The 1988 version of X.509 lists properties that RSA keys must satisfy to be secure given current knowledge about the difficulty of factoring large numbers. The discussion concludes with a constraint on the public exponent and the modulus :

It must be ensured that e > log₂(n) to prevent attack by taking the eth root mod to disclose the plaintext.

Although the constraint is correct, the reason given for requiring it is incorrect.What is wrong with the reason given and what is the correct reason?