vulcan-cryptanalysis

cannot be independently verified and must therefore be considered mere conjecture.
Nonetheless, every claim in this paper is made in good faith, in that we
believe the information to be as accurate as possible, and we are not trying to
deliberately deceive or mislead anyone. We certainly welcome any corrections
or additions to the information presented in this paper.
We are absolutely confident in our technical reporting of Vulcan because
that information was determined solely via reverse-engineering. Furthermore,
we verified the accuracy of our analysis by comparing simulation data to that
obtained from genuine DVP hardware.
1.2 Terminology
Unless specifically stated to the contrary, all mathematical operations described
in this paper are on the Galois Field GF (2). Accordingly, multiplication corresponds
to a logical AND function and addition corresponds to a logical XOR
function. Additionally, we adopt digital filter terminology for shift register delay
elements, using the notation z −1 to indicate a delay of one bit.
In our figures, we use a square with the text z −1 inside it to denote a 1-
bit delay element that is equivalent to a D flip-flop. We use a circle with a
multiplication sign in to denote multiplication over GF (2), which is equivalent
to a logical AND gate. We use a circle with a plus sign in it to denote addition
over GF (2), which is equivalent to a logical XOR gate.
As much as possible, we try to use cryptographic vocabulary appropriate
for stream ciphers in this paper. As such, we prefer that the term key refer to
the keystream generated by the stream cipher instead of the user-selected secret
key, which we prefer to identify with the term cryptovariable (CV).
Thus when we refer to a bit of key, we are referring to a keystream bit
that is added modulo-2 to plaintext to produce ciphertext during an encrypt
operation, and added modulo-2 to ciphertext to produce plaintext during a
decrypt operation. Likewise when we refer to a bit of cryptovariable, we are
referring to a bit of the (preferably randomly selected) secret that must be
shared amongst all users of the cryptosystem in order for secure communication
to be possible.
Additionally, we prefer the term ciphertext autokey (or the equivalent acronym
CTAK) instead of cipher feedback (CFB) for describing the mode in which the
Vulcan stream cipher operates. Ciphertext autokey implies that the keystream
is generated automatically based solely upon prior ciphertext and the cryptovariable.
This provides a self-synchronizing property that is advantageous for
channels that do not easily provide for cryptographic synchronization [1].
As we previously mentioned, Vulcan is the internal code name by which
the cipher presented in this paper is known to Motorola. Motorola marketed
products based on the Vulcan cipher as DVP. We use the terms Vulcan and
DVP more or less interchangeably in this paper, but observe the convention
that Vulcan can only refer to the cipher itself, whereas DVP can refer to either
the cipher or a product that incorporates it.
2