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my PC found a factor for (2^2048)-1 in under a second...so does that make RSA-2048 less secure right? No. Factoring numbers with special forms like that is easy. You have a Mersenne number, $n = 2^e - 1$, whose exponent $e = 2048$ is composite. Whenever $e = u v$, we have $2^u - 1 \mid (2^u)^v - 1 = 2^e - 1$, since in general $x - 1 \mid x^k - 1$. (...
estimate the probability that Pollard's p-1 factorization in its two-stages variant finds a factor of an RSA modulus product of $k$ random $b$-bit primes, as a function of the bounds $B_1$ and $B_2$ used TLDR: GMP-ECM is fine. Modify it for $b$ bits rather than n decimal digits, and use $k\;P$ where $P$ is the low probability shown. The way GMP-ECM ecm -v -...