02/22/2005 Joint Seminer Satoshi Koga Information Technology & Security Lab. Kyushu Univ. A Distributed Online Certificate Status Protocol with Low Communication Costs A preliminary version of this paper is presented at PKC 2004

2 Background Public Key Infrastructure (PKI) –secure , authentication system etc.. Certificate revocation problem –The certificate must be revoked if The user’s private key is compromised User’s personal information is changed –The verifier must check the revocation information

3 Certificate revocation Compromise of private key, or changing personal information –The certificate must be revoked If a certificate is revoked… –Certificate owner sends a revocation requests to the CA who issues certificates –The CA should publish revocation information –The certificate verifier should check the status of certificate Is this certificate valid? or revoked? Certificate verifier

4 Certificate revocation systems Certificate Revocation List (CRL) The list of revoked certificates The size of the CRL is long High communication costs Online Certificate Status Protocol (OCSP) Provide the up-to-date response to certificate status queries Low Communication costs

5 Online Certificate Status Protocol (OCSP) Responder checks the status of a certificate instead of users –User requests the status of a certificate –Responder sends the response including the status of requested certificate –Mitigate the load of user –Reduce the communication costs, compared with CRL CA Responder User request response Revocation information Back

6 OCSP (cont’d) Security –Responses are signed by OCSP responder Communication costs –A user receives response –Independent on number of revoked certificates problem –High computation costs of OCSP responder  It is vulnerable to Denial-of-Service (DoS) attacks

7 Motivation Centralized OCSP Compromise of responder’s private key affects the entire system Protection of the private key Hardware Security Module (FIPS140-2 by NIST) Threshold cryptography :each server holds a shared private key and a predetermined number of servers must cooperate in order to perform the operation unavoidablePrivate key exposures appear to be unavoidable

8 Distributed OCSP Minimize the damage caused by responder’s key exposures A Distributed OCSP(D-OCSP) composed of the multiple responders –Each responder has the different private key  If a responder’s private key is compromised, the others are not derived

9 Traditional D-OCSP CA responder’s certificate CA’s certificate User response + signature responder 1 responder n responder 2 PK 1, SK 1 PK 2, SK 2 PK n, SK n To eliminate the validation of certificate revocation, the CA issues responder’s certificate with short lifetime

10 Challenging issue Responder’s certificate with a short lifetime  In case that the client receives the response, she must download responder’s certificate  Communication costs is inefficient Responder’s certificate with a long lifetime  The client needs to obtain the different responder’s certificates  The client must store the multiple certificates

11 Our Proposed Distributed OCSP To mitigate the damage caused by responder’s private key exposure A distributed OCSP (D-OCSP) Propose an efficient D-OCSP –The client can verify any responses by using a single public key  The client just obtains a single certificate

12 Our idea To generate the responder’s private keys –Use the Key-Insulated Signature scheme (KIS) [DO03] –Each responder has the different private key, but corresponding public key remains fixed –The client can verify any responses by using a single public key To validate responder’s private key –Use the NOVOMODO [M02] [DO03] Y. Dodis et al., “ Strong Key-Insulated Signature Schemes”, PKC [M02] S. Micali, “NOVOMODO”, 1 st Annual PKI Research Workshop, 2002.

13 The lifetime of protocol is divided into short time periods The beginning of period i, a private key is updated The private key is updated frequently, but the corresponding public key is fixed Even if SK i is exposed, the attacker cannot forge signature for any time periods (key-insulated security) SK 1 Lifetime Period 1Period 2 SK T Period T SK 2 Key-insulated signature scheme (KIS) Period i SK i PK

14 The master key SK * is stored on the secure device The Secure-device computes the partial key SK i ’ The user derives Sk i+1 using partial key SK i ’ and SK i Once Sk i+1 is derived, SK i is deleted If an attacker can know SK i, she cannot derive any other private keys (as long as SK* is secure) Secure device SK* SK 1 ’ SK T ’ SK 1 Lifetime Period 1Period 2 SK T Period T SK 2 Update algorithm in KIS signer

15 All signatures can be verified by using a fixed public key Key-insulated security Responder’s private keys are generated using Key-Insulated signature scheme n (= the number of responders) private keys are generated at first stage Our method

16 The CA stores the master key The CA generates n private keys using key update algorithm in KIS The CA delivers a private key to each responder securely CA responder 1 responder n Decentralization Method Reponder’s public key responder 2 SK 1 SK 2 SK n The user must check that responder’s private key is not revoked

17 Use the NOVOMODO [M02] –Using one-way hash function h –Generating the following hash-chain –At period t, the verifier checks the following equation X Input value h XTXT hh X0X0 Validation of responder’s private key X T-1 h

18 The CA produces n hash-chains and stores them securely The CA issues responder’s certificate D: certificate data Responder 1 Responder n Issuance of responder’s certificate X T,1 h X T-1, 1 hh X 0, 1 X T-2, 1 h X T,2 h X T-1, 2 hh X 0,2 X T-2, 2 h X T,n h X T-1, n hh X 0, n X T-2, n h Responder 2 C res =Sig CA (D, PK res, X 0, 1, X 0, 2, …, X 0, n )

19 If responder’s private key is valid at period t, the CA delivers the hash value to responder The responder sends both the signed response and this hash value The user checks the following equation at period t –The user can verify the responder’s private key using hash function CA responder i Validation process X t, i X 0, i = h t (X t, i )

20 CA responder’s certificate CA’s certificate User Our Proposed D-OCSP responder 1 responder n responder 2 SK 1 SK 2 SK n Response + X t, i X t,1 X t,2 X t,i

21 Discussions Security –If one private key is exposed, the attacker can not derive the others (Key-insulated security) –If the attacker obtains the hash value, she cannot derive the next hash value (one-way function) Minimize the impact of responder’s private key exposure

22 Discussions (cont’d) Communication costs –The client can check any responses using a single public key –The client simply obtains one responder’s certificate  the communication cost is efficient –The client only stores one certificate  the memory space is small Computational costs –Signing cost and verification cost are less efficient

23 Efficiency Traditional D-OCSP (DSA) Our proposed D-OCSP (KIS) Size of a response bytes bytes Verification costs (# of multiplications) 3+EX|q|t+2+3EX|q| Signature costs (# of multiplications) 2+EX|q|2+2EX|q| ・ OpenSSL ・ CA’s key size ： 2048 bit ・ Responder’s key size ： 1024 bit ・ EX ： # of multiplication required to compute a exponentiation ・ |q| =160 ・ t = (# of responders)

24 Conclusion Centralized OCSP –Compromise of private key affects the entire system –Mitigate the damage caused by compromise of responder Efficient distributed OCSP –Apply key-insulated signature scheme and NOVOMODO –Any responses can be checked by using fixed public key