MA28140 Datasheet, PDF (17/72 Pages) Dynex Semiconductor – Packet Telecommand Decoder

x (i)
P₀ (i) P₁ (i) P₂ (i) P₃ (i)
C₀
C₁
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MA28140
P₅₉(i)
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Figure 8: Realisation of the Hashing Function
THE AUTHENTICATION KEY
The Authentication Key consists of:
60 x 48-bit Hard Knapsack Weights = 2880 bits = 360 octets
60 x 1-bit Hashing Function coefficients = 60 bits = 8 octets
Full Authentication Key =
2940 bits = 368 octets
The system includes two such 2940-bit keys:
- a fixed, mission-unique Authentication Key, called the
Fixed Key;
- an in-flight programmable Authentication Key, called the
Programmable Key.
(a) Fixed Key
The Fixed Key is required for start-up and emergency
(recovery) operations. The Fixed Key is stored in the external
ROM as part of the Mission-Specific Data.
(b) Programmable Key
The Programmable Key is required for all normal
operations. The contents of the Programmable Key reside in
the RAM where it can be modified by means of Authentication
Control Commands specifically defined for that purpose. The
format of these Change Programmable Key Block Control
Commands, which are specified in the section on AU Control
Commands (page 18), allows any 5-octet block to be modified
starting at any of the 368 octet boundaries.
The Supervisor
The Supervisor consists of four main parts:
- the Logical Authentication Channel (LAC) Registers;
- the Final Authorisation Function;
- the Control Command Processor;
- the Deletion Function.
They are briefly described in the next four subsections.
THE LAC COUNTERS
A LAC Counter is basically a 30-bit counter which is used
to associate every TC segment with an authentication
sequence number. The purpose of this number is to protect the
system against attacks by ensuring that identical TC segments
will not have the same signature except at very large intervals
of time. The LAC counter is incremented by one every time a
TC segment is successfully authenticated (and only then). The
LAC counter value used for authenticating each TC segment is
uplinked with each signature.
Three LAC Registers are provided:
- one Principal LAC register (LAC ID = 00);
- one Auxiliary LAC register (LAC ID = 01);
- one Recovery LAC register (LAC ID = 10).
Bits 0 and 1 of the LAC are fixed in order to select the LAC
Register to be used for the final authorisation of a TC
Segment. For what concerns the 30 bits of LAC Count (Bits 2
through 31, where the LSB is Bit 31), they are implemented as
follows:
- The Principal and Auxiliary LAC counters have 30 bits.
- The Recovery LAC counter has 8 bits (the LSBs 24-31)
whereas the remaining 22 bits (2-23) are permanently
set to 1.
THE FINAL AUTHORISATION FUNCTION
When the received signature s of a TC Segment compares
with the onboard-generated signature S, the contents of the
received LAC Count field is compared with the contents of the
indicated LAC Register. If both contents are found equal, there
are two cases:
- The TC Segment was transferred on a MAP to be
authenticated with a MAP ID lower or equal to the MAP ID
pointer. In this case, the TC Segment is authorised for
transfer to the Segmentation Layer.
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