Correct handling of gcd(p - 1, e) in RSA keygen
Correct handling of gcd(p - 1, e) in RSA keygen

We were calling inverse mod but because p - 1 > e the binary
extended euclidean algorithm was used instead of the const time
version.

Use the fact that e is odd (for RSA keys) to remove the factors of
2 from p - 1 and then check coprimality that way, since it allows
using our const time algo.

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