The global telephone network is relied upon daily by billions worldwide for reliable communications. Beyond their use for communications, telephones are being used as a solution to identify users on Internet because almost every person globally has at least one phone number. Unfortunately, telephones are also plagued with fraud and abuse, making this use and many others insecure. This abuse is ultimately caused by the fact that the phone network o.ers no strong guarantees of identity, and addressing this problem is complicated by the fact that the network is composed of many di.erent and largely incompatible technologies.

In this study, we examine the poor state of authentication in telephone networks and provide new mechanisms to authenticate callers to each other. We begin by examining how the telephone network---specifically, text messaging---is being used to bolster claims of identity and authentication in Internet systems, finding that public gateways negate many of the supposed advantages of these techniques. We then turn our attention to interconnect bypass fraud, showing that while telephone networks cannot e.ectively determine the true origin of a phone call, we can provide mechanisms based on in-call audio measurements to detect so-called "simboxing fraud." Finally, we develop two new systems, Authloop and Authenticall, to address call authentication. Both systems provide strong cryptographic authentication of callers. Authloop transmits this information through call audio, while Authenticall uses an auxiliary data channel to authenticate both call end points and call content. In total, this thesis provides mechanisms to prevent robocalling, phone phishing, interconnect bypass fraud, preventing billions of dollars in fraud and restoring trust and confidence in the phone network.