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CMSC 414 Computer (and Network) Security Lecture 15

CMSC 414 Computer (and Network) Security Lecture 15. Jonathan Katz. Review of cryptography…. Private-key (key shared in advance) Private-key encryption Message authentication codes (MACs) Public-key (PK distributed/SK secret) Public-key encryption Signature schemes.

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CMSC 414 Computer (and Network) Security Lecture 15

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  1. CMSC 414Computer (and Network) SecurityLecture 15 Jonathan Katz

  2. Review of cryptography… • Private-key (key shared in advance) • Private-key encryption • Message authentication codes (MACs) • Public-key (PK distributed/SK secret) • Public-key encryption • Signature schemes

  3. Review of cryptography… • Encryption does not provide integrity • Signatures/MACs do not provide secrecy • Signing is not the same as (public key) encryption/decryption • A “checksum” is not the same as a MAC • Deterministic encryption is not secure • CBC-MAC is not the same as CBC encryption

  4. Midterm stats • Average: 65 • (Roughly:) • 80-100: A • 60-80: B • 45-60: C • <45: D/F

  5. Administrative items • HW3 • Project coming soon…

  6. Representing Identity (Chapter 14)

  7. Identity • An identity specifies a principal (a unique entity) • Authentication binds a principal to a (representation of an) identity • Identities are used for, e.g., accountability and access control (among others)

  8. Example: files and objects • Note: the name of an object may depend on the context • E.g., a filename for human use, a file descriptor for process use, and a file allocation entry used by the kernel • E.g., user with different accounts

  9. Example: groups • An “entity” may be a set of entities, i.e., a group • Two implementations of groups • Group is an alias for a set of principals; principals stay in their groups • Principals can change groups; rights depend upon current group membership

  10. Roles • A role is a group that ties membership to function • When a principal assumes a role, the principal is given the rights belonging to that role

  11. Naming and certificates • Identifiers correspond to principals • Must uniquely identify the principal • (Real) names alone are not enough!

  12. E.g., X.509 certificates • Distinguished names identify a principal • Series of fields, each with key and value • E.g. /O=University of Maryland/OU=College Park/OU=Computer Science/CN=J. Katz • “O” - organization; “OU” - organizational unit; “CN” = common name

  13. Certificates • Certification authorities vouch for the identity of the principal to whom a certificate is issued • CA authentication policy determines the level of authentication needed to identify the principal before the certificate is issued • CA issuance policy describes the principals to whom the CA will issue certificates • A single CA can “act” as multiple CAs, each with their own policies…

  14. Example: Verisign (1996) • Three levels of authentication • Verification of valid email address • Verification of name/address • Background check • Different authentication policies; same issuance policy (individuals) • Another issuance policy was for issuing certificates to web servers

  15. Certificate infrastructure • Hierarchical structure of CAs • Nodes correspond to CAs • Children of a CA are constrained by the policies of their parents • Example… • We will revisit cert. infrastructures later…

  16. Example • Internet Policy Registration Authority (IPRA) issues certificates for policy certification authorities (PCAs) • PCAs certify other CAs • Note that their policies cannot conflict with those of the IPRA

  17. Conflicts • What if a single CA issues certificates under different policies? • What if a CA issues a certificate tied to an email address, but the owner of this address changes? • What if two CAs have the same dist. name? • What if two different CAs issue certificates for the same distinguished name (to different principals)?

  18. Easy solution • For organizational certificates, the last type of conflict can be prevented by incorporating CA name into distinguished name • Does not solve the other problems, in general…

  19. Handling conflicts • Conflict detection database… • Before a PCA may issue a certificate to a CA, it checks for a conflict in the database • Sends a hash of the CAs dist. name, the CAs public key, and the dist. name of the PCA • If first two fields conflict with a database entry, the two PCAs must resolve the conflict • Note that this only ensures uniqueness of (DN, PK) pairs

  20. Handling conflicts (in action) • Two CAs with same dist. name? • Will have different public keys… • Same CA with two different policies? • Will use different public keys for each

  21. What does identity mean? • Ultimately, identity is proved using physical means • Driver’s license, fingerprints, etc. • If these are compromised, then certificates are irrelevant! • Certificate is just a binding between external identity and (DN, PK)

  22. Anonymity vs. pseudonymity • Anonymity • No one can identify the source of any messages • Can be achieved via the use of “persona” certificates (with “meaningless” DNs) • Pseudonymity • No one can identify the source of a set of messages… • …but they can tell that they all came from the same person

  23. Levels of anonymity • There is a scale of anonymity • Ranges from no anonymity (complete identification), to partial anonymity (e.g., crowds),to complete anonymity • Pseudonymity is an orthogonal issue…

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