Cisco Systems Linksys PAP2, RT31P2

- Expiration Date (12B)

- Public Key (512b or 64B)

- Signature (1024b or 512B)

The signing agent is implicit and must be the same for all PHONE ADAPTER’s that intended to

communicate securely with each other. The public key of the signing agent is pre-configured into the

PHONE ADAPTER’s by the administrator and will be used by the PHONE ADAPTER to verify the

Mini-Certificate of its peer. The Mini-Certificate is valid if a) it has not expired, and b) its signature

checks out.

User Interface

The PHONE ADAPTER can be set up such that all outbound calls are secure calls by default, or not

secure by default. If outbound calls are secure by default, user has the option to disable security

when making the next call by dialing *19 before dialing the target number. If outbound calls are not

secure by default, user has the option to make the next outbound call secure by dialing *18 before

dialing the target number. On the other hand, user cannot force inbound calls to be secure or not

secure; it is at the mercy of the caller whether he/she enables security or not for that call.

If the call successfully switches to the secure mode, both parties will hear the “Secure Call Indication

Tone” for a short while and the CID will be updated with the Name and Number extracted from the

Mini-Certificate sent by the other party, provided CIDCW service and equipment are available: the

CID Name in this case will have a ‘$’ sign inserted at the beginning. The callee should check the

name and number again to ensure the identity of the caller. The caller should also double check the

name and number of the callee to make sure this is what he/she expects. Note that the PHONE

ADAPTER will not switch to secure mode if the callee’s CID Number from its Mini-Certificate does not

agree with the user-id used in making the outbound call: the caller’s PHONE ADAPTER will perform

this check after receiving the callee’s Mini-Certificate.

Service Provider Requirements

The PHONE ADAPTER Mini-Certificate (MC) has a 512-bit public key used for establishing secure

calls. The administrator must provision each subscriber of the secure call service with an MC and the

corresponding 512-bit private key. The MC is signed with a 1024-bit private key of the service

provider who acts as the CA of the MC. The 1024-bit public key of the CA signing the MC must also

be provisioned to each subscriber. The CA public key is used by the PHONE ADAPTER to verify the

MC received from the other end. If the MC is invalid, the PHONE ADAPTER will not switch to secure

mode. The MC and the 1024-bit CA public key are concatenated and base64 encoded into the single

parameter <Mini Certificate>. The 512-bit private key is base64 encoded into the <SRTP Private

Key> parameter, which should be hidden from the PHONE ADAPTER’s web interface like a

password.

Since the secure call establishment relies on exchange of information embedded in message bodies

of SIP INFO requests/responses, the service provider must maker sure that their infrastructure will

allow the SIP INFO messages to pass through with the message body unmodified.

Linksys provides a configuration tool called gen_mc for the generation of MC and private keys with

the following syntax:

gen_mc <ca-key> <user-name> <user-id> <expire-date>

Where:

- ca-key is a text file with the base64 encoded 1024-bit CA private/public key pairs for

signing/verifying the MC, such as

9CC9aYU1X5lJuU+EBZmi3AmcqE9U1LxEOGwopaGyGOh3VyhKgi6JaVtQZt87PiJINKW8XQj3B9Qq