gnutls-min-prime-bits

gnutls-min-prime-bits is a variable defined in `gnutls.el'.
Its value is

256

Documentation:
Minimum number of prime bits accepted by GnuTLS for key exchange.
During a Diffie-Hellman handshake, if the server sends a prime
number with fewer than this number of bits, the handshake is
rejected. (The smaller the prime number, the less secure the
key exchange is against man-in-the-middle attacks.)

A value of nil says to use the default GnuTLS value.

You can customize this variable.