Caesar's cipher

print("""== Caesar's encryption/decryption algorythm ==
To switch between encryption/decryption modes, enter a single 'e' or 'd'.
To input encryption key, enter a number 1...25.
To quit, enter 0.""")

def cipher(t):
    d = {chr(ord('a')+i):chr(ord('a')+(i+key*encrypt)%26) for i in range(26)}
    d.update({chr(ord('A')+i):chr(ord('A')+(i+key*encrypt)%26) for i in range(26)})
    return ''.join([d[c] if c.isalpha() else c for c in t])

go_on = True
encrypt = 1
key = 0

while go_on:
    u = input()
    if u in [str(i+1) for i in range(25)]:
        key = int(u)
        print('<Encryption key switched to %d>'%key)
    elif u == '0':
        go_on = False
        print('<Quit>')
    elif u == 'e':
        encrypt = 1
        print('<Switched to encryption mode>')
    elif u == 'd':
        encrypt = -1
        print('<Switched to decryption mode>')
    else:
        print(cipher(u))