MA28140 Datasheet, PDF (15/72 Pages) Dynex Semiconductor – Packet Telecommand Decoder

MA28140
4.3 AUTHENTICATION LAYER
Structure of the Authenticated Segments
The TC segment is the protocol data unit of the
Segmentation Layer. The general format of an authenticated
TC Segment is specified in Section 10 of ref.1. The particular
format of an authenticated TC segment for the PTD is the
following:
(a) The length of the signature field of the Authentication
Tail is 5 octets.
(b) The length of the Authentication Tail is 9 octets (5 octets
for the signature + 4 octets for the LAC); the maximum length
of the TC Segment is 249 octets (Segment Header (1 octet) +
Segment Data Field (239 octets) + Authentication Tail (9
octets)), and its minimum length 10 octets (Segment Header (1
octet) + Authentication Tail (9 octets)).
SEGMENT SEGMENT
SEGMENT HEADER
DATA FIELD TRAILER
Sequence
MAP
(optional)
Flags
Identifier
2 bits
6 bits
variable
9 octets
<----------------1 octet ------------><----- from 9 to 248 octets ------>
The segment trailer is optional and has a fixed length of 9
octets. The following table summarizes the management of the
Segment Trailer.
Type of
Authentication
Internal AU
External AU
Ty pe of Fra me
Authenticated frame
Not authenticated frame
Authenticated frame
AU disable
Not authenticated frame
All
S e gme nt Tra ile r
segment trailer (9 octets length)
no segment trailer
segment trailer (9 octets length)
if AuTsl=0,
no segment trailer if AuTsl=1
no segment trailer
no segment trailer
The selection of MAPs that are deemed to carry
authenticated TC segments takes into account the possibility
to associate MAP IDs in pairs when packet re-assembly is
required. Therefore, authenticated MAPs are selected by
pairs, using the 5 LSBs of the MAP identifier field of the
Segment Header. The selection mechanism is such that it will
point at the last pair of MAP identifiers (counting upwards from
MAP 0) that carries authenticated segments. The value
identifying this particular pair of identifiers is called the
Authenticated MAP ID Pointer and is stored in ROM.
For example, selecting MAP 4 (i.e. Authenticated MAP ID
Pointer = 4) means that the first 5 pairs of MAPs (i.e. MAP 0
and 32, MAP 1 and 33, MAP 2 and 34, MAP 3 and 35, MAP 4
and 36) are expected to carry authenticated TC segments.
Overview of the Layer
This optional layer is implemented on-chip but a
connection to an external Authentication Unit is also
implemented in case another implementation is desired. The
choice of the AU is done by means of a dedicated
configuration input AUEXT:
• AUEXT = 1: the internal AU is disabled and the external
AU is used,
• AUEXT = 0: the internal AU is used and the external AU
is disabled.
MAP 63 is reserved for AU configuration commands when
authentication is disabled. It is possible to bypass this layer
(when no authentication is required) by means of a dedicated
configuration input AUDIS. In this case, segments are passed
directly to the segmentation layer .The values of the AUDIS pin
are:
• AUDIS = 1: the internal or external AU is disabled,
• AUDIS = 0: the internal or external AU is enabled.
When the AU is disabled, the TC segment does not have
an AU tail (the last nine octets are not deleted), the
Authenticated MAP ID Pointer has no meaning and MAP 63 is
considered as a standard MAP (the data is output on MAP
number 63 without removing the AU tail).
An 80 bit length status, AUS, is generated by this block and
fetched by the telemetry system in order to send it back to the
ground segment.
The Authentication Processor
The authentication method specified in references 1 and 2
consists of generating a 40-bit digital signature using a
transformation under a secret key applied to the TC Segment.
This authentication signature is appended to the TC segment
and guarantees to the recipient that the TC Segment is
authentic with respect to its sender and its contents.
An incoming TC Segment is authenticated by performing
the same transformation made by the transmitting end, and by
comparing the received signature with the onboard-generated
one. A functional diagram of the Authentication Processor is
shown below. There are four main parts:
- the Hashing Function;
- the Hard Knapsack;
- the Deletion Box;
- the Signature Comparator.
They are described in the next four subsections. Not
apparent on the functional diagram of Figure 7 is the
organisation of the secret Authentication Keys stored in the
Authentication Processor. This is described in the section on
AU Control Commands on page 18.
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