diff --git a/39.md b/39.md
index c819e43b..8b05a7d7 100644
--- a/39.md
+++ b/39.md
@@ -13,26 +13,34 @@ Nostr protocol users may have other online identities such as usernames, profile
 ## `i` tag on a metadata event
 
 A new optional `i` tag is introduced for `kind 0` metadata event contents in addition to name, about, picture fields as included in [NIP-01](https://github.com/nostr-protocol/nips/blob/master/01.md):
-```json
+
+```jsonc
 {
+  "kind": 0,
   "tags": [
     ["i", "github:semisol", "9721ce4ee4fceb91c9711ca2a6c9a5ab"],
     ["i", "twitter:semisol_public", "1619358434134196225"],
-    ["i", "mastodon:bitcoinhackers.org/@semisol", "109775066355589974"]
-    ["i", "telegram:1087295469", "nostrdirectory/770"]
+    ["i", "mastodon:bitcoinhackers.org/@semisol", "109775066355589974"],
+    ["i", "telegram:1087295469", "nostrdirectory/770"],
+    ["i", "openpgp4fpr:1A04E0F1A78D982BD8885B7EB325A9C5F70849D0", "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", "LS0tLS1CRUdJTiBQR1AgUFVCTElDIEtFWSBCTE9DSy0tLS0tCgptRE1FWmdIZE94WUpLd1lCQkFIYVJ3OEJBUWRBNnBaZ2RzbUR2aVJGNDVDYys5TUJiK3VkUURZZ2ZHRjZrQkJVCk1CeUd4S2EwRjJaeVlXNTZZWEFnUEY5QVpuSmhibnBoY0M1amIyMCtpSk1FRXhZS0FEc1dJUVFhQk9EeHA0MlkKSzlpSVczNnpKYW5GOXdoSjBBVUNaZ0hkT3dJYkF3VUxDUWdIQWdJaUFnWVZDZ2tJQ3dJRUZnSURBUUllQndJWApnQUFLQ1JDekphbkY5d2hKMEw5R0FRQ3VFUFF0aW9EdGlKaWNVSGw4S0RqU0dmRU1laEFWdjZsTGJuZDBTRjN6Ci9BRCtQZWJCcFk3YkwvakY4QnViaU8rVm9nbUhsa2NGZ3h5V2ZEMDYwMDAwb0FvPQo9OWRBUAotLS0tLUVORCBQR1AgUFVCTElDIEtFWSBCTE9DSy0tLS0tCg"]
   ],
-  ...
+  // ...
 }
 ```
 
-An `i` tag will have two parameters, which are defined as the following:
-1. `platform:identity`: This is the platform name (for example `github`) and the identity on that platform (for example `semisol`) joined together with `:`.
-2. `proof`: String or object that points to the proof of owning this identity.
+An `i` tag will have two parameters and an optional third parameter, which are defined as the following:
+1. `type:identity`: This is the platform name (for example `github`) or the linked cryptographic identity type (for example `openpgp4fpr`), and the identity on that platform (for example `semisol`) or public key fingerprint (for example `1A04E0F1A78D982BD8885B7EB325A9C5F70849D0`) joined together with `:`.
+2. `proof`: String that points to the proof of owning this identity or is the proof itself (crytographic signature).
+3. The full public key in the case of a cryptographic identity.
 
-Clients SHOULD process any `i` tags with more than 2 values for future extensibility.
+Clients SHOULD process any `i` tags with more than 3 values for future extensibility.
 Identity provider names SHOULD only include `a-z`, `0-9` and the characters `._-/` and MUST NOT include `:`.
 Identity names SHOULD be normalized if possible by replacing uppercase letters with lowercase letters, and if there are multiple aliases for an entity the primary one should be used.
 
+## Revocation
+
+The event MAY be updated by removing `i` tags that no longer apply.
+
 ## Claim types
 
 ### `github`
@@ -62,3 +70,79 @@ Identity: A Telegram user ID.
 
 Proof: A string in the format `<ref>/<id>` which points to a message published in the public channel or group with name `<ref>` and message ID `<id>`. This message should be sent by user ID `<identity>` and have the text `Verifying that I control the following Nostr public key: "<npub encoded public key>"`.
 This can be located at `https://t.me/<proof>`.
+
+### `openpgp4fpr`
+
+Identity: A PGP fingerprint as described in the [openpgp4fpr URI scheme](https://metacode.biz/openpgp/openpgp4fpr), uppercase.
+
+Proof: A signature of the text `Verifying that I control the following Nostr public key: "<npub encoded public key>"` in base64 format, unwrapped.
+
+Public key material: The full PGP public key in base64 format, unwrapped.
+
+#### Example
+
+```jsonc
+{
+  "kind": 0,
+  "tags": [
+    ["i", "pgp:1A04E0F1A78D982BD8885B7EB325A9C5F70849D0", "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", "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"],
+    // ...
+  ],
+  // ...
+}
+```
+
+Example command to obtain a signature:
+
+```bash
+echo 'Verifying that I control the following Nostr public key: "<npub encoded public key>"' | gpg --sign --armor | base64
+```
+
+Example command to export the public key:
+
+```bash
+gpg --armor --export name@example.com | base64
+```
+
+### `x509`
+
+Identity: The X.509 certificate's fingerprint, uppercase.
+
+Proof: A signature of the text `Verifying that I control the following Nostr public key: "<npub encoded public key>"` in base64 format, unwrapped.
+
+Public key material: The full public key of the certificate in base64 format, unwrapped.
+
+#### Example
+
+```jsonc
+{
+  "kind": 0,
+  "tags": [
+    ["i", "x509:3220C353A73CFBD0C2F3052471C445324CF452BCBA26DE1C473A52FE5C44E1D6", "DRhQobBXnYFijFjJFZNWiAstqDv+8OACMQIzh+KKw0XS1PW869alYSW4erTUx8xlymlpMoC9et5+kLcfkOXf9jF2UVv5R1JCEGwD5L3/04OtT97h9CftPy1pvuYV6mhja3Ccv2RKGg7Hk99VlpQtFjtgbmCTxaXONGNhYbR/EL6aCEvKfyuuYyodtWTyo1Ys/R6eF7dqFPrDQ/rQ5W14Jmfxhvgn0SW83WqFy/d1A6AQqrku1ZWcHqk41Xaj72A5VmYY5bIe5NTTPTFVz6WhHkx452iZ9w6YlEz2PGzYlUAwq0arpXsD8BJrfBrTj9t+PMRvTV+Sw50Hgu4Ajl9waw==", "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"],
+    // ...
+  ],
+  // ...
+}
+```
+
+Example command to find the SHA-256 fingerprint of the certificate from a keystore: 
+
+```bash
+keytool -list -keystore example.keystore
+```
+
+Example commands to extract the private key and sign the message:
+
+```bash
+keytool -importkeystore -srckeystore example.keystore -destkeystore example.p12 -deststoretype pkcs12
+
+openssl pkcs12 -in example.p12 -nocerts -nodes -out privatekey.pem
+
+echo 'Verifying that I control the following Nostr public key: "<npub encoded public key>"' | openssl dgst -sha256 -sign privatekey.pem | openssl base64 -A
+```
+
+Example command to extract the public key:
+
+```bash
+openssl rsa -in privatekey.pem -pubout | base64
+```
\ No newline at end of file