CMSC 414Computer (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

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

Midterm stats

Average: 65

(Roughly:) – 80-100: A– 60-80: B– 45-60: C– <45: D/F

Administrative items

HW3

Project coming soon…

Representing Identity(Chapter 14)

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)

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

Example: groups

An “entity” may be a set of entities, i.e., a group

Two implementations of groups1. Group is an alias for a set of principals;

principals stay in their groups

2. Principals can change groups; rights depend upon current group membership

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

Naming and certificates

Identifiers correspond to principals– Must uniquely identify the principal– (Real) names alone are not enough!

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

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…

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

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…

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

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)?

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…

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

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

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)

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

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…