The Basics
AWS KMS is a Key Management Service that let you create Cryptographic keys that you can use to encrypt and decrypt data and also other keys. You can read more about it here.
Important points about Keys
Please note that the customer master keys(CMK) generated can only be used to encrypt small amount of data like passwords, RSA key. You can use AWS KMS CMKs to generate, encrypt, and decrypt data keys. However, AWS KMS does not store, manage, or track your data keys, or perform cryptographic operations with data keys.
You must use and manage data keys outside of AWS KMS. KMS API uses AWS KMS CMK in the encryption operations and they cannot accept more than 4 KB (4096 bytes) of data. To encrypt application data, use the server-side encryption features of an AWS service, or a client-side encryption library, such as the AWS Encryption SDK or the Amazon S3 encryption client.
Scenario
We want to create signup and login forms for a website.
Passwords should be encrypted and stored in DynamoDB database.
What do we need?
Lets Implement it as Serverless Application Model (SAM)!
Lets first create the Key that we will use to encrypt and decrypt password.
KmsKey:
Type: AWS::KMS::Key
Properties:
Description: CMK for encrypting and decrypting
KeyPolicy:
Version: '2012-10-17'
Id: key-default-1
Statement:
- Sid: Enable IAM User Permissions
Effect: Allow
Principal:
AWS: !Sub arn:aws:iam::${AWS::AccountId}:root
Action: kms:*
Resource: '*'
- Sid: Allow administration of the key
Effect: Allow
Principal:
AWS: !Sub arn:aws:iam::${AWS::AccountId}:user/${KeyAdmin}
Action:
- kms:Create*
- kms:Describe*
- kms:Enable*
- kms:List*
- kms:Put*
- kms:Update*
- kms:Revoke*
- kms:Disable*
- kms:Get*
- kms:Delete*
- kms:ScheduleKeyDeletion
- kms:CancelKeyDeletion
Resource: '*'
- Sid: Allow use of the key
Effect: Allow
Principal:
AWS: !Sub arn:aws:iam::${AWS::AccountId}:user/${KeyUser}
Action:
- kms:DescribeKey
- kms:Encrypt
- kms:Decrypt
- kms:ReEncrypt*
- kms:GenerateDataKey
- kms:GenerateDataKeyWithoutPlaintext
Resource: '*'
The important thing in above snippet is the KeyPolicy. KMS requires a Key Administrator and Key User. As a best practice your Key Administrator and Key User should be 2 separate user in your Organisation. We are allowing all permissions to the root users.
So if your key Administrator leaves the organisation, the root user will be able to delete this key. As you can see **KeyAdmin **can manage the key but not use it and KeyUser can only use the key. ${KeyAdmin} and **${KeyUser} **are parameters in the SAM template.
You would be asked to provide values for these parameters during SAM Deploy.
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