Privad Overview and Private
Auctions
Paul Francis (MPI-SWS)
Ruichuan Chen (MPI-SWS)
Bin Cheng (NEC Research)
Alexey Reznichenko (MPI-SWS)
Saikat Guha (MSR India)
2
Can we replace current
advertising systems with one
that is private enough, and
targets at least as well?
Can we replace current
advertising systems with one
that is private enough, and
targets
at today’s
least as
well?model
• Follows
business
• Advertisers bid for ad space, pay
for clicks
• Publishers provide ad space, get
paid for clicks
• Deal with click fraud
• Scales adequately
Can we replace current
advertising systems with one
that is private enough , and
targets at least as well?
• Most users don’t care about privacy
• But privacy advocates do, and so do
governments
• Privacy advocates need to be
convinced
Can we replace current
advertising systems with one
that is private enough , and
targets at least as well?
Our approach:
• “As private as possible”
• While still satisfying other goals
• Hope that this is good enough
Can we replace current
advertising systems with one
that is private enough, and
targets at least as well?
A principle: Increased privacy
begets better targeting
Today’s advertising model
Trackers
(simplified)
Publishers
Advertisers
Broker (Ad
exchange)
Client
Trackers
U
U
U
Publishers
Advertisers
Broker (Ad
exchange)
Trackers track users
Compile user profile
Client
Trackers
U
U
U
Publishers
Advertisers
U
U
U
Broker (Ad
exchange)
Trackers may share
profiles with advertisers?
Client
Trackers
U
U
U
Publishers
Advertisers
U
U
U
Broker (Ad
exchange)
Client gets
webpage with
adbox
Client
Trackers
U
U
U
Publishers
Advertisers
U
U
U
Broker (Ad
Client tells
broker of page exchange)
Client
Trackers
Publishers
U
U
U
Advertisers
U
U
U
Broker (Ad
exchange)
Broker launches auction
(for given user visiting
Client
given webpage ….)
Also does clickfraud etc.
Trackers
Publishers
U
U
U
Advertisers
U
U
U
Broker (Ad
exchange)
(alternatively the
publisher could
Client
have
launched the
auction)
Trackers
U
U
U
Publishers
Advertisers
U
U
U
Broker (Ad
exchange)
Client
Advertisers
present bids
and ads
Trackers
U
U
U
Publishers
Advertisers
U
U
U
Broker (Ad
exchange)
Client
Broker picks
winners,
delivers ads
Trackers
U
U
U
Publishers
Advertisers
U
U
U
Broker (Ad
exchange)
Client
User waits for
this exchange
Trackers
U
U
U
Publishers
Advertisers
U
U
U
Broker (Ad
exchange)
Client
Various reporting
of results . . . .
Publishers
Advertisers
Broker
Dealer
U
SA
Clients
Privad Basic
Architecture
Publishers
A
Advertisers
Broker
Dealer
U
SA
Clients
Learn
interest in
tennis shoes
Publishers
A
Advertisers
Broker
Dealer
Clients
U
SA
A
Anonymous
request for tennis
shoes
Publishers
A
Advertisers
Broker
Dealer
Clients
U
SA
A
Relevant and
non-relevant ads
stored locally
Chan: {Interest, Region, Language}
Ad: {AdID, AdvID, Content, Targeting, . . . .}
Key K unique to this request
Dealer knows Client requests some channel
Broker knows some Client requests this channel
Dealer cannot link requests
ICCCN 2010
23
Publishers
A
Broker
Dealer
Webpage
with
adbox
Clients
U
SA
A
Advertisers
Publishers
A
Advertisers
Broker
Dealer
Clients
U
SA
A
Ad is delivered
locally
Minimal delay
May or may not be
related to page
context
Publishers
A
Advertisers
Broker
Dealer
Clients
U
SA
A
View or click is
reported to Broker
via Dealer
Report: {AdID, PubID, EvType}
Dealer learns client X clicked on some ad
Broker learns some client clicked on ad Y
At Broker, multiple clicks from same client appear
as clicks from multiple clients
ICCCN 2010
27
List of suspected rid’s
rid: Report ID
Unique for every report
Used to (indirectly) inform Dealer of
suspected attacking Clients
Dealer remembers rid↔Client mappings
Client with many reported rid’s is suspect
ICCCN 2010
28
Many interesting challenges
Click fraud and auction fraud
2nd-price, pay-per-click auction
How to do profiling
Protecting user from malicious advertisers
….and still have good targeting
Gathering usage statistics and correlations
Accommodating multiple clients
Dynamic bidding for ad boxes
Co-existing with today’s systems
Many interesting challenges
Click fraud and auction fraud
2nd-price, pay-per-click auction
How to do profiling
Protecting user from malicious advertisers
….and still have good targeting
Gathering usage statistics and correlations
Accommodating multiple clients
Dynamic bidding for ad boxes
Co-existing with today’s systems
Advertising auctions today
• Almost all auctions are second
price
• Most auctions are Pay Per Click
(PPC)
Bid3:
Bid1:
Bid2:
Bid2:
Bid1:
Bid3:
$6
$2
$7
$3
$5
$1
$4
Second Price Auction
Bid2: $7
Bid3: $6
Bid1: $5
• Winner pays bid+δ of
next ranked bidder
• Bidders can safely bid
maximum from the start
Second Price Auction
Bid2: $7 ($9)
Maximum bid
Bid3: $6 ($6)
Bid1: $5 ($5)
• Winner pays bid+δ of
next ranked bidder
• Bidders can safely bid
maximum from the start
Second Price Auction
Bid2: ($9)
Bid3: ($6)
Bid1: ($5)
• Bidder 2 is 1st
ranked
– Pays $6+1¢=$6.01
• Bidder 3 is 2nd
ranked
– Pays $5+1¢=$5.01
What about PPC (pay per
click)?
Bid2: $9
P(C)=0.1
Bid3: $6
P(C)=0.1
Bid1: $5
P(C)=0.4
Click
Probabilities
What about PPC (pay per
click)?
Bid2: $9
P(C)=0.1
$0.9
Bid3: $6
P(C)=0.1
$0.6
Bid1: $5
P(C)=0.4
$2.0
Expected
Revenue
Expected Revenue = Bid X Click Probability
What about PPC (pay per
click)?
Bid1: $5
P(C)=0.4
$2.0
Bid2: $9
P(C)=0.1
$0.9
Bid3: $6
P(C)=0.1
$0.6
Ad Rank= Bid X Click Probability
What does bidder 1 pay???
Bid1: $5
P(C)=0.4
Bid2: $9
P(C)=0.1
Bid3: $6
P(C)=0.1
What does bidder 1 pay???
Bid1: $5
P(C)=0.4
Bid2: $9
P(C)=0.1
Bid3: $6
P(C)=0.1
Certainly not $9+1¢=$9.01
Google Second Price Auction
Bid1: $5
P(C)=0.4
Bid2: $9
P(C)=0.1
Bid3: $6
P(C)=0.1
Ad Rank= Bid X Click Probability
P(C) next
CPC = Bid next
P(C) clicked
Google Second Price Auction
Bid1: $5
P(C)=0.4
$2.26
Bid2: $9
P(C)=0.1
$6.01
Bid3: $6
P(C)=0.1
$?
Ad Rank= Bid X Click Probability
P(C) next
CPC = Bid next
P(C) clicked
What is the Click Probability???
What is the Click Probability???
• Historical click
performance of the ad
• Landing page quality
• Relevance to the user
• User click through rates
• …….
What is the Click Probability???
• Historical click
performance of the ad
• Landing page quality
• Relevance to the user
• User click through rates
• …….
Today all this
is known by
the broker
(ad network)
What is the Click Probability???
• Historical click
performance of the ad
• Landing page quality
• Relevance to the user
• User click through rates
• …….
In a non-tracking advertising system,
the broker knows nothing about the
user!
What is the Click Probability???
• Historical click
performance of the ad
• Landing page quality
• …….
• Relevance to the user
• User click through rates
• …….
Known at
broker
(call it G)
Known at user
(call it U)
Second price auction with broker
and user components
• Ranking by revenue potential:
– Assume that Click Probability = G x U
• Second-Price cost per click:
Non-tracking advertising
revisited
• User profile at client
• Privacy goals at broker:
– Anonymity: No user identifier tied to any user
profile attributes
– Unlinkability: Individual user profile attributes
cannot be linked
Finally: Problem Statement
• Satisfy anonymity and unlinkability goals in a
system that runs this auction:
• Where Bid and G are known at broker
• And U is known at client
Basic Architecture
Two questions
• Where do we do the ranking?
• Where do we do the CPC computation?
Two
questions
Do CPC at
Broker:
• Don’t want to reveal
• Where
do we doBid
the ranking?
advertiser’s
• Fraud
• Where do we do the CPC computation?
Three flavors of Non-Tracking
auctions
Broker
(Bid, G)
Client
(U)
Rank@Client
Bid, G
Rank@Broker
U
Rank@3rdParty
U
3
party
Bid, G
Three flavors of Non-Tracking
auctions
Broker
(Bid, G)
Client
(U)
Rank@Client
Bid, G
Rank@Broker
U
Rank@3rdParty
U
3
party
Bid, G
Client
(U)
Computes ranking:
(B × G) × U
Broker
(Bid, G)
A - the ad ID,
Value of (B × G),
E[B,G],
(+ targeting etc.)
Client
(U)
Computes ranking:
(B × G) × U
Broker
(Bid, G)
A - the ad ID,
Value of (B × G),
E[B,G],
(+ targeting etc.)
Client
(U)
Computes ranking:
(B × G) × U
Tim
e
Ac - clicked ad ID
((Bn × Gn) × Un / Uc)
E[Bc, Gc]
Broker
(Bid, G)
A - the ad ID,
Value of (B × G),
E[B,G],
(+ targeting etc.)
Decrypts E[Bc, Gc]
Computes CPC:
((Bn × Gn) × Un / Uc) / Gc
Checks that CPC ≤ Bc
Client
(U)
Broker
(Bid, G)
Decrypts E[Bc, Gc]
Computes CPC:
((Bn × Gn) × Un / Uc) / Gc
Checks that CPC ≤ Bc
Client
(U)
Computes ranking:
(B × G) × U
Broker
(Bid, G)
User information
A - the by
adhiding
ID,
obscured
withinofthis
Value
(B × G),
composite value
E[B,G],
(+ targeting etc.)
Ac - clicked ad ID
((Bn × Gn) × Un / Uc)
E[Bc, Gc]
Decrypts E[Bc, Gc]
Computes CPC:
((Bn × Gn) × Un / Uc) / Gc
Checks that CPC ≤ Bc
Client
(U)
Computes ranking:
(B × G) × U
Broker
(Bid, G)
A - the ad ID,
Value of (B × G),
E[B,G],
(+ targeting etc.)
Ac - clicked ad ID
((Bn × Gn) × Un / Uc)
E[Bc, Gc]
Decrypts E[Bc, Gc]
Computes CPC:
((Bn × Gn) × Un / Uc) / Gc
Checks that CPC ≤ Bc
Client
Bc and
(U) Gc may
have changed
between ranking
Computesand
ranking:
CPC
(B × G)calculation
×U
Broker
(Bid, G)
A - the ad ID,
Value of (B × G),
E[B,G],
(+ targeting etc.)
Ac - clicked ad ID
((Bn × Gn) × Un / Uc)
E[Bc, Gc]
Decrypts E[Bc, Gc]
Computes CPC:
((Bn × Gn) × Un / Uc) / Gc
Checks that CPC ≤ Bc
All three auction designs introduce
various system delays
• precompute and cache ranking
• use out-of-date bid information
• do not immediately reflect changes in bids
Changes in bids constitute main
source of churn
Advertisers constantly update their bids to
• show ads in a preferred position
• meet target number of impressions
• respond to market changes
How detrimental are auction
delays?
Broker perspective:
• How much revenue is lost due to these
delays?
Advertiser perspective:
• How they affect advertisers’ rankings?
Bing’s Auction log
• 2TB of log data spanning 48 hours
• 150M auctions with 18M ads
• Trace record for an auction includes:
– All participating ads
– Bids and quality scores
– Whether ad was shown and clicked
Understanding effect of churn
on revenue
Idea:
• Simulate auctions with stale bid
information
• Compute auctions at time t using bids
recorded at time t-x
• Compare generated revenue to auctions
with up-to-date bid information
We cannot predict changes in
clicking behavior when rankings
change
Trace
A
B
Simulation
Click
B
Same
position
click?
A
Same
ad
click?
We simulate five click models
1.
2.
3.
4.
5.
100% same position
75% same position, 25% same ad
50%-50%
25% same position, 75% same ad
100% same ad
Bid staleness and change in
revenue
Same
position
0.1%
Change in
revenue
75%-25%
50%-50%
0%
25%-75%
Same ad
-0.1%
1m
5m
30m
2h
6h
Bid staleness
1d
2d
Average fraction of auctions affected for advertiser (%)
20
Rank increased
Rank decreased
Became visible
Became invisible
Bid staleness and change in
ranking
Average %
of
auctions
affected
per
advertiser
18
16
14
12
Rank increased
Rank decreased
Became visible
Became invisible
1010
88
66
44
22
00
1m
1m
2m
5m
5m
15m
30m
30m
1h
2h
2h
Staleness
3h
6h
6h
Bid staleness
12h
1d
1d
36h
2d
2d
Computing U
So far, we assume we know user
component of click probability
Hard to compute purely at client
Not enough history
Unlinkably gather click stats from clients,
compute U, feed back to clients
Assume a set of factors X={x1, x2, …, xL}
Level of interest in ad’s product/service
Targeting/user match quality
Webpage context
User’s historic CTR
…….
Clients report {Ad-ID, X, click}
Broker computes U = f(X), delivers f(X)
along with ad
Problem if X={x1, x2, …, xL} fingerprints user
Possible mitigating factors:
Level of interest
Many interests change,
many interests don’t correlate that well
Targeting match quality
Different ads have different targeting
Webpage context
Can be course-grained
User’s historic CTR
Can be course-grained
Future work
So far, designs appear practical, but:
• Can we accurately compute user score U?
– And without violating privacy….
• Are there new forms of click fraud?
• Need experience in practice….
User Statistics
Broker and advertiser want to know deep
statistical information about users
What kind of targeting works best?
When should ads be shown?
Are users interested in A also interested in B?
How can conversion rates be improved?
Centralized systems have full knowledge
How can Privad privately provide this
information?
Differential Privacy
Differential Privacy adds noise to answers
of DB queries
Such that presence or absence of single DB
element cannot be determined
Normally modeled as a single trusted DB
Query
DB
Trusted
True
Answer
Add
Noise
Noisy
Answer
Distributed Differential Privacy
DB
Dealer
DB
DB
DB
Query
(cleartext)
Distributed Differential Privacy
DB
Dealer
DB
DB
DB
True Answers
(encrypted)
Noisy Answers
(encrypted)
A couple URLs
adresearch.mpi-sws.org
trackingfree.org
• Backups, trust model
ICCCN 2010
81
Publishers
Advertisers
Broker
Dealer
Software
Agent
U
SA
Clients
Generate user
profiles locally
at the client
In other words,
Adware!
Publishers
Advertisers
Broker
Dealer
U
SA
Clients
Anonymizes
client-broker
communications
Cannot
eavesdrop
Helps with
clickfraud
Publishers
Advertisers
Broker
Client/broker
messages:
Dealer
Contain minimal
info (no PII)
U
SA
Clients
Cannot be
linked to same
client
Publishers
Advertisers
Broker
Dealer
U
SA
Clients
Unlinkability
and
anonymity
Publishers
Advertisers
Broker
Dealer
Browser sandbox
Encrypted
U
SA
Clients
Trusted, open
Cleartext
Reference
Monitor
Untrusted Software
Black-box Agent
U
Publishers
Advertisers
Broker
Possibly
malicious
Dealer
“Pretty
Honest but
honest but
tempted
very curious”
Doesn’t
collude
Clients
Browser sandbox
Encrypted
U
SA
Trusted, open
Cleartext
Reference
Monitor
Untrusted Software
Black-box Agent
U
Privacy and threat models???
Honest but curious isn’t quite right
We expect the broker to do “what it
can get away with”, but cautiously
Plus we need to make privacy
advocates comfortable
No formal privacy model
Formal models are too narrow and
restrictive
Dealer
U
SA
Clients
Dealer and Software Agent
are new components
How are they incentivized?
Dealer
U
SA
Clients
Dealer:
Legally bound to follow
protocols, not collude
Execute open-source
software, open to
inspection
Dealer
Client:
Various options:
U
SA
Clients
Provide benefit: free
software, content, …..
Like adware!
Bundle with browser or
OS
• Local threats…
ICCCN 2010
92
Publishers
Advertisers
Broker
Dealer
U
SA
Clients
Please suspend
disbelief, imagine
that we succeed……
Publishers
A
Broker
Perfectly
Private
Advertising
System
Dealer
U
SA
Clients
Advertisers
Publishers
Advertisers
A
Broker
Perfectly
Private
Advertising
System
Dealer
Ad
U
SA
Clients
Ad
Ad targeted to:
“Man” AND
“Married” AND
“Has girlfriend”
Publishers
Advertisers
A
Broker
Perfectly
Private
Advertising
System
Dealer
U
SA
Clients
Click
Advertiser gets
(very) personal
information
about users
Honey, why are
you getting ads
for sexy lingerie?
ICCCN 2010
97

More
???
Privad
Privacy
Less

Worse
Targeting
Better