phc-discussions - Re: [PHC] Are password trailing 0's a problem?

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Message-ID: <op.xcfg8lzryldrnw@laptop-air>
Date: Sat, 08 Mar 2014 16:06:11 -0800
From: "Jeremy Spilman" <jeremy@...link.co>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Are password trailing 0's a problem?

Just restating what CiC said in bullet form, but seeing it in pseudo-code  
always helps me. When using password as the HMAC key, the way that PBKDF2  
does, you end up with:

if (password.length < block_size)
   password = password.pad(0x00, password.length - block_size)
else if (password.length > block_size)
   password = hash(password)

I guess it should be apparent that you can pass the hash of a password  
longer than the block size in place of the password, but I bet 99.9% of  
people using PBKDF2 would be "shocked" to learn this.