Re: [stir] RFC8224 / Use of a=fingerprint

[Chris Wendt <[e-mail]>]

MSRP is tcp/tls so doesn’t apply to intended the use of a=fingerprint/mky

> On Mar 22, 2018, at 8:14 AM, Asveren, Tolga <[e-mail]> wrote:
> 
> A few doubts regarding use of a=fingerprint for RFC8224:
> 
> 4.1.  PASSporT Construction
> ....
> 
>   o  Fourth, if the request contains a Session Description Protocol
>      (SDP) message body and if that SDP contains one or more
>      "a=fingerprint" attributes, then the JSON key "mky" MUST appear
>      with the algorithm(s) and value(s) of the fingerprint attributes
>      (if they differ), following the format given in [RFC8225],
>      Section 5.2.2.
> 
> 
> 12.1.  Protected Request Fields
> ....
>  When signing a request that contains a fingerprint of keying material
>   in SDP for DTLS-SRTP [RFC5763], this mechanism always provides a
>   signature over that fingerprint.  This signature prevents certain
>   classes of impersonation attacks in which an attacker forwards or
>   cut-and-pastes a legitimate request.  Although the target of the
>   attack may accept the request, the attacker will be unable to
>   exchange media with the target, as they will not possess a key
>   corresponding to the fingerprint.  For example, there are some
>   baiting attacks, launched with the REFER method or through social
>   engineering, where the attacker receives a request from the target
>   and reoriginates it to a third party.  These might not be prevented
>   by only a signature over the From, To, and Date, but they could be
>   prevented by securing a fingerprint for DTLS-SRTP.  While this is a
>   different form of impersonation than is commonly used for
>   robocalling, ultimately there is little purpose in establishing the
>   identity of the user that originated a SIP request if this assurance
>   is not coupled with a comparable assurance over the contents of the
>   subsequent media communication.  This signature also reduces the
>   potential for active eavesdropping attacks against the SIP media.  In
>   environments where DTLS-SRTP is unsupported, however, no field is
>   signed and no protections are provided.
> 
> i- (with lawyer hat on)
> Which one of these statements prevails? I assume it is the former as it is using normative language as "MUST" therefore "a=fingerprint" must be used when it is present.
> 
> ii- (with technical hat on)
> Wouldn't the attack vector mentioned in 12.1 be applicable for connection oriented media, e.g. a=fingerprint in SDP is used while establishing a MSRP session (and possibly for other cases) as well?
> 
> Thanks,
> Tolga
> 
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