diff --git a/IETF-RFC.md b/IETF-RFC.md
index 78dc24a..edd8a5e 100644
--- a/IETF-RFC.md
+++ b/IETF-RFC.md
@@ -657,6 +657,9 @@ contain the following information about its OCM API:
   for a short-lived bearer token.
   _ `"invite-wayf"` - to indicate that this OCM Server exposes a WAYF
   Page to facilitate the Invite flow.
+  _ `"http-sig"` - to indicate that this OCM Server supports
+  [RFC9421] HTTP Message Signatures and advertises public keys in the
+  `publicKeys` array for signature verification.
 * OPTIONAL: criteria (array of string) - The criteria for accepting a
   Share Creation Notification.
   As all Receiving Servers SHOULD require the use of TLS in API
@@ -674,19 +677,37 @@ contain the following information about its OCM API:
   address \* `"invite"` - an invite MUST have been exchanged between the
   sender and the receiver before a Share Creation Notification can be
   sent
-* OPTIONAL: publicKey (object) - The signatory used to sign outgoing
-  request to confirm its origin.
-   The signatory is optional, but if present, it MUST contain
-  two string fields, `id` and `publicKeyPem`.
+* OPTIONAL: publicKey (object) - DEPRECATED: Use publicKeys array
+  instead for RFC 9421 support.
+  Legacy field for draft-cavage HTTP Signatures (RSA only).
+  Maintained for backward compatibility with existing deployments.
+  The signatory is optional, but if present, it MUST contain
+  two string fields, `keyId` and `publicKeyPem`.
   properties:
   - REQUIRED keyId (string) unique id of the key in URI format.  The
     hostname set the origin of the request and MUST be
     identical to the current discovery endpoint.
     Example: https://my-cloud-storage.org/ocm#signature
-  - REQUIRED publicKeyPem (string) - PEM-encoded version of the public
-    key.
+  - REQUIRED publicKeyPem (string) - PEM-encoded RSA public key for
+    draft-cavage signatures.
     Example:
     "----BEGIN PUBLIC KEY----\n...\n----END PUBLIC KEY----\n"
+* OPTIONAL: publicKeys (array of objects) - Array of public keys for
+  [RFC9421] HTTP Message Signatures.
+  Servers advertising the "http-sig" capability MUST provide this
+  field. Clients SHOULD prefer [RFC9421] signatures when this capability
+  is present. Each object in the array MUST contain:
+  - REQUIRED keyId (string) - Unique identifier for this key in URI
+    format. Hostname MUST match the discovery endpoint hostname.
+    Example: https://my-cloud-storage.org/ocm#key-1
+  - REQUIRED publicKeyPem (string) - PEM-encoded public key for
+    [RFC9421] signatures.
+    Example:
+    "----BEGIN PUBLIC KEY----\nMCowBQYDK...\n----END PUBLIC KEY----\n"
+  - REQUIRED algorithm (string) - Cryptographic algorithm identifier
+    from the IANA HTTP Signature Algorithms Registry as defined in
+    [RFC9421] Section 6.2.
+    Example: "ed25519"
 * OPTIONAL: inviteAcceptDialog (string) - URL path of a web page where
   a user can accept an invite, when query parameters `"token"` and
   `"providerDomain"` are provided.  Implementations that offer the
diff --git a/spec.yaml b/spec.yaml
index 41a7ee6..da9b222 100644
--- a/spec.yaml
+++ b/spec.yaml
@@ -420,9 +420,11 @@ components:
               - protocol-object
               - invites
               - exchange-token
+              - http-sig
           example:
             - webdav-uri
             - protocol-object
+            - http-sig
         criteria:
           type: array
           description: |
@@ -442,26 +444,61 @@ components:
             - invite
         publicKey:
           type: object
+          deprecated: true
           description: >
-            The signatory used to sign outgoing request to confirm its origin.
-            The 
-            signatory is optional but it MUST contain `id` and `publicKeyPem`.
+            DEPRECATED: Use publicKeys array instead for RFC 9421 support.
+            Legacy field for draft-cavage HTTP Signatures (RSA only).
+            Maintained for backward compatibility with existing deployments.
+            The signatory is optional but it MUST contain `keyId` and `publicKeyPem`.
           properties:
             keyId:
               type: string
               description: >
                 unique id of the key in URI format. The hostname set the origin
-                of the 
-                request and MUST be identical to the current discovery endpoint.
+                of the request and MUST be identical to the current discovery endpoint.
               example: https://my-cloud-storage.org/ocm#signature
             publicKeyPem:
               type: string
               description: |
-                PEM-encoded version of the public key.
+                PEM-encoded RSA public key for draft-cavage signatures.
               example: |
                 -----BEGIN PUBLIC KEY-----
                 MII...QDD
                 -----END PUBLIC KEY-----
+        publicKeys:
+          type: array
+          description: >
+            Array of public keys for RFC 9421 HTTP Message Signatures.
+            Servers advertising the "rfc-http-sig" capability MUST provide this field.
+            Clients SHOULD prefer RFC 9421 signatures when this capability is present.
+          items:
+            type: object
+            required:
+              - keyId
+              - publicKeyPem
+              - algorithm
+            properties:
+              keyId:
+                type: string
+                format: uri
+                description: >
+                  Unique identifier for this key in URI format.
+                  Hostname MUST match the discovery endpoint hostname.
+                example: https://my-cloud-storage.org/ocm#key-1
+              publicKeyPem:
+                type: string
+                description: |
+                  PEM-encoded public key for RFC 9421 signatures.
+                example: |
+                  -----BEGIN PUBLIC KEY-----
+                  MCowBQYDK2VwAyEA...
+                  -----END PUBLIC KEY-----
+              algorithm:
+                type: string
+                description: >
+                  Cryptographic algorithm identifier from the IANA HTTP Signature
+                  Algorithms Registry as defined in RFC 9421 Section 6.2.
+                example: ed25519
         tokenEndPoint:
           type: string
           description: >