The problem is smaller: the attacker needs to predict only the password; some packets are predictable for 100%. The password is the thing that you put at the end of a command: "ip ospf message-digest-key 1 md5 c1$c0" How long could the password be? 10-12 letters are considered to be a good password (if upper case, lower case, special letters, numbers), but people typically use just words (only 100k words are typically well-known). Then some APTs have billions of typical passwords in the database (prioritized for probability). They could just try them all on a good GPU. It is needed for hash to be slow. Hence, for example, SHA-2 consists of "And, Xor, Or, Rot, Shr, Add (mod 2^32)" in 64 or 80 rounds! (for every block of 512bits). Hence, a few milliseconds twice per every hop for a rather small control plane message.
It is strange for me that nobody cares about this latency. Eduard -----Original Message----- From: Jay Acuna <[email protected]> Sent: Monday, September 8, 2025 22:26 To: North American Network Operators Group <[email protected]> Cc: Saku Ytti <[email protected]>; Dan Collins <[email protected]>; Vasilenko Eduard <[email protected]> Subject: Re: MD5 is slow On Mon, Sep 8, 2025 at 3:00 AM Vasilenko Eduard via NANOG <[email protected]> wrote: > Your comments on the performance are very important. > I still believe that any Hash must be slow enough, because if it were > fast, then the attacker could take a big GPU and brute force it only the > password is not known, but could be tested from the dictionary). They do not require high latency. 0.1ms per call is still just fine. And the concept of brute force a hashing algorithm should resist involves many orders of magnitude more possibilities than contained in a password. Put it this way: MD5 has a block size of 512 bit. The MD5 algorithm has not failed in its security purpose for a hashing algorithm: If one is able to reverse an input by directly trying every possible input that contains a number of unpredictable bits 447 or less. Exactly the same way as it's not a MD5 problem if you have a 1-byte password, and someone tried all 255 possibilities. You really need a bare minimum of least a block of input; if not more to properly use MD5 and similar secure hashing algorithms. Predictable bits also don't help against guessing, so you should consider this as 512 bits of entropy on the input or more to safely use a hashing procedure calculated on 512-bit blocks. I also read MD5 input shorter than 512 must be padded congruent to 448 modulo 512. SHA-2 is similar. You need more bits than a typical password would contain. Standard secure hashing algorithms are not designed to save you in case your input contains fewer random bits. You have the option of using a key-stretching algorithm instead of a straight hash. PBKDF2, as mentioned before. Multiple rounds of hashing chained in a certain manner can cause delays for brute force guessing. A dictionary word contains about 4-bits worth of entropy. If the 512 bits are not filled, the input is to be padded with bits in front of your password, which are predictable, so they don't count. That is far from enough unpredictable bits to directly use MD5. From a randomized password; you get approximately 6 to 7 bits worth of entropy per character, so a good password length input for MD5 would be at about 85 random characters. -- -JA _______________________________________________ NANOG mailing list https://lists.nanog.org/archives/list/[email protected]/message/4LFVVQ7VGFM76EPKKFRSEAAXHKBGCREV/
