1. Tamper-Evident Digital Signatures: Protecting Certification Authorities Against Malware Jong Youl Choi Dept. of Computer Science Indiana University at Bloomington Philippe Golle Palo Alto Research Center Markus Jakobsson School of Informatics Indiana University at Bloomington jychoi@cs.indiana.edupgolle@parc.commarkus@indiana.edu

2. Page 1 Threats to Certificate Authorities Stealing private key –Malicious attack such as Trojan horse, virus –Leaking CA’s private key via covert-channel Hidden communication channel –CAs use lots of random numbers –Hard to prove randomness since it is directly related to privacy

3. Page 2 What is a covert channel? Hidden communication channel Steganography – Information hiding Original ImageExtracted Image

4. Page 3 Prisoners' problem [Simmons,’93] Two prisoners want to exchange messages, but must do so through the warden Subliminal channel in DSA What Plan? Plan A

5. Page 4 Leaking attack on RSA-PSS A random salt is used as a padding string in a signature In verification process, the salt is extracted from the message Hidden information can be embedded in the salt RSA-PSS : PKCS #1 V2.1

6. Page 5 Approaches Need an observer to detect leaking An observer investigates outputs from CA mkmk Pseudo Random Number Generator Sig k Something hidden? Certificate Authority Malicious attack Replacement of function

7. Page 6 Hindsight Observing is not easy because of a random number –looking innocuous –Not revealing any state Fine as long as a random number is generated in a designated way Using hindsight, we detect abnormal behavior generating a random number

8. Page 7 Weakness of an observer An observer can be attacked, causing a single point of failure mkmk Pseudo Random Number Generator Sig k Something hidden? Certificate Authority  Public verifiability with multiple observers

9. Page 8 Undercover observer CA outputs non-interactive proof as well as signature Ambushes until verification is invalid mkmk Pseudo Random Number Generator Sig k

10. Page 9 Tamper-evident Chain Predefined set of random values in lieu of random number on the fly Hash chain verification s1s1 s2s2 s3s3 …. snsn Seed Sig 1 Sig 2 …. Sig n h() ? s 1 =h(s 2 ) ? s n-1 =h(s n ) s’ 3 Sig’ 3 ? s 2 =h(s 3 ) ? s 0 =h(s 1 ) s0s0 h()

11. Page 10 DSA Signature Scheme Gen : x  y = g x mod p Sign : m  (s, r) where r = (g k mod p) mod q and s = k -1 (h(m) + x r) for random value k Verify : For given signature (s, r), u 1 = h(m) s -1 u 2 = r s -1 and check r=g u 1 y u 2 mod p mod q

12. Page 11 Hash chain construction k1k1 k2k2 k3k3 …. knkn PRNG Sig 1 Sig 2 …. Sig n h() ? w 1 =h(r 2 ||w 2 ) ? w n-1 =h(r n ||w n ) k’ 3 Sig’ 3 ? w 2 =h(r 3 ||w 3 ) r 1 =g k 1 r 2 =g k 2 …. r n =g k n r 3 =g k 3 w1w1 w2w2 …. wnwn w3w3 r 3 ’=g k 3 w0w0 ? w 0 =h(r 1 ||w 1 ) Seed

13. Page 12 Conclusion Any leakage from CAs is dangerous CAs are not strong enough from malicious attacks We need observers which are under-cover A small additional cost for proofs Or, Send me emails : jychoi@cs.indiana.edu