An IAM user receives an Access Denied message when the user attempts to access objects...

WithSSE-KMS, authorization is a two-part check: the caller must have S3 permissions to read the objectandthe caller must be allowed to use the KMS key for decryption. Even if an IAM policy grants kms:Decrypt, the request will still fail if theKMS key policydoes not allow the principal (or does not allow the account to delegate use of the key). KMS key policies are authoritative: they can prevent key usage even when IAM policies appear to allow it.

A common misconfiguration is editing the key policy and removing the statement that grants the AWS account (or key administrators) the ability to manage and delegate permissions for the key—often described as removing “Enable IAM user permissions” or otherwise blocking the account from using IAM policies to authorize key usage. In that case, the IAM user’s kms:Decrypt permission in IAM is not sufficient because the key policy no longer permits it, resulting in Access Denied when S3 attempts to call KMS on the user’s behalf during GetObject.

Option A is not required for decrypting data (DescribeKey is useful for discovery but not necessary for GetObject). Option B would not inherently cause access denied if permissions align. Option C is incorrect because same-account S3 access can be granted purely via IAM without a bucket policy. Therefore, the key policy change is a valid reason.