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

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

IETF47, Mar 2000, Adelaide S/MIME Certificate Usage Verification – check signer’s address against sender’s address Encryption – obtaining “encryptee’s” public key certificate cn=Alice Attribute Certificates & S/MIME

IETF47, Mar 2000, Adelaide S/MIME Problems Multiple addresses –User has multiple addresses Maintenance of addresses –Change company name (and Internet domain) Security Proxy –a proxy signs and decrypts on behalf of many users Privacy/Spam Attribute Certificates & S/MIME

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

IETF47, Mar 2000, Adelaide Solution Criteria Cryptographically bound association between an e- mail address and a public key Unambiguous reference from 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

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

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

IETF47, Mar 2000, Adelaide Cryptographically bind addresses with Gateway’s PK certificate cn=Gateway AC AC Attribute Certificates Attribute Certificates & S/MIME

IETF47, Mar 2000, Adelaide Attribute Certificates Cryptographically bind addresses with entity’s PK certificate cn=Alice AC AC Attribute Certificates & S/MIME

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

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 address Validity & Revocation –Validity: as long as the PKC & address remain valid –Revocation: use available standards Attribute Certificates & S/MIME

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 address 4.Retrieve PK certificates referenced by selected attribute certificates cn=Alice certificate= attributeCertificate= Alice’s new LDAP entry Attribute Certificates & S/MIME

IETF47, Mar 2000, Adelaide Practical Implementation (4/5) Message Verification Walkthrough –Retrieve 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

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

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 ACs and PKCs Security –Need to ensure that content of AC is valid Attribute Certificates & S/MIME

IETF47, Mar 2000, Adelaide Comparison with existing Infrastructure Existing Multiple addresses in certificate Re-issue keys in new certificate with new 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 addresses  Contra to legislation in some countries Proposed Store address in AC, which references PKC Issue AC’s as required Flexible method for maintaining 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

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