Leakage would have allowed malware to be written into the software.
Nokia was forced to pay millions of dollars to criminals who threatened to release a core part of the code of Symbian operating system in 2008.
The phone company contacted the police after the threat and delivered the cash to blackmailers at a parking lot in Tampere, central Finland. Police attempted to follow the criminals but lost track of them, reported Finnish TV station MTV.
The case is still open with the police, Reuters reported. Detective Chief Inspector Tero Haapala told the news agency, "We are investigating felony blackmail, with Nokia the injured party."
Leaking of the encryption key of Symbian software would have allowed outsiders to write additional code, including malware which would have been undetectable.
Nokia has subsequently moved to Windows OS for its phones, and was acquired by Microsoft this year for $7.6bn.