diff --git a/fep/521a/fep-521a.feature b/fep/521a/fep-521a.feature
new file mode 100644
index 0000000..2992e2c
--- /dev/null
+++ b/fep/521a/fep-521a.feature
@@ -0,0 +1,29 @@
+Feature: FEP-521a: Representing actor's public keys
+
+    @fep-521a
+    Scenario: Representing Ed25519 public key
+        Given the Ed25519 public key as a hex string "2e6fcce36701dc791488e0d0b1745cc1e33a4c1c9fcc41c63bd343dbbe0970e6"
+        And actor ID "https://server.example/users/alice"
+        Then Multikey object is
+            """
+            {
+                "id": "https://server.example/users/alice#ed25519-key",
+                "type": "Multikey",
+                "controller": "https://server.example/users/alice",
+                "publicKeyMultibase": "z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK"
+            }
+            """
+
+    @fep-521a
+    Scenario: Representing RSA public key
+        Given the RSA public key in PKCS#1 DER format as a hex string "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"
+        And actor ID "https://server.example/users/alice"
+        Then Multikey object is
+            """
+            {
+                "id": "https://server.example/users/alice#rsa-key",
+                "type": "Multikey",
+                "controller": "https://server.example/users/alice",
+                "publicKeyMultibase": "z4MXj1wBzi9jUstyPMS4jQqB6KdJaiatPkAtVtGc6bQEQEEsKTic4G7Rou3iBf9vPmT5dbkm9qsZsuVNjq8HCuW1w24nhBFGkRE4cd2Uf2tfrB3N7h4mnyPp1BF3ZttHTYv3DLUPi1zMdkULiow3M1GfXkoC6DoxDUm1jmN6GBj22SjVsr6dxezRVQc7aj9TxE7JLbMH1wh5X3kA58H3DFW8rnYMakFGbca5CB2Jf6CnGQZmL7o5uJAdTwXfy2iiiyPxXEGerMhHwhjTA1mKYobyk2CpeEcmvynADfNZ5MBvcCS7m3XkFCMNUYBS9NQ3fze6vMSUPsNa6GVYmKx2x6JrdEjCk3qRMMmyjnjCMfR4pXbRMZa3i"
+            }
+            """
diff --git a/fep/521a/fep-521a.md b/fep/521a/fep-521a.md
index dfa4551..881b30e 100644
--- a/fep/521a/fep-521a.md
+++ b/fep/521a/fep-521a.md
@@ -25,13 +25,22 @@ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "S
 
 Actor's public key MUST be represented as an object with `Multikey` type, as defined in section **2.3.1.2 Multikey** of [Data Integrity](https://www.w3.org/TR/vc-data-integrity/#multikey) specification. This object MUST have the following properties:
 
-- `id`: the unique global identifier of the public key. Identifiers SHOULD be generated by appending a URL fragment to the actor ID. Different public keys of the same actor MUST be identified using different fragments.
+- `id`: the unique global identifier of the public key.
 - `type`: the value of this property MUST contain the string `Multikey`.
 - `controller`: the value of this property MUST match actor ID.
 - `publicKeyMultibase`: a [Multibase](https://www.w3.org/TR/vc-data-integrity/#multibase-0) encoded value of a [Multicodec](https://github.com/multiformats/multicodec/) prefix and the key. Implementations MUST use the `base-58-btc` alphabet.
 
 The `Multikey` object MAY contain the `expires` property indicating the expiration date of the key. Implementations MUST NOT not accept a signature created with a key that has been expired.
 
+### Key IDs
+
+Identifiers SHOULD be generated by appending a fragment identifier to the actor ID. Different public keys of the same actor MUST be identified using different fragment IDs.
+
+Resolution of a URI containing a fragment identifier is performed in two steps:
+
+1. Resolve URI without a fragment identifier.
+2. Find a nested object where `id` (or `@id`) property contains the full URI.
+
 ### Key types
 
 Implementers can use cryptographic keys of any type for which [Multicodec](https://github.com/multiformats/multicodec/) prefix is registered.
@@ -74,6 +83,15 @@ Other use cases are currently out of scope of this proposal.
 
 This proposal describes how to represent actor's public keys. The corresponding private keys are controlled by the server.
 
+## Test vectors
+
+See [fep-521a.feature](./fep-521a.feature)
+
+## Implementations
+
+- Mitra
+- streams
+
 ## References
 
 - [ActivityPub] Christine Lemmer Webber, Jessica Tallon, [ActivityPub](https://www.w3.org/TR/activitypub/), 2018