Implementing Attribute-Based Access Control (ABAC) with AWS KMS

Introduction

Managing access to sensitive information and resources is paramount for any organization in today's digital age. AWS KMS offers a robust solution for creating and managing cryptographic keys that secure our data. One of the powerful features of AWS KMS is its support for Attribute-Based Access Control (ABAC), which allows for fine-grained access control. This blog post aims to demystify ABAC in AWS KMS, providing a straightforward implementation guide with real-world examples and implementation details.

Understanding ABAC in AWS KMS

Attribute-Based Access Control (ABAC) is an access control paradigm that defines access rights based on attributes. In AWS KMS, these attributes can be tags assigned to IAM roles or users and aliases or tags assigned to KMS keys. Unlike traditional role-based access control (RBAC), which grants access based on predefined roles, ABAC allows for more dynamic and granular access control, considering multiple attributes.

Key Concepts:

• KMS Key: A cryptographic key managed by AWS KMS used to encrypt and decrypt data.
• Alias: A friendly name that points to a KMS key. Useful for managing keys without exposing their actual ID.
• Tag: A key-value pair that can be attached to both IAM roles/users and KMS keys for categorization and control purposes.

Real-World Example: Secure Document Storage

Imagine a company, SecureDocs Inc., that uses AWS to store sensitive documents. They have multiple departments (e.g., HR, Finance, Operations) each requiring access to its specific documents. SecureDocs Inc. decides to use AWS KMS for encrypting these documents and ABAC to control access.

Step 1: Tagging IAM Roles and KMS Keys

• IAM Roles/Users: Assign tags based on department, e.g., {"Department": "HR"}, {"Department": "Finance"}.
• KMS Keys: Each key encrypts documents for a specific department, tagged accordingly, e.g., KMS key for HR documents tagged as {"Department": "HR"}.

Step 2: Creating Key Policies

AWS KMS keys come with key policies that specify who can use the key and how. SecureDocs Inc. modifies these policies to include conditions that check for matching tags between the IAM role/user and the KMS key.

For example, the key policy for the HR documents' KMS key includes a condition that allows access only if the requesting IAM role/user has the tag {"Department": "HR"}.

Implementation Details

1. Tagging IAM Roles/Users and KMS Keys:

// Example tag for an IAM role
{
"Key": "Department",
"Value": "HR"
}

// Example tag for a KMS key
{
"Key": "Department",
"Value": "HR"
}

2. Modifying the Key Policy:

{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Principal": {"AWS": "arn:aws:iam::123456789012:role/HRUserRole"},
"Action": "kms:Decrypt",
"Resource": "*",
"Condition": {
"StringEquals": {
"aws:RequestTag/Department": "HR",
"kms:ResourceTag/Department": "HR"
}
}
}
]
}

This policy ensures that only entities (IAM roles/users) with the Department tag set to HR can decrypt data using the HR department's KMS key.

Benefits of Using ABAC with AWS KMS

• Granular Access Control: ABAC allows for fine-grained access control based on multiple attributes, providing a more flexible and precise access control mechanism.
• Dynamic Access Control: Attributes can be changed without updating policies, making it easier to manage access as your organization evolves.
• Simplified Management: By using tags and aliases, administrators can simplify the management of keys and access policies.

Conclusion

Implementing ABAC with AWS KMS offers a powerful way to manage access to encrypted data. Organizations can create dynamic and granular access control policies that adapt to changing needs by leveraging tags and aliases. The example of SecureDocs Inc. illustrates how ABAC can be implemented in a real-world scenario, ensuring that sensitive documents are accessible only to the appropriate departments. 

See also

Read more about ABAC and RBAC at secdops.com/blog/rbac-vs-abac-a-beginners-guide-to-permissions-management.