Many articles in the Ukrainian press today report that Iran is ignoring the requests by the Ukrainian authorities. In particular Polina Chizh (Полина Чиж), representative of Ukraine's Attorney general's Office, stated that they sent 2 official requests to Iran (on January 8th and on January 15th), asking to have access to the flight recorders, and to let their (i.e. Ukrainian) experts decrypt them, but that they "didn't receive any answer". The Office also offered to decrypt the flight recorders in Ukraine, and unofficially agreed to have an Iranian expert inspect the Ukranian laboratory yesterday (January 15th) to get final approval, but the much-awaited Iranian expert never arrived (Polina Chizh: "Now they say, that their expert, as it was promised, today - January 15th, is not coming").
Interesting detail: in the second official request to the Iranian authorities the Office asks also to 'gain access' to the passangers' seats (I remember someone on Pprune noticing that they weren't visible in the crash site pictures).