# Can one reverse a hash with partial plaintext knowledge?

First off, please forgive my ignorance because I am not as well versed in cryptography and mathematics as I would like to be. I may say something obviously wrong/dumb; please point it out!

Is there any technique which may reduce the difficulty of recovering plain text from a hash if the attacker knows part of the plain text?

Here is an example. Alice writes this letter:

Dear Bob,
Run!
Yours Truly,
Alice


Alice sends the plaintext to Bob over channel X and the hash over channel Y.

Eve intercepts the hash but not the plaintext.

Eve knows Alice starts the message with "Dear Bob," and ends with "Yours Truly, Alice". Eve also knows that Alice sends messages in fixed blocks and will pad the body content with whitespace to meet the required block length.

Can Eve use her knowledge of 1)fixed message size and 2)partial plaintext knowledge to reduce the computational difficulty of reversing the hash?

I would appreciate any help on this topic, even if your answer is just "read this whitepaper".

• @Kaveh: Probably "cryptographically-secure hash functions." May 28 '11 at 14:09
• "partial plaintext knowledge" sounds to me similar to salt in the context of crypto. May 28 '11 at 17:00
• @Kaveh: Collision-resistance and one-wayness are orthogonal but related properties. "Cryptographically-secure hash functions" is an umbrella term for both. However, I believe this question is more concerned with one-wayness (i.e. the hardness of finding one preimage). May 28 '11 at 19:37
• @Sadeq, I added a link to wiki page of cryptographically-secure hash functions. May 29 '11 at 7:28
• The answer depends upon the hash; the question is ill-specified. What hash function are you using? Alternatively, what can be assumed about the hash function?
– D.W.
May 30 '11 at 4:21