Exponentiation with short, public exponents doesn't use a
precomputation table. Building the table would take more time
that it would eventually save.
However without explicit parenthesis the test for that parsed as
"(public and e.len < 3) or (e.len == 3 and top_byte <= 0x0f)"
and not "public and (e.len < 3 or...)" as intended.
Not a practical issue since a secret exponent is never going to be
short, but we're still supposed to use the constant-time path for
non-public exponents.
Frank Denis
1ea73060bb