Identity theft is probably as old as the existence of the socio-legal concept of identity. Have you ever heard of Mesopotamian cylinder seals? These were small cylindrical (obviously) carvings in clay, stone, or metal that would've been owned by every individual with need of legal proof of identity. The carving on each one was unique, and could vary from simple geometric patterns to complex religious or pastoral scenery, or even contain information about the owner's name, profession, and residence. Because of the individual nature of each seal, they could be used as a semi-secure proof of identify. The cylinder would be rolled across a wet piece of clay (the common writing material of the time) to make an inverted impression of its surface. They were employed as signatures on legal contracts and bills of sale, indicators of witnesses for important events, seals for letters and shipping/storage containers, marks of ownership, and a hundred other things. There was even an ingenious use as "locks", whereby you might fasten shut the door of your storage room with a metal hook and clasp, which would then be covered in clay and stamped with your seal. It wouldn't prevent someone from breaking the lock and stealing your stuff, but it did provide incontrovertible proof of tampering if the seal was disturbed.

Anyways, due to the complex and often litigious nature of Mesopotamian legal life (not to mention the pervasiveness of bureaucracy), a seal was a powerful personal identifier. Therefore the theft of a seal was a serious and problematic affair. Legal codes required that any such theft or loss be reported to the palace bureaucrats immediately, and there was even a special office in the Middle Babylonian period for investigating stolen or forged seals! Obviously you could wreck small-scale havoc with someone else's seal, much like the theft of a social security number or the like today - you might gain fraudulent access to court records, take out loans against someone else's name, dispute their ownership of property, or goodness knows what else. It definitely sounds like identity theft to me!

One tangential fact that I enjoy: some very poor and materially-deprived people couldn't afford a seal, and in such cases if it was required for them to make a mark upon legal documents, a fingernail might be used! Not nearly as secure, which speaks to the long-standing lack of regard for the security and legal standing of the lower classes.