The blackmailer had gotten hold of the Symbian encryption key used for signing. The code is a few kilobytes in size. Had the key been leaked Nokia would not have been able to ensure that the phones accept only applications approved by the company.
jppiiroinen writes: I find it very odd that back in the days 2007-2008 when Nokia had a huge market share with Symbian devices, that they did not disclose the information that somebody had stolen their encryption keys. Being a listed company after all. They did even ended up paying millions of euros and the local Finnish police manage to fail to investigate who was behind it.