ruby, 80 bytes

Result

AC

Duration

577ms

Code [DL]

print gets.chop.chars.each_with_index.map{ |i,c| ((i.ord+c-64)%26+65).chr }.join

stdin

ZMUGBUSUCWIWWNCBRVGTYFZARN

stdout

AOXKGAZCLGTIJBRRINZNTBWYQN

stderr

strace