diff --git a/Cargo.lock b/Cargo.lock
index 90759a6..9f8ca0c 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1707,6 +1707,7 @@ dependencies = [
  "rust-argon2",
  "secp256k1",
  "serde",
+ "serde_jcs",
  "serde_json",
  "serde_yaml",
  "serial_test",
@@ -2742,6 +2743,12 @@ version = "1.0.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "71d301d4193d031abdd79ff7e3dd721168a9572ef3fe51a1517aba235bd8f86e"
 
+[[package]]
+name = "ryu-js"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6518fc26bced4d53678a22d6e423e9d8716377def84545fe328236e3af070e7f"
+
 [[package]]
 name = "same-file"
 version = "1.0.6"
@@ -2850,6 +2857,17 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "serde_jcs"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cacecf649bc1a7c5f0e299cc813977c6a78116abda2b93b1ee01735b71ead9a8"
+dependencies = [
+ "ryu-js",
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "serde_json"
 version = "1.0.64"
diff --git a/Cargo.toml b/Cargo.toml
index 73b139a..a9b22ad 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -62,6 +62,8 @@ secp256k1 = { version = "0.21.3", features = ["rand", "rand-std"] }
 # Used for serialization/deserialization
 serde = { version = "1.0.136", features = ["derive"] }
 serde_json = "1.0"
+# Used to create JCS representations
+serde_jcs = "0.1.0"
 # Used to parse config file
 serde_yaml = "0.8.17"
 # Used to calculate SHA2 hashes
diff --git a/src/activitypub/deliverer.rs b/src/activitypub/deliverer.rs
index 14078e1..5610d99 100644
--- a/src/activitypub/deliverer.rs
+++ b/src/activitypub/deliverer.rs
@@ -12,6 +12,10 @@ use crate::http_signatures::create::{
     create_http_signature,
     HttpSignatureError,
 };
+use crate::json_signatures::create::{
+    sign_object,
+    JsonSignatureError,
+};
 use crate::models::users::types::User;
 use crate::utils::crypto::deserialize_private_key;
 use crate::utils::urls::get_hostname;
@@ -27,6 +31,9 @@ pub enum DelivererError {
     #[error(transparent)]
     HttpSignatureError(#[from] HttpSignatureError),
 
+    #[error(transparent)]
+    JsonSignatureError(#[from] JsonSignatureError),
+
     #[error("activity serialization error")]
     SerializationError(#[from] serde_json::Error),
 
@@ -115,7 +122,8 @@ async fn deliver_activity_worker(
         ),
         ACTOR_KEY_SUFFIX,
     );
-    let activity_json = serde_json::to_string(&activity)?;
+    let activity_signed = sign_object(&activity, &actor_key, &actor_key_id)?;
+    let activity_json = serde_json::to_string(&activity_signed)?;
     if recipients.is_empty() {
         return Ok(());
     };
diff --git a/src/activitypub/signatures.rs b/src/activitypub/signatures.rs
new file mode 100644
index 0000000..8b13789
--- /dev/null
+++ b/src/activitypub/signatures.rs
@@ -0,0 +1 @@
+
diff --git a/src/json_signatures/create.rs b/src/json_signatures/create.rs
new file mode 100644
index 0000000..8199667
--- /dev/null
+++ b/src/json_signatures/create.rs
@@ -0,0 +1,94 @@
+use rsa::RsaPrivateKey;
+use serde::Serialize;
+use serde_json::Value;
+
+use crate::utils::crypto::sign_message;
+
+/// Data Integrity Proof
+/// https://w3c.github.io/vc-data-integrity/
+#[derive(Serialize)]
+#[serde(rename_all = "camelCase")]
+struct Proof {
+    #[serde(rename = "type")]
+    proof_type: String,
+    proof_purpose: String,
+    verification_method: String,
+    proof_value: String,
+}
+
+// Similar to https://identity.foundation/JcsEd25519Signature2020/
+// - Canonicalization algorithm: JCS
+// - Digest algorithm: SHA-256
+// - Signature algorithm: RSASSA-PKCS1-v1_5
+const PROOF_TYPE: &str = "JcsRsaSignature2022";
+
+const PROOF_PURPOSE: &str = "assertionMethod";
+
+#[derive(thiserror::Error, Debug)]
+pub enum JsonSignatureError {
+    #[error(transparent)]
+    JsonError(#[from] serde_json::Error),
+
+    #[error("signing error")]
+    SigningError(#[from] rsa::errors::Error),
+
+    #[error("invalid value")]
+    InvalidValue,
+}
+
+pub fn sign_object(
+    object: &impl Serialize,
+    signer_key: &RsaPrivateKey,
+    signer_key_id: &str,
+) -> Result<Value, JsonSignatureError> {
+    // Canonicalize
+    // JCS: https://www.rfc-editor.org/rfc/rfc8785
+    let object_str = serde_jcs::to_string(object)?;
+    // Sign
+    let signature_b64 = sign_message(signer_key, &object_str)?;
+    // Insert proof
+    let proof = Proof {
+        proof_type: PROOF_TYPE.to_string(),
+        proof_purpose: PROOF_PURPOSE.to_string(),
+        verification_method: signer_key_id.to_string(),
+        proof_value: signature_b64,
+    };
+    let proof_value = serde_json::to_value(proof)?;
+    let mut object_value = serde_json::to_value(object)?;
+    let object_map = object_value.as_object_mut()
+        .ok_or(JsonSignatureError::InvalidValue)?;
+    object_map.insert("proof".to_string(), proof_value);
+    Ok(object_value)
+}
+
+#[cfg(test)]
+mod tests {
+    use serde_json::json;
+    use crate::utils::crypto::generate_weak_private_key;
+    use super::*;
+
+    #[test]
+    fn test_sign_object() {
+        let signer_key = generate_weak_private_key().unwrap();
+        let signer_key_id = "https://example.org/users/test#main-key";
+        let object = json!({
+            "type": "Create",
+            "actor": "https://example.org/users/test",
+            "id": "https://example.org/objects/1",
+            "to": [
+                "https://example.org/users/yyy",
+                "https://example.org/users/xxx",
+            ],
+            "object": {
+                "type": "Note",
+                "content": "test",
+            },
+        });
+        let result = sign_object(&object, &signer_key, signer_key_id).unwrap();
+        let result_str = serde_json::to_string(&result).unwrap();
+        assert_eq!(
+            result_str,
+            r#"{"actor":"https://example.org/users/test","id":"https://example.org/objects/1","object":{"content":"test","type":"Note"},"proof":{"proofPurpose":"assertionMethod","proofValue":"P4ye1hDvrGQCCClzHfCU9xobMAeqlUfgEWGlZfOTE3WmjH8JC/OJwlsjUMOUwTVlyKStp+AY+zzU4z6mjZN0Ug==","type":"JcsRsaSignature2022","verificationMethod":"https://example.org/users/test#main-key"},"to":["https://example.org/users/yyy","https://example.org/users/xxx"],"type":"Create"}"#,
+        );
+    }
+}
diff --git a/src/json_signatures/mod.rs b/src/json_signatures/mod.rs
new file mode 100644
index 0000000..c5fb369
--- /dev/null
+++ b/src/json_signatures/mod.rs
@@ -0,0 +1 @@
+pub mod create;
diff --git a/src/lib.rs b/src/lib.rs
index 2a92622..89819f2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -9,6 +9,7 @@ mod frontend;
 pub mod http;
 mod http_signatures;
 mod ipfs;
+mod json_signatures;
 pub mod logger;
 pub mod mastodon_api;
 mod models;