Erroring on a symmetric-key type would actually be an extra line of
code.
In theory we could try to save that one line of code, but it is
premature optimisation at this point. Also, this is a predominantly
asymmetric crypto feature, it is less confusing/more user friendly if we
don't allow symmetric keys here.
Signed-off-by: Janos Follath <janos.follath@arm.com>
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