[OTR-dev] Fragmenting proposal

[Ian Goldberg]

Different IM networks have different maximum lengths that messages can
be.  Can you guys look over this proposal for message fragmenting, and
see if there's something I'm missing?  [This will become part of the
Protocol documentation.]

Thanks,

   - Ian

Fragments
---------

[Remember when reading this section that the network model assumes
in-order delivery, but that some messages may not get delivered at all
(for example, if the user disconnects).  And, of course, there's the
possibility of an active attacker, who is allowed to perform a Denial of
Service attack, but not to learn contents of messages.]

Transmitting Fragments:

    If you have information about the maximum size of message you are
    able to send (the different IM networks have different limits), you
    can fragment an OTR message as follows:

    - Start with the OTR message as you would normally transmit it.  For
      example, an OTR Data Message would start with "?OTR:AAED" and end
      with ".".
    - Break it up into sufficiently small pieces.  Let the number of
      pieces be (n), and the pieces be piece[1],piece[2],...,piece[n].
    - Transmit (n) messages with the following (printf-like) structure
      (as k runs from 1 to n inclusive):

	  "?OTR,%hu,%hu,%s," , k , n , piece[k]

    - Note that k and n are unsigned short ints (2 bytes), and each has
      a maximum value of 65535.

Receiving Fragments:

    If you receive a message containing "?OTR," (note that you'll need
    to check for this _before_ checking for any of the other "?OTR:"
    markers):

    - Parse it as the printf statement above into k, n, and piece.
    - Let (K,N) be your currently stored fragment number, and F be your
      currently stored fragment.  [If you have no currently stored
      fragment, then K = N = 0 and F = "".]

    - If k == 0 or n == 0 or k > n, discard this (illegal) fragment.

    - If k == 1:
      - Forget any stored fragment you may have
      - Store (piece) as F.
      - Store (k,n) as (K,N).

    - If n == N and k == K+1:
      - Append (piece) to F.
      - Store (k,n) as (K,N).

    - Otherwise:
      - Forget any stored fragment you may have
      - Store "" as F.
      - Store (0,0) as (K,N).

    After this, if N > 0 and K == N, treat F as the received message.

Example:

    Here is an OTR Key Exchange Message we would like to transmit over a
    network with an unreasonably small maximum message size:

	?OTR:AAEKAQAAAICGmmRMlmuq4gY7Ro0GiYAJKWwVZyITNyifFP9VRIVgyxxGxwV
	bFjoGMhO9XE0xFisuO6M27DPkX7hCtIXZM2glDszmTklQO5hJPu0g/RgDZ84q0ee
	Q5AvexW3Hmp/VHUPTpZfJPep/Ctiqn0oE2y/2yRPyYQjpZCL440sM5i7B1wAAABT
	zzL9WbuaxOK8rfrtaw4Lx/iLxeQAAAIAWaGpchsVOV1D6xK5cS5QNANelTvyVHre
	XPSRjU0NFKIHrNDiFwa8lXcIBH/E8MHoQDzw+J2AuU6MuICPT8GMJYBcSZq0OM7x
	gmfNlt1viUXxJXbYRpD82ki7QsMA1I7aQo/OqMryKlW5W8UqEjVcCsTOjEyQphLY
	ENG6St9+ivgAAAIBgUjzleG1+VYCXZszTj+x5gNNidVVNKI+MG5elHMcsg2Guef3
	DBYEsor6YGeqJLAfhk28Tg7tktMQwGN5GXR1ZNkwkoFIOyVRq3lfabfHtsTp+Hkx
	5e8OrhTZ1G+ScDeqYbbTtUj631LhXUoyp+7pllVtpyLgqk5z9JYu6Kw0ZkQAAAAE
	AAADASZH/uq17EVRo6dBZIL12x9JLx4gpEjgovfNLoORa6E+sMMuG7Z+zfLQVodX
	H5shi/dvPzwbVrA/Iw72XHSYtld8lK/FLtjsI5mzancvRAEs1ZDBoBJRLW1X54eF
	HpN/peDi6fBbdXyGahWYyF9MCJxDFCRqAHvEMZbfdyEtkXbFUZM2lJM2SJJG9zGZ
	LCvd2/gF/VOgMlvdus+8TFW0k7cBhAgm/rb+EUeovkWXy2BiVpInXKCCH+M6EVpU
	YNG7BPtH44ABwUw6Y5n5sSb6dtout34NGz+dspXMajffkZxFOAcabRwKIpw==.

    We could fragment this message into (for example) three pieces:

	?OTR,1,3,?OTR:AAEKAQAAAICGmmRMlmuq4gY7Ro0GiYAJKWwVZyITNyifFP9VRI
	VgyxxGxwVbFjoGMhO9XE0xFisuO6M27DPkX7hCtIXZM2glDszmTklQO5hJPu0g/R
	gDZ84q0eeQ5AvexW3Hmp/VHUPTpZfJPep/Ctiqn0oE2y/2yRPyYQjpZCL440sM5i
	7B1wAAABTzzL9WbuaxOK8rfrtaw4Lx/iLxeQAAAIAWaGpchsVOV1D6xK5cS5QNAN
	elTvyVHreXPSRjU0NFKIHrNDiFwa8lXcIBH/E8MHoQDzw+J2AuU6MuICPT8GMJYB
	cSZq0OM7xgmfNlt1viUXxJXbYRpD82ki7QsMA1I7aQo/OqMryKlW5W8UqEjVcCsT
	OjEyQphLY,

	?OTR,2,3,ENG6St9+ivgAAAIBgUjzleG1+VYCXZszTj+x5gNNidVVNKI+MG5elHM
	csg2Guef3DBYEsor6YGeqJLAfhk28Tg7tktMQwGN5GXR1ZNkwkoFIOyVRq3lfabf
	HtsTp+Hkx5e8OrhTZ1G+ScDeqYbbTtUj631LhXUoyp+7pllVtpyLgqk5z9JYu6Kw
	0ZkQAAAAEAAADASZH/uq17EVRo6dBZIL12x9JLx4gpEjgovfNLoORa6E+sMMuG7Z
	+zfLQVodXH5shi/dvPzwbVrA/Iw72XHSYtld8lK/FLtjsI5mzancvRAEs1ZDBoBJ
	RLW1X54eFHpN/peDi6fBbdXyGahWYyF9MCJxDFCRqAHvEMZbfdyEtkXbFUZM2lJM
	2SJJG9zGZ,

	?OTR,3,3,LCvd2/gF/VOgMlvdus+8TFW0k7cBhAgm/rb+EUeovkWXy2BiVpInXKC
	CH+M6EVpUYNG7BPtH44ABwUw6Y5n5sSb6dtout34NGz+dspXMajffkZxFOAcabRw
	KIpw==.,