SIP TLS and SRTP-RTP internetworking on CUBE using IOS CA

Introduction

This document describes the basics of Session Initiation Protocol (SIP) Transport Layer Security (TLS) and Secure Real-time Transport Protocol (SRTP) over Cisco Unified Border Element (CUBE) with a configuration example.

Secure voice communication over CUBE can be divided into two parts:

• Secure signaling – CUBE uses TLS to secure signaling over SIP and Internet Protocol Security (IPSec) in order to secure signaling over H.323
• Secure Media – SRTP

Prerequisites

Requirements

Cisco recommends that you have knowledge of these topics:

• Cisco Unified Communications Manager (CUCM) Certificate Trust List (CTL) files are created for Mixed-mode
• IP Phones are registered in Secure mode (Encryption)
• CUBE basic voice service voip and dial-peer configuration is done

Components Used

The information in this document is based on these software and hardware versions:

• CUCM 10.5
• CUBE – 3925E with IOS 15.3(3)M3
• Cisco IP Communicator (CIPC)

Background Information

• TLS - TLS and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide communication security over the Internet.

        In Open Systems Interconnection (OSI) model equivalences, TLS/SSL is initialized at layer 5 (the session layer) and then works at layer 6 (the presentation  layer). In both the models, TLS and SSL work on behalf of the underlying transport layer, whose segments carry encrypted data.

• Certificate Authority (CA) - Trusted entity that issues certificates: Cisco or a third-party entity.

• Device Authentication - Process that validates the identity of the device and ensures that the entity is what it claims to be before a connection is made.

• Encryption - Process of translating data into ciphertext that ensures the confidentiality of the information. Only the intended recipient can read the data. It requires an encryption algorithm and encryption key.

• Public/Private Keys - Keys that are used in encryption. Public keys are widely available, but private keys are held by their respective owners. Asymmetrical encryption combines both types.

Configure

Network Diagram

In this image, the configuration example for setting up SIP TLS and SRTP between CUCM/IP phone and CUBE is shown. CUBE internetworks between SRTP and Real-time Transport Protocol (RTP). CUBE acts as IOS CA and CUCM would use self-signed certificates.

CUBE Configuration

1. Configure clock and enable HTTP server

Synchronize the clocks in the CA server and the client trustpoints (CUBE/OGW/TGW). Otherwise, there are issues with the validity of the certificates issued by the CA server.

Secure-CUBE#clock set <hh:mm:ss> < Day of the month> <MONTH> <Year>
Or
Ntp server <IP Address>

Client trustpoints use HTTP to receive certificate from CA.

Secure-CUBE(config)#ip http server

2. Generate an RSA Keypair

This step generates Private and Public keys.

In this example, CUBE is just a label. It can be anything.

Secure-CUBE(config)#crypto key generate rsa general-keys label CUBE modulus 1024
The name for the keys will be: CUBE
% The key modulus size is 1024 bits
% Generating 1024 bit RSA keys, keys will be non-exportable...
[OK] (elapsed time was 0 seconds)
Secure-CUBE(config)#

3. Configure IOS CA Server

In this example, CA Server is named cube-ca.

crypto pki server cube-ca
 database level complete
 no database archive
 grant auto
 lifetime certificate 1800

Secure-CUBE(cs-server)#no shut
%Some server settings cannot be changed after CA certificate generation.
% Please enter a passphrase to protect the private key
% or type Return to exit

Password:

Re-enter password:

% Generating 1024 bit RSA keys, keys will be non-exportable...
[OK] (elapsed time was 0 seconds)

% Certificate Server enabled.

Secure-CUBE(cs-server)#

4. Create PKI trustpoints for cube for TLS communication.

In this example, trustpoint name for CUBE is CUBE-TLS. IP address used in enrollment url must be local interface on CUBE. Subject name used in this step must match on X.509 Subject Name on CUCM SIP Trunk security profile. The best practice is to use host-name with domain name (if domain name is enabled).

Associate RSA key pair created in Step 2.

crypto pki trustpoint CUBE-TLS
 enrollment url http://X.X.X.X:80    
 serial-number none
 fqdn none
 ip-address none
 subject-name CN=Secure-CUBE             
 revocation-check none
 rsakeypair CUBE

 5. Authenticate the trustpoint with CA server and accept certificate of CA.

Secure-CUBE(config)#crypto pki authenticate CUBE-TLS
Certificate has the following attributes:
       Fingerprint MD5: BCEBB5A1 1AC882F7 24BE476D 06537711
     Fingerprint SHA1: CE2FEEA5 42515B33 3EF6A8F6 7E31D6DF 8E32BEB6

% Do you accept this certificate? [yes/no]: yes
Trustpoint CA certificate accepted.
Secure-CUBE(config)#

6. Enroll the trustpoint with CA server.

In this step the CUBE receives a signed certificate from CA.

Secure-CUBE(config)#crypto pki enroll CUBE-TLS
%
% Start certificate enrollment ..
% Create a challenge password. You will need to verbally provide this
   password to the CA Administrator in order to revoke your certificate.
   For security reasons your password will not be saved in the configuration.
   Please make a note of it.

Password:
Re-enter password:


% The subject name in the certificate will include: CN=Secure-CUBE
% The fully-qualified domain name will not be included in the certificate
Request certificate from CA? [yes/no]: yes
% Certificate request sent to Certificate Authority
% The 'show crypto pki certificate verbose CUBE-TLS' commandwill show the fingerprint.

Secure-CUBE(config)#

7. Create the trustpoint for the CUCM.

If CallManager group has multiple CM servers, then trustpoint needs to be created for all servers, otherwise failover not works.

crypto pki trustpoint cucmpub
 enrollment terminal
 revocation-check none

crypto pki trustpoint cucmsub
 enrollment terminal
 revocation-check none

8. Enroll the CUCM certificate to CUBE.

Step 1. Log in to CUCM OS admin.

Step 2. Navigate to Security > Certificate Management > Find.

 Step 3. Click the CallManager certificate, then download and save .PEM file as shown in this image.

Step 4. Open the file in the notepad and copy the content from BEGIN CERTIFICATE to END CERTIFICATE.

Step 5. Paste this certificate in CUBE as shown.

Secure-CUBE(config)#crypto pki authenticate cucmpub
 
Enter the base 64 encoded CA certificate.
End with a blank line or the word "quit" on a line by itself
 
-----BEGIN CERTIFICATE-----
MIICojCCAgugAwIBAgIQaqCuzslHvcr8xyIxDugyJDANBgkqhkiG9w0BAQUFADBj
MQswCQYDVQQGEwJJTjEOMAwGA1UEChMFY2lzY28xDDAKBgNVBAsTA3RhYzEOMAwG
A1UEAxMFY21wdWIxEjAQBgNVBAgTCWthcm5hdGFrYTESMBAGA1UEBxMJYmFuZ2Fs
b3JlMB4XDTE1MDIwNzE3MDkyMloXDTIwMDIwNjE3MDkyMVowYzELMAkGA1UEBhMC
SU4xDjAMBgNVBAoTBWNpc2NvMQwwCgYDVQQLEwN0YWMxDjAMBgNVBAMTBWNtcHVi
MRIwEAYDVQQIEwlrYXJuYXRha2ExEjAQBgNVBAcTCWJhbmdhbG9yZTCBnzANBgkq
hkiG9w0BAQEFAAOBjQAwgYkCgYEA0hkaJtUpBK4Uw7brGidgfVyk2FJRA32xkUHn
aQbSz89dyjCX//Vpt8GblwXediTKRBYX1J4I7iGl1cuPNYOh9giSeLlxgzthMt1M
d+XoGGby9DhrwWJSZY5b8MN8uETfilOn3ANN/yJf5xJ7D7qIq5ZhfQHDAm8QTuoS
SSqFciUCAwEAAaNXMFUwCwYDVR0PBAQDAgK8MCcGA1UdJQQgMB4GCCsGAQUFBwMB
BggrBgEFBQcDAgYIKwYBBQUHAwUwHQYDVR0OBBYEFDGq0WCT/OnqwePSnhaknzR0
BconMA0GCSqGSIb3DQEBBQUAA4GBACb9gC0u/piCQrv7BeLk2/qFmZl/zVuXPDOn
wqz4yBMsa7Nk6QmpP5zXKJJfXb3iKJPsMRWuUNEe+Df+sx0rUit3oGcF4ce/1ZfV
RKvt461TvA5r9HGXO+KaI8v7BaWeeROBfTboRpkvqRjFt6eIHEtn7+uUIcumDASp
SkXO8/Ar
-----END CERTIFICATE-----
 
Certificate has the following attributes:
 Fingerprint MD5: 92DA2B5B A888784D C53B6C29 2E2B6A3C
 Fingerprint SHA1: 5D31BEF0 DF2DCA7E 64D40246 89E564DD 9A7F8A01
 
% Do you accept this certificate? [yes/no]: yes
Trustpoint CA certificate accepted.
% Certificate successfully imported
 
Secure-CUBE(config)#

Step 6. Follow same procedure for the other CUCM servers.

9. Configure TCP TLS as transport protocol.

This can be done either at a global or at a dial-peer level.

voice service voip
sip
session transport tcp tls

10. Assign trustpoint for sip-ua, this trustpoint is used for all SIP signaling between CUBE and CUCM

sip-ua
crypto signaling remote-addr <cucm pub ip address> 255.255.255.255 trustpoint CUBE-TLS
crypto signaling remote-addr <cucm sub ip address> 255.255.255.255 trustpoint CUBE-TLS

or default trustpoint can be configured for all SIP signaling from CUBE.

sip-ua
crypto signaling default trustpoint CUBE-TLS

11. Enable SRTP.

Voice service voip
srtp fallback

12. For SRTP and RTP internetworking, secure transcoder is required.

If IOS version is 15.2.2T (CUBE 9.0) or later then, LTI transcoder can be configure to minimize the configuration.

LTI transcoder doesn't need PKI trustpoint configuration for SRTP-RTP calls

dspfarm profile 1 transcode universal security
codec g711ulaw
codec g711alaw
codec g729ar8
codec g729abr8
maximum sessions 10
associate application CUBE

If IOS is below 15.2.2T, then configure sccp transcoder.

Skinny Call Control Protocol (SCCP) transcoder would need trustpoint for signaling however if same router is used to host the transcoder then same trustpoint(CUBE-TLS) can be used for CUBE as well as transcoder.

sccp local GigabitEthernet0/0
sccp ccm 10.106.95.155 identifier 1 priority 1 version 7.0
sccp
!
sccp ccm group 1
bind interface GigabitEthernet0/0
associate ccm 1 priority 1
associate profile 2 register secxcode
!
dspfarm profile 2 transcode universal security
trustpoint CUBE-TLS
codec g711ulaw
codec g711alaw
codec g729ar8
codec g729abr8
maximum sessions 10
associate application SCCP
!
telephony-service
secure-signaling trustpoint CUBE-TLS
sdspfarm units 1
sdspfarm transcode sessions 10
sdspfarm tag 1 secxcode
max-ephones 1
max-dn 1
ip source-address 10.106.95.155 port 2000
max-conferences 8 gain -6
transfer-system full-consult

CUCM Configuration

1. Export CUBE IOS certificate to CUCM.

Step 1. Export IOS certificate. Copy self-signed CA certificate and save as .PEM file for example,  Secure-CUBE.pem

 Secure-CUBE(config)#crypto pki export CUBE-TLS pem terminal
% CA certificate:
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
 
% General Purpose Certificate:
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
 
Secure-CUBE(config)#

Step 2. Upload IOS CA certificate on CUCM as CallManager-trust.

Step 3. Navigate to CM OS Administration > Security > Certificate Management > Upload Certificate/Certificate Chain

Step 4. Upload .PEM file as shown in this image.

2. Create New SIP Trunk Security Profile

Step 1. On CM Administration navigate to System > Security > SIP Trunk Security Profiles > File.

Step 2. Copy the existing Non Secure SIP Trunk Profile in order to create new secure profile as shown in this image.

3. Create SIP trunk to the CUBE

Step 1. Enable SRTP on SIP trunk as shown in this image.

Step 2. Configure Destination Port 5061 (TLS) and apply New Secure SIP trunk Security profile on the SIP trunk as shown in this image.

Verify

Secure-CUBE#show sip-ua connections tcp tls detail
Total active connections : 2
No. of send failures : 0
No. of remote closures : 13
No. of conn. failures : 0
No. of inactive conn. ageouts : 0
TLS client handshake failures : 0
TLS server handshake failures : 0
 
---------Printing Detailed Connection Report---------
Note:
** Tuples with no matching socket entry
 - Do 'clear sip <tcp[tls]/udp> conn t ipv4:<addr>:<port>'
 to overcome this error condition
++ Tuples with mismatched address/port entry
 - Do 'clear sip <tcp[tls]/udp> conn t ipv4:<addr>:<port> id <connid>'
 to overcome this error condition
 
Remote-Agent:10.106.95.151, Connections-Count:2
Remote-Port Conn-Id Conn-State WriteQ-Size Local-Address
=========== ======= =========== =========== ===========
     5061       16  Established     0       10.106.95.155
     57396      17  Established     0       10.106.95.155
 
 
-------------- SIP Transport Layer Listen Sockets ---------------
Conn-Id Local-Address
=========== =============================
 2           [10.106.95.155]:5061

Output of show call active voice brief is captured when LTI transcoder is used.

Secure-CUBE#show call active voice brief 
Telephony call-legs: 0
SIP call-legs: 2
H323 call-legs: 0
Call agent controlled call-legs: 0
SCCP call-legs: 0
Multicast call-legs: 0
Total call-legs: 2
1283 : 33 357052840ms.1 (23:57:23.929 IST Sun Feb 15 2015) +2270 pid:3 Answer 3001 active
dur 00:00:08 tx:383/61280 rx:371/59360 dscp:0 media:0 audio tos:0xB8 video tos:0x0
IP 10.106.95.132:17172 SRTP: off rtt:0ms pl:0/0ms lost:0/0/0 delay:0/0/0ms g711ulaw TextRelay: off Transcoded: Yes
media inactive detected:n media contrl rcvd:n/a timestamp:n/a
long duration call detected:n long duration call duration:n/a timestamp:n/a
LostPacketRate:0.00 OutOfOrderRate:0.00
 
1283 : 34 357052840ms.2 (23:57:23.929 IST Sun Feb 15 2015) +2270 pid:1 Originate 2001 active
dur 00:00:08 tx:371/60844 rx:383/62812 dscp:0 media:0 audio tos:0xB8 video tos:0x0
IP 10.65.58.24:24584 SRTP: on rtt:0ms pl:0/0ms lost:0/0/0 delay:0/0/0ms g711ulaw TextRelay: off Transcoded: Yes
media inactive detected:n media contrl rcvd:n/a timestamp:n/a
long duration call detected:n long duration call duration:n/a timestamp:n/a
LostPacketRate:0.00 OutOfOrderRate:0.00

Also when an SRTP encrypted call is made between Cisco IP phone and CUBE or Gateway, a lock icon is displayed on the IP phone.

Troubleshoot

These debugs are helpful for troubleshooting PKI/TLS/SIP/SRTP issues.

debug crypto pki{ API | callbacks | messages | scep | server | transactions | validation }
debug ssl openssl { errors | ext | msg | states }
debug srtp {api | events }
debug ccsip {messages | error | events | states | all }
debug voip ccapi inout