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Here is the details: I found a web api while pentesting for a company which I cannot disclose and while doing so I found a API which signs a request of the users choice, but it adds a tag to the top then SHA1's the message then signs it with RSA textbook scheme with no padding. And I'm trying to attack another API that uses the same key keypair for verification, but it cannot have that tag that's appended on the top for use.

What type of attacks are practical for this situation (If there is any)?

What I'm trying to do: Is forge a signature with my selection of the message/SHA1 hash.

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    $\begingroup$ Your description of the signature scheme is very unclear to me. Could you maybe be more precise? $\endgroup$
    – Maeher
    Nov 27, 2017 at 22:30
  • $\begingroup$ @Maeher Let me rewrite It, I just reread it, and I agree with you. I was like up around 2:00 AM writing this. $\endgroup$ Nov 29, 2017 at 14:59
  • $\begingroup$ maybe related crypto.stackexchange.com/questions/20085/… $\endgroup$
    – gusto2
    Nov 29, 2017 at 15:58
  • $\begingroup$ @gusto2 I just added a bit more description in OP $\endgroup$ Nov 29, 2017 at 16:54

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