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 Feb 8 awarded Autobiographer Feb 7 comment How to find roots of equation $f(x)=0 \pmod p$, where $p$ is prime number? How are F and f related. Is f in the title the sames a s f in the text? What have you tried and what is your problem? Please improve your question. Jan 31 comment Is $H'(M) = 5M + 9H(M) \pmod{2^n}$ a secure hash function? is this question on topic her? I dont think so: Do we accept questions asking for cryptanalysis of your cipher (hash function, ...) design? Jan 30 comment How to calculate plaintext from modpower cipher So I think answers should concentrate on the fact that e is small an not on well known requirements of the RSA. Jan 30 comment How to calculate plaintext from modpower cipher This is related to the RSA cryptosystem en.wikipedia.org/wiki/RSA_(cryptosystem)#Key_generation . You can decrypt a message if you can factor the modulus, which is easy to factor if it has only small prime factors. If k is the number of prime factors then the smallest prime factor cannot have more than #bits(n)/k bits. So k=2 is optimal. The advantage to use a small exponent e is that encryption is very fast. But does a small e have drawbacks? It seems so: check footnote 8 of the wiki article or paragraph 4 of crypto.stanford.edu/~dabo/abstracts/RSAattack-survey.html . Jan 14 awarded Critic Jan 14 awarded Citizen Patrol Jan 10 awarded Teacher Jan 7 comment Hash function that allows to decide if A > B if you only have hash(A) and hash(B)? I think that prove does not really make sense but it is to long to describe this in a comment so I post an answer crypto.stackexchange.com/a/31765/1915 Jan 7 answered Hash function that allows to decide if A > B if you only have hash(A) and hash(B)? Jan 7 comment Hash function that allows to decide if A > B if you only have hash(A) and hash(B)? No, that does not mean that $H(a)H(b)$. It means that you have a function $f$ and $f(H(a),H(b))$ returns 'true' if \$a B if you only have hash(A) and hash(B)? That is wrong. The OP does not require that H(A)