Android forensics: Interpretation of timestamps

Abstract

Interpretation of traces found on Android devices is an important aspect of mobile forensics. This is especially true for timestamps encountered on the device under investigation. In the presence of both naive and UTC timestamps, some form of timestamp normalisation is required. In addition, the investigator needs to gain some understanding of potential clock skew that may exist, especially when evidence from the device under investigation has to be correlated to real world events or evidence from other devices. A case study is presented where the time zone on the Android device was set incorrectly, while the clock was set to correspond to the time zone where the device was actually located. Initially, the fact that both time zones enforced daylight saving time (DST) at different periods was expected to complicate the timestamps normalisation. However, it was found that the version of the Time Zone Database on the device was outdated and did not correspond to the actual time zone rules for the given period. After the case study, the results of experiments on a broader range of devices are presented. Among other things, these results demonstrate a method to detect clock skew based on the mmssms.db database. However, it was also found that the applicability of this method is highly dependent on specific implementation choices made by different vendors.