Re: MD5 is slow

[Dan Collins via NANOG <nanog () lists nanog org>]

On Fri, Sep 5, 2025, 03:22 Vasilenko Eduard via NANOG <nanog () lists nanog org>
wrote:

> Any hash MUST be slow (by design) to withstand brute force. In the network
> device case, it is about 5ms for SHA-2 (of course, dependent on the control
> plane processor).


It is not necessary for a cryptographic hash function to be slow (on the
order of milliseconds), nor is it true that SHA2 is designed to be slow, or
that it is that slow on short inputs. Even relatively weak processors can
compute millions of hashes per second with SHA2 hash functions. If you
could find a way to compute hashes a million times faster, and used that in
a brute-force preimage attack, you'd be shaving off the equivalent of 20
bits of difficulty - not near enough to make a brute force attack on a
256-bit hash tractible.

The case where you do care about speed is when you're dealing with password
hashing, for example. The threat is a dictionary attack, since testing even
the most common 10,000 passwords against a database of user accounts is
very likely to compromise a good percentage of them. Using pbkdf2, or
bcrypt, or argon2, allows you to slow that down. But that's only because we
know that a lot of people use simple or common passwords. You're likely to
get some hits even in the first few thousand attempts.

For signing or message integrity checking, that isn't the case. If you're
trying to find a second public key (or any other message) corresponding to
a specific predetermined hash value, by brute force, the probability of
each attempt succeeding is 2^-n. Even if you could try a million hashes per
microsecond on every CPU that exists on the planet Earth, we're talking
about something like 10^40 years to succeed at a sha-256 preimage attack.
128 bit hashes would be faster but still not remotely possible.

>
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