571 B | 571 chars
import sys
from math import sqrt, pi, sin, factorial
inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = " " : [*map(int,inp().split(x))]
mfi = lambda x = " " : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def solve():
a, b = input().split()
ret = ""
for i in range(len(b)):
k = ord(a[i % len(a)]) - 65
ret += chr((ord(b[i]) - 65 + k + 26) % 26 + 65)
return f"Ciphertext: {ret}"
if __name__ == "__main__":
tc = ii()
for t in range(1, tc+1):
ret = solve()
p(ret)