publishing all these events at once.
+
+If required to also provide proof of freshness, a spammer is forced to perform
+all their proof of work in near real time, making them unable to perform such a
+flooding attack.
+
+Another use case of proof of freshness is to prove that a message was not signed
+ahead of time.
+
+A single-letter tag `f` is used in order to allow filtering on tag values in
+subscriptions, as described in `NIP-12`, thus protecting subscriptions from spam
+attacks.
+
+
+Choice of Block Hashes
+----------------------
+
+In the interest of reducing the subscription filter size for time ranges, block
+hashes should be chosen from a limited set.
+
+The first `f` tag provided in an event SHOULD contain the hash of the latest
+safe block whose height is divisible by 144. A block is considered safe if it's
+in the best chain with at least 5 blocks on top of it.
+
+144 blocks is roughly the number of blocks mined in 24 hours. An event MAY
+provide additional `f` tags with different divisibility choices.
+
+
+Relay Behavior
+--------------
+
+A relay MAY reject an event with an `f` tag referencing a block with a timestamp
+more than 2 hours ahead of the event's `created_at` timestamp.
+
+A relay MAY reject an event if it does not recognize the hash given in an `f`
+tag, as long as the relay has an up-to-date view of the best Bitcoin chain as of
+the event's `created_at` timestamp.
+
+
+Querying Example
+-------------------
+
+The following filter contains the hashes for blocks 777744 and 777888.
+Assuming events follow the rules given above for choice of block hashes, the
+filter subscribes to events with a certain proof of work made between
+approximately 2023-02-22 03:46 and 2023-02-24 00:18:
+
+```json
+["REQ", "subid",
+ {"ids": ["000000000"],
+  "#f": ["00000000000000000004744ecc695175c314a076bd888ca990140605ce02cf62",
+         "00000000000000000000470a063ad158dfb60983e6b0b81363aafc35dd5fcf77"]}]
+```
+
+By filtering this way, the client is guaranteed to only receive events whose
+proof of work has been generated after the time of the earliest block hash
+provided.