crypto.py 文件源码

def decrypt(cls, key, keyusage, ciphertext):
        if len(ciphertext) < 24:
            raise ValueError('ciphertext too short')
        cksum, basic_ctext = ciphertext[:16], ciphertext[16:]
        ki = HMAC.new(key.contents, cls.usage_str(keyusage), MD5).digest()
        ke = HMAC.new(ki, cksum, MD5).digest()
        basic_plaintext = ARC4.new(ke).decrypt(basic_ctext)
        exp_cksum = HMAC.new(ki, basic_plaintext, MD5).digest()
        ok = _mac_equal(cksum, exp_cksum)
        if not ok and keyusage == 9:
            # Try again with usage 8, due to RFC 4757 errata.
            ki = HMAC.new(key.contents, pack('<I', 8), MD5).digest()
            exp_cksum = HMAC.new(ki, basic_plaintext, MD5).digest()
            ok = _mac_equal(cksum, exp_cksum)
        if not ok:
            raise InvalidChecksum('ciphertext integrity failure')
        # Discard the confounder.
        return basic_plaintext[8:]