Chilkat2-Python

Create a DKIM-Signature for a MIME Message

Demonstrates the Chilkat Dkim.DkimSign method, which creates a DKIM-Signature header and prepends it to a complete MIME message held in a BinData, modifying it in place. The DkimAlg, DkimCanon, DkimDomain, DkimSelector, DkimHeaders, and DkimBodyLengthCount properties configure the generated signature.

Note: The file paths are relative to the application's current working directory. Absolute paths may also be used. Supply the paths appropriate to your own environment.

Background: The signature is computed over a hash of the body plus a chosen set of headers, so the message must be completely built — every header, encoding, and boundary — before signing; any later change invalidates it. The d= (domain) and s= (selector) tags tell a verifier where to find the public key: at selector._domainkey.domain in DNS. relaxed/relaxed canonicalization tolerates the minor whitespace changes mail systems make in transit, which is why it is the common choice.

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import sys
import chilkat2

success = False

# Demonstrates the Dkim.DkimSign method, which creates a DKIM-Signature header and prepends it to
# a complete MIME message.  The only argument is a BinData holding the MIME, which is modified in
# place.
# 
# The DkimAlg, DkimCanon, DkimDomain, DkimSelector, DkimHeaders, and DkimBodyLengthCount
# properties configure the signature that is generated.

dkim = chilkat2.Dkim()

# Configure the DKIM signature.
dkim.DkimDomain = "example.com"
dkim.DkimSelector = "myselector"

# Signing algorithm written to the a= tag.
dkim.DkimAlg = "rsa-sha256"

# Canonicalization for headers and body, written to the c= tag.
dkim.DkimCanon = "relaxed/relaxed"

# Colon-separated list of header fields to sign, written to the h= tag.
dkim.DkimHeaders = "From:To:Subject:Date:Message-ID"

# Maximum number of canonicalized body bytes to hash.  0 means the entire body.
dkim.DkimBodyLengthCount = 0

# Load the RSA private key and give it to the Dkim object.
privKey = chilkat2.PrivateKey()
success = privKey.LoadPemFile("qa_data/dkim_private.pem")
if (success == False):
    print(privKey.LastErrorText)
    sys.exit()

success = dkim.SetDkimPrivateKey(privKey)
if (success == False):
    print(dkim.LastErrorText)
    sys.exit()

# Load the complete MIME message to be signed.  It must already contain all headers, transfer
# encodings, MIME boundaries, and body bytes.
mimeData = chilkat2.BinData()
success = mimeData.LoadFile("qa_data/message.eml")
if (success == False):
    print(mimeData.LastErrorText)
    sys.exit()

# Sign.  The DKIM-Signature header is prepended to the MIME in place.
success = dkim.DkimSign(mimeData)
if (success == False):
    print(dkim.LastErrorText)
    sys.exit()

# Save the signed message.
success = mimeData.WriteFile("qa_output/signed.eml")
if (success == False):
    print(mimeData.LastErrorText)
    sys.exit()

print("Message signed.")