When doing the public key exchange, the number of prime bits should be high <ins>enough</ins> to ensure that the channel can't be eavesdropped on by third parties.  If this number is too low, <del>you</del> <ins>Emacs</ins> will <del>be warned.</del> <ins>warn you.  (This is the @code{diffie-hellman-prime-bits} check in @code{network-security-protocol-checks}).</ins>