I. Introduction
An Online Identity (OID) refers to an account that Internet users establish on web platforms, such as social networks, online communities, email systems, and online stores. Most web platforms allow individuals to register new OIDs easily without a thorough verification of personal information. This has created opportunities to engage in malicious activities. Abuses of OIDs, which can involve significant losses, have increased dramatically over the last decade [1]. Experienced criminals can create an unlimited number of OIDs on different web platforms to serve their disparate needs [2]. Therefore, OID Linkage (OL), i.e., recognizing all OIDs used by a criminal (hereafter referred to as “multiple OIDs”) among a number of suspect OIDs, has become a crucial task for law enforcement agencies (LEA) when investigating OID-involved criminal cases.