[Dovecot] Custom password encryption scheme, how to do it?

[Madison Kelly]

Ed W wrote:
> 
>> - Generate an initial SHA256 hash out of the password+salt.
>> - Re-hash the initial SHA256 hash many thousands of times.
> 
> As an aside you should do some research to determine if the second of 
> these steps adds any value.  I don't believe that there is a known way 
> to reverse an SHA256 hash, and if one is discovered it's not immediately 
> obvious that the technique would not break the case of it being applied 
> multiple times...

The value it adds is that it slows down a brute force attackers by 
reducing the number of keys they can try per second (modest systems can 
try ~65,000 keys per second). So by re-encrypting the keys (say 65,000 
times) reduces the number of keys an attacker can try per second from 
about 65,000/sec to 1/sec.

<... looks for the article ...>

Found it:

http://en.wikipedia.org/wiki/Key_strengthening

> Also the keyspace of a password with say 8 alphanumeric chars is very 
> much smaller than an SHA256 space, so you have a big bruteforce issue 
> already

I will be the first to acknowledge that my encryption scheme is probably 
a healthy way into "overkill". As it is, the salt is a 32-byte string of 
alternating mixed-case letters, numbers and other characters (like 
space, '/', '\', '!', etc...).

The reason for the strength is that I use the underlying password scheme 
for multiple projects, some of which contain medical and financial 
information. Dovecot itself isn't such a big concern, but I like to 
standardize.

> Basically it's not immediately obvious that step 2 adds any or at least 
> significant value.  Perhaps instead use a larger salt?

It's just to slow down brute force attacks and to help reduce the 
usefulness of rainbow tables that much further.

> If you are using sql lookups then of course you can code all kinds of 
> stuff as part of the lookup...
> 
> Good luck

Thanks kindly for your reply! I make *no* claims to being a security 
expert, so I quite welcome any feedback on my scheme. :)

Madi