Problems of Forensic Sciences 2016 Vol. 107 (CVII) 515-536
METHODS OF INVESTIGATING THE AUTHENTICITY OF RECORDINGS OF TELEPHONE CONVERSATIONS
Rafał KORYCKI
Forensic Laboratory, Internal Security Agency, Warsaw, Poland
Streszczenie
This article presents an authenticity investigation of GSM phone call recordings. The presented approach consists of an investigation of file structure, as well as a global analysis, which is conducted on the file as a whole, and local analysis, which gives an indication of authenticity related to time. It also implements recommendations described in recent papers and an SWGDE document concerning best practices for digital audio authentication. The lack of or incomplete audio background in GSM phone call recordings caused by voice oriented compression algorithms usually results in abrupt changes in the audio background. Therefore, there is a possibility of alterations that defy aural detection or time-frequency analysis. Due to the lack of approved methods dedicated to GSM phone call recordings, this article addresses some relevant techniques which may be used in authenticity examinations, e.g. scrutiny of wrongly stored WAVE header, decoding the time of recording from the file footer or system date analysis. Moreover, the use of statistical modelling of DC offset as well as noise spectral analysis – in verifying or excluding an investigated recorder given the same recording chain and/or environment – is described. The research shows that despite a lack of relevant information about the signal processing chain in GSM systems, one may still draw conclusions based on several essential findings.
Słowa kluczowe
GSM recordings; Authenticity; File format investigation; ENF; DC offset; System date.