Request: Add example how to sign a mail

[ikreb7]

Hello,

I try to sign a mail with cryptography but I have still problems. There was the issue #4488. But at that time, S/MIME support was not yet so advanced.

This is my code, but I have still the problem, that Thunderbird show that the mail isn't signed and outlook shows that the mail is signed, but don't open the mail.

import smtplib
from email.header import Header
from email.mime.application import MIMEApplication
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from email.utils import formataddr, formatdate, parseaddr

from cryptography import x509
from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.serialization import pkcs7

from_addr = "sender@github.com"
to_addr = "receiver@github.com"

message = MIMEMultipart()
message["Subject"] = Header("Signed Mail", "utf-8").encode()
message["From"] = formataddr(parseaddr(f"Github <{from_addr}>"))
message["To"] = to_addr
message["Date"] = formatdate(localtime=True)
message["Auto-Submitted"] = "auto-generated"
message.preamble = "This is an S/MIME signed message"

body = "This a plain text body!"
message.attach(MIMEText(body, "utf-8"))

ca_cert = open("cert.pem", "rb").read()
ca_key = open("key.pem", "rb").read()

cert = x509.load_pem_x509_certificate(ca_cert)
key = serialization.load_pem_private_key(ca_key, None)
options = [pkcs7.PKCS7Options.DetachedSignature]

signed_message = (
    pkcs7.PKCS7SignatureBuilder()
    .set_data(message.as_bytes())
    .add_signer(cert, key, hashes.SHA256())
    .sign(serialization.Encoding.SMIME, options)
)

server_name = "localhost"
with smtplib.SMTP() as server:
    server.set_debuglevel(True)
    server.connect(server_name, port=25)
    server.sendmail(from_addr, to_addr, signed_message)

[alex]

I'm pretty sure that you're double-SMIME-encoding the data.

[ikreb7]

I'm pretty sure that you're double-SMIME-encoding the data.

Thanks for your response! What do you mean exactly?

[alex]

I mean that you're both using the SMIME encoding option with the PKCS7SignatureBuilder and also doing your own SMIME encoding. I think this may be duplicating portions of it.

[ikreb7]

I mean that you're both using the SMIME encoding option with the PKCS7SignatureBuilder and also doing your own SMIME encoding. I think this may be duplicating portions of it.

Right. I updated the code. But it still doesn't work.

[ikreb7]

OK. I make the example more simple, but here are the From, To and Subject headers invisible.

message = EmailMessage()
message["From"] = formataddr(parseaddr(f"Github <{from_addr}>"))
message["To"] = to_addr
message["Date"] = formatdate(localtime=True)
message["Auto-Submitted"] = "auto-generated"
message["Subject"] = Header("Signed Mail", "utf-8").encode()
message.preamble = "This is an S/MIME signed message"

body = "This a plain text body!"
message.set_content(body)

ca_cert = open("cert.pem", "rb").read()
ca_key = open("key.pem", "rb").read()

cert = x509.load_pem_x509_certificate(ca_cert)
key = serialization.load_pem_private_key(ca_key, None)
options = [pkcs7.PKCS7Options.DetachedSignature]

signed_text = (
    pkcs7.PKCS7SignatureBuilder()
    .set_data(message.as_bytes())
    .add_signer(cert, key, hashes.SHA256())
    .sign(serialization.Encoding.SMIME, options)
)

server_name = "localhost"
with smtplib.SMTP() as server:
    server.set_debuglevel(True)
    server.connect(server_name, port=25)
    server.sendmail(from_addr, to_addr, signed_text)

This is the mail text

MIME-Version: 1.0
Content-Type: multipart/signed; protocol="application/x-pkcs7-signature"; micalg="sha-256"; boundary="===============5864291155697480492=="

# the headers have to be here. Then it works.

This is an S/MIME signed message

--===============5864291155697480492==
From: No-reply <noreply@github.com>
To: receiver@github.com
Date: Mon, 26 Aug 2024 20:58:25 +0100
Auto-Submitted: auto-generated

This is the same problem like #10664.

[ikreb7]

The problem is that the header is in the body of the message and is then not taken into account by the mail program. So this solves the problem but seems to be a dirty hack.

header = f"""From: {formataddr(parseaddr(f"Github <{from_addr}>"))}
To: {to_addr}
Date: {formatdate(localtime=True)}
Auto-Submitted: auto-generated
Subject: {Header("Signed Mail", "utf-8").encode()}
"""

signed_text = header.encode() + signed_text