读取文件，给定密钥SECURITY，打印出加密矩阵，实现 Playfair 密码的解密算法，并生成一个包含明文的文本文件

很高兴为您解答这个问题！以下是实现 Playfair 密码的解密算法的代码：

def generate_matrix(key):
    # 生成 Playfair 密码加密矩阵
    matrix = []
    for c in key:
        if c not in matrix and c.isalpha():
            matrix.append(c)
    for c in "ABCDEFGHIKLMNOPQRSTUVWXYZ":
        if c not in matrix:
            matrix.append(c)
    return matrix

def decrypt(ciphertext, key):
    # 解密 Playfair 密码
    matrix = generate_matrix(key)
    plaintext = ""
    for i in range(0, len(ciphertext), 2):
        c1 = ciphertext[i]
        c2 = ciphertext[i+1]
        r1, c1 = divmod(matrix.index(c1), 5)
        r2, c2 = divmod(matrix.index(c2), 5)
        if r1 == r2:
            plaintext += matrix[r1*5+(c1-1)%5]
            plaintext += matrix[r2*5+(c2-1)%5]
        elif c1 == c2:
            plaintext += matrix[((r1-1)%5)*5+c1]
            plaintext += matrix[((r2-1)%5)*5+c2]
        else:
            plaintext += matrix[r1*5+c2]
            plaintext += matrix[r2*5+c1]
    return plaintext

# 读取文件并解密
with open("ciphertext.txt", "r") as f:
    ciphertext = f.read()
plaintext = decrypt(ciphertext, "SECURITY")

# 生成包含明文的文本文件
with open("plaintext.txt", "w") as f:
    f.write(plaintext)

请注意，这段代码只是一个示例，实际使用时需要根据具体需求进行修改和优化。