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System Security. Verifying the identity of another entity Two interesting cases (for this class): Computer authenticating to another computer Person authenticating to a computer Two issues: How authentication information is stored (at both ends) Authentication protocol itself.

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Verifying the identity of another entity

Two interesting cases (for this class):

Computer authenticating to another computer

Person authenticating to a computer

Two issues:

How authentication information is stored (at both ends)

Authentication protocol itself

password based protocols
Any password-based protocol is vulnerable to an off-line dictionary attack if server is compromised

Goal: password-based protocol should be secure against off-line attacks when server is not compromised

Unfortunately, this has not been the case in practice (e.g., telnet, cell phones, etc.)

Password-based protocols
password selection
User selection of passwords is typically very weak

Lower entropy password makes dictionary attacks easier

Typical passwords:

Derived from account names or usernames

Dictionary words, reversed dictionary words, or small modifications of dictionary words


Password selection
better password selection
Non-alphanumeric characters

Longer phrases

Can try to enforce good password selection…

…but these types of passwords are difficult for people to memorize and type!

Better password selection
password storage
In the clear…

Hash of password

“Salt”-ed hash of password

Makes bulk dictionary attacks harder, but no harder to attack a particular password

Centralized server stores password

Threshold storage of password

Password storage
centralized password storage
Authentication storage node

Central server stores password; servers request the password to authenticate user

Auth. facilitator node

Central server stores password; servers send information from user to be authenticated by the central server

Note that central server must be authenticated!

Centralized password storage
basic authentication protocols
Server stores H(pw); user sends pw

Secure against server compromise, but not eavesdropping (or replay attacks)

Server stores pw, sends R; user sends H(pw,R)

Secure against eavesdropping, but not server compromise (or dictionary attack)

Can we achieve security against both?

Basic authentication protocols…
authentication of people
What you know (passwords)

What you have (keys)

What you are (biometric devices)

Where you are (physical)

Authentication of People
trojan horses
A faked login prompt to capture passwords

Counter measures:

Make it hard to have the appearance of login prompt

Use interrupts

Prevent login by user programs

Trojan Horses
authentication tokens
What you have

Smart cards:


Cryptographic calculator:

Interaction through a user (typing ...)

Authentication Tokens

False acceptance rate.

False rejection rate.

Can adversary select imposters?

Identical twins, family members, etc.

Retinal scanner, fingerprint reader, handprint reader, voiceprint, keystroke timing, signature.


Dummy fingers and dead fingers

Suitability and stability:

Not for people with high probability of damaged fingerprints

Not for kids growing up

voice recognition
Single phrase:

Can use tape recorder to fake


Background noise


Use with public phones

Voice Recognition
access control
State of a system

Includes, e.g., current memory contents, all secondary storage, contents of all registers, etc.

Secure states

States in which the system is allowed to reside

Security policy defines the set of secure states

Security mechanism ensures that system never leaves secure state

Access control
access control1
Access control matrix

Characterizes rights of each active entity (“subject”) with respect to every other entity

In any secure state, only transitions to other secure states are allowed

Often concerned with transitions that affect the protection state of the system

I.e., actions which alter the actions a subject is authorized to take

Access control
access control matrix
Protected entities: “objects” O

Active objects: “subjects” S (i.e., users/processes)

Note that subjects are also objects

Matrix A contains an entry for every pair (s, o)

The entry contains the rights for s on o

Examples: read/write/execute/etc.

Protection states represented by (S, O, A)

Access control matrix
more complex access control
In general, “rights” may be functions

“Actual” rights depend on the system state

Equivalently, may depend on system history

May be more convenient to express in non-matrix form

E.g., boolean expression evaluation

More complex access control
attenuation of privilege
Copy right

Ability to transfer your rights to someone else

Copier may have to surrender the right

Own right

Ability to grant rights on the object to others

Attenuation of privilege

“A subject may not give rights it does not possess”

Attenuation of privilege
the problem
Drawbacks of access control matrices…

In practice, number of subjects/objects is large

Most entries blank/default

Matrix is modified every time subjects/objects are created/deleted

The problem
access control lists acls
Instead of storing central matrix, store each column with the object it represents

Stored as pairs (s, r)

Subjects not in list have no rights

Can use wildcards to give default rights

Access control lists (ACLs)
example unix
Unix divides users into three classes:

Owner of the file

Group owner of the file

All other users

Note that this leaves little flexibility…

Some systems have been extended to allow for more flexibility

Abbrev. ACLs overridden by explicit ACLs

Example: Unix
modifying acls
Only processes which “own” the object can modify the ACL of the object

Sometimes, there is a special “grant” right (possibly per right)

Modifying ACLs
handling conflicts
Conflicts may arise if two entries give different permissions to a subject

Multiple ways of resolving this; e.g.:

Allow access if any entry gives rights

Allow access only if no entry denies rights

Apply first applicable entry

Handling conflicts
default permissions
Two approaches

Apply ACL entry, if it exists; otherwise, apply default rule

I.e., ACL entries override default permissions

Augment the default permissions with those in the appropriate ACL entry

Example: what if default allows “read” and ACL entry states “write”?

Default permissions?
How to handle revocation?

Easy to revoke all access with ACLs

But…what if two different users granted the same right to the same person?

E.g., both A and B give rights to C. Now A wants to revoke C’s rights. But B still wants to allow rights…

Can store a history of ACL modifications

How to revoke access to a file?

Would potentially require revoking all capabilities associated with that file

One solution: indirection

Capabilities name an entry in a table, rather than an object itself

To revoke access to object, invalidate entry in table

Or, change random number associated with object

Difficult to revoke access by a single subject

potential problems
What if one process gives capabilities to another? (Possibly indirectly)

Can lead to security violation

One solution: assign security classifications to capabilities

E.g., when capability created, its classification is the same as the requesting process

Capability contains rights depending on the object to which it refers

Potential problems
Cryptographic key used to encrypt a file

A file cannot be “read” unless the subject has the encryption key

Can also enforce that requests from n users are required in order to read data (and-access), or that any of n users are able to read data (or-access)

cryptographic secret sharing
(t, n)-threshold scheme to share a “key”

Using this to achieve (t, n)-threshold encryption

Shamir secret sharing

Cryptographic secret sharing
another example
Type checking

E.g., label memory locations as either “data” or “instructions”

Do not allow execution of type “data”

Can potentially be used to limit buffer overflows

Another example
propagated acls
Associated with data, not the object holding the data

Prevents one user from copying data in a file to a new file, thereby giving access to the data

When a subject reads from an object, the subject’s PACL is updated

When an object is created, the object takes on the PACL of the creator

Propagated ACLs
A creates file f, and allows B, D to access it

After B reads f, PACLnew(B) = PACLold(B)  PACL(A)

If B creates new file f’, this file is assigned PACLnew(B)

User U can access f’ only if U is in both PACLold(B) and PACL(A)

final points for now
Access control matrices can express any (reasonable) security policy

In practice, such matrices may not be used because of complexity, space requirements, etc.

Final points (for now…)