Fragment Length, Fragment Offset Non-conformance bug

[bathooman]

Description

• Type: Non-conformance Bug
• Priority: Minor

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Non-conformance Bug

Version: development branch

Expected behavior
The DTLS RFC specifies the following requirement regarding the fragment offset and fragment length when fragmentation is not present:

An unfragmented message is a degenerate case with fragment_offset=0 and fragment_length=length

Actual behavior
One can complete a handshake with the following values for the fragment_offset and fragment_length:

CKE.message_length = 17
CKE.fragment_length = 16777198
CKE.fragment_offset = 255

The problem also exists when the server sends records with non-compliant Fragment_offset and fragment_length.

I have attached the handshake trace for the mentioned non-conformance. You clearly see that the handshake proceeds while the Fragment_offset and fragment_length are invalid.
capture.zip

[mrdeep1]

Your code base is not the current develop branch as you indicate (fragment_offset is not used). If #64 is merged into the develop branch, then the non-conformance you mention will get caught. Even with #62 I am not sure where the non conformance is (24 bit masking) that allows the bad packet to get through.. The packet capture is telling me Fatal Alert: Handshake Failure has happened (sent back by the Server), but am unable to decrypt the encrypted handshakes.

[bathooman]

The problem is solved now. Thank you.