I know this applies to secure ciphers, but does this also apply to hashes?

  • 4
    $\begingroup$ That depends on what's hashed. If the first half of the SHA-1 hash of a password is EC461B5480380ECF863D, I bet the second half is 9802EDBE70152AEE1C46. Search the concatenation to see why. When the present page will have been indexed, searching the first half will likely yield the second. $\endgroup$
    – fgrieu
    Mar 22, 2016 at 7:33
  • $\begingroup$ While the question may not be a duplicate, the answer is the same as the one given to this question $\endgroup$
    – malexmave
    Mar 22, 2016 at 8:03
  • $\begingroup$ Could you explain how this "applies to secure ciphers"? $\endgroup$
    – Maarten Bodewes
    Mar 25, 2016 at 15:17
  • $\begingroup$ @malexmave In this case I think the question is too far away from the one linked to. The explanation may be similar but it could well be that answers on this question will have a different structure. $\endgroup$
    – Maarten Bodewes
    Mar 25, 2016 at 15:50
  • $\begingroup$ @MaartenBodewes Yes. This wasn't to suggest that this should be closed as duplicate, only to give a pointer to existing knowledge on the site for this topic. $\endgroup$
    – malexmave
    Mar 26, 2016 at 8:36

1 Answer 1


Not without knowing anything about the message.

Each 32-bit word of the hash is a function of the current state plus one 32-bit word of the expanded message.

If the message is longer than 448 bits (512-bit block less 64 bits for length) then you would also need to know the output of the previous block, since it gets added to the output of the current block.


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