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Using AWS SSM Parameter Store to Retrieve Secrets Encrypted by KMS Using Python

Today we will use Amazon Web Services SSM Service to store secrets in their Parameter Store which we will encyrpt using KMS.

Then we will read the data from SSM and decrypt using our KMS key. We will then end it off by writing a Python Script that reads the AWS credentials, authenticates with SSM and then read the secret values that we stored.

The Do List:

We will break up this post in the following topics:

  • Create a KMS Key which will use to Encrypt/Decrypt the Parameter in SSM
  • Create the IAM Policy which will be used to authorize the Encrypt/Decrypt by the KMS ID
  • Create the KMS Alias
  • Create the Parameter using PutParameter as a SecureString to use Encryption with KMS
  • Describe the Parameters
  • Read the Parameter with and without Decryption to determine the difference using GetParameter
  • Read the Parameters using GetParameters
  • Environment Variable Example

Create the KMS Key:

As the administrator, or root account, create the KMS Key:

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>>> import boto3
>>> session = boto3.Session(region_name='eu-west-1', profile_name='personal')
>>> iam = session.client('iam')
>>> kms = session.client('kms')
>>> response = kms.create_key(
    Description='Ruan Test Key',
    KeyUsage='ENCRYPT_DECRYPT',
    Origin='AWS_KMS',
    BypassPolicyLockoutSafetyCheck=False,
    Tags=[{'TagKey': 'Name', 'TagValue': 'RuanTestKey'}]
)

>>> print(response['KeyMetadata']['KeyId'])
foobar-2162-4363-ba02-a953729e5ce6

Create the IAM Policy:

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>>> response = iam.create_policy(
    PolicyName='ruan-kms-test-policy',
    PolicyDocument='{
        "Version": "2012-10-17",
        "Statement": [{
            "Sid": "Stmt1517212478199",
            "Action": [
                "kms:Decrypt",
                "kms:Encrypt"
            ],
            "Effect": "Allow",
            "Resource": "arn:aws:kms:eu-west-1:0123456789012:key/foobar-2162-4363-ba02-a953729e5ce6"
        }]
    }', 
    Description='Ruan KMS Test Policy'
)
>>> print(response['Policy']['Arn'])
arn:aws:iam::0123456789012:policy/ruan-kms-test-policy

Create the KMS Alias:

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>>> response = kms.create_alias(AliasName='alias/ruan-test-kms', TargetKeyId='foobar-2162-4363-ba02-a953729e5ce6')

Publish the Secrets to SSM:

As the administrator, write the secret values to the parameter store in SSM. We will publish a secret with the Parameter: /test/ruan/mysql/db01/mysql_hostname and the Value: db01.eu-west-1.mycompany.com:

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>>> from getpass import getpass
>>> secretvalue = getpass()
Password:

>>> print(secretvalue)
db01.eu-west-1.mycompany.com

>>> response = ssm.put_parameter(
    Name='/test/ruan/mysql/db01/mysql_hostname',
    Description='RuanTest MySQL Hostname',
    Value=secretvalue,
    Type='SecureString',
    KeyId='foobar-2162-4363-ba02-a953729e5ce6',
    Overwrite=False
)

Describe Parameters

Describe the Parameter that we written to SSM:

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>>> response = ssm.describe_parameters(
    Filters=[{'Key': 'Name', 'Values': ['/test/ruan/mysql/db01/mysql_hostname']}]
)
>>> print(response['ResponseMetadata']['Parameters'][0]['Name'])
'/test/ruan/mysql/db01/mysql_hostname'

Reading from SSM:

Read the Parameter value from SSM without using decryption via KMS:

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>>> response = ssm.get_parameter(Name='/test/ruan/mysql/db01/mysql_hostname')
>>> print(response['Parameter']['Value'])
AQICAHh7jazUUBgNxMQbYFeve2/p+UWTuyAd5F3ZJkZkf9+hwgF+H+kSABfPCTEarjXqYBaJAAAAejB4BgkqhkiG9w0BBwagazBpAgEAMGQGCSqGSIb3DQEHATAeBglghkgBZQMEAS4wEQQMJUEuT8wDGCQ3zRBmAgEQgDc8LhLgFe+Rutgi0hOKnjTEVQa2lKTy3MmTDZEeLy3Tlr5VUl6AVJNBpd4IWJTbj5YuqrrAAWWJ

As you can see the value is encrypted, this time read the parameter value with specifying decryption via KMS:

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>>> response = ssm.get_parameter(Name='/test/ruan/mysql/db01/mysql_hostname', WithDecryption=True)
>>> print(response['Parameter']['Value'])
db01.eu-west-1.mycompany.com

Grant Permissions to Instance Profile:

Now we will create a policy that can only decrypt and read values from SSM that matches the path: /test/ruan/mysql/db01/mysql_*. This policy will be associated to a instance profile role, which will be used by EC2, where our application will read the values from.

Our policy will look like this:

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{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "Stmt1517398919242",
      "Action": [
        "kms:Decrypt"
      ],
      "Effect": "Allow",
      "Resource": "arn:aws:kms:eu-west-1:0123456789012:key/foobar-2162-4363-ba02-a953729e5ce6"
    },
    {
      "Sid": "Stmt1517399021096",
      "Action": [
        "ssm:GetParameter"
      ],
      "Effect": "Allow",
      "Resource": "arn:aws:ssm:eu-west-1:0123456789012:parameter/test/ruan/mysql/db01/mysql_*"
    }
  ]
}

Create the Policy:

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>>> pol = '{"Version": "2012-10-17","Statement": [{"Sid": "Stmt1517398919242","Action": ["kms:Decrypt"],"Effect": "Allow","Resource": "arn:aws:kms:eu-west-1:0123456789012:key/foobar-2162-4363-ba02-a953729e5ce6"},{"Sid": "Stmt1517399021096","Action": ["ssm:GetParameter"],"Effect": "Allow","Resource": "arn:aws:ssm:eu-west-1:0123456789012:parameter/test/ruan/mysql/db01/mysql_*"}]}'
>>> response = iam.create_policy(PolicyName='RuanGetSSM-Policy', PolicyDocument=pol, Description='Test Policy to Get SSM Parameters')

Create the instance profile:

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>>> response = iam.create_instance_profile(InstanceProfileName='RuanTestSSMInstanceProfile')

Create the Role:

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>>> response = iam.create_role(RoleName='RuanTestGetSSM-Role', AssumeRolePolicyDocument='{"Version": "2012-10-17","Statement": [{"Sid": "","Effect": "Allow","Principal": {"Service": "ec2.amazonaws.com"},"Action": "sts:AssumeRole"}]}')

Associate the Role and Instance Profile:

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>>> response = iam.add_role_to_instance_profile(InstanceProfileName='RuanTestSSMInstanceProfile', RoleName='RuanTestGetSSM-Role')

Attach the Policy to the Role:

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>>> response = iam.put_role_policy(RoleName='RuanTestGetSSM-Role', PolicyName='RuanTestGetSSMPolicy1', PolicyDocument=pol')

Launch the EC2 instance with the above mentioned Role. Create the get_ssm.py and run it to decrypt and read the value from SSM:

get_ssm.py
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import boto3
session = boto3.Session(region_name='eu-west-1')
ssm = session.client('ssm')
hostname = ssm.get_parameter(Name='/test/ruan/mysql/db01/mysql_hostname', WithDecryption=True)
print(hostname['Parameter']['Value'])

Run it:

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$ python get_ssm.py
db01.eu-west-1.mycompany.com

Reading with GetParameters:

So say that we created more than one parameter in the path that we allowed, lets use GetParameters to read more than one Parameter:

get_parameters.py
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import boto3
session = boto3.Session(region_name='eu-west-1')
ssm = session.client('ssm')
response = ssm.get_parameters(
    Names=[
        '/test/ruan/mysql/db01/mysql_hostname',
        '/test/ruan/mysql/db01/mysql_user'
    ],
    WithDecryption=True
)

for secrets in response['Parameters']:
    if secrets['Name'] == '/test/ruan/mysql/db01/mysql_hostname':
        print("Hostname: {}".format(secrets['Value']))
    if secrets['Name'] == '/test/ruan/mysql/db01/mysql_user':
        print("Username: {}".format(secrets['Value']))

Run it:

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$ python get_parameters.py
Hostname: db01.eu-west-1.mycompany.com
Username: super_dba

Environment Variable Example from an Application:

Set the Environment Variable value to the SSM key:

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$ export MYSQL_HOSTNAME="/test/ruan/mysql/db01/mysql_hostname"
$ export MYSQL_USERNAME="/test/ruan/mysql/db01/mysql_user"

The application code:

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import os
import boto3

session = boto3.Session(region_name='eu-west-1')
ssm = session.client('ssm')

MYSQL_HOSTNAME = os.environ.get('MYSQL_HOSTNAME')
MYSQL_USERNAME = os.environ.get('MYSQL_USERNAME')

hostname = ssm.get_parameter(Name=MYSQL_HOSTNAME, WithDecryption=True)
username = ssm.get_parameter(Name=MYSQL_USERNAME, WithDecryption=True)

print("Hostname: {}".format(hostname['Parameter']['Value']))
print("Username: {}".format(username['Parameter']['Value']))

Let the application transform the key to the SSM Value:

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$ python app.py
Hostname: db01.eu-west-1.mycompany.com
Username: super_dba

Resources:

Great thanks to the following resources:

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