diff --git a/cruciverb-server/src/activitypub/signatures.rs b/cruciverb-server/src/activitypub/signatures.rs
new file mode 100644
index 0000000..4adb805
--- /dev/null
+++ b/cruciverb-server/src/activitypub/signatures.rs
@@ -0,0 +1,135 @@
+use rsa::{RsaPrivateKey, RsaPublicKey, Pkcs1v15Sign};
+use rsa::pkcs1::DecodeRsaPublicKey;
+use rsa::pkcs8::DecodePublicKey;
+use sha2::{Sha256, Digest};
+use base64::Engine;
+use base64::engine::general_purpose::STANDARD as B64;
+
+pub fn sign_request(
+    private_key: &RsaPrivateKey,
+    key_id: &str,
+    method: &str,
+    path: &str,
+    host: &str,
+    body: Option<&[u8]>,
+) -> Vec<(String, String)> {
+    let date = chrono::Utc::now().format("%a, %d %b %Y %H:%M:%S GMT").to_string();
+    let mut headers_to_sign = vec![
+        format!("(request-target): {} {}", method.to_lowercase(), path),
+        format!("host: {}", host),
+        format!("date: {}", date),
+    ];
+
+    let mut result = vec![
+        ("Host".into(), host.into()),
+        ("Date".into(), date.clone()),
+    ];
+
+    let signed_headers;
+    if let Some(body) = body {
+        let digest = B64.encode(Sha256::digest(body));
+        let digest_header = format!("SHA-256={}", digest);
+        headers_to_sign.push(format!("digest: {}", digest_header));
+        result.push(("Digest".into(), digest_header));
+        signed_headers = "(request-target) host date digest";
+    } else {
+        signed_headers = "(request-target) host date";
+    }
+
+    let sign_string = headers_to_sign.join("\n");
+    let hash = Sha256::digest(sign_string.as_bytes());
+    let mut digest_info = vec![0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 0x00, 0x04, 0x20];
+    digest_info.extend_from_slice(&hash);
+    let padding = Pkcs1v15Sign::new_unprefixed();
+    let signature = private_key.sign(padding, &digest_info).expect("signing failed");
+    let sig_b64 = B64.encode(&signature);
+
+    let sig_header = format!(
+        "keyId=\"{}\",algorithm=\"rsa-sha256\",headers=\"{}\",signature=\"{}\"",
+        key_id, signed_headers, sig_b64
+    );
+    result.push(("Signature".into(), sig_header));
+
+    result
+}
+
+pub fn verify_request(
+    public_key_pem: &str,
+    method: &str,
+    path: &str,
+    headers: &[(String, String)],
+    body: Option<&[u8]>,
+) -> Result<(), String> {
+    let sig_header = headers.iter()
+        .find(|(k, _)| k.to_lowercase() == "signature")
+        .map(|(_, v)| v.as_str())
+        .ok_or("missing Signature header")?;
+
+    let mut _key_id = "";
+    let mut signed_headers_str = "";
+    let mut signature_b64 = "";
+    let mut algorithm = "";
+
+    for part in sig_header.split(',') {
+        let part = part.trim();
+        if let Some(v) = part.strip_prefix("keyId=\"").and_then(|s| s.strip_suffix('"')) {
+            _key_id = v;
+        } else if let Some(v) = part.strip_prefix("headers=\"").and_then(|s| s.strip_suffix('"')) {
+            signed_headers_str = v;
+        } else if let Some(v) = part.strip_prefix("signature=\"").and_then(|s| s.strip_suffix('"')) {
+            signature_b64 = v;
+        } else if let Some(v) = part.strip_prefix("algorithm=\"").and_then(|s| s.strip_suffix('"')) {
+            algorithm = v;
+        }
+    }
+
+    if signature_b64.is_empty() || signed_headers_str.is_empty() {
+        return Err("malformed Signature header".into());
+    }
+
+    if !algorithm.is_empty() && algorithm != "rsa-sha256" {
+        return Err(format!("unsupported algorithm: {}", algorithm));
+    }
+
+    let header_map: std::collections::HashMap<String, String> = headers.iter()
+        .map(|(k, v)| (k.to_lowercase(), v.clone()))
+        .collect();
+
+    let mut sign_parts = Vec::new();
+    for h in signed_headers_str.split(' ') {
+        let value = if h == "(request-target)" {
+            format!("{} {}", method.to_lowercase(), path)
+        } else if let Some(v) = header_map.get(h) {
+            v.clone()
+        } else {
+            return Err(format!("missing signed header: {}", h));
+        };
+        sign_parts.push(format!("{}: {}", h, value));
+    }
+    let sign_string = sign_parts.join("\n");
+
+    if signed_headers_str.contains("digest") {
+        let digest_val = header_map.get("digest")
+            .ok_or("signed headers include digest but Digest header is missing")?;
+        let body = body.ok_or("signed headers include digest but request body is empty")?;
+        let expected = format!("SHA-256={}", B64.encode(Sha256::digest(body)));
+        if digest_val != &expected {
+            return Err("digest mismatch".into());
+        }
+    }
+
+    let public_key = RsaPublicKey::from_pkcs1_pem(public_key_pem)
+        .or_else(|_| RsaPublicKey::from_public_key_pem(public_key_pem))
+        .map_err(|e| format!("invalid public key: {}", e))?;
+    let sig_bytes = B64.decode(signature_b64)
+        .map_err(|_| "invalid signature base64")?;
+    let hash = Sha256::digest(sign_string.as_bytes());
+    let mut digest_info = vec![0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05, 0x00, 0x04, 0x20];
+    digest_info.extend_from_slice(&hash);
+    let padding = Pkcs1v15Sign::new_unprefixed();
+
+    public_key.verify(padding, &digest_info, &sig_bytes)
+        .map_err(|e| format!("signature verification failed: {}", e))?;
+
+    Ok(())
+}