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.