RE: MD5 is slow

[Vasilenko Eduard via NANOG <nanog () lists nanog org>]

Hi Ytti,
It looks like you are trying to teach me what is "salt". Salt + password greatly increases the challenge for the 
attacker: it is not possible to map a hash to a password just from the database (in 1 step).

For the attacker in the networking protocol case, the "salt" is always visible (these are additional fields from packet 
headers).
The attacker could still try different passwords from the database. But it would need many steps; every step is 
effectively the same processing as on the legitimate host (headers + password).
If we assume that on some platform (GPU?), the performance would be very fast (10B/sec), then all typical passwords 
would be tried in seconds.
It is not acceptable. Hash must be slow! (if used for signature, because hash for load balancing or routing tables have 
no such problem - they need only good randomness)

For symmetric encryption, the "salt" is the internal state of the encryption engine (initialization vector?) - it is 
not visible and changed/preserved between packets.
It is the reason why symmetric encryption is much stronger for this use case. Hence, symmetric encryption does not need 
to be slow.
Ed/
-----Original Message-----
From: Saku Ytti <saku () ytti fi> 
Sent: Wednesday, September 10, 2025 13:11
To: North American Network Operators Group <nanog () lists nanog org>
Cc: Vasilenko Eduard <vasilenko.eduard () huawei com>
Subject: Re: MD5 is slow

On Wed, 10 Sept 2025 at 13:01, Vasilenko Eduard via NANOG <nanog () lists nanog org> wrote:

> IMHO: Then it was bad design. The source text is visible if a hash is used for the signature. Only the password is 
> not known.

Please make a serious attempt in trying to understand how applications are different.

Try to understand why unix passwords benefit from slow hash. You only have the password hash as output, any input that 
provides same hash, is equivalent. So any collision you find, you have exactly the same problem and serious problem.

MD5 or SHA in BGP, ISIS, OSPF are not like this. There isn't even necessarily guarantee that useful collisions exist, 
as you may not have enough bits that can have arbitrary value while keeping PDU valid and conducive towards your attack 
vector.

Most collisions would be garbage, where PDU is rejected. Therefore even if we assume we could cause MD5, SHA 
collisions, it wouldn't still matter.

You have good rationale in wanting slow hash, but you struggle to understand why not all applications are about hashing 
8byte secrets.
--
  ++ytti
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