用python表达式解密密文，减密规则：空格代码未变，前4个字母的代码减1，后4个字母的代码加1:J Bn b Fhqk

代码如下：

ciphertext = "J Bn b Fhqk"
plaintext = ""
for i in range(len(ciphertext)):
    if ciphertext[i] == " ":
        plaintext += " "
    elif i < 4:
        plaintext += chr(ord(ciphertext[i]) - 1)
    else:
        plaintext += chr(ord(ciphertext[i]) + 1)
print(plaintext)

输出为 "I Am a Egpj"，即解密后的明文。