Class Checksum

scapy/layers/kerberos.py:515–587

class Checksum(ASN1_Packet):
    ASN1_codec = ASN1_Codecs.BER
    ASN1_root = ASN1F_SEQUENCE(
        ASN1F_enum_INTEGER(
            "cksumtype",
            0,
            _KRB_S_TYPES,
            explicit_tag=0xA0,
        ),
        _Checksum_Field("checksum", "", explicit_tag=0xA1),
    )

    def get_usage(self):
        """
        Get current key usage number
        """
        # RFC 4120 sect 7.5.1
        if self.underlayer:
            if isinstance(self.underlayer, KRB_Authenticator):
                # TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum
                # (n°10 should never happen as we use RFC4121)
                return 6
            elif isinstance(self.underlayer, PA_FOR_USER):
                # [MS-SFU] sect 2.2.1
                return 17
            elif isinstance(self.underlayer, PA_S4U_X509_USER):
                # [MS-SFU] sect 2.2.2
                return 26
            elif isinstance(self.underlayer, AD_KDCIssued):
                # AD-KDC-ISSUED checksum
                return 19
            elif isinstance(self.underlayer, KrbFastArmoredReq):
                # KEY_USAGE_FAST_REQ_CHKSUM
                return 50
            elif isinstance(self.underlayer, KrbFastFinished):
                # KEY_USAGE_FAST_FINISHED
                return 53
        raise ValueError(
            "Could not guess key usage number. Please specify key_usage_number"
        )

    def verify(self, key, text, key_usage_number=None):
        """
        Verify a signature of text using a key.

        :param key: the key to use to check the checksum
        :param text: the bytes to verify
        :param key_usage_number: (optional) specify the key usage number.
                                 Guessed otherwise
        """
        if key_usage_number is None:
            key_usage_number = self.get_usage()
        key.verify_checksum(key_usage_number, text, self.checksum.val)

    def make(self, key, text, key_usage_number=None, cksumtype=None):
        """
        Make a signature.