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Application of Attribute Certificates in S/MIME

Application of Attribute Certificates in S/MIME. 47 th IETF Conference Adelaide, March 2000. Greg Colla & Michael Zolotarev Baltimore Technologies. Attribute Certificates & S/MIME. Overview. S/MIME and PK Certificates S/MIME Problems Secure mail requirements Possible solutions

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Application of Attribute Certificates in S/MIME

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

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

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

  4. alice@foo.com alice@dev.foo.com • alice@bar.com Attribute Certificates & S/MIME 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 IETF47, Mar 2000, Adelaide

  5. 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 Essential Requirements IETF47, Mar 2000, Adelaide

  6. Attribute Certificates & S/MIME 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 IETF47, Mar 2000, Adelaide

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

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

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

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

  11. Attribute Certificates & S/MIME 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) IETF47, Mar 2000, Adelaide

  12. Attribute Certificates & S/MIME 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 IETF47, Mar 2000, Adelaide

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

  14. Attribute Certificates & S/MIME 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 IETF47, Mar 2000, Adelaide

  15. Attribute Certificates & S/MIME 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) IETF47, Mar 2000, Adelaide

  16. Attribute Certificates & S/MIME 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 IETF47, Mar 2000, Adelaide

  17. 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 Comparison with existing Infrastructure IETF47, Mar 2000, Adelaide

  18. Attribute Certificates & S/MIME 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 IETF47, Mar 2000, Adelaide

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