Authentication and Integrity

Cisco CallManager allows authentication of calls. When you are configuring devices for authenticated calls, two services are provided:

  • Device authentication for the IP phone and the server Achieved by using device certificates and digital signatures
  • Authentication and integrity of signaling messages Achieved by using TLS Secure Hash Algorithm 1 (SHA-1) Hash-based Message Authentication Code (HMAC) (with symmetric keys)

Certificate Exchange in TLS

At the beginning of a TLS session, the Cisco CallManager server and the IP phone exchange certificates using the messages shown in Figure 26-11.

Figure 26-11. Certificate Exchange Process

The certificate exchange process occurs as follows:

  1. The IP phone and the Cisco CallManager server negotiate the cryptographic algorithms in the IP phone Hello and Cisco CallManager Hello messages.
  2. The server sends its (self-signed) certificate to the IP phone.
  3. The server requests a certificate from the IP phone.
  4. The IP phone sends its certificate to the server.

At this point, both the IP phone and the server validate the certificates they just received over the network:

  • The IP phone simply looks up the certificate of the server in its local certificate store. The received certificate must be found locally because it must have been sent in the CTL. If it is not included in the CTL, the session is dropped. If it is found, the public key of the server is extracted from the certificate.
  • The server looks up the IP phone in the local device database to see if this IP phone is known and authorized to connect via TLS. Then, the certificate of the IP phone is validated using the locally available CAPF public key (from the CAPF certificate), and if valid, the public key of the IP phone is extracted from the IP phone certificate.

Server-to-Phone Authentication

The next stage of the TLS handshake is authentication of the server by the IP phone. A simplified version of the authentication steps is shown in Figure 26-12.

Figure 26-12. Server-to-Phone Authentication

The CallManager-to-Phone authentication occurs as follows:

  1. The IP phone generates a random challenge string and sends it to the server, requesting that the server sign it with the private RSA key of the server.
  2. The server signs the message with its private RSA key and returns the result (response) to the IP phone.
  3. The IP phone verifies the signature using the public key of the server.

Phone-to-Server Authentication

After the server has authenticated to the IP phone, the IP phone needs to authenticate to the server. A simplified version of the authentication steps is shown in Figure 26-13.

Figure 26-13. Phone-to-Server Authentication

The Phone-to-CallManager authentication occurs as follows:

  1. The server generates a random challenge string and sends it to the IP phone, requesting that the IP phone sign it with the private RSA key of the IP phone.
  2. The IP phone signs the message with its private RSA key and returns the result (response) to the server.
  3. The server verifies the signature with the public key of the IP phone.

Note

In the certificate of the IP phone, the public key of the IP phone is tied to the identity of the IP phone. Because Cisco CallManager identifies an IP phone by MAC address and not by IP address or name, the MAC address of the phone is used as the identifier in the certificate of the IP phone.

 

TLS SHA-1 Session Key Exchange

After the bidirectional authentication, a SHA-1 session key is exchanged using these steps:

1.

The IP phone generates a session key for SHA-1 hashing.
 

2.

The IP phone encrypts it using the public RSA key of the server and sends it to the server.
 

3.

The server decrypts the message and thus also knows which key to use for SHA-1 hashing of the TLS packets.
 

The IP phone and the server can now exchange signaling messages over authenticated TLS packets, ensuring the integrity and authenticity of each signaling message exchanged between the two.

Encryption

Part I: Cisco CallManager Fundamentals

Introduction to Cisco Unified Communications and Cisco Unified CallManager

Cisco Unified CallManager Clustering and Deployment Options

Cisco Unified CallManager Installation and Upgrades

Part II: IPT Devices and Users

Cisco IP Phones and Other User Devices

Configuring Cisco Unified CallManager to Support IP Phones

Cisco IP Telephony Users

Cisco Bulk Administration Tool

Part III: IPT Network Integration and Route Plan

Cisco Catalyst Switches

Configuring Cisco Gateways and Trunks

Cisco Unified CallManager Route Plan Basics

Cisco Unified CallManager Advanced Route Plans

Configuring Hunt Groups and Call Coverage

Implementing Telephony Call Restrictions and Control

Implementing Multiple-Site Deployments

Part IV: VoIP Features

Media Resources

Configuring User Features, Part 1

Configuring User Features, Part 2

Configuring Cisco Unified CallManager Attendant Console

Configuring Cisco IP Manager Assistant

Part V: IPT Security

Securing the Windows Operating System

Securing Cisco Unified CallManager Administration

Preventing Toll Fraud

Hardening the IP Phone

Understanding Cryptographic Fundamentals

Understanding the Public Key Infrastructure

Understanding Cisco IP Telephony Authentication and Encryption Fundamentals

Configuring Cisco IP Telephony Authentication and Encryption

Part VI: IP Video

Introducing IP Video Telephony

Configuring Cisco VT Advantage

Part VII: IPT Management

Introducing Database Tools and Cisco Unified CallManager Serviceability

Monitoring Performance

Configuring Alarms and Traces

Configuring CAR

Using Additional Management and Monitoring Tools

Part VIII: Appendix

Appendix A. Answers to Review Questions

Index

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Authorized Self-Study Guide Cisco IP Telephony (CIPT)
Cisco IP Telephony (CIPT) (Authorized Self-Study) (2nd Edition)
ISBN: 158705261X
EAN: 2147483647
Year: 2004
Pages: 329
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