import boto3
import json
import logging
import os

logger = logging.getLogger()
logger.setLevel(logging.INFO)


def lambda_handler(event, context):
    """Secrets Manager Rotation Template

    This is a template for creating an AWS Secrets Manager rotation lambda

    Args:
        event (dict): Lambda dictionary of event parameters. These keys must include the following:
            - SecretId: The secret ARN or identifier
            - ClientRequestToken: The ClientRequestToken of the secret version
            - Step: The rotation step (one of createSecret, setSecret, testSecret, or finishSecret)

        context (LambdaContext): The Lambda runtime information

    Raises:
        ResourceNotFoundException: If the secret with the specified arn and stage does not exist

        ValueError: If the secret is not properly configured for rotation

        KeyError: If the event parameters do not contain the expected keys

    """
    arn = event['SecretId']
    token = event['ClientRequestToken']
    step = event['Step']

    # Setup the client
    service_client = boto3.client('secretsmanager', endpoint_url=os.environ['SECRETS_MANAGER_ENDPOINT'])

    # Make sure the version is staged correctly
    metadata = service_client.describe_secret(SecretId=arn)
    if not metadata['RotationEnabled']:
        logger.error("Secret %s is not enabled for rotation" % arn)
        raise ValueError("Secret %s is not enabled for rotation" % arn)
    versions = metadata['VersionIdsToStages']
    if token not in versions:
        logger.error("Secret version %s has no stage for rotation of secret %s." % (token, arn))
        raise ValueError("Secret version %s has no stage for rotation of secret %s." % (token, arn))
    if "AWSCURRENT" in versions[token]:
        logger.info("Secret version %s already set as AWSCURRENT for secret %s." % (token, arn))
        return
    elif "AWSPENDING" not in versions[token]:
        logger.error("Secret version %s not set as AWSPENDING for rotation of secret %s." % (token, arn))
        raise ValueError("Secret version %s not set as AWSPENDING for rotation of secret %s." % (token, arn))

    if step == "createSecret":
        create_secret(service_client, arn, token)

    elif step == "setSecret":
        set_secret(service_client, arn, token)

    elif step == "testSecret":
        test_secret(service_client, arn, token)

    elif step == "finishSecret":
        finish_secret(service_client, arn, token)

    else:
        raise ValueError("Invalid step parameter")


def create_secret(service_client, arn, token):
    """Create the secret

    This method first checks for the existence of a secret for the passed in token. If one does not exist, it will generate a
    new secret and put it with the passed in token.

    Args:
        service_client (client): The secrets manager service client

        arn (string): The secret ARN or other identifier

        token (string): The ClientRequestToken associated with the secret version

    Raises:
        ResourceNotFoundException: If the secret with the specified arn and stage does not exist

    """
    # Make sure the current secret exists
    current_dict = get_secret_dict(service_client, arn, "AWSCURRENT")

    # Now try to get the secret version, if that fails, put a new secret
    try:
        get_secret_dict(service_client, arn, "AWSPENDING")
        logger.info("createSecret: Successfully retrieved secret for %s." % arn)
    except service_client.exceptions.ResourceNotFoundException:
        iam_user = current_dict['smtpUser']
        iam = boto3.client('iam')

        # Remove the oldest key if there are multiple
        logger.debug("Retrieving keys for user %s" % iam_user)
        keys = iam.list_access_keys(UserName=iam_user)['AccessKeyMetadata']
        logger.debug("Found %s key(s)" % len(keys))

        if len(keys) > 1:
            oldest_key = keys[0]
            for key in keys:
                if key['CreateDate'] < oldest_key['CreateDate']:
                    logger.debug("%s is older than %s" % (key, oldest_key))
                    oldest_key = key
            key_id_to_delete = oldest_key['AccessKeyId']
            key_date_to_delete = oldest_key['CreateDate']
            iam.delete_access_key(UserName=iam_user, AccessKeyId=key_id_to_delete)
            logger.info("Deleted old key %s from %s" % (key_id_to_delete, key_date_to_delete))

        # Generate a new access key
        logger.info("createSecret: Creating new access key for %s." % iam_user)
        newKey = iam.create_access_key(UserName=iam_user)['AccessKey']
        current_dict['smtpAccessKeyId'] = newKey['AccessKeyId']
        current_dict['smtpSecretKey'] = newKey['SecretAccessKey']
        current_dict['smtpUsername'] = newKey['AccessKeyId']
        current_dict['smtpPassword'] = generate_ses_password(newKey['SecretAccessKey'])

        # Put the secret
        service_client.put_secret_value(SecretId=arn, ClientRequestToken=token, SecretString=json.dumps(current_dict), VersionStages=['AWSPENDING'])
        logger.info("createSecret: Successfully put secret for ARN %s and version %s." % (arn, token))

def generate_ses_password(secret_key, charset='utf-8'):
    import hmac    #required to compute the HMAC key
    import hashlib #required to create a SHA256 hash
    import base64  #required to encode the computed key

    # These variables are used when calculating the SMTP password. You shouldn't
    # change them.
    message = 'SendRawEmail'
    version = '\x02'

    # Compute an HMAC-SHA256 key from the AWS secret access key.
    signatureInBytes = hmac.new(secret_key.encode(charset),message.encode(charset),hashlib.sha256).digest()
    # Prepend the version number to the signature.
    signatureAndVersion = version.encode(charset) + signatureInBytes
    # Base64-encode the string that contains the version number and signature.
    smtpPassword = base64.b64encode(signatureAndVersion)
    # Decode the string and return it
    return smtpPassword.decode(charset)

def set_secret(service_client, arn, token):
    """Set the secret

    This method should set the AWSPENDING secret in the service that the secret belongs to. For example, if the secret is a database
    credential, this method should take the value of the AWSPENDING secret and set the user's password to this value in the database.

    Args:
        service_client (client): The secrets manager service client

        arn (string): The secret ARN or other identifier

        token (string): The ClientRequestToken associated with the secret version

    """
    # This is where the secret should be set in the service
    # NB This is not applicable for access keys, since they are created and set in one operation

def test_secret(service_client, arn, token):
    """Test the secret

    This method should validate that the AWSPENDING secret works in the service that the secret belongs to.

    Args:
        service_client (client): The secrets manager service client

        arn (string): The secret ARN or other identifier

        token (string): The ClientRequestToken associated with the secret version

    """
    # This is where the secret should be tested against the service
    pending_dict = get_secret_dict(service_client, arn, "AWSPENDING")
    iam_user = pending_dict['smtpUser']
    logger.info("Checking that the pending key is valid for user %s" % iam_user)
    iam = boto3.client('iam')
    logger.debug("Retrieving keys for user %s" % iam_user)
    keys = iam.list_access_keys(UserName=iam_user)['AccessKeyMetadata']
    logger.debug("Found %s key(s)" % len(keys))
    pending_key = pending_dict['smtpAccessKeyId']
    for key in keys:
        if key["AccessKeyId"] == pending_key:
            logger.info("Pending access key %s was found on the user" % pending_key)
            return
    raise ValueError("Unable to find pending access key on user %s" % iam_user)

def finish_secret(service_client, arn, token):
    """Finish the secret

    This method finalizes the rotation process by marking the secret version passed in as the AWSCURRENT secret.

    Args:
        service_client (client): The secrets manager service client

        arn (string): The secret ARN or other identifier

        token (string): The ClientRequestToken associated with the secret version

    Raises:
        ResourceNotFoundException: If the secret with the specified arn does not exist

    """
    # First describe the secret to get the current version
    metadata = service_client.describe_secret(SecretId=arn)
    current_version = None
    for version in metadata["VersionIdsToStages"]:
        if "AWSCURRENT" in metadata["VersionIdsToStages"][version]:
            if version == token:
                # The correct version is already marked as current, return
                logger.info("finishSecret: Version %s already marked as AWSCURRENT for %s" % (version, arn))
                publish_update(service_client, arn)
                return
            current_version = version
            break

    # Finalize by staging the secret version current
    service_client.update_secret_version_stage(SecretId=arn, VersionStage="AWSCURRENT", MoveToVersionId=token, RemoveFromVersionId=current_version)
    logger.info("finishSecret: Successfully set AWSCURRENT stage to version %s for secret %s." % (version, arn))

    publish_update(service_client, arn)

def publish_update(service_client, arn):
    logger.info("Notifying consumers that the secret has updated")

    current_dict = get_secret_dict(service_client, arn, "AWSCURRENT")
    topic_arn = current_dict['snsTopic']
    if not topic_arn:
        logger.warn("No topic ARN provided; unable to notify")
        return
    sns = boto3.client('sns')
    result = sns.publish(TopicArn=topic_arn, Message='Updated %s' % arn)
    logger.info("Notification sent as message %s" % result['MessageId'])

def get_secret_dict(service_client, arn, stage, token=None):
    """Gets the secret dictionary corresponding for the secret arn, stage, and token

    This helper function gets credentials for the arn and stage passed in and returns the dictionary by parsing the JSON string

    Args:
        service_client (client): The secrets manager service client

        arn (string): The secret ARN or other identifier

        token (string): The ClientRequestToken associated with the secret version, or None if no validation is desired

        stage (string): The stage identifying the secret version

    Returns:
        SecretDictionary: Secret dictionary

    Raises:
        ResourceNotFoundException: If the secret with the specified arn and stage does not exist

        ValueError: If the secret is not valid JSON

    """
    required_fields = ['smtpUser', 'smtpAccessKeyId', 'smtpSecretKey']

    # Only do VersionId validation against the stage if a token is passed in
    if token:
        secret = service_client.get_secret_value(SecretId=arn, VersionId=token, VersionStage=stage)
    else:
        secret = service_client.get_secret_value(SecretId=arn, VersionStage=stage)
    plaintext = secret['SecretString']
    secret_dict = json.loads(plaintext)

    # Run validations against the secret
    for field in required_fields:
        if field not in secret_dict:
            raise KeyError("%s key is missing from secret JSON" % field)

    # Parse and return the secret JSON string
    return secret_dict