Generate credentials to connect to an Amazon SES SMTP endpoint

#!/usr/bin/env python3

import hmac
import hashlib
import base64
import argparse

SMTP_REGIONS = [
    "us-east-2",  # US East (Ohio)
    "us-east-1",  # US East (N. Virginia)
    "us-west-2",  # US West (Oregon)
    "ap-south-1",  # Asia Pacific (Mumbai)
    "ap-northeast-2",  # Asia Pacific (Seoul)
    "ap-southeast-1",  # Asia Pacific (Singapore)
    "ap-southeast-2",  # Asia Pacific (Sydney)
    "ap-northeast-1",  # Asia Pacific (Tokyo)
    "ca-central-1",  # Canada (Central)
    "eu-central-1",  # Europe (Frankfurt)
    "eu-west-1",  # Europe (Ireland)
    "eu-west-2",  # Europe (London)
    "eu-south-1",  # Europe (Milan)
    "eu-north-1",  # Europe (Stockholm)
    "sa-east-1",  # South America (Sao Paulo)
    "us-gov-west-1",  # AWS GovCloud (US)
    "us-gov-east-1",  # AWS GovCloud (US)
]

# These values are required to calculate the signature. Do not change them.
DATE = "11111111"
SERVICE = "ses"
MESSAGE = "SendRawEmail"
TERMINAL = "aws4_request"
VERSION = 0x04


def sign(key, msg):
    return hmac.new(key, msg.encode("utf-8"), hashlib.sha256).digest()


def calculate_key(secret_access_key, region):
    if region not in SMTP_REGIONS:
        raise ValueError(f"The {region} Region doesn't have an SMTP endpoint.")

    signature = sign(("AWS4" + secret_access_key).encode("utf-8"), DATE)
    signature = sign(signature, region)
    signature = sign(signature, SERVICE)
    signature = sign(signature, TERMINAL)
    signature = sign(signature, MESSAGE)
    signature_and_version = bytes([VERSION]) + signature
    smtp_password = base64.b64encode(signature_and_version)
    return smtp_password.decode("utf-8")


def main():
    parser = argparse.ArgumentParser(
        description="Convert a Secret Access Key to an SMTP password."
    )
    parser.add_argument("secret", help="The Secret Access Key to convert.")
    parser.add_argument(
        "region",
        help="The AWS Region where the SMTP password will be used.",
        choices=SMTP_REGIONS,
    )
    args = parser.parse_args()
    print(calculate_key(args.secret, args.region))


if __name__ == "__main__":
    main()