Add digest header

Author: timmot

README.md

@@ -247,6 +247,36 @@ follow_request_message = {
 r = requests.post(recipient_inbox, headers=headers, json=follow_request_message)
 ```
 
+### 7. Mastodon now requires the Digest header
+Thanks @Yoxem for the info.
+
+```python
+import json
+import hashlib
+
+follow_request_message = { ... } # as above
+follow_request_json = json.dumps(follow_request_message)
+digest = base64.b64encode(hashlib.sha256(follow_request_json.encode('utf-8')).digest())
+
+# signature information is now
+signature_text = b'(request-target): post %s\ndigest: SHA-256=%s\nhost: %s\ndate: %s' % (recipient_path.encode('utf-8'), digest, recipient_host.encode('utf-8'), current_date.encode('utf-8'))
+
+raw_signature = private_key.sign(
+    signature_text,
+    padding.PKCS1v15(),
+    hashes.SHA256()
+)
+
+signature_header = 'keyId="%s",algorithm="rsa-sha256",headers="(request-target) digest host date",signature="%s"' % sender_key, base64.b64encode(raw_signature).decode('utf-8')
+
+headers = {
+    'Date': current_date,
+    'Content-Type': 'application/activity+json',
+    'Host': recipient_host,
+    'Digest': "SHA-256="+digest.decode('utf-8'),
+    'Signature': signature_header
+}
+```
 
 ## Standards
 * [ActivityPub](https://www.w3.org/TR/activitypub/)