Subject: [Asrg] Fighting SPIT on a cell phone

From: "Pars Mutaf" <[email-masked]>
Date: Tue, 8 Jan 2008 15:43:55 +0100

[sorry for cross-posting]

Hello,

I want to leave my cell phone number (SIP URI) on a discussion forum, or
web page, blog, craigslist etc. But wish to avoid SPIT (SPam over Internet
Telephony). A solution is presented below (with variations called weak, strong
and indirect).

Comments are appreciated.

Regards,
Pars Mutaf

1. Weak solution

I leave the IP address of my cell phone but not a SIP URI. Interested
party sends a
request to my phone. My phone generates a random SIP URI and returns a
different
SIP URI to each querier.

If I receive SPIT to the SIP URI 'x', then I can cancel it. Since each
querier is returned
a different SIP URI, legitimate parties can continue to call me or send SMS.

Since the SIP URI 'x' was canceled, a SPITer can request another one
and still send
me SPIT. To avoid this attack, the querier can be requested to solve a
hard challenge
e.g. a CAPTCHA. A SIP URI will be returned only after the querier user
provided the
solution. The difficulty of the CAPTCHA can be adaptively tuned by the
target host.

When done, i.e. the desired phone call is received, the target user
can stop receiving
requests to the indicated IP address.

2. Strong solution

I leave the IP address of my phone but not a SIP URI. I want to
receive phone calls or
SMS only from people that I know. Interested party sends a request to
my phone. My
phone displays a message with the requester's name e.g.:

    "Alice Collins requested phone number. Accept? [YES/NO]"

If I accept, my phone generates a random SIP URI and returns it to the querier.

This solution requires human name certification.

An attacker can send continuous bogus requests to the target IP
address and make
the target phone continuously display the above message, annoying the
target user.
This attack can be defeated by requesting the querier user to solve a
hard CAPTCHA
before his request can be displayed at the target host's screen. The
difficulty of the
CAPTCHA can be adaptively tuned by the target host.

3. Indirect solution (using e-mail)

I leave the IP address of my cell phone and a randomly generated
e-mail address. The
mobile host (cell phone) is its own mail server. The mail is routed to
the e-mail address at
the indicated IP address.

The querier can send me an e-mail with a brief text explaining why a
SIP URI is requested.
The e-mail content will be limited to several lines, reducing space
for spam. E-mails
containing an URL can be dropped by the host since the querier is not
supposed to indicate
an URL to request a SIP URI. Similarly, emails containing an image can
also be dropped.

==
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---

From: "Pars Mutaf" <[email-masked]>
Date: Tue, 8 Jan 2008 15:53:54 +0100

Hello,

I want to leave my cell phone number (SIP URI) on a discussion forum, or
web page, blog, craigslist etc. But wish to avoid SPIT (SPam over Internet
Telephony). A solution is presented below (with variations called weak,
strong and indirect).

Regards,
Pars Mutaf

1. Weak solution

I leave the IP address of my cell phone but not a SIP URI. Interested
party sends a request to my phone. My phone generates a random SIP URI
and returns a different SIP URI to each querier.

If I receive SPIT to the SIP URI 'x', then I can cancel it. Since each
querier is returned a different SIP URI, legitimate parties can continue
to call me or send SMS.

Since the SIP URI 'x' was canceled, a SPITer can request another one
and still send me SPIT. To avoid this attack, the querier can be requested
to solve a hard challenge e.g. a CAPTCHA. A SIP URI will be returned only
after the querier user provided the solution. The difficulty of the CAPTCHA
can be adaptively tuned by the target host.

When done, i.e. the desired phone call is received, the target user
can stop receiving requests to the indicated IP address.

2. Strong solution

I leave the IP address of my phone but not a SIP URI. I want to
receive phone calls or SMS only from people that I know. Interested party
sends a request to my phone. My phone displays a message with the
requester's name e.g.:

If I accept, my phone generates a random SIP URI and returns it to the
querier.

An attacker can send continuous bogus requests to the target IP
address and make the target phone continuously display the above message,
annoying the target user. This attack can be defeated by requesting the
querier user to solve a hard CAPTCHA before his request can be displayed
at the target host's screen. The difficulty of the CAPTCHA can be
adaptively tuned by the target host.

I leave the IP address of my cell phone and a randomly generated
e-mail address. The mobile host (cell phone) is its own mail server. The
mail is routed to the e-mail address at the indicated IP address.

The querier can send me an e-mail with a brief text explaining why a
SIP URI is requested. The e-mail content will be limited to several lines,
reducing space for spam. E-mails containing an URL can be dropped by the
host since the querier is not supposed to indicate an URL to request a SIP
URI. Similarly, emails containing an image can also be dropped.

==