Elliptic Curve Cryptography, decrypting returns false result
I would imagine your calculator doesn't know how to handle the point at infinity because it relies fully on affine points. You can compute that

\begin{align*}
    C_2 &amp;= P + k*e_2 \\
        &amp;= (45,48) + 47*53*(3,25) \\
        &amp;= 5*(3,25) + 47*53*(3,25) &amp; (\text{since }(45,48) = 5*(3,25)) \\
        &amp;= 2496*(3,25) \\
        &amp;= \infty &amp; (\text{since }2496\equiv0\bmod 64)
\end{align*}
This computation seems to go wrong somehow, possibly because it can't represent $\infty$ internally.