# Special-purpose keys {% hint style="info" %} This page was generated from content adapted from the [AWS Developer Guide](https://github.com/awsdocs/aws-kms-developer-guide.git) {% endhint %} ## HMAC keys * **Tip**\ Best practices recommend that you limit the time during which any signing mechanism, including an HMAC, is effective. This deters an attack where the actor uses a signed message to establish validity repeatedly or long after the message is superseded. HMAC tags do not include a timestamp, but you can include a timestamp in the token or message to help you detect when its time to refresh the HMAC. ## Imported key material * **Note**\ AWS KMS does not support decrypting any AWS KMS ciphertext outside of AWS KMS, even if the ciphertext was encrypted under a KMS key with imported key material. AWS KMS does not publish the ciphertext format this task requires, and the format might change without notice. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://awsnotes.dendron.so/cryptography-and-pki/aws-key-management-service-aws-kms/topics/special-purpose-keys.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.