1. No Spam at Any (CPU) Speed Mike Burrows Cynthia Dwork Andrew Goldberg MSR Silicon Valley

2. Computational Spam Fighting If I don’t know you: –Prove you spent ten seconds computing, –just for me, and just for this message User Experience: –Automatically and in the background –Checking proof extremely easy “Proof” is solution to crypto puzzle Memory latencies more uniform than cpu speeds –Factor of 15 versus factor of 5 –Design puzzles requiring many random memory accesses

3. Point-to-Point Architecture (Ideal Message Flow) (Ideal Message Flow) –“Send and forget” –Cache tags, discard duplicates; one puzzle buys one message Sender client S Sender client S Recipient client R Recipient client R m, tag = f(S,R,date,nonce)

4. Here to There (and There ’ ) Three e-mail messages R’s mail client caches m, S, R, t, nonce Bounce (viral marketing opportunity) –html attachment with Java Script for f –contains parameters for f (S, R, t,nonce) –clicking on link causes computation, sending e-mail –(optional) link for download of client software Ignorant Sender S Ignorant Sender S Spam-Protected Recipient R Spam-Protected Recipient R m bounce release m

5. Run Times Machine Time x/IBMCPU IBM Server 10.6 1.0 1.3 GHz P3 Laptop 11.0 1.04 1.2 GHz P4 Laptop 14.3 1.25 2.0 GHz Mac Laptop 17.4 1.64 800 MHz P2 Desktop 23.8 2.24 366 MHz Sun Server 25.5 2.41 450 MHz Set-top 48.4 4.57 233 MHz PDA 53.6 5.06 200 MHz  Time = total time to solve puzzle  x/IBM = ratio to IBM Server of solution speed

6. One Implementation: Proxies Sender Proxy computes tags Receiver Proxy checks tags –wrongly tagged mail placed in special folder and/or bounced –bounce message can contain link to code download site Other possibilities include built-in functionality –eg in Outlook/Exchange, Hotmail Sender Client PROXYPROXY Server Receiver Client PROXYPROXY Server