chore(secrets-management): expand a little

knowledge base/cloud computing/aws/kms.md

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+# AWS Key Management Service
+
+AWS' native encryption keys management service.
+
+1. [TL;DR](#tldr)
+1. [Aliases](#aliases)
+1. [Further readings](#further-readings)
+   1. [Sources](#sources)
+
+## TL;DR
+
+Creates and controls encryption keys one can use to encrypt data.<br/>
+Keys created with KMS are protected by FIPS 140-3 Security Level 3 validated HSMs.<br/>
+They are created, managed, used, and deleted entirely **within** the managed service. They **never** leave KMS
+unencrypted. To use or manage keys in KMS, one **must** interact with the service.
+
+The service costs $0.03 to $12 per 10,000 API calls, depending on the action and type of key used.<br/>
+Refer [Pricing].
+
+Key policies are the **primary** way to control access to KMS keys.<br/>
+Every KMS key must have **exactly one** key policy.<br/>
+Statements in such policies determine **who** has permission to use KMS keys, and **how** they can use it. One _can_
+configure **additional** [IAM] policies and grants for keys.<br/>
+Key policies are Regional.
+
+> [!important]
+> IAM policies manage access to a KMS key **only** if the key policy **explicitly** allows it.<br/>
+> Without permission from the key policy, IAM policies have no effect.<br/>
+> The default key policy enables IAM policies.
+
+**No** AWS principal, **including** the account root user and the key creator, has **any** permissions to a key until
+a key policy, IAM policy, or grant **explicitly** allows, and never denies, access to it.
+
+Keys created by customers are referred to as _customer managed keys_.<br/>
+They are recommended when wanting **full control** over the lifecycle and usage of the keys.<br/>
+Customer managed keys incur in both management and usage costs.
+
+_AWS managed keys_ are keys that exists in an account, but can only be used in the context of an AWS service and only
+in the same account. One **cannot** share resources encrypted under an AWS managed key with other accounts.<br/>
+They do **not** allow managing anything about their lifecycle or permissions.<br/>
+AWS managed keys do not have management costs, but incur in usage costs.<br/>
+These keys use an alias in the form `aws/<service code>`, e.g. `aws/ebs`.
+
+AWS managed keys are a legacy key type, and are no longer being created for new AWS services as of 2021. Instead,
+services are now using _AWS owned keys_ to encrypt customer data by default.<br/>
+AWS owned keys are stored in an AWS account managed by the related AWS service. Only the service's operators can manage
+the keys' lifecycle and usage permissions.<br/>
+By using AWS owned keys, AWS services can transparently encrypt data and allow for cross-account or cross-region sharing
+of data.<br/>
+Customers are **not** charged for the keys' existence **nor** their usage, but they cannot change their policies, audit
+activities on these keys, nor delete them.
+
+KMS can provide encryption keys for protecting data in other AWS services (e.g., [EBS], [RDS], [S3]).
+AWS services that integrates with KMS only use _symmetric_ encryption keys to encrypt data.<br/>
+These services do **not** support encryption with _asymmetric_ keys.
+
+Asymmetric keys are related public key and private key pairs.<br/>
+The **private** key is created in KMS and never leaves the service unencrypted. To use the private key, one **must**
+interact with KMS.<br/>
+One can use the **public** key by calling the AWS APIs, or download it and use it outside of KMS.
+
+Use a **symmetric** encryption KMS key to encrypt the data one stores or manages in an AWS service.
+
+## Aliases
+
+Refer [Aliases in AWS KMS].
+
+Each key is represented by its key ID, but can have one or more aliases associated.<br/>
+Aliases allow using a human-friendly name to identify the key they are associated to in _some_ AWS operations.<br/>
+They are **not** a property of a key, and actions on the alias do **not** affect the associated key. However, all
+aliases associated with a key are deleted when that key is deleted.
+
+> [!important]
+> Specifying an alias as resource in an IAM policy will make the policy refer **to the alias**, not to the key it is
+> associated with.
+
+## Further readings
+
+- [Secrets management]
+
+### Sources
+
+- [AWS Key Management Service]
+
+<!--
+  Reference
+  ═╬═Time══
+  -->
+
+<!-- In-article sections -->
+<!-- Knowledge base -->
+[EBS]: ebs.md
+[IAM]: iam.md
+[RDS]: rds.md
+[S3]: s3.md
+[Secrets management]: ../../secrets%20management.md
+
+<!-- Upstream -->
+[AWS Key Management Service]: https://docs.aws.amazon.com/kms/latest/developerguide/overview.html
+[Pricing]: https://aws.amazon.com/kms/pricing/
+[Aliases in AWS KMS]: https://docs.aws.amazon.com/kms/latest/developerguide/kms-alias.html
+
+<!-- Others -->