General brute force attack question

Robert J. Hansen rjh at
Wed Jun 17 01:45:57 CEST 2015

> Is this a correct interpretation?

Pretty close.

> My understanding of en-/decryption is that there is no indication of 
> progress toward finding a successful key match of a given
> encryption.

Not quite.  If you're doing a brute-force attack it's easy to figure out
what fraction of the possible number of keys you've tried, and to
present that as a progress bar -- when the progress bar is half done,
you've searched half the possible keys, and thus there's a 50% chance of
finding the key by then.  So yes, it's possible to come up with a pretty
good estimate of how long it'll take to brute-force a cipher, and that
lets you do things like status bars... it's just that the amount of time
is, for any good system, ludicrously big.

> Related to this is the oft-repeated request to avoid identifiable 
> information (initials, birth date, etc.) in a cryptographic key. I 
> presume this gives an attacker a preferred set of characters to 
> attempt before moving on to truly random combinations.

Called "cribs", yes.  Even then, this is rarely used in the key itself.
 Usually it's used as the input to a key derivation function, which
accepts something nice and English-like as input and yields a garbled
mess for output.

> Finally, a brute force attack requires potentially billions of 
> attempts.

Add *many* more zeroes on to this.  :)

> How does an attacker then perform a brute force attack? Does he cadge
> a block of encrypted text and hammer on that until success?

Without getting into high levels of detail, all I can say is "it will
vary from system to system."

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