Distributed Computing for cracking a long passwd

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oclhashcat cannot crack passwords > 16 chars.

True, but oclHashcat-lite and hashcat CPU can. The problem is that the first 4 chars can not be static or you will loose all the performance.

In hashcat CPU you can also use this format:

hashalt

where salt is the fixed string you know and then use an appropriate salted algorithm type (if it exists).