How I became a password cracker

http://arstechnica.com/security/2013/03/how-i-became-a-password-cracker/

by shawndumas • 13 years ago

95 34 13 years ago

shanelja5• 13 years ago

Regarding your tips on passwords, one technique I used to employ in password security was the following:

When the user signs up, store a Timestamp for creation date, then take their email address and use it as a prefix and suffix to salt the password, for instance:

If the users password is shanelja, then the routine would be as such:

TIMESTAMPshaneljaEMAIL

or in my case:

147182994718shaneljashanea93@hotmail.co.uk

This produces a ridiculously hard password to brute force.

While this may be easy to compute if you know the rule, the general rule of thumb is that if your server has been attacked in such a way that the attacker has a copy of your database, they will most likely know the rule in any case, so it makes sense to have a good one.

Outside of this, no one should be using MD5 in this day and age, I would even recommend against Sha256 and other variants in that family, though I have a lot of respect for Blowfish, etc.

sparky• 13 years ago

Just use bcrypt [1] (or scrypt [2])

[1] http://codahale.com/how-to-safely-store-a-password/ [2] http://www.tarsnap.com/scrypt.html

shanelja5• 13 years ago

Bcrypt uses the blowfish encryption algorithm published by Bruce Schneier in 1993. [1]

I do.

[1] http://bcrypt.sourceforge.net/

shared4you2• 13 years ago

Someone else explicitly says "Don't use bcrypt" [1]

[1] http://www.unlimitednovelty.com/2012/03/dont-use-bcrypt.html

raverbashing3• 13 years ago

Using bcrypt or scrypt or PBKDF2 is useless if you hash only the password with it.

It's trivial to build a lookup table like with md5, it just takes longer.

Salting is essential. I don't know why the parent was downvoted (probably because his salt is predictable?) so good salting is needed.

Example: WPA2 uses the SSID as the salt and PBKDF2 to derive the encryption key.

pluies_public• 13 years ago

Uh? What's the advantage of this technique over a randomly generated salt?

shanelja5• 13 years ago

To be honest, there isn't any, when I implemented it I was the tender age of 16 and thought there was some validity in the method, but in itself, once you get to these kind of 30-ish letter long passwords, they become damn near impossible to brute force.

The only redeeming factor I suppose would be that you have to store your random string somewhere anyway, at least this method required code traversal rather than just an SQL field named "password_salt".

zobzu2• 13 years ago

well, its a salt, too. just not so random. ;-)

jasey• 13 years ago

Because part of the puzzle is in the source code, and if the attacker got the encrypted user table from the common method of SQL injection they don't know the rule which is defined only in the function which checks validation of the pw and generates the pw hash.

What I do similar is,

Pw_hash = hash('f4/$$er3@' + salt + plain_text_pw);

If the attacker only gets the database (which has the hash and the salt) and not the source code they don't have the 'f4/$$er3@' value needed to perform any attacks

jlgaddis• 13 years ago

> I would even recommend against Sha256 and other variants [...]

Why?

shanelja5• 13 years ago

Mostly because it's very quick to compute millions of hashes, meaning that it lessens the time required to crack a bunch of passwords - of course, this might seem like the last of your problems if your system has been attacked, but most of your users probably have the same password for every site they use, including Paypal, etc.

Something which can be scaled to take a lot longer to hash can make it expensive and practically unfeasible for someone to crack more than the simplest passwords.

chickopozo5• 13 years ago

>Outside of this, no one should be using MD5 in this day and age, I would even recommend against Sha256 and other variants in that family, though I have a lot of respect for Blowfish, etc.

@shanelija SHA-256 is a secure cryptographically secure hash function. It is not and was never intended nor should be used as a key derivation function.

Given time and email are not unique, they should not be used as a salt. Concatenation of a salt is not the correct method either, not for KDF and not for message authentication.

One of the aims of a hash functions is to be fast, really fast, super fast while making it computational infeasible to: make a message that will hash to a given value alter a message that when hashed will produce the same or a give value. Find two different messages with the same hash value.

bcrypt, scrypt, PBKDF2 are perfect for password storage.

YOU are not off the hook yet until you repeat these 2 sentences 10 times: A secure crypto algorithm/system/function/whatever MUST continue to be secure even if the algorithm/system/function/whatever is public.

It is perfectly fine to homebrew crypto given one key condition, you never ever ever ever use it for things that matter.

dhimes• 13 years ago

shanelija said s/he uses bcrypt. Apparently s/he just makes the passwords stronger than the user makes them before hashing.

zobzu2• 13 years ago

Meh, I had to read all 3 pages to read that the author just didn't load the word list properly :(

chickopozo5• 13 years ago

I want my time back, three pages on basically nothing. I broke into my house last year because I lost my keys -> brb writing an article titled "How I became a burglar and locksmith"*

*I took my keys from in the house so... by ars standard that's serious theft.

Rather than reading that article might I suggest having a gander at moxie's https://www.cloudcracker.com and read his blog too for all the goodies.

chickopozo5• 13 years ago

Blog: https://www.cloudcracker.com/blog/

enraged_camel• 13 years ago

>>I want my time back, three pages on basically nothing.

Yeah, it's shameful that such a poorly written article is being touted as a "feature story." Then again that's what happens when your editor-in-chief demands articles of a certain length and you don't have much to say, so you fill three pages with bullshit. Kind of like in college!

How I became a password cracker

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