TQP Development, LLC v. Sling Media, Inc.

Filing 1

COMPLAINT against Sling Media, Inc. ( Filing fee $ 350 receipt number 0540-3825100.), filed by TQP Development, LLC. (Attachments: # 1 Exhibit A, # 2 Civil Cover Sheet, # 3 Civil Cover Sheet Attachment)(Fenster, Marc)

EXHIBIT A

111111111111111111111111111111111111111111111111111111111111111111111111111
USOO5412730A

United States Patent

[11]

[19]

Jones
[54]

[45]

ENCRYPTED DATA TRANSMISSION
SYSTEM EMPLOYING MEANS FOR
RANDOMLY ALTERING THE ENCRYPTION
KEYS

Patent Number:
Date of Patent:

4,985,919

Michael F. Jones, Nashua, N.H.

[73]

Assignee:

Telequip Corporation, Hollis, N.H.

[21]

Appl. No.: 872,674

[22]

Filed:

Apr. 23, 1992
Related U.S. Application Data

[63]

Continuation-in-part of Ser. No. 418,178, Oct. 6, 1989,
abandoned.

[51] Int. Cl.6 ............................................... H04L 9/00
[52] U.S. Cl•.......................................... 380/46; 380/9;
380/21; 380/29; 380/49
[58] Field of Search ................... 380/9, 20, 21, 43, 44,
380/49,50,18,46,29
[56]

References Cited

May 2, 1995

1/1991 Naruse et aI ....................... 380/9 X

Primary Examiner-Bernarr E. Gregory
Attorney, Agent, or Firm-Allegretti & Witcoff, Ltd.
[57]

[75] Inventor:

5,412,730

ABSTRACT

A modem suitable for transmitting encrypted data over
voice-grade telephone line. The modem is implemented
by the combination of integrated circuit components
including a microprocessor, a serial communications
controller which communicates with connected data
terminal equipment, and a modulator/demodulator for
translating between voice band tone signals and digital
data. Pseudo random number generators are employed
at both the transmitting and receiving stations to supply
identical sequences of encryption keys to a transmitting
encoder and a receiving decoder. An initial random
number seed value is made available to both stations.
The random number generators are advanced at times
determined by predetermined characteristics of the data
being transmitted so that, after transmission has taken
place, the common encryption key can be known only
to the transmitting and receiving stations.
2 Claims, 5 Drawing Sheets

U.S. PATENT DOCUMENTS

Microfiche Appendix Included
(2 Microfiche, 119 Pages)

4,809,327 2/1989 Shima .................................... 380/44
4,876,716 10/1989 Okamoto ............................... 380/21

r---------------------------------------------------------------------------------TRANSMITTING STATION, 11
...
,

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SOURCE

,

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t

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'--

--------------------~-------------------------~--------------------------- ~
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-------------------~--------------------------~-------------------------_.
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---.

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'I'

DATA
UTILIZATION
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"-- 33

I

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L

DECRYPTOR

..

31
RECEIVING STATION, 12

~----------------------------------------------------------------------------------

u.s. Patent

May 2, 1995

5,412,730

Sheet 1 of 5

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.
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u.s. Patent

May 2, 1995

5,412,730

Sheet 5 of 5

-----------------------------------------------------------------------_._---------------DATA
SOURCE

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._---------------------------------------------------------------------------------------~

Fig. 4

1

5,412,730

ENCRYPTED DATA TRANSMISSION SYSTEM
EMPLOYING MEANS FOR RANDOMLY
ALTERING THE ENCRYPTION KEYS
BACKGROUND OF THE INVENTION

5

This is a continuation-in-part of application Ser. No.
07/418,178 filed on Oct. 6, 1989.
A computer program listing is submitted herewith as
a microfiche appendix having 119 frames on 2 micro- 10
fiche.
This invention relates to data transmissions systems
and, more particularly, to systems for transmitting enciphered data.
Data encryption provides security for transmitted 15
data by scrambling the "clear text" data into "cipher
text". Typically, the transmitted data is scrambled in a
manner selected by a unique key value (such as a 56-bit
binary number) and unscrambled, at the receiving station, by a reverse process that requires the same key 20
value be known.
For increased data security, the encryption key value
may be changed frequently to further reduce the likelihood that an unauthorized party may decipher the data.
In such systems, new key values are sent at intervals 25
from the transmitting station to the receiving station.
The keys may be generated by a random number generator located at the transmitting end, encrypted in accordance with the currently active key, and transmitted
along with the other data. At the receiving station, the 30
encrypted key is extracted from the data stream, deciphered, and substituted at a designated time for the
prior key. In such a system, if any of the transmitted
keys are deciphered, the successive keys may be deciphered as well, so that all ofthe transmitted information 35
may be decoded.
In accordance with a principle feature of the present
invention, pseudo-random number generators are employed at both the transmitting and receiving stations to
supply a like sequence of encryption keys to both the 40
encryptor and decryptor, without these keys being
transmitted in any form over the transmission facility.
In accordance with the invention, to permit the two
stations to communicate, each supplied in advance with
a random number seed value whiCh exclusively deter- 45
mines the numerical content of the sequence of numeric
values generated by each of the two pseudo-random
generators. In order that the two generators switch
from one output key value to the next in synchronism,
means are employed at both the transmitting and receiv- 50
ing stations to monitor the flow of transmitted data and
to advance the random number generator each time the
transmitted data satisfies a predetermined condition.
The monitoring function can advantageously be performed simply by counting the units of data being trans- 55
mitted and by advancing each pseudo-random key generator each time the count reaches an agreed-upon interval number. In this way, no additional synchronization information needs to be added to the data stream.
For even greater security, the interval number (which 60
must be reached before the key is switched) may itself
be a changing value generated by a random number
generator, so that the duration during which a given
key is active changes from key to key at times which are
predictable only by the authorized recipient.
65
In accordance with still another feature of the invention, different random number seed values and different
interval numbers (or different random number seed

2

values for the generator of the interval numbers) may be
associated with each of a plurality of remote locations
with whom secured communication is required, so that
the data on any given link is decipherable only by the
authorized receiving station, even though other stations
may have identical communication and decryption
hardware.
As a consequence of the foregoing feature, the invention may be used to advantage to control communications within a network. A key memory is provided
which permits a unique serial number identifying a
remote unit to be stored along with the current encryption key value, the serial number and key value being
stored on command from the local unit or by being
downloaded from a remote unit which serves as the
host or network supervisor. Once the host station has
supplied the initial seed value keys to the units forming
the two terminal locations for a given link and transmission over that link begins, the host is no longer "knows"
the encryption key values since they are dependent
upon the nature of the transmissions over the link. Consequently, link security cannot be compromised even by
an "insider" who is in possession of the initial key values
supplied by the host"
As contemplated by still another feature of the invention, the encryption and decryption may advantageously be accomplished within a modem unit which
also performs data compression and decompression, as
well as error-handling functions. Advantageously, the
compression, encryption and error-coding functions
may all be performed (in that sequence) at the transmitting station by the same processor, while a like processor at the receiving end is suitably programmed to provide, in sequence, the error control, decryption, and
decompression functions.
The principles of the invention may be applied to
advantage in terminals connected as part of a secured
communication network operating under central control. A key memory at each terminal may be loaded, by
a secure communication from the central control, with
encryption keys associated with other terminals with
which secured communication is authorized. In this
way, the central control can selectively permit or prohibit any terminal from decoding communications from
any other terminal on a dynamically changing basis.
This and other features and advantages of the invention may be more clearly understood by considering the
following detailed description of specific embodiments
of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
In the course of the detailed description to follow,
reference will frequently be made to the attached drawings, in which:
FIG. 1 is a functional block diagram illustrating the
basic signal processing steps which embody the invention;
FIG. 2 is a hardware block diagram which shows a
modem apparatus of the type contemplated by the invention;
FIGS. 3A, 3B and 3C are schematic representations
which, in combination, illustrate a preferred embodiment of the modem apparatus of FIG. 2; and
FIG. 4 is a functional block diagram illustrating enhanced signal processing capabilities used in the preferred embodiment of the invention.

3

5,412,730

DESCRIPTION OF THE PREFERRED
EMBODIMENT
FIG. I-Basic Processing
FIG. 1 illustrates the manner in which the data being
transmitted is subjected to a sequence of signal processing steps as contemplated by the present invention.
These processing steps are executed at a transmitting
station 11 and at a receiving station 12 connected to
opposite ends of a communications channel 13.
At the transmitting station 11, a source of data 15
supplies a serial data stream to the data input of an
encryptor 17. The data from source 15 may take substantially any form, such as a file of text characters, each
encoded as a 8-bit byte, or a file of numerical binary
information expressed in l6-bit or 32-bit words. A block
counter 21 monitors the stream of data from the source
15 and generates an "advance signal" each time the data
meets a predetermined condition. Advantageously, the
block counter 21 may simply count the number of bytes
(characters), words or blocks of data being transmitted,
compare the current count with a predetermined 37
interval number" and produce an advance signal each
time the current count reaches the interval number (at
which time the current count is reset to 0).
The advance signal produced by block counter 21 is
supplied to the advance input of a pseudo-random number generator 23 which supplies a sequence of encryption key values to the key input of the encryptor 17. The
content of the key sequence is predetermined by the
combination of (1) the internal makeup of the generator
23 and by (2) a supplied random number seed value
which initializes the generator 23. The generator 23
responds to each advance signal from block counter 21
by changing its output to the next successive encryption
key value. Thus, for example, the combination of
counter 21 and generator 23 operate to change the encryption key each time total number of bytes transmitted is an exact multiple of the predetermined interval
number.
The encryptor 17 translates fixed length segments of
the data from source 15 ("clear text") into fixed-length
"cipher text" output segments, each segment translation
taking place in a manner uniquely determined by the
encryption key currently supplied by the pseudo-random number generator 23. The encryptor 17 (and the
decryptor 19, to be discussed) may advantageously
employ the accepted NBIS Data Encryption Standard
(DES), which codes and decodes data in 64-bit (8 byte)
units in accordance with a 56-bit key. The block
counter 21 need not supply advance signals on boundaries between encryption units, nor does the generator
23 need to provide new key value precisely on encryption unit boundaries. Instead, the encryptor 17 may
buffer the new keys temporarily, using it for the first
time on the next successive encryption unit of data.
At the receiving station 12, the incoming cipher text
is applied to the data input of the decryptor 31 whose
key input is connected to receive a sequence of keys
from the pseudo-random number generator 27. The
clear text output from the decryptor 31 is applied to a
data utilization device 33 and is monitored by a block
counter 29 which supplies advance signals to the number generator 27. Block counter 29 performs the identical function as that performed by the counter 21 at the
transmitting station 11 and hence supplies advance signals to the generator 27 at precisely the same times
(relative to the data stream) that counter 21 advances

5

10

15

20

25

4

generator 23. Each time the current count reaches the
interval number, the pseudo-random number generator
27 is advanced. Since the internal makeup of random
number generator 27 is identical to that of generator 23,
and since it is supplied with the same seed value, and
since block counter 29 is supplied with the same interval
number value as that supplied to the block counter 21,
exactly the same sequence of keys will be supplied to
the random number generators 23 and 27, and the keys
will change at precisely the same time (relative to the
data stream) to accurately decipher the transmitted
data.
Of course, in order for the receiving station to successfully deciphe'r the incoming cipher text, the receiving station 12 must be provided (in some fashion) with
both the correct seed value and the correct interval
number. These values are supplied to the receiving
station in advance of the transmission by any secure
means. However, once the receiver possesses these
values, no further information is required to decipher
the transmissions. No key values, key verification values, or key synchronization signals need accompany the
transmitted ciphered text to control or coordinate the
encryption or decryption processing, even though the
encryption keys are continuously changing to enhance
security.
FIG. 2-Hardware

30

35

40

45

50

55

60

65

The principles of the present invention may be advantageously implemented in a data communications
modem having a hardware architecture of the type
generally depicted in FIG. 2 of the drawings. As
shown, the modem operates under the supervisory control of a microprocessor 101 such as the model 80188
microprocessor available from Intel Corporation. The
instructions and data operated on by the microprocessor 101 are stored in a memory subsystem 103 which is
composed of both read-only memory (advantageously
implemented as EPROM memory) and random access
memory (RAM). Memory subsystem 103 is coupled to
the microprocessor 101 by a memory address bus 105
and a data bus 107.
The data bus 107 also provides a data path to three
peripheral devices: a display 109, a serial communications controller (SCC) 111, and a modem module 113.
The SCC 111 may take the form of an integrated circuit
such as the model 82530 controller manufactured by
Intel Corporation. The modem module 113 may be
constructed using a modem unit 115, model R9696 chip
set available from Rockwell International Corporation,
a cooperating set of integrated circuits capable of performing trellis-coded modulation and demodulation
meeting the V.32 9600 baud communications protocol
standard, as well as the V.22 bis standard, and further
includes analog/digital conversion circuits which provide an interface to a direct access adapter (DAA) 117.
The adapter 117 may take the form of a type CH1828
integrated circuit DAA available from Cermetek Microelectronics, Inc.
The modem hardware shown in FIG. 2 is used at
both ends of the communications channel. At the transmitting end, data to be transmitted is supplied by the
connected data terminal equipment (DTE) via the serial
port 121 (e.g., a RS-232c or RS-422 standard port). The
asynchronous serial interface with the DTE typically
operates under the combined control of the microprocessor 101 and the SCC 111 in accordance with a

5

5,412,730

standard interface protocol,,(e.g., the V.42 standard
protocol). The DTE (data teiminal equipment) may be
any terminal or computer adapted to communicate via
this standard port using the selected serial protocol.
The encryption/decryption processing is essentially
"transparent" to the DTE; that is, the data is enciphered
and deciphered without effecting the content of the
data sent by or received by the DTE. However, it is
desirable to permit the connected DTE to send commands (such as extensions to the standard "AT command set") which will control encryption processing,
turning encryption ON and OFF, and accepting seed
values and interval numbers entered as "passwords"
directly from the connected DTE.
Data signals from the DTE which are to be transmitted are encrypted as described above and shown in
FIG. 1, the random number seed values and the interval
number values being pre-supplied to the microprocessor 101 and stored in memory subsystem 103. At the
receiving end, the modem module 113 shown in FIG. 2
receives the incoming data (typically as a 9600 baud
trellis-coded signal adapted for transmission over the
analog telephone link) and converts that incoming signal into data which is processed by microprocessor 101
and supplied via the SCC 111 to the connected DTE. In
the receiving mode, microprocessor 101 decrypts the
data as illustrated by the receiving station 12 in FIG. 1.
FIGS. 3A, 3B and 3C-Preferred Embodiment
FIGS. 3A, 3B and 3C, in combination, illustrate a
preferred embodiment of the modem hardware shown
in FIG. 2. It should be noted that the devices illustrated
in FIGS. 3A, 3B and 3C which are common to the
functional units in FIG. 2 are designated by identical
"Drawing Reference Characters".
FIG. 3A illustrates the microprocessor 101, the memory subsystem 103, and various supporting logic. The
crystal inputs Xl and X2 to the microprocessor 101
provides an external connection for a fundamental
mode parallel resonant crystal 157 (Y1). The resonant
crystal 157 is utilized by the internal oscillator of the
microprocessor 101 to generate the clock signal,
CLKOUT. In a preferred embodiment, the resonant
crystal 157 has a fundamental frequency of 14.7456
Mhz.
The memory subsystem 103 includes memory devices
103a-d. The "jumper points" are designated as "E1-E12". The "jumper points" facilitate implementing various types of memory devices and memory configurations employed by the memory subsystem 103. The
memory subsystem 103 is discussed in detail below.
FIG. 3B illustrates the serial communications controller (SCC) 111, the serial port 121, the data terminal
equipment (DTE), and various supporting logic. The
integrated circuits 123, 124, 125, 127, 129, 131, 135 and
137 are utilized to time delay various signals from the
microprocessor 101 and thereby provide orderly operation between the microprocessor 101, the SCC 111 and
the modem unit 115. These integrated circuits also generate the synchronous ready signal and asynchronous
ready signal, SRDY and ARDY, respectively.
As mentioned above, the data terminal equipment
(DTE) communicates with the modem hardware over
the serial port 121 (e.g., a RS-232c or a RS-422 standard
port). FIG. 3B illustrates interface configurations to
implement both a RS-232 interface standard and a RS422 interface standard. The differential line drivers 149
(U25) and the differential line receivers 151 (U26) are

5

10

15

20

25

6

employed to implement the RS-422 interface standard.
The line drivers 139 (U20), 141 (U21), and 147 (U24) are
employed to implement the RS-232 interface standard.
It will be readily appreciated by those skilled in the art
that although two types of interfaces are depicted, only
one interface is actually utilized at one time. The integrated circuits employed to implement the interface
standard not used are unnecessary.
FIG. 3C illustrates the modem unit 115, the direct
access adapter (DAA) 117, the telephone interface 120,
and various supporting logic. The micro-switch bank
155 (SW2) provide the user the option of "hard-wiring"
the baud rate settings for the modem unit 115. In operation, the switch settings of the micro-switch bank 155
are passed to the modem unit 115, via the octal buffer
145 (U23), when the modem hardware is powered-up or
reset.
TABLE 1, below, provides information on the model
or type of devices employed in the modem hardware
illustrated in FIGS. 3A, 3B, and 3C. TABLE 1 also
provides one of several manufactures and/or suppliers
of the devices. TABLE 1 references the devices by
"Drawing Reference Characters" and "Device Reference Designations" consistent with their usage in FIGS.
3A, 3B, and 3C.
TABLE 1

Drawing
Ref.
30 Char.

Device
Ref.
Desig. Type

Manufacturer

----......;;~-..:..:......------------101
Motorola, Inc.
Ut
80188

35

40

45

50

103a
103b
103c
103d
119
123
124
125
127
129
l3l
133
135
137
111
115
117
139
141
143
145
147
149
151
153
155
157

U2
U3

U4
U5
U6
U8
U9
Uto
Uti
Ut2
Ut3
U!4
Ut5
U!6
Ut7
Ut8
Ut9
U20
U21
U22
U23
U24
U25
U26
B
SW2
YI

27C256
27C256
JEDEC
JEDEC
74ALS373
74HCI63
74HCI63
74HCI63
24ASOO
74AS04
74AS04
DSI232
74ASOO
74ASOO
82530
R9696-DP
CHl828
74HCI26
DSI4C88
74HC374
74HC540
26LS31
26LS32
CMB06

Intel Corporation
Intel Corporation
Intel Corporation
Intel Corporation
Texas Instruments, Inc.
Texas Instruments, Inc.
Texas Instruments, Inc.
Texas Instruments, Inc.
Texas Instruments, Inc.
Texas Instruments, Inc.
Texas Instruments, Inc.
Dallas Semiconductor
Texas Instruments, Inc.
Texas Instruments, Inc.
Intel Corporation
Rockwell International Corp.
Cermetek Microelectronics, Inc.
Motorola, Inc.
Motorola, Inc.
Texas Instruments, Inc.
Texas Instruments, Inc.
Motorola, Inc.
Motorola, Inc.
Motorola, Inc.
Star Micronics

55
The manufacturer specification sheets, commonly
known as "Data Sheets", for the device model or type'
indicated above are hereby incorporated by reference.
As mentioned above, the memory subsystem 103
60 includes memory devices 103a-d. The memory subsystem 103 may be implemented using RAM, ROM and/or
PROM type memory devices. TABLE 1 indicates the
memory devices 103a and 103b are type 27C256, ultra
violet erasable programmable read only memories (UV
65 PROMs) manufactured by Intel Corporation. When
either of memory devices 103a or 103b is a PROM type
device, then both 103a and 103b should be implemented
using PROM type devices.

5,412,730

7

Memory devices 103a and 103b may also be type
HM62256LP-SL series or HM62256LFP-SL series
CMOS static RAM manufactured by Hitachi America,
Ltd. Further, memory devices 103c and 103d may also
be type HM62256LP-SL series or HM62256LFP-SL
series CMOS static RAM. It will be appreciated by one
skilled in the art that a JEDEC type RAM, memory
devices 103c and 103d, is a RAM that conforms to the
industry standards regarding the integrated circuit
packaging. The type HM62256LP-SL series or
HM62256LFP-SL series CMOS static RAM devices
meet JEDEC standards.
The storage capacity of the memory subsystem 103
may be increased or decreased in relation to modem
hardware and/or modem software requirements. For
example, memory subsystem 103 may be configured
using 128K type, 256K type, and/or 512K type
RAMs/ROMs/PROMs. The "jumper points" facilitate
implementing various memory device types and memory configurations of the memory subsystem 103. The
manufacturer, by way of a memory device's data sheet,
provides configuration instructions. These instructions
dictate "jumper point" usage.
The power and ground pins for the integrated circuits, the buzzer 153, and the micro-switch bank 155 of
the modem hardware are not depicted in FIGS. 3A, 3B
and 3C. TABLE 2, below, provides power and ground
pin connections for the devices used to implement the
modem hardware illustrated in FIGS. 3A, 3B and 3C.
In particular, TABLE 2 indicates the pin connections to
+5V, digital ground, analog ground, and +/ -12V.
TABLE 2 references the devices by the "Drawing
Reference Characters" and "Device Reference Designations" consistent with their usage in FIGS. 3A, 3B,
and 3C.

8
FIG.4-Enhancements

5

10

15

20

25

30

35

The principles of the invention may be advantageously employed to encipher and decipher data which
is also compressed for enhanced transmission efficiency
and combined with error detection/correction coding.
Moreover, the invention may utilize a key storage system to store unique keys for different called and calling
parties, and may employ means for varying the interval
number in a random fashion so that the time durations
during which particular encryption keys are active
varies in unpredictable ways. These further enhancements to the system are depicted in FIG. 4 of the drawings which illustrates the preferred embodiment of the
invention.
If the data signals are to be "compressed" for increased transmission efficiency (e.g., by Huffman encoding or the like), the compression processing of the
data should precede encryption, because the encryption
process inherently randomizes the data, eliminating the
redundancy upon which efficient compression depends.
On the other hand, error control processing (such as
adding cyclic redundancy check (CRC) block checking
codes) is best done after encryption in accordance with
the invention, because successful synchronization of the
advance signals from the block counters 21 and 29 requires substantially error-free data transmission (which
the error-checking protocols insure).
As contemplated by the present invention, data compression, data encryption, and error control functions
may all be performed by a single control processor.
Thus, when a modem of the class shown in FIG. 2 of
the drawings is employed, the microprocessor 101 operates on the outgoing data stream by first performing
data compression, then performing the encryption step,

TABLE 2
Drawing
Reference Character

Device
Reference Designation

101
103a
103b
103c
103d
119
123
124
125
127
129
131
133
135
137
III
I IS
117
139
141
143
145
147
149
151
153
155

Ul
U2
U3
U4
US
U6
U8
U9
UlO
Ull
Ul2
U13
Ul4
Ul5
Ul6
Ul7
Ul8
Ul9
U20
U2I
U22
U23
U24
U25
U26
B
SW2

+5 V Pin(s)

Digital
Ground Pin(s)

9,21,43
28
28
28
28
20
1,4, 5, 7, 10, 16
6,10,16
1,4, 10, 13, 14
14
14
14
8
14
14
7,8,9
1,45,61

26, 46, 47, 50, 60
14
14
14
14
1,10
3,6,8
3,4,5,8
7
7
7
7
3,4
7
7
31
29,37,53

- 12 V

30,31
3,4

32

36
10

14

I

7,12,13

20
20
14
16
16
Power
16

+ 12 V

7

14

Analog
Ground Pin(s)

1,10
10
7
8, 12, IS
8,12
Ground
9

and finally performing the error detection/protection
The bypass capacitors are not illustrated in FIGS.
processing before forwarding the data on to the modem
3A, 3B and 3C. It will be appreciated by those skilled in
the art that these capacitors have a first terminal con- 65 module 113 for trellis coding and digital-to-analog conversion for transmission over the telephone network.
nected to the power pin of the integrated circuit and a
second terminal to ground. These type capacitors may
The signal processing functions used in this enhanced
arrangement are shown in FIG. 4 of the drawings. In
have a capacitance value of about 0.01 microfarad.

9

5,412,730

FIG. 4, the functional units employed in the basic system shown in FIG. 1 are designated by the same numerals used in FIG. 1, and the description of those units
need not be repeated.
A data compressor 34 is shown connected between 5
the data source 15 and the encryptor 17. In the hardware as seen in FIG. 2, data compression may be conveniently performed by the microprocessor 101 on the
data from the DTE obtained via the SCC 111. At the
receiving station 12 as seen in FIG. 4, a data decompres- 10
sor 35 is connected between the decryptor 31 and the
data utilization device 33. Note also that, as depicted in
FIG. 4, the data is monitored by the block counter 21
prior to compression, rather than afterwards. Correspondingly, at the receiving station 12, the block 15
counter 29 monitors the data flow after it is decompressed. In this way, both counters monitor the same
data stream. Both could be reconnected to monitor the
compressed data stream if desired, however.
Error control processing is done by the error control 20
coder 36 which, for example, might add cyclic redundancy check data to the data being transmitted to permit data correction in the error detector/corrector 37 at
the receiving end, or to initiate a retransmission under
the active error correction protocol. This error correc- 25
tion processing (at both ends) may be advantageously
performed by the same microprocessor that performs
the data compression and encryption functions.
To further enhance the security of the transmission,
the duration of the interval during which each given 30
key is active may be changed in a pseudo-random fashion. For this purpose, a pseudo-random number generator 38 is used at the transmitting station 11 to supply the
interval numbers to the block counter 21. The generator
38 is advanced to a new number each time an advance 35
signal is received from the output of block counter 21
over line 39 (so that a new interval number is supplied
to the block counter 21 each time it advances the encryption key generator 23). Block counter 21 may simply load the interval number from generator 38 into an 40
accumulator which is then decremented toward zero
when it emits-the advance signal to generator 23, at
which time it is loaded with a new and different interval
number from generator 38. At the receiving station 12,
a pseudo-random generator 40 (which performs the 45
same pseudo-random number generating process as the
generator 38 at the transmitting station 11) supplies a
sequence of interval numbers to counter 29. Generator
40 is advanced by the advance signals from counter 29
which also advance the encryption key generator 27. 50
The random number generators 23 and 38 at the
transmitting station obtain their seed values from a key
memory 50. Key memory 50 stores the random number
keys indexed by destination (along with telephone dialup numbers for automatic dialing). Similarly, at the- 55
receiving station, the seed values for the remote terminals from which the receiving station is authorized to
receive information are stored in a key memory 60
connected to supply seed values to the generators 27
and 40. The key memories eliminates the need for au- 60
thorized users to remember and enter keys before each
transmission or reception.
In addition, the use of key memories allows the stations to be operated as terminals in a secure network
under the control of a central station which, in separate 65
transmissions over different secure links, enters (and
erases) the keys needed by authorized sending and receiving stations connected to the network. In this way,

10

the central station permits one network user to transmit
to a single other user, or to "broadcast" to selected,
authorized users on the network only, while enabling all
terminals to use the network for unsecured transmissions.
The key memory within each station modem unit
includes a lookup table comprising a plurality of entries,
each of which stores control information concerning
another station in the network. Advantageously, each
table entry specifies:
(1) the serial number which identifies a remote hardware unit (and which corresponds to a serial number stored in the non-volatile memory of that remote unit);
(2) the current encryption key value (e.g. an 8 digit
DES encryption key) to be uS@<! for communications to and from that remote station; and
(3) an optional dial-up telephone number (or comparable routing information used by the network
switching system).
A switch operated by a physical key is also advantageously included in each station unit and has "security
enabled" and "security disabled" positions. The key
memory can only be loaded with values identifying one
or more remote units with whom communications are
authorized when the switch is in the "security disabled"
position (typically when the unit is being set up by an
authorized operator who has the physical key needed to
disable the security switch). At that time, the table can
be loaded either from a remote (host) station or by a
local command which takes the form of an extension to
the standard modem AT command set. That load command take the form:
AT JSN KDESKEY PHONENUM

where AT is the AT command prefix, JSN is the letter
"J" immediately followed by the serial number of the
remote station with which communications is authorized, KDESKEY is the letter "K" immediately followed by an 8 character DES encryption key, and
PHONENUM is the standard routing code (e.g. dial-up
phone number string). In the preferred embodiment, up
to 1000 serial numbers and keys, and up to 100 optional
dial-up phone number strings (each with up to 39 digits)
may stored in the key memory lookup table.
To make a secured transmission, the calling station
uses PHONENUM to establish the connection, normal
modem handshaking procedures are executed to establish a working data connection, including standard parameter negotiations (e.g. the V.42 parameters if that
protocol is being used). If the security key is enabled,
and a secure transmission is being requested by the
caller, the answering modem will not send its parameter
message (the V.42 XID frame) until it receives one from
the originator, this initial message including the (unencrypted) originator's serial number. The answering
modem uses the received serial number to select the
locally stored encryption key corresponding to that
serial number, and encrypts its responsive XID frame
using the fetched key. Thereafter, all transmissions between the originating and answering modems are encrypted and the encryption keys at each end of the
secure ling are thereafter altered in accordance with the
encryption algorithm as heretofore described.
In accordance with an important feature of this arrangement, the host system may initially authorize communication between two connected units by supplying

11

5,412,730

12

the appropriate serial numbers and initial key values
"MODEM.H" is a "c" computer language program
(unique to an authorized link), but as soon as transmisthat provides definitions and parameters for the firmsion begins between the two units over the authorized
ware of the modem unit 115. The "SCC.ASM" is an
link, the encryption keys are changed in ways that are
assembly language program that facilitates interfacing
unknowable to the host. As a consequence, knowledge 5 with the SCC 111. The "STARTUP.ASM" is an assemof the initial seed values supplied by the host are of no
bly language program that performs initial setup and
further value and cannot be used to monitor ongoing
POST routines. The "TEST.C" is a "c" computer
communications over the authorized link.
language routine that performs self-test and power-on
self-check. The "UTIL.C" is a "c" computer language
Programming
10 routine that performs utility routines for the modem
The encryption and decryption operations may be
unit 115.
performed by special purpose devices, such as those
The computer program modules written in the "c"
widely sold to implement the DES standard encryption
computer language are specifically designed for "MImethod. As noted, however, the encryption function
CROSOFT C 5.1". It should be noted that computer
can be less expensively added by suitable programming 15 languages other than "c" may be employed to perform
of the microprocessor 101 to perform this function as
the function for which the "c" computer language
well as the control, compression, and error handling
modules were so designed.
functions.
Various preferred embodiments of the present invenWorking computer programs for use with an 80188
tion have been described. It is understood, however,
microprocessor appear in the computer program listing 20 that changes and modifications can be made without
in the microfiche appendix. These computer programs
departing from the true scope and spirit of the present
perform the encryption, decryption, control, compresinvention as defined by the following claims, which are
sion, and error handling functions. The computer proto be interpreted in view of the foregoing.
gram listing appearing in the microfiche appendix inWhat is claimed is:
cludes 14 modules, arranged alphabetically therein, 25
1. A method for transmitting data comprising a senamed: (1) "COMM.C"; (2) "COMMAND.C"; (3)
quence of blocks in encrypted form over a communica"DISPLAY.ASM"; (4) "DISPLAY.H"; (5) "ENtion link from a transmitter to a receiver comprising, in
CRYPT.ASM"; (6) "EQUS. INC"; (7) "KEY_SCHD.
combination, the steps of:
ASM"; (8) "MAIN. C"; (9) "MODEM. ASM"; (10)
providing a seed value to both said transmitter and
"MODEM.H"; (11) "SCC.ASM"; (12) "START- 30
receiver,
.ASM"; (13)"TEST.C"; and (14) "UTIL. C".
generating a first sequence of pseudo-random key
In the microfiche appendix, each computer program
values based on said seed value at said transmitter,
module includes a comment section followed by a comeach new key value in said sequence being proputer program listing. Comments are also interspersed
duced at a time dependent upon a predetermined
within the program listing. Each program is briefly 35
characteristic of the data being transmitted over
described below.
said link,
The "COMM.C" is· a "c" computer language proencrypting the data sent over said link at said transgram that performs communication routines. The
mitter in accordance with said first sequence,
"COMMAND.C" is also a "c" computer language
generating a second sequence of pseudo-random key
program that performs command processing routines. 40
values based on said seed value at said receiver,
The "DISPLAY.ASM" is an assembly language proeach new key value in said sequence being program that performs display initialization and display
duced at a time dependent upon said predetermined
interface. The "DISPLAY.H" is a "c" computer lancharacteristic of said data transmitted over said link
guage program whose function is to output LCD dissuch that said first and second sequences are identiplay strings. The "ENCRYPT.ASM" is an assembly 45
cal to one another a new one of said key values in
language program having the function "KEY_SCHD
said first and said second sequences being produced
ASM" which calculates a sequence of 16 key-related
each time a predetermined number of said blocks
values required in the DES algorithm. This sequence is
are transmitted over said link, and
pre-calculated when the DES key is changed to indecrypting the data sent over said link at said receiver
50
crease the speed of encryption and/or decryption.
in accordance with said second sequence.
The "EQUS.INC" is an assembly language listing
that defines assembly language names used in the sys2. The method as set forth in claim 1 further including
the step of altering said predetermined number of blocks
tem. The "MAIN.C" is a "c" computer language program that performs modem supervisory control. The
each time said new key value in said first and said sec"MODEM.ASM" is an assembly language program 55 ond sequences is produced.
that facilitates interfacing with the modem unit 115. The
* * * * *

60

65