Generate dkim keys and a TXT record file for AWS Route 53

gen_dkim.py

#!/usr/bin/env python

# Usage
# ./gen_dkim.py mail.yourdomain.tld

import sys
from subprocess import call
from os import devnull

if len(sys.argv) < 2:
    raise Exception('Missing domain parameter')

domain = sys.argv[1]

dev_null = open(devnull, 'w')
with dev_null:
    print('Generating private DKIM key for %s' % domain)
    command = ['openssl', 'genrsa', '-out', 'dkim_%s.priv' % domain, '2048']
    call(command, stdout=dev_null)

output_file = open('dkim_%s.pub' % domain, 'w')
with output_file:
    print('Generating public DKIM key for %s' % domain)
    command = ['openssl', 'rsa', '-in', 'dkim_%s.priv' % domain, '-pubout']
    call(command, stdout=output_file)

print('Generating txt record file for %s' % domain)

print('Generating %s' % 'dkim_%s.aws.txt' % domain)
pub_key_file = open('dkim_%s.pub' % domain, 'r')
with pub_key_file:
    content = pub_key_file.read()
    key_part = 'p=' + ''.join(content.split("\n")[1:-2]) # strips first and last lines (banners) and joins

    # split the key part into blocks of 252 chars
    key_part_size = 252
    res = [key_part[y-key_part_size:y] for y in range(key_part_size, len(key_part)+key_part_size,key_part_size)]
    public_key = ' '.join(map(lambda x: '"%s"' % x, res))

    dkim = '"v=DKIM1; h=sha256; k=rsa; " %s' % public_key

    dkim_file = open('dkim_%s.aws.txt' % domain, 'w')
    with dkim_file:
        dkim_file.write(dkim)