Module: Rex::Proto::Kerberos::Crypto::Asn1Utils

Included in:
Des3CbcSha1, DesCbcMd5
Defined in:
lib/rex/proto/kerberos/crypto/asn1_utils.rb

Instance Method Summary collapse

Instance Method Details

#truncate_nulls_after_asn1(input) ⇒ Object

Some crypto schemes just decide to add a bunch of null bytes as padding, and leave it up to the application to decide how many of those null bytes to remove. We can’t just remove all zeroes from the end of the data, because some of them may actually be part of the data. The assumption here is that the information about how many bytes to use comes from the ASN1 data structure. So here we ask the ASN1 parser’s enclosing (first) element “How many bytes do you take up?”

Raises:



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# File 'lib/rex/proto/kerberos/crypto/asn1_utils.rb', line 12

def truncate_nulls_after_asn1(input)
   valid_until = 0
   OpenSSL::ASN1.traverse(input) do | depth, offset, header_len, length, constructed, tag_class, tag|
     valid_until = offset + length + header_len
     break
   end
   
   # For this to be a valid result, we expect this byte, and all following it, to be zeroes. Alternatively, there could be no padding at all (e.g. block multiple)
   suffix = input[valid_until, input.length]
   expected_result = suffix == "" || suffix.unpack('C*').all? {|char| char == 0}
   raise ::Rex::Proto::Kerberos::Model::Error::KerberosDecodingError, 'Failed to truncate decrypted data' unless expected_result

   return input[0,valid_until]
end