1. Application of Attribute Certificates in S/MIME Greg Colla & Michael Zolotarev Baltimore Technologies 47 th IETF Conference Adelaide, March 2000

2. IETF47, Mar 2000, Adelaide Overview S/MIME and PK Certificates S/MIME Problems Secure mail requirements Possible solutions E-mail Attribute Certificates Practical Implementation Issues Attribute Certificates & S/MIME

3. IETF47, Mar 2000, Adelaide S/MIME Certificate Usage alice@foo.com Verification – check signer’s e-mail address against sender’s address Encryption – obtaining “encryptee’s” public key certificate cn=Alice subjAltName=alice@foo.com Attribute Certificates & S/MIME

4. IETF47, Mar 2000, Adelaide S/MIME Problems Multiple e-mail addresses –User has multiple e-mail addresses Maintenance of e-mail addresses –Change company name (and Internet domain) Security Proxy –a proxy signs and decrypts on behalf of many users Privacy/Spam alice@foo.com alice@dev.foo.com alice@bar.com Attribute Certificates & S/MIME

5. IETF47, Mar 2000, Adelaide Essential Requirements Address Aliasing : Associate a single entity with multiple e-mail addresses, with a single PKC. Secure Proxying: Associate multiple entities, each with their own e-mail address, with a common PKC. Address Sharing: Associate multiple entities, each with their own PKC, with a single e-mail address. Attribute Certificates & S/MIME

6. IETF47, Mar 2000, Adelaide Solution Criteria Cryptographically bound association between an e- mail address and a public key Unambiguous reference from e-mail address to PK certificate(s) Dynamic extension of address set Practical aspects –Generation, distribution, publication, retrieval, verification Minimum of changes to current standards Utilize existing infrastructure Attribute Certificates & S/MIME

7. IETF47, Mar 2000, Adelaide Overview of Possible Solutions 1.Embed e-mail address into entity’s cert a)One e-mail address per certificate, each with same public key b)One certificate with multiple e-mail addresses 2.Address  PKC association signed by entity –Authenticated attributes 3.Address  PKC association signed by TTP –Attribute Certificate Attribute Certificates & S/MIME

8. IETF47, Mar 2000, Adelaide Attribute Certificates Flexible Scalable Standards Based Available Infrastructure TTP (AA) Owner E-mail address Signature Other Attributes Attribute Certificates & S/MIME

9. IETF47, Mar 2000, Adelaide Cryptographically bind e-mail addresses with Gateway’s PK certificate alice@foo.com bob@foo.com cn=Gateway AC alice@foo.com bob@foo.com AC E-mail Attribute Certificates Attribute Certificates & S/MIME

10. IETF47, Mar 2000, Adelaide E-mail Attribute Certificates Cryptographically bind e-mail addresses with entity’s PK certificate alice@foo.com alice@dev.foo.com cn=Alice AC alice@foo.com alice@dev.foo.com AC Attribute Certificates & S/MIME

11. IETF47, Mar 2000, Adelaide Practical Implementation (1/5) Generation –Generation by an Attribute Authority(AA) TTP attests that the address is associated with the entity Request –By or on behalf of entity –Automatically by security proxy –By relying party (LAAP) Attribute Certificates & S/MIME

12. IETF47, Mar 2000, Adelaide Practical Implementation (2/5) Distribution & Retrieval –Generate by AA, publish in LDAP –Distribute as part of signed message –Retrieval based on e-mail address Validity & Revocation –Validity: as long as the PKC & e-mail address remain valid –Revocation: use available standards Attribute Certificates & S/MIME

13. IETF47, Mar 2000, Adelaide Practical Implementation (3/5) Retrieving attribute and PK certificates from LDAP 1.Use the from: or to: address from message as a search index 2.Request the directory to retrieve all attribute certificates from the matching entries 3.Out of all returned attribute certificates, select those with required e-mail address 4.Retrieve PK certificates referenced by selected attribute certificates cn=Alice mail=alice@foo.com mail=alice@dev.foo.com certificate= attributeCertificate= Alice’s new LDAP entry Attribute Certificates & S/MIME

14. IETF47, Mar 2000, Adelaide Practical Implementation (4/5) Message Verification Walkthrough –Retrieve e-mail AC(s) using sender’s address as index –Retrieve PKC(s) referenced by AC(s) –Identify signing certificate –Validate... –Validate the message Attribute Certificates & S/MIME

15. IETF47, Mar 2000, Adelaide Practical Implementation (5/5) Message Encryption Walkthrough –Retrieve e-mail AC(s) using recipient’s address as index –Validate... –Retrieve PKC(s) referenced by valid e-mail AC(s) –Validate... –Encrypt the message using valid encryption certificate(s) Attribute Certificates & S/MIME

16. IETF47, Mar 2000, Adelaide Other Considerations Privacy –Remove private information from PK certificate –Different access control on PK certificate than e- mail AC in directory –Different directories for email ACs and PKCs Security –Need to ensure that content of e-mail AC is valid bill.clinton@whitehouse.gov Attribute Certificates & S/MIME

17. IETF47, Mar 2000, Adelaide Comparison with existing Infrastructure Existing Multiple addresses in certificate Re-issue keys in new certificate with new e-mail address Supported by existing PK and S/MIME infrastructure  Difficult for large number of e- mail addresses (ie security proxies)  Difficult to separate internal and external e-mail addresses  Contra to legislation in some countries Proposed Store E-mail address in e-mail AC, which references PKC Issue e-mail AC’s as required Flexible method for maintaining e-mail addresses Infrastructure available Supplements current S/MIME infrastructure Supports security proxies Defined mechanism to retrieve PKC’s from directory, AA  Additions required to processing module’s logic Attribute Certificates & S/MIME

18. IETF47, Mar 2000, Adelaide Summary Maintenance of e-mail addresses limits S/MIME usability Attribute Certificates cryptographically bind e-mail addresses with PK certificates E-mail Attribute Certificates provide a flexible solution for maintaining e-mail addresses Supplements current infrastructure Localized modifications required to S/MIME components to utilize E-mail ACs E-mail ACs can be used to solve other S/MIME limitations Attribute Certificates & S/MIME