Broadband access networks—including those built with advanced HFC, wireless, and fiber technologies—have unique network security concerns. This paper analyzes the security threats present in such networks, and it develops a general security threat model for broadband access networks. Significant threats include masquerade and eavesdropping. The paper then examines cryptographic techniques appropriate for countering these threats, focusing on authentication and encryption. It considers the strengths and costs of various alternatives. The final section discusses practical implementation issues, particularly those that arise because of the potential size of access networks and because of the data rates at which they operate. These characteristics demand security measures that are very scalable and capable of very high speed operation.