I'm an engineer with experience in applied cryptography, in particular in Smart Card systems.
Feel free to edit my answers to fix obvious errors, or improve them on expression (I'm not a native English speaker).
7 Find the area of quadrilateral formed by $4$ (not consecutive) vertices of a $12$-gon inscribed in a circle. Apr 5 '14
6 A concise distance problem May 6 '13
4 Solving $x^e\equiv a\pmod n$ Mar 3 '14
3 Properties of finite magmas $(S,\cdot)$ with $\forall(x,y,z)\in S^3, x\cdot(y\cdot z)=y\cdot(x\cdot z)$? Jul 8 '12
2 Odds that a random $m$-bit integer has an at-least $n$-bit divisor with all its factors at most $k$ bits Jul 3 '13
2 How many commutative block ciphers are there? Apr 4 '14
2 System of linear diophantine modular inequalities Dec 22 '14