Mark Needham

Thoughts on Software Development

AWS Lambda: Encrypted environment variables

with 3 comments

Continuing on from my post showing how to create a ‘Hello World’ AWS lambda function I wanted to pass encrypted environment variables to my function.

The following function takes in both an encrypted and unencrypted variable and prints them out.

Don’t print out encrypted variables in a real function, this is just so we can see the example working!

import boto3
import os
 
from base64 import b64decode
 
def lambda_handler(event, context):
    encrypted = os.environ['ENCRYPTED_VALUE']
    decrypted = boto3.client('kms').decrypt(CiphertextBlob=b64decode(encrypted))['Plaintext']
 
    # Don't print out your decrypted value in a real function! This is just to show how it works.
    print("Decrypted value:", decrypted)
 
    plain_text = os.environ["PLAIN_TEXT_VALUE"]
    print("Plain text:", plain_text)

Now we’ll zip up our function into HelloWorldEncrypted.zip, ready to send to AWS.

zip HelloWorldEncrypted.zip HelloWorldEncrypted.py

Now it’s time to upload our function to AWS and create the associated environment variables.

If you’re using a Python editor then you’ll need to install boto3 locally to keep the editor happy but you don’t need to include boto3 in the code you send to AWS Lambda – it comes pre-installed.

Now we write the following code to automate the creation of our Lambda function:

import boto3
from base64 import b64encode
 
fn_name = "HelloWorldEncrypted"
kms_key = "arn:aws:kms:[aws-zone]:[your-aws-id]:key/[your-kms-key-id]"
fn_role = 'arn:aws:iam::[your-aws-id]:role/lambda_basic_execution'
 
lambda_client = boto3.client('lambda')
kms_client = boto3.client('kms')
 
encrypt_me = "abcdefg"
encrypted = b64encode(kms_client.encrypt(Plaintext=encrypt_me, KeyId=kms_key)["CiphertextBlob"])
 
plain_text = 'hijklmno'
 
lambda_client.create_function(
        FunctionName=fn_name,
        Runtime='python2.7',
        Role=fn_role,
        Handler="{0}.lambda_handler".format(fn_name),
        Code={ 'ZipFile': open("{0}.zip".format(fn_name), 'rb').read(),},
        Environment={
            'Variables': {
                'ENCRYPTED_VALUE': encrypted,
                'PLAIN_TEXT_VALUE': plain_text,
            }
        },
        KMSKeyArn=kms_key
)

The tricky bit for me here was figuring out that I needed to pass the value that I wanted to base 64 encode the output of the value encrypted by the KMS client. The KMS client relies on a KMS key that we need to setup. We can see a list of all our KMS keys by running the following command:

$ aws kms list-keys

The format of these keys is arn:aws:kms:[zone]:[account-id]:key/[key-id].

Now let’s try executing our Lambda function from the AWS console:

$ python CreateHelloWorldEncrypted.py

Let’s check it got created:

$ aws lambda list-functions --query "Functions[*].FunctionName"
[
    "HelloWorldEncrypted", 
]

And now let’s execute the function:

$ aws lambda invoke --function-name HelloWorldEncrypted --invocation-type RequestResponse --log-type Tail /tmp/out | jq ".LogResult"
"U1RBUlQgUmVxdWVzdElkOiA5YmNlM2E1MC0xODMwLTExZTctYjFlNi1hZjQxZDYzMzYxZDkgVmVyc2lvbjogJExBVEVTVAooJ0RlY3J5cHRlZCB2YWx1ZTonLCAnYWJjZGVmZycpCignUGxhaW4gdGV4dDonLCAnaGlqa2xtbm8nKQpFTkQgUmVxdWVzdElkOiA5YmNlM2E1MC0xODMwLTExZTctYjFlNi1hZjQxZDYzMzYxZDkKUkVQT1JUIFJlcXVlc3RJZDogOWJjZTNhNTAtMTgzMC0xMWU3LWIxZTYtYWY0MWQ2MzM2MWQ5CUR1cmF0aW9uOiAzNjAuMDQgbXMJQmlsbGVkIER1cmF0aW9uOiA0MDAgbXMgCU1lbW9yeSBTaXplOiAxMjggTUIJTWF4IE1lbW9yeSBVc2VkOiAyNCBNQgkK"

That’s a bit hard to read, some decoding is needed:

$ echo "U1RBUlQgUmVxdWVzdElkOiA5YmNlM2E1MC0xODMwLTExZTctYjFlNi1hZjQxZDYzMzYxZDkgVmVyc2lvbjogJExBVEVTVAooJ0RlY3J5cHRlZCB2YWx1ZTonLCAnYWJjZGVmZycpCignUGxhaW4gdGV4dDonLCAnaGlqa2xtbm8nKQpFTkQgUmVxdWVzdElkOiA5YmNlM2E1MC0xODMwLTExZTctYjFlNi1hZjQxZDYzMzYxZDkKUkVQT1JUIFJlcXVlc3RJZDogOWJjZTNhNTAtMTgzMC0xMWU3LWIxZTYtYWY0MWQ2MzM2MWQ5CUR1cmF0aW9uOiAzNjAuMDQgbXMJQmlsbGVkIER1cmF0aW9uOiA0MDAgbXMgCU1lbW9yeSBTaXplOiAxMjggTUIJTWF4IE1lbW9yeSBVc2VkOiAyNCBNQgkK" | base64 --decode
START RequestId: 9bce3a50-1830-11e7-b1e6-af41d63361d9 Version: $LATEST
('Decrypted value:', 'abcdefg')
('Plain text:', 'hijklmno')
END RequestId: 9bce3a50-1830-11e7-b1e6-af41d63361d9
REPORT RequestId: 9bce3a50-1830-11e7-b1e6-af41d63361d9	Duration: 360.04 ms	Billed Duration: 400 ms 	Memory Size: 128 MB	Max Memory Used: 24 MB

And it worked, hoorah!

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Written by Mark Needham

April 3rd, 2017 at 5:49 am

Posted in Software Development

Tagged with ,

  • Ben

    what is the benefit of encrypting env variables? cloud functions are anyway kept far from the public

  • One, benefit could be knowing that malicious transitive dependencies can not get access to an environment variable that contains a secret in plaintext.

  • Stephen Paulger

    The lines where you are decrypting the env vars can/should live outside of your handler. That way the decryption doesn’t have to be done every time the handler is called, which will be faster and will save you some money.