A recent compiler complains about the sprintf() in authenticate() being able to
write up to 1023 bytes into challenge, which is not possible given that getChallenge()
returns a string 10 bytes smaller than whatever string is in its 1024 byte buffer.
Reducing the size of the buffer in getChallenge() quiets the warning.  It should not
affect anything else, as the challenge string is used as a salt that must be <= 96 bits.