vulcan-cryptanalysis

4.1 Description and Operation
Our T3010AX KVL has a TRN6777B cryptographic hybrid in it. This hybrid
contains the SC76807 Vulcan IC and some additional non-cryptographic circuitry
that is of no interest here. It is the presence of the TRN6777B crypto
hybrid within the KVL that allows us to gather a known triplet of cryptovariable,
plaintext and ciphertext that we can use to verify the correctness of our
software simulations of the Vulcan cipher.
The KVL is a relatively simple device that allows an end user to manually
enter (via a membrane keypad) a cryptovariable into the key loader and then
transfer this cryptovariable into a DVP-equipped radio. Unlike more modern key
loaders (e.g. the KVL4000), the T3010 cannot generate a random cryptovariable
(this fact alone is a rather serious security flaw). Instead, the user must manually
enter 24 octal digits, with the final digit being restricted to values 0 to 3. Key
entry proceeds in four groups of six octal digits each, with the intermediate
result being displayed on a LED numeric display. The T3010 thus provides the
user the ability to enter a 71-bit cryptovariable even though the Vulcan cipher
itself requires a 138-bit cryptovariable.
Internally, the KVL is based on a Motorola 6802 microprocessor that performs
all the required functions except for encryption. Encryption is handled
exclusively by the embedded TRN6777B hybrid. The 6802 processor primarily
performs user interface functions such as handling the membrane keypad inputs
and numeric LED display outputs.
Interestingly, when transferring a cryptovariable from the KVL to a target
radio (a process commonly referred to as key loading), communication is strictly
one-way: from the KVL to the target. No provision exists for the target to
communicate to the KVL. This means that the KVL cannot be certain that a
key load operation was successful. Later generations of Securenet key loaders
implemented bidirectional communications between the KVL and the target
device.
To overcome the problem of not knowing if a key loading process was successful
or not, the KVL sends a short burst of encrypted CVSD immediately
following the cryptovariable transfer. If the CV was successfully transferred,
the target radio will correctly decrypt this ciphertext and play the resulting
plaintext (a tone) on the radio speaker. Therefore, if the user hears a beep from
the radio immediately after key loading, that means the CV was successfully
loaded. If no beep occurs, the key load process failed and only static will be
heard.
If it were not for this unusual form of verification, the KVL would not require
an internal crypto hybrid. The only reason the T3010 KVL has an internal
TRN6777B hybrid is so that it can generate the correct ciphertext to send to
the target radio to make it beep after a successful key load. We exploited this
operation by capturing data with a logic analyzer and then used that data to
confirm the validity of our software simulations of Vulcan.
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