Which of the following is NOT a characteristic of a cryptographic hash
function, H (m), where m denotes the message being hashed by the
function H?

A.
H (m) is a one-way function.

B.
H (m) is difficult to compute for any given m.

C.
The output is of fixed length.

D.
H (m) is collision free.

Explanation:
For a cryptographic hash function, H (m) is relatively easy to compute
for a given m.
* Answer “H (m) is collision free” is a characteristic of a good cryptographic
hash function, in that collision free means that for a given message, M,
that produces H (M) = Z, it is computationally infeasible to find another
message, M1, such that H (M1) = Z.
* Answer “The output is of fixed length” is part of the definition of
a hash function since it generates a fixed-length result that is independent
of the length of the input message. This characteristic is useful for
generating digital signatures since the signature can be applied to the
fixed-length hash that is uniquely characteristic of the message instead
of to the entire message, which is usually much longer than the hash.
* Answer “H (m) is a one-way function” relates to answer “H (m) is difficult to compute for any
given m” in that a one-way function is difficult or impossible to invert. This means that for a hash
function H (M) = Z, it is
computationally infeasible to reverse the process and find M given the
hash Z and the function H.