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 (2664019) 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.