Quiz 1 key

1)	Stream ciphers are best used in applications which require
          quick encryption where the data may come in bursts of
          unknown size.  This is because stream ciphers encrypt one
          byte of plaintext immediately into one byte of ciphertext,
          so no padding is needed.  Stream ciphers are useful over
          noisy connections because they can usually recover
          completely from corrupted bits with little or no error
          propagation.

Block ciphers encrypt blocks of plaintext into ciphertext, not single
characters.  The block size is independent of the byte size, so this
method gives high diffusion.  Block ciphers are also immune to
inserted symbols because insertion would probably change the block
size, and the insertion would be detected in decryption.

2)	The Bell-La Padula model is a data access scheme that is
          designed around access of data.  The system consists of
          subjects, S, and objects, O.  Each subject and object is
          given a security class, denoted C(S) and C(O).  Classes are
          ordered by the less-than/equal-to relation.  There are two
          access rules that are defined for the classes.  The first is
          the Simple Security Property, which states that S may have
          read access to O only if C(O) is less than or equal to C(S).
          The second, called the *-Property, states that a subject S
          who already has read access to O may have write access to
          O' only if C(O) is less than or equal to C(O').

The Biba model is very similar to the Bell-La Padula model in design
because it also orders objects by security class.  The difference is
that Biba is concerned with data integrity instead of access, so the
classification I(O) identify the trustworthiness of objects, not
sensitivity of objects like C(O) did.  Biba uses the
greater-than/equal-to operator instead of the less-than-equal-to one.
It also has two basic properties based upon subjects and objects.  The
first is the Simple Integrity Property and states that S can modify O
only if I(S) is greater that or equal to I(O).  The second, called the
Integrity *-Property, states that if S has read access to O with
integrity I(O), S can modify O' only if I(O) is greater than or
equal to I(O').

3)	Refer to the homework 1 key, problem 2.