1. Matej Bel University Cascaded signatures Ladislav Huraj Department of Computer Science Faculty of Natural Sciences Matthias Bel University Banska Bystrica Slovak republic huraj@fpv.umb.sk

2. Matej Bel University Cascaded signatures A cascaded signature is a signature over a document and other signatures over the same document that had been issued a priori Some real applications of this structure How cascaded signatures improve efficiency of Public Key Infrastructure

3. Matej Bel University Terminology Public Key Infrastructure (PKI) enables to exchange data securely and privately Digital signature integrity and non-repudiation Certificate a binding, through a cryptographic digital signature, of a public key, a validity interval and/or conditions, and identity, authorization, or other information

4. Matej Bel University Terminology Certification Authority (CA) is a trusted third party, which issues certificates to subjects by signing it Certification Path a set of certificates that provide a chain of trust that together with the public key of the initial object in the path can be processed to obtain the final object in the path.

5. Matej Bel University Using of cascaded signature The models

6. Matej Bel University SDSI/SPKI Certificates are defined as signed objects A cascaded signature is what we get if we sign the signed object at the end of a previously signed list ( Signed: ( Object-Hash: ( SHA1: &345678 ) ) ( Date: 1996-02-14T11:46:05.046-0500 ) ( Signer: ( Principal: ( Global-Name: VeriSign!!’s "Bob" )... ) ) ( Signature: &8dff4123 ) ( Signed: ) ( Object-Hash: ( SHA1: &86731b) ( Date: 1996-03-19T07:00:11.341-0500 ) ( Signer: ( Principal: ( Global-Name: VeriSign!!’s "Alice" )...)) ( Signature: &7830ca12 ) ) )

7. Matej Bel University SDSI/SPKI digital time-stamping running the correct program reconfirmation –SDSI does not have CRL - Certificate Revocation List (CRL) - a data structure that enumerates digital certificates that have been invalidated by their issuer prior –the signer can specify the reconfirmation period that is appropriate for that signature

8. Matej Bel University S/MIME Secure Multipurpose Internet Mail Extensions (S/MIME) is a protocol for sending secure e-mail Signing certificates are one extension in S/MIME They are useful in any environment where certificates might be transmitted with signed messages This attribute is used to thwart attacks based on falsification of certificates

9. Matej Bel University Security Embassy If trustworthy authority masquerades it makes it difficult for the client to prove that this has happened One solution Security Embassy SE builds electronic evidence on behalf of her owner in a remote (untrusted) domain SE received in input some signed data, performs a signature verification with the public key configured, then SE sings these data using its private key. Signed data serve for audit trail

10. Matej Bel University Chained protocol A delegates rights to B, which acts on D using the delegated rights A  B: where T A = A A gives rights Pr a to B, for duration limited by t a B can sent itself signed request to D, request token chaining with delegated token B  D:, where T A = A T B = B Delegation of rights B,T B A,T A

11. Matej Bel University Delegation of rights Nested protocol The second delegation protocol based on nested tokens The single stage delegation is exactly the same as that given in the case of chained tokens A  B: where T A = A Here is the message for a second stage delegation, from B to C B  C: > B, A> TATA B,C,Pr b,r b,t b

12. Matej Bel University Delegation of rights Misplaced assumptions of trust E.g. A trusts B but does not trust C in delegation. Therefore B does not pass any delegation from A to C. The delegation sequence A-B-C will not be allowed under the correct behaviour of A and B because A does not trust C and A trusts B to behave correctly (in that B will not pass any delegation from A to C). In the chained protocol, if C can get hold of tokens T A-B and T B-C from the system using say wiretapping, as these are valid delegation tokens, C can form a chain giving the appearance that the delegation chain A-B-C is in force, even when B has not actually such a delegation.

13. Matej Bel University Nested certificates Nested certificates are used to guarantee the integrity and correctness of the signature over a subject certificate A nested certificate is considered as a certificate for another certificate The two requirements of a nested certificate are: 1) to certify that the subject certificate content has been signed by the claimed CA and 2) to certify that the subject certificate content has not been maliciously modified

14. Matej Bel University Nested certificate contains: -the existing signature over the subject certificate content -the hash of its subject certificate content Subject certificate verification: the actual hash and the actual signature over the subject certificate must be compared with the ones stored in the nested certificate. Nested certificates are used to guarantee the integrity and correctness of the signature over a subject certificate. Nested certificates Nested certificateSubject certificate Content Signature of SC Hash of Content Signature Signature of SC

15. Matej Bel University Nested certificates Subject certificate verification method does not employ public key cryptosystem operations. Subject certificate verification is faster. A large number of nested certificates must be issued in order to have nested certificate paths in the global certificate network. But the certificates are issued only once. Preferred in many systems for which minimization of the time complexity for verifiers is more important.

16. Matej Bel University Conclusion The principle of cascaded signature itself is the same in all these systems. However, the purpose, for which it was created, differs A cascaded signature was created to: –a) extend and complete the information digital time stamp and confirmation of the correct program –b) confirm validity reconfirmation of signatures of authorization certificates, issuing cascaded proxies, issuing cascaded Dsig signature label, Security Embassy principle, S/MIME signing certificates and nested certificates

17. Matej Bel University Conclusion Implementing of the cascaded signature into infrastructure improves the efficiency of certificate verification when delegating rights among the entities, as well as when accelerating the verification process. Thank you for your attention