README.md

@@ -1,7 +1,14 @@
 # gpgmymail
 
-Takes an email from stdin and encrypts it using the recipient's PGP key,
-provided as an argument when calling the script.
+Takes an email from stdin and encrypts it to stdout using the recipient's PGP
+key, provided as an argument when calling the script.
+
+Leaves a `X-gpgmymail-Status` header on the email, which has the following
+statuses:
+
+* `entered` - the email has entered the encryption function, but not been
+              encrypted
+* `encrypted` - the encryption function has encrypted the email
 
 Written to be a Sieve filter to be used with `sieve_extprograms`. Can be used
 in a Sieve filter e.g.:

gpgmymail

@@ -24,6 +24,12 @@ works well for emails created with this tool. When encrypting, the tool
 preserves all headers in the original email in the encrypted part, and
 copies relevant headers to the output. When decrypting, any headers are
 ignored, and only the encrypted headers are restored.
+
+Emails exiting this script will have the 'X-gpgmymail-Status' header, which has
+the following options:
+
+- entered: the email has entered the encrypt() function
+- encrypted: the email has been encrypted
 """
 
 import argparse
@@ -42,6 +48,7 @@ import gnupg
 
 # constants
 DEFAULT_ENCODING='utf-8'  # default is latin-1 which fails w some unicode chars
+PEP_SUBJECT='=?utf-8?Q?p=E2=89=A1p?='
 
 def is_message_encrypted(message: email.message.Message) -> bool:
     """Determines whether or not an email message is encrypted.
@@ -169,6 +176,27 @@ def decode_email(message: email.message.Message) -> email.message.Message:
     
     return decoded_bytes_to_return_value(decoded_bytes)
 
+def set_email_header(
+    message: email.message.Message,
+    name: str,
+    value: str
+) -> None:
+    """
+    Set the header of an email Message. Will either replace the first instance
+    of the header, or if the header is not present, will add the header.
+
+    Note: Python passes objects as references, so there is no need for a return
+    value.
+
+    :param message: the Message object to be modified
+    :param name: the email header to set
+    :param value: the value to set the header to
+    """
+    if message.get(name):
+        message.replace_header(name, value)
+    else:
+        message.add_header(name, value)
+
 def encrypt(
         message: email.message.Message,
         recipients: typing.List[str],
@@ -198,6 +226,9 @@ def encrypt(
 
     :return: The encrypted email as a string"""
 
+    # mark the email as having passed through us
+    set_email_header(message, 'X-gpgmymail-Status', 'entered')
+    
     # exclusion criteria:
     # some mail clients like Thunderbird don't like twice-encrypted emails,
     # so we return the message as-is if it's already encrypted
@@ -254,6 +285,13 @@ def encrypt(
         if key.lower() not in headers_not_to_override:
             encmsg[key] = value
 
+    # mark as confirming to pEp: https://blog.jak-linux.org/2019/06/13/encrypted-email-storage/#pretty-easy-privacy-pp
+    set_email_header(encmsg, 'Subject', PEP_SUBJECT)
+    set_email_header(encmsg, 'X-pEp-Version', '2.1')
+
+    # we have encrypted the email, set our gpgmymail header appropriately
+    set_email_header(encmsg, 'X-gpgmymail-Status', 'encrypted')
+
     return encmsg.as_string()
 
 def decrypt(message: email.message.Message, *, encoding: str = DEFAULT_ENCODING) -> str: