Whenever possible, use key policies to give users permission to call the
Decrypt operation on the CMK, instead of IAM policies. Otherwise, you
might create an IAM user policy that gives the user Decrypt permission
on all CMKs. This user could decrypt ciphertext that was encrypted by
CMKs in other accounts if the key policy for the cross-account CMK
permits it. If you must use an IAM policy for Decrypt
permissions,
limit the user to particular CMKs or particular trusted accounts.
The result of this operation varies with the key state of the CMK. For
details, see How Key State Affects Use of a Customer Master Key
in the AWS Key Management Service Developer Guide.