If the UserName is not specified, the IAM user name is determined
implicitly based on the AWS access key ID used to sign the request. This
operation works for access keys under the AWS account. Consequently, you
can use this operation to manage AWS account root user credentials even
if the AWS account has no associated users.
Because the body of an X.509 certificate can be large, you should use
POST rather than GET when calling UploadSigningCertificate. For
information about setting up signatures and authorization through the
API, go to Signing AWS API Requests
in the AWS General Reference. For general information about using the
Query API with IAM, go to Making Query Requests
in the IAM User Guide.