Security Mechanisms

5886

From the threats described above, we gather that the fundamental security services required for the SIP

5887

protocol are: preserving the confidentiality and integrity of messaging, preventing replay attacks or message

5888

spoofing, providing for the authentication and privacy of the participants in a session, and preventing

denial-5889

of-service attacks. Bodies within SIP messages separately require the security services of confidentiality,

5890

integrity, and authentication.

5891

Rather than defining new security mechanisms specific to SIP, SIP reuses wherever possible existing

5892

security models derived from the HTTP and SMTP space.

5893

Full encryption of messages provides the best means to preserve the confidentiality of signaling - it

5894

can also guarantee that messages are not modified by any malicious intermediaries. However, SIP requests

5895

and responses cannot be naively encrypted end-to-end in their entirety because message fields such as the

5896

Request-URI, Route, and Via need to be visible to proxies in most network architectures so that SIP

5897

requests are routed correctly. Note that proxy servers need to modify some features of messages as well (such

5898

as addingViaheader field values) in order for SIP to function. Proxy servers must therefore be trusted, to

5899

some degree, by SIP UAs. To this purpose, low-layer security mechanisms for SIP are recommended, which

5900

encrypt the entire SIP requests or responses on the wire on a hop-by-hop basis, and that allow endpoints to

5901

verify the identity of proxy servers to whom they send requests.

5902

SIP entities also have a need to identify one another in a secure fashion. When a SIP endpoint asserts

5903

the identity of its user to a peer UA or to a proxy server, that identity should in some way be verifiable. A

5904

cryptographic authentication mechanism is provided in SIP to address this requirement.

5905

An independent security mechanism for SIP message bodies supplies an alternative means of end-to-end

5906

mutual authentication, as well as providing a limit on the degree to which user agents must trust

intermedi-5907

aries.

5908

26.2.1 Transport and Network Layer Security

5909

Transport or network layer security encrypts signaling traffic, guaranteeing message confidentiality and

5910

integrity.

5911

Oftentimes, certificates are used in the establishment of lower-layer security, and these certificates can

5912

also be used to provide a means of authentication in many architectures.

5913

Two popular alternatives for providing security at the transport and network layer are, respectively,

5914

TLS [25] and IPSec [26].

5915

IPSec is a set of network-layer protocol tools that collectively can be used as a secure replacement for

5916

traditional IP (Internet Protocol). IPSec is most commonly used in architectures in which a set of hosts or

5917

administrative domains have an existing trust relationship with one another. IPSec is usually implemented

5918

at the operating system level in a host, or on a security gateway that provides confidentiality and integrity

5919

for all traffic it receives from a particular interface (as in a VPN architecture). IPSec can also be used on a

5920

hop-by-hop basis.

5921

In many architectures IPSec does not require integration with SIP applications; IPSec is perhaps best

5922

suited to deployments in which adding security directly to SIP hosts would be arduous. UAs that have a

5923

pre-shared keying relationship with their first-hop proxy server are also good candidates to use IPSec. Any

5924

deployment of IPSec for SIP would require an IPSec profile describing the protocol tools that would be

5925

required to secure SIP. No such profile is given in this document.

5926

TLS provides transport-layer security over connection-oriented protocols (for the purposes of this

docu-5927

ment, TCP); “tls” (signifying TLS over TCP) can be specified as the desired transport protocol within aVia

5928

header field value or a SIP-URI. TLS is most suited to architectures in which hop-by-hop security is required

5929

between hosts with no pre-existing trust association. For example, Alice trusts her local proxy server, which

5930

after a certificate exchange decides to trust Bob’s local proxy server, which Bob trusts, hence Bob and Alice

5931

can communicate securely.

5932

TLS must be tightly coupled with a SIP application. Note that transport mechanisms are specified on

5933

a hop-by-hop basis in SIP, thus a UA that sends requests over TLS to a proxy server has no assurance that

5934

TLS will be used end-to-end.

5935

The TLS RSA WITH AES 128 CBC SHA ciphersuite MUST be supported at a minimum by

imple-5936

menters when TLS is used in a SIP application. For purposes of backwards compatibility, proxy servers,

5937

redirect servers, and registrars SHOULD support TLS RSA WITH 3DES EDE CBC SHA. Implementers

5938

MAYalso support any other ciphersuite.

5939

26.2.2 SIPS URI Scheme

5940

The SIPS URI scheme adheres to the syntax of the SIP URI (described in 19), although the scheme string is

5941

”sips” rather than ”sip”. The semantics of SIPS are very different from the SIP URI, however. SIPS allows

5942

resources to specify that they should be reached securely.

5943

A SIPS URI can be used as an address-of-record for a particular user - the URI by which the user is

5944

canonically known (on their business cards, in theFromheader field of their requests, in theToheader field

5945

ofREGISTER requests). When used as the Request-URIof a request, the SIPS scheme signifies that

5946

each hop over which the request is forwarded, until the request reaches the SIP entity responsible for the

5947

domain portion of theRequest-URI, must be secured with TLS; once it reaches the domain in question it

5948

is handled in accordance with local security and routing policy, quite possibly using TLS for any last hop to

5949

a UAS. When used by the originator of a request (as would be the case if they employed a SIPS URI as the

5950

address-of-record of the target), SIPS dictates that the entire request path to the target domain be so secured.

5951

The SIPS scheme is applicable to many of the other ways in which SIP URIs are used in SIP today in

5952

addition to theRequest-URI, including in addresses-of-record, contact addresses (the contents ofContact

5953

headers, including those ofREGISTER methods), andRoute headers. In each instance, the SIPS URI

5954

scheme allows these existing fields to designate secure resources. The manner in which a SIPS URI is

5955

dereferenced in any of these contexts has its own security properties which are detailed in [4].

5956

The use of SIPS in particular entails that mutual TLS authentication SHOULDbe employed, asSHOULD

5957

the ciphersuite TLS RSA WITH AES 128 CBC SHA. Certificates received in the authentication process

5958

SHOULDbe validated with root certificates held by the client; failure to validate a certificateSHOULDresult

5959

in the failure of the request.

5960

motivationNote that in the SIPS URI scheme, transport is independent of TLS, and thus “sips:[email protected];transport

5961

and “sips:[email protected];transport=sctp” are both valid (although note that UDP is not a valid transport

5962

for SIPS). The use of “transport=tls” has consequently been deprecated, partly because it was specific to a

5963

single hop of the request. This is a change since RFC 2543.

5964

Users that distribute a SIPS URI as an address-of-record may elect to operate devices that refuse requests

5965

over insecure transports.

5966

26.2.3 HTTP Authentication

5967

SIP provides a challenge capability, based on HTTP authentication, that relies on the 401 and 407 response

5968

codes as well as header fields for carrying challenges and credentials. Without significant modification, the

5969

reuse of the HTTP Digest authentication scheme in SIP allows for replay protection and one-way

authenti-5970

cation.

5971

The usage of Digest authentication in SIP is detailed in Section 22.

5972

26.2.4 S/MIME

5973

As is discussed above, encrypting entire SIP messages end-to-end for the purpose of confidentiality is not

5974

appropriate because network intermediaries (like proxy servers) need to view certain header fields in order

5975

to route messages correctly, and if these intermediaries are excluded from security associations, then SIP

5976

messages will essentially be non-routable.

5977

However, S/MIME allows SIP UAs to encrypt MIME bodies within SIP, securing these bodies

end-to-5978

end without affecting message headers. S/MIME can provide end-to-end confidentiality and integrity for

5979

message bodies, as well as mutual authentication. It is also possible to use S/MIME to provide a form of

5980

integrity and confidentiality for SIP header fields through SIP message tunneling.

5981

The usage of S/MIME in SIP is detailed in Section 23.

5982

In document SIP: Session Initiation Protocol (Page 173-176)