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A database file is encrypted with 256bits AES and the key is the first 7 characters (from the character set 0-9, a - f, all small letter) of MD5 hash . This implies 2^28 bits (268,435,456) possible passwords. How long will take to brute force such a password?

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    It depends how long it takes to verify a password but cryptopp.com/benchmarks.html indicates that you can perform about 255 million MD5 hash per second on a relatively weak CPU which, more or less, means walking through the whole keyspace in a bit over one second. – Stephane May 30 '16 at 12:22
  • You have to factor into account hardware, as it stands your question is too broad and unlikely to allow us to provide a good answer. Are you asking about the time just out of curiosity? or do you want to know because you are assessing a specific security implementation? – Purefan May 30 '16 at 13:17
  • @Purefan, I am writing a report on decrypting a 256BITS AES encrypted database. I don't have the facility to test out the brute forcing scenarios, however, it would help my report for knowledge and documentation purpose. – e.kenny67 May 30 '16 at 13:23
  • Then the most accurate answer I guess would be to figure out the logarithmic complexity, otherwise you're looking at answers like "a lot" which are not really useful – Purefan May 30 '16 at 14:44
  • @Stephane that sounds like an answer – Neil Smithline May 30 '16 at 15:46
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To clarify and expand what I said in the comment:

A GPU-based cracker can go through a LOT of MD5 hash per seconds. According to this, one based on the (old) NVIDIA Geforce 8800 Ultra can compute about 200 million hashes per seconds. Since your keyspace is a bit above 2 millions entry large, it means that you can get through ALL of it in about one second and a half.

That doesn't, however, tell you the whole story: you're not trying to generate a collision here but you're trying to find the key used in an encryption scheme. This means that each of these hashes must then be tested. It is very possible - even likely - for this step to take much longer that simply generating a hash. Exactly how long depends on how the chosen algo is used exactly and at what stage you can decide if the key is valid or not.

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