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I'm just trying to understand how a Weak/broken hash can break RSA-PSS or any other signature/encryption that uses it. I know you can forge another message that has the same hash but that message probably doesn't mean anything, sending random numbers isn't really worth the trouble.

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  • $\begingroup$ Welcome to Cryptography. If you are interested in Weak/broken hash function that can cause a break in RSA-PSS then it is a fine question, if you asking for any other signature/encryption then it is a broad question. This seems like a survey task, is it? $\endgroup$ – kelalaka Jun 7 at 16:31
  • $\begingroup$ Also, which version What is the difference between RSASSA-PSS signing and RSA-PSS signing? $\endgroup$ – kelalaka Jun 7 at 16:39
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    $\begingroup$ "I know you can forge another message that has the same hash but that message probably doesn't mean anything"; actually, if the hash is vulnerable to a chosen prefix collision attack (which would include MD5 and SHA1), then the prefix of both messages can be perfectly legitimate; there will be some segment of data which is likely random looking (the part that the attack selects); however depending on the format the data is in, that might be unremarkable. $\endgroup$ – poncho Jun 7 at 18:11
  • $\begingroup$ I'm interested in RSA-PSS and why a hash function works and why it's necessary for it to be collision resistant. how can a hacker for example exploit such weakness? like hashing with an MD5 or any other weak one like a hash function where H(2M)=4H(M) $\endgroup$ – Ahmad Wahbi Jun 8 at 12:03

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