def lookup(pubkey)
      ptr = ffi_delegate.lookup(pubkey)
      return nil if ptr.null?
      Certificate.from_ffi_delegate(ptr)
    end
def lookup(pubkey)
      ptr = ffi_delegate.lookup(pubkey)
      return nil if ptr.null?
      Certificate.from_ffi_delegate(ptr)
    end
Initializes a new certificate store.

 @param location [String, #to_s, nil] location the path to the
   directories to load certificates from, or nil if no certificates need
   to be loaded from the disk
 Looks up a certificate in the store by its public key.

 @param pubkey [String] the public key in question, in Z85 format
 @return [Certificate] the matching certificate, if found
 @return [nil] if no matching certificate was found