> For the complete documentation index, see [llms.txt](https://awsnotes.dendron.so/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://awsnotes.dendron.so/cryptography-and-pki/aws-key-management-service-aws-kms/topics/special-purpose-keys.md).

# Special-purpose keys

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This page was generated from content adapted from the [AWS Developer Guide](https://github.com/awsdocs/aws-kms-developer-guide.git)
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## 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deters an attack where the actor uses a signed message to establish validity repeatedly or long after the message is superseded.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.