Authentication and access control for AWS KMS - AWS Key Management Service

Authentication and access control for AWS KMS

To use AWS KMS, you must have credentials that AWS can use to authenticate your requests. The credentials must include permissions to access AWS resources: AWS KMS keys and aliases. No AWS principal has any permissions to a KMS key unless that permission is provided explicitly and never denied. There are no implicit or automatic permission to use or manage a KMS key.

The primary way to manage access to your AWS KMS resources is with policies. Policies are documents that describe which principals can access which resources. Policies attached to an IAM identity are called identity-based policies (or IAM policies), and policies attached to other kinds of resources are called resource policies. AWS KMS resource policies for KMS keys are called key policies. All KMS keys have a key policy.

To control access to your AWS KMS aliases, use IAM policies. To allow principals to create aliases, you must provide the permission to the alias in an IAM policy and permission to the key in a key policy. For details, see Controlling access to aliases.

To control access to your KMS keys, you can use the following policy mechanisms.

  • Key policy – Every KMS key has a key policy. It is the primary mechanism for controlling access to a KMS key. You can use the key policy alone to control access, which means the full scope of access to the KMS key is defined in a single document (the key policy). For more information about using key policies, see Key policies.

  • IAM policies – You can use IAM policies in combination with the key policy and grants to control access to a KMS key. Controlling access this way enables you to manage all of the permissions for your IAM identities in IAM. To use an IAM policy to allow access to a KMS key, the key policy must explicitly allow it. For more information about using IAM policies, see IAM policies.

  • Grants – You can use grants in combination with the key policy and IAM policies to allow access to a KMS key. Controlling access this way enables you to allow access to the KMS key in the key policy, and to allow identities to delegate their access to others. For more information about using grants, see Grants in AWS KMS.

KMS keys belong to the AWS account in which they were created. However, no identity or principal, including the AWS account root user, has permission to use or manage a KMS key unless that permission is explicitly provided in a key policy, IAM policy or grant. The IAM identity who creates a KMS key is not considered to be the key owner and they don't automatically have permission to use or manage the KMS key that they created. Like any other identity, the key creator needs to get permission through a key policy, IAM policy, or grant. However, identities who have the kms:CreateKey permission can set the initial key policy and give themselves permission to use or manage the key.

The following topics provide details about how you can use AWS Identity and Access Management (IAM) and AWS KMS permissions to help secure your resources by controlling who can access them.

Concepts in AWS KMS access control

Learn the concepts used in discussions of access control in AWS KMS.

Authentication

Authentication is the process of verifying your identity. To send a request to AWS KMS, you must sign into AWS using your AWS credentials.

Authorization

Authorization provides the permission to send requests to create, manage, or use AWS KMS resources. For example, you must be authorized to use a KMS key in a cryptographic operation.

To control access to your AWS KMS resources, use key policies, IAM policies, and grants. Every KMS key must have a key policy. If the key policy allows it, you can also use IAM policies and grants to give principals access to the KMS key. To refine your authorization, you can use condition keys that allow or deny access only when a request or resource meets the conditions you specify. You can also allow access to principals you trust in other AWS accounts.

Authenticating with identities

Authentication is how you sign in to AWS using your identity credentials. You must be authenticated (signed in to AWS) as the AWS account root user, as an IAM user, or by assuming an IAM role.

You can sign in to AWS as a federated identity by using credentials provided through an identity source. AWS IAM Identity Center (IAM Identity Center) users, your company's single sign-on authentication, and your Google or Facebook credentials are examples of federated identities. When you sign in as a federated identity, your administrator previously set up identity federation using IAM roles. When you access AWS by using federation, you are indirectly assuming a role.

Depending on the type of user you are, you can sign in to the AWS Management Console or the AWS access portal. For more information about signing in to AWS, see How to sign in to your AWS account in the AWS Sign-In User Guide.

If you access AWS programmatically, AWS provides a software development kit (SDK) and a command line interface (CLI) to cryptographically sign your requests by using your credentials. If you don't use AWS tools, you must sign requests yourself. For more information about using the recommended method to sign requests yourself, see Signing AWS API requests in the IAM User Guide.

Regardless of the authentication method that you use, you might be required to provide additional security information. For example, AWS recommends that you use multi-factor authentication (MFA) to increase the security of your account. To learn more, see Multi-factor authentication in the AWS IAM Identity Center User Guide and Using multi-factor authentication (MFA) in AWS in the IAM User Guide.

AWS account root user

When you create an AWS account, you begin with one sign-in identity that has complete access to all AWS services and resources in the account. This identity is called the AWS account root user and is accessed by signing in with the email address and password that you used to create the account. We strongly recommend that you don't use the root user for your everyday tasks. Safeguard your root user credentials and use them to perform the tasks that only the root user can perform. For the complete list of tasks that require you to sign in as the root user, see Tasks that require root user credentials in the IAM User Guide.

Federated identity

As a best practice, require human users, including users that require administrator access, to use federation with an identity provider to access AWS services by using temporary credentials.

A federated identity is a user from your enterprise user directory, a web identity provider, the AWS Directory Service, the Identity Center directory, or any user that accesses AWS services by using credentials provided through an identity source. When federated identities access AWS accounts, they assume roles, and the roles provide temporary credentials.

For centralized access management, we recommend that you use AWS IAM Identity Center. You can create users and groups in IAM Identity Center, or you can connect and synchronize to a set of users and groups in your own identity source for use across all your AWS accounts and applications. For information about IAM Identity Center, see What is IAM Identity Center? in the AWS IAM Identity Center User Guide.

IAM users and groups

An IAM user is an identity within your AWS account that has specific permissions for a single person or application. Where possible, we recommend relying on temporary credentials instead of creating IAM users who have long-term credentials such as passwords and access keys. However, if you have specific use cases that require long-term credentials with IAM users, we recommend that you rotate access keys. For more information, see Rotate access keys regularly for use cases that require long-term credentials in the IAM User Guide.

An IAM group is an identity that specifies a collection of IAM users. You can't sign in as a group. You can use groups to specify permissions for multiple users at a time. Groups make permissions easier to manage for large sets of users. For example, you could have a group named IAMAdmins and give that group permissions to administer IAM resources.

Users are different from roles. A user is uniquely associated with one person or application, but a role is intended to be assumable by anyone who needs it. Users have permanent long-term credentials, but roles provide temporary credentials. To learn more, see When to create an IAM user (instead of a role) in the IAM User Guide.

IAM roles

An IAM role is an identity within your AWS account that has specific permissions. It is similar to an IAM user, but is not associated with a specific person. You can temporarily assume an IAM role in the AWS Management Console by switching roles. You can assume a role by calling an AWS CLI or AWS API operation or by using a custom URL. For more information about methods for using roles, see Using IAM roles in the IAM User Guide.

IAM roles with temporary credentials are useful in the following situations:

  • Federated user access – To assign permissions to a federated identity, you create a role and define permissions for the role. When a federated identity authenticates, the identity is associated with the role and is granted the permissions that are defined by the role. For information about roles for federation, see Creating a role for a third-party Identity Provider in the IAM User Guide. If you use IAM Identity Center, you configure a permission set. To control what your identities can access after they authenticate, IAM Identity Center correlates the permission set to a role in IAM. For information about permissions sets, see Permission sets in the AWS IAM Identity Center User Guide.

  • Temporary IAM user permissions – An IAM user or role can assume an IAM role to temporarily take on different permissions for a specific task.

  • Cross-account access – You can use an IAM role to allow someone (a trusted principal) in a different account to access resources in your account. Roles are the primary way to grant cross-account access. However, with some AWS services, you can attach a policy directly to a resource (instead of using a role as a proxy). To learn the difference between roles and resource-based policies for cross-account access, see How IAM roles differ from resource-based policies in the IAM User Guide.

  • Cross-service access – Some AWS services use features in other AWS services. For example, when you make a call in a service, it's common for that service to run applications in Amazon EC2 or store objects in Amazon S3. A service might do this using the calling principal's permissions, using a service role, or using a service-linked role.

    • Forward access sessions (FAS) – When you use an IAM user or role to perform actions in AWS, you are considered a principal. When you use some services, you might perform an action that then initiates another action in a different service. FAS uses the permissions of the principal calling an AWS service, combined with the requesting AWS service to make requests to downstream services. FAS requests are only made when a service receives a request that requires interactions with other AWS services or resources to complete. In this case, you must have permissions to perform both actions. For policy details when making FAS requests, see Forward access sessions.

    • Service role – A service role is an IAM role that a service assumes to perform actions on your behalf. An IAM administrator can create, modify, and delete a service role from within IAM. For more information, see Creating a role to delegate permissions to an AWS service in the IAM User Guide.

    • Service-linked role – A service-linked role is a type of service role that is linked to an AWS service. The service can assume the role to perform an action on your behalf. Service-linked roles appear in your AWS account and are owned by the service. An IAM administrator can view, but not edit the permissions for service-linked roles.

  • Applications running on Amazon EC2 – You can use an IAM role to manage temporary credentials for applications that are running on an EC2 instance and making AWS CLI or AWS API requests. This is preferable to storing access keys within the EC2 instance. To assign an AWS role to an EC2 instance and make it available to all of its applications, you create an instance profile that is attached to the instance. An instance profile contains the role and enables programs that are running on the EC2 instance to get temporary credentials. For more information, see Using an IAM role to grant permissions to applications running on Amazon EC2 instances in the IAM User Guide.

To learn whether to use IAM roles or IAM users, see When to create an IAM role (instead of a user) in the IAM User Guide.

Managing access using policies

You control access in AWS by creating policies and attaching them to AWS identities or resources. A policy is an object in AWS that, when associated with an identity or resource, defines their permissions. AWS evaluates these policies when a principal (user, root user, or role session) makes a request. Permissions in the policies determine whether the request is allowed or denied. Most policies are stored in AWS as JSON documents. For more information about the structure and contents of JSON policy documents, see Overview of JSON policies in the IAM User Guide.

Administrators can use AWS JSON policies to specify who has access to what. That is, which principal can perform actions on what resources, and under what conditions.

By default, users and roles have no permissions. To grant users permission to perform actions on the resources that they need, an IAM administrator can create IAM policies. The administrator can then add the IAM policies to roles, and users can assume the roles.

IAM policies define permissions for an action regardless of the method that you use to perform the operation. For example, suppose that you have a policy that allows the iam:GetRole action. A user with that policy can get role information from the AWS Management Console, the AWS CLI, or the AWS API.

Identity-based policies

Identity-based policies are JSON permissions policy documents that you can attach to an identity, such as an IAM user, group of users, or role. These policies control what actions users and roles can perform, on which resources, and under what conditions. To learn how to create an identity-based policy, see Creating IAM policies in the IAM User Guide.

Identity-based policies can be further categorized as inline policies or managed policies. Inline policies are embedded directly into a single user, group, or role. Managed policies are standalone policies that you can attach to multiple users, groups, and roles in your AWS account. Managed policies include AWS managed policies and customer managed policies. To learn how to choose between a managed policy or an inline policy, see Choosing between managed policies and inline policies in the IAM User Guide.

Resource-based policies

An AWS KMS key policy is a resource-based policy that controls access to a KMS key. Every KMS key must have a key policy. You can use other authorization mechanism to allow access to the KMS key, but only if the key policy allows it. (You can use an IAM policy to deny access to a KMS key even if the key policy doesn't explicitly permit it.)

Resource-based policies are JSON policy documents that you attach to a resource, such as a KMS key, to control access to the specific resource. The resource-based policy defines the actions that a specified principal can perform on that resource and under what conditions. You don't specify the resource in a resource-based policy, but you must specify a principal, such as accounts, users, roles, federated users, or AWS services. Resource-based policies are inline policies that are located in that service that manages the resource. You can't use AWS managed policies from IAM, such as the AWSKeyManagementServicePowerUser managed policy, in a resource-based policy.

Access control lists (ACLs)

Access control lists (ACLs) control which principals (account members, users, or roles) have permissions to access a resource. ACLs are similar to resource-based policies, although they do not use the JSON policy document format.

Amazon S3, AWS WAF, and Amazon VPC are examples of services that support ACLs. To learn more about ACLs, see Access control list (ACL) overview in the Amazon Simple Storage Service Developer Guide.

AWS KMS does not support ACLs.

Other policy types

AWS supports additional, less-common policy types. These policy types can set the maximum permissions granted to you by the more common policy types.

  • Permissions boundaries – A permissions boundary is an advanced feature in which you set the maximum permissions that an identity-based policy can grant to an IAM entity (IAM user or role). You can set a permissions boundary for an entity. The resulting permissions are the intersection of an entity's identity-based policies and its permissions boundaries. Resource-based policies that specify the user or role in the Principal field are not limited by the permissions boundary. An explicit deny in any of these policies overrides the allow. For more information about permissions boundaries, see Permissions boundaries for IAM entities in the IAM User Guide.

  • Service control policies (SCPs) – SCPs are JSON policies that specify the maximum permissions for an organization or organizational unit (OU) in AWS Organizations. AWS Organizations is a service for grouping and centrally managing multiple AWS accounts that your business owns. If you enable all features in an organization, then you can apply service control policies (SCPs) to any or all of your accounts. The SCP limits permissions for entities in member accounts, including each AWS account root user. For more information about Organizations and SCPs, see How SCPs work in the AWS Organizations User Guide.

  • Session policies – Session policies are advanced policies that you pass as a parameter when you programmatically create a temporary session for a role or federated user. The resulting session's permissions are the intersection of the user or role's identity-based policies and the session policies. Permissions can also come from a resource-based policy. An explicit deny in any of these policies overrides the allow. For more information, see Session policies in the IAM User Guide.

Multiple policy types

When multiple types of policies apply to a request, the resulting permissions are more complicated to understand. To learn how AWS determines whether to allow a request when multiple policy types are involved, see Policy evaluation logic in the IAM User Guide.

AWS KMS resources

In AWS KMS, the primary resource is an AWS KMS key. AWS KMS also supports an alias, an independent resource that provides a friendly name for a KMS key. Some AWS KMS operations allow you to use an alias to identify a KMS key.

Each instance of a KMS key or alias has a unique Amazon Resource Name (ARN) with a standard format. In AWS KMS resources, the AWS service name is kms.

  • AWS KMS key

    ARN format:

    arn:AWS partition name:AWS service name:AWS Region:AWS account ID:key/key ID

    Example ARN:

    arn:aws:kms:us-west-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab

  • Alias

    ARN format:

    arn:AWS partition name:AWS service name:AWS Region:AWS account ID:alias/alias name

    Example ARN:

    arn:aws:kms:us-west-2:111122223333:alias/example-alias

AWS KMS provides a set of API operations to work with your AWS KMS resources. For more information about identifying KMS keys in the AWS Management Console and AWS KMS API operations, see Key identifiers (KeyId). For a list of AWS KMS operations, see the AWS Key Management Service API Reference.