diff --git a/src/BirdsiteLive.Domain/ActivityPubService.cs b/src/BirdsiteLive.Domain/ActivityPubService.cs
index 7101c23..21ee7fd 100644
--- a/src/BirdsiteLive.Domain/ActivityPubService.cs
+++ b/src/BirdsiteLive.Domain/ActivityPubService.cs
@@ -2,6 +2,7 @@
 using System.Linq;
 using System.Net;
 using System.Net.Http;
+using System.Security.Cryptography;
 using System.Text;
 using System.Threading.Tasks;
 using BirdsiteLive.ActivityPub;
@@ -88,9 +89,10 @@ namespace BirdsiteLive.Domain
 
             var date = DateTime.UtcNow.ToUniversalTime();
             var httpDate = date.ToString("r");
-            var signature = _cryptoService.SignAndGetSignatureHeader(date, actorUrl, targetHost, usedInbox);
 
-            
+            var digest = ComputeSha256Hash(json);
+
+            var signature = _cryptoService.SignAndGetSignatureHeader(date, actorUrl, targetHost, digest, usedInbox);
 
             var client = new HttpClient();
             var httpRequestMessage = new HttpRequestMessage
@@ -101,7 +103,8 @@ namespace BirdsiteLive.Domain
                 {
                     {"Host", targetHost},
                     {"Date", httpDate},
-                    {"Signature", signature}
+                    {"Signature", signature},
+                    {"Digest", $"SHA-256={digest}"}
                 },
                 Content = new StringContent(json, Encoding.UTF8, "application/ld+json")
             };
@@ -109,5 +112,16 @@ namespace BirdsiteLive.Domain
             var response = await client.SendAsync(httpRequestMessage);
             return response.StatusCode;
         }
+
+        static string ComputeSha256Hash(string rawData)
+        {
+            // Create a SHA256   
+            using (SHA256 sha256Hash = SHA256.Create())
+            {
+                // ComputeHash - returns byte array  
+                byte[] bytes = sha256Hash.ComputeHash(Encoding.UTF8.GetBytes(rawData));
+                return Convert.ToBase64String(bytes);
+            }
+        }
     }
 }
\ No newline at end of file
diff --git a/src/BirdsiteLive.Domain/CryptoService.cs b/src/BirdsiteLive.Domain/CryptoService.cs
index ed62a59..837922e 100644
--- a/src/BirdsiteLive.Domain/CryptoService.cs
+++ b/src/BirdsiteLive.Domain/CryptoService.cs
@@ -7,7 +7,7 @@ namespace BirdsiteLive.Domain
     public interface ICryptoService
     {
         string GetUserPem(string id);
-        string SignAndGetSignatureHeader(DateTime date, string actor, string host, string inbox = null);
+        string SignAndGetSignatureHeader(DateTime date, string actor, string host, string digest, string inbox);
     }
 
     public class CryptoService : ICryptoService
@@ -33,7 +33,7 @@ namespace BirdsiteLive.Domain
         /// <param name="actor">in the form of https://domain.io/actor</param>
         /// <param name="host">in the form of domain.io</param>
         /// <returns></returns>
-        public string SignAndGetSignatureHeader(DateTime date, string actor, string targethost, string inbox = null)
+        public string SignAndGetSignatureHeader(DateTime date, string actor, string targethost, string digest, string inbox)
         {
             var usedInbox = "/inbox";
             if (!string.IsNullOrWhiteSpace(inbox))
@@ -41,12 +41,12 @@ namespace BirdsiteLive.Domain
 
             var httpDate = date.ToString("r");
 
-            var signedString = $"(request-target): post {usedInbox}\nhost: {targethost}\ndate: {httpDate}";
+            var signedString = $"(request-target): post {usedInbox}\nhost: {targethost}\ndate: {httpDate}\ndigest: SHA-256={digest}";
             var signedStringBytes = Encoding.UTF8.GetBytes(signedString);
             var signature = _magicKeyFactory.GetMagicKey().Sign(signedStringBytes);
             var sig64 = Convert.ToBase64String(signature);
 
-            var header = "keyId=\"" + actor + "\",headers=\"(request-target) host date\",signature=\"" + sig64 + "\"";
+            var header = "keyId=\"" + actor + "\",algorithm=\"rsa-sha256\",headers=\"(request-target) host date digest\",signature=\"" + sig64 + "\"";
             return header;
         }
     }