Encryption/decryption apparatus with non-accessible table of keys

1 2

anyone, even those operating the encryption/decrypENCRYPTION/DECRYPTION APPARATUS WITH tion apparatus. Various techniques have been develNON-ACCESSIBLE TABLE OF KEYS oped to access encryption keys stored in an electronic

memory for this purpose. For example, a new encrypFIELD OF THE INVENTION 5 tion key can be selected for subsequent encryption of

The present invention generally pertains to apparatus communications between stations based on the last enfor encrypting and decrypting data, and more specifi- cryption key that was used, by applying a secret forcally, to apparatus for implementing the encryption and rnula to generate the new key. However, if the formula decryption process with secret encryption keys. is discovered or otherwise becomes known by someone

„ _ 10 who is outside the organizational network, security of

BACKGROUND OF THE INVENTION the encryption system is breached, since that person can

Procedures for encrypting and decrypting data for generate the encryption keys that will subsequently be transmission over non-secure radio or telephone links used, simply by applying the formula to any previously have been highly refined to meet the needs of the mili- discovered key.

tary and industry. An encryption algorithm that is virtu- 15 Clearly, it would be preferable to randomly generate ally unbreakable in any reasonable time frame, by even the encryption key that is used to encrypt data transmitthe most powerful of high-speed computers, has been ted to another station each time that communications developed and published by U.S. National Bureau of are initiated. Yet, random generation of an encryption Standards and sanctioned for use by industry in this key at one station inherently renders the receiving stacountry as an acceptable method for protecting com- 20 tion unable to decrypt the message, because it does not puterized data conveyed over non-secure channels. In have the encryption key used. What is therefore refact, integrated circuits designed specifically for en- quired ^ means for transmitting the encryption key cryption and decryption of data in accordance with this from one station t0 another in an encrypted form, with Data Encryption Algorithm (DEA) are readily avail- some provision that enables the receiving station to able from several vendors, such as Western Digital TM. 25 d { ^ en tion key Prior ^ encryption/deThe algorithm, like most encryption schemes, uses an tion apparatus do not rovide means t0 accomplish

encryption key to encrypt data. Successful use of the thjs ^ m m effident manner that is nQt ^ cirum. DtA and almost any other encryption/decryption ventgd . . £xch . (KEK) ^

is used in

algorithm commonly employed, requires that the sta- & ss Qf transferri^ an encryption key for en

tion receiving the encrypted transmission have the same 30 , , .. X .

, j . . if J « • J * J * cryptmg and decrypting the message to the other sta

key used to encrypt the data in order to decrypt it. ® , ?, . . ^ „,

Accordingly, no unauthorized party should know or tlon.,m"st * avmlable l°A*° h statl0ns' but can_ not be have access to the encryption key that is being used. avallabie to any°ne outslde the secure network, °J sta" Unfortunately, for any prior art encryption/decryp- tlons- Even * the encryption apparatus is available to tion system using the DEA or similar algorithms, exten- 35 someone outside the organization, it should be virtually sive security measures are required for managing and TMP°ssible to discover the KEKs used by stations cornperiodically changing the encryption keys that are used. pnsing the network, if secure communications are to be Any third party that gains access to the encryption key maintained.

being used to encrypt data can tap into a non-secure line ^ foregoing aspects and many of the attendant over which encrypted messages are transmitted and 40 advantages of this invention over the prior art will then use the key to decrypt messages that are inter- become more readily appreciated as the same becomes cepted. Even if knowledge of the encryption key used is better understood by reference to the following detailed limited to those operating the encryption/decryption description, when taken in conjunction with the accomequipment, there can be no assurance that others out- panying drawings.

side an organization will not breach security and learn 45 SUMMARY OF THE INVENTION

the encryption key due to failure of someone in the

organization to follow security procedures. As the size In accordance with the present invention, encrypof a network over which secure communications must tion/decryption apparatus for ensuring secure commube maintained expands, the difficulty in managing the nications between two stations include encryption proencryption keys used on the network grows exponen- 50 cessor means for encrypting and decrypting data using tially. a session data encryption key (DEK) that is input

Since any person with access to the encryption keys thereto. Control means coupled to the encryption procan breach the security of encrypted communications cessor means, are provided for controlling the operabetween members of the network, encryption keys must tion of the encryption processor means. The control be changed on a regular basis. Frequent changes in the 55 means supply the encryption processor means with the encryption keys in use rninimizes the risk of disclosure data for encryption and decryption and with an encrypby individuals that previously had access to the keys. tion key for use in encrypting and decrypting the data to However, any such change requires that the new en- produce an output signal in response to programmed cryption keys be distributed to all stations in the net- instructions. These programmed instructions cause the work. Typically, the new encryption keys are hand 60 control means to automatically randomly select a part carried to each station site by bonded couriers; never- of a session DEK and to combine it with another part of theless, it is possible that a courier may compromise the session DEK received from the other station to security. Even if a security breach does not occur, the determine the session DEK that will be used by the cost of regularly distributing encryption keys to each encryption processor means to encrypt data. Nonstation of a large network in this manner may be prohib- 65 volatile memory means that are coupled to the control itive. means store a plurality of key encryption keys that are

For these reasons, it is preferable to use encryption used by the encryption processor means in encrypting a keys at each station in a network that are not known to part of the session DEK for transmission to the other 3 4

station. The control means select the key encryption tion network is distributing secure DEKs to each stakey from the plurality of key encryption keys as a func- tion in the network on a regular basis. In FIG. 1, a tion of a check value determined with the part of the simple network for carrying out encrypted communicasession key. tions is shown generally at reference numeral 10. NetWithin the non-volatile memory means is disposed an 5 work 10 is shown simply as two stations, including a internal power source that provides electrical power to station 12 and a station 14, but it will be appreciated that maintain storage of the plurality of key encryption keys. the network can comprise many other such stations. Potting means encapsulate the encryption processor Both stations 12 and 14 use similar components for means, the control means, and the non-volatile memory encrypting and decrypting communications. For exammeans in a radio and light wave opaque material that is 10 pie, station 12 includes a data device 16, which may, for sufficiently hard and resistant to dissolution by solvents example, comprise a fascimile machine or personal comto prevent its removal without damage to interconnec- puter (neither shown separately). Data device 16 is tions coupling the non-volatile memory means to the connected through lines 18 to an EDU A 20. Station 12 control means and damage to interconnections supply- uses EDU A 20 to establish secure communications ing electrical power to the non-volatile memory means 15 over a non-secure line (or radio link) 22 with station 14, from the internal power source. Such damage causes which includes an EDU B 28. EDU B 28 is connected erasure of the plurality of key encryption keys stored in to a data device 24 over lines 26. Data device 24 is the the non-volatile memory means. In addition, the control same type of device as data device 16. Thus, if data means respond to any attempt to externally interrogate devices 16 and 24 are fascimile machines, communicathe non-volatile memory means by causing erasure of 20 tions network 10 permits secure communication of facthe key encryption keys stored therein. simile information in an encrypted form between sta

Multiplexer means are coupled to the control means tions 12 and 14 over non-secure line 22.

to receive a data signal and a select signal therefrom, Because of the manner in which secure communica

and are also coupled to the encryption processor means, tions are established between EDU A 20 and EDU B

an output port, and the memory means; the multiplexer 25 28, tapping into non-secure line 22 using a similar EDU

means selectively convey the data signal to one of the (not shown) would NOT enable a third party to breach

encryption processor means, the output port, and the secure communications between stations 12 and 14. In

memory means, in response to the select signal. The the preferred form of the present invention, communi

control means include a non-volatile memory for retain- cations between EDU A 20 and EDU B 28 are carried

ing program steps and a unique identification code that 30 out using a session encryption key that is changed with

identifies a specific encryption/decryption apparatus. each session and comprises two parts, one part ran

In addition, the control means include means for lock- domly selected by EDU A 20, and the other part ran

ing the control means and its non-volatile memory to domly selected by EDU B 28. Thus, the present inven

prevent data and program steps from being read exter- tion comprises the EDU at each of the communicating

nally after storage of the program steps in the non- 35 stations 12 and 14. In establishing secure communica

volatile memory is complete. The means for locking tions between two stations 12 and 14, the EDU at each

include means for encrypting data and memory ad- station randomly select its respective portion of the

dresses defining memory storage locations within the session encryption key, encrypts that portion of the

non-volatile memory of the control means and within session encryption key, and transmits the encrypted

the non-volatile memory means. 40 respective portion of the session encryption key to the

BRIEF DESCRIPTION OF THE DRAWINGS

other station. Once both station 12 and station 14 have decrypted the portion of the session encryption key

FIG. 1 is a block diagram of a communications net- developed by the other station, the two portions are

work comprising two stations, each provided with an logically combined at each station to produce the com

encryption/decryption unit (EDU) in accordance with 45 plete or final session encryption key used for encrypting

the present invention, thereby enabling the stations to data transmitted between stations 12 and 14 during the

establish secure communications over a non-secure line current session. In addition, the EDUs are prepro

or radio link; grammed to ensure that the intended station in a two

FIG. 2 is a schematic block diagram of one of the way communication link is actually receiving or trans

EDUs shown in FIG. 1; 50 mitting the encrypted data, to guard against a third

FIG. 3 is a flow chart illustrating the logical steps party tapping into non-secure line 22 with another

implemented at one station by the EDU in selecting and EDU. The EDUs also ensure that the two portions of

encrypting a first portion of a session encryption key for the session encryption key that are exchanged between

transmittal to another station; stations 12 and 14 are correctly received and decrypted,

FIG. 4 is a flow chart illustrating the logical steps 55 thereby protecting against data errors that might have

implemented by the EDU at the other station in de- arisen in the transmission of the encrypted portions of

crypting the first portion of the session encryption key, the session encryption key between the two stations or

and in selecting and encrypting a second portion of the in their decryption.

session encryption key for transmittal to the one station; A block diagram of EDU 20 is shown in FIG. 2;

and 60 EDU 28 is exactly the same, except for having a differ

FIG. 5 is a flow chart illustrating the logical steps ent EDU identification number stored within it. EDU

implemented by the EDU at the one station to decrypt 20 includes a potted module 30 and an external input

the second portion of the session encryption key. /output (I/O) bus 32 for providing interconnections

... nr Tuc between the EDU and the data device (or to other

DETMLED DESCWPTION OF THE 65 componentS) if the EDU is used „ an element of a more

PKfchfcKKfcD fcMBODlMEN 1 extensive data encryption apparatus) that will provide

As noted above, one of the more difficult problems in the data to be encrypted or will receive the data that is

establishing and maintaining an encrypted communica- decrypted by the EDU. Module 30, which comprises 5 6

virtually the entire EDU, is encapsulated within a radio therein are preserved in the absence of an externally opaque and light opaque potting compound 34 to pre- applied voltage for up to 10 years. In addition, the interyen t discovery of the internal circuitry and to prevent nal data registers and key configuration registers of the forced electromagnetic or visual tapping, monitoring, DS 5000 integrated circuit are non-volatile. Data stored or other forms of penetration that might be attempted to 5 within the embedded RAM that comprise program uncover encryption keys and other information in- steps carried out by CPU 36 in establishing secure comcluded therein. The potting compound is sufficiently munications can be modified after encapsulation of hard and resistant to abrasion to prevent its removal module 30 has been accomplished with potting material without damaging the components comprising the 34; however, initial loading of the embedded RAM EDU or at least causing loss of important data stored 10 within the DS 5000 microchip comprising CPU 36 is therein. Of greatest sensitivity to maintaining the secu- accomplished with a conventional universal asynchrority of communications between EDUs comprising a nous receiver/transmitter (UART) interface (not network is the need to protect against discovery of shown) that is connected through external I/O bus 32 KEKs that are encrypted using a key that is unique to by lines 76. In addition, control lines 50 connect CPU 36 each EDU and is assigned to it when it is initialized. The 15 to external I/O bus 32 and convey write, read, interencrypted KEKs are stored as tables within each EDU rupt, and signals for ports 0-3 (P1.0-P1.3) of the CPU. and are utilized for encrypting portions of the session Data lines (D0-D7) 54 interconnect CPU 36 with encryption key that are exchanged between two sta- RAM 42 and with a buffer 46. Buffer 46 comprises an tions and subsequently logically combined at each EDU SN 74HCT245 octal bus transceiver with a three-state to produce a session DEK that is used for encryption of 20 output that is used to block external access to internal data exchanged over non-secure line 22. To avoid data transfers occurring within module 30, thereby breaching the security of communications on network preventing an external device from accessing KEKs 10, it is absolutely imperative that these KEKs not be- stored in RAM 42 and other data transferred between come publicly known. components of the module. Buffer 46 is enabled via

In the preferred form of module 30, two sets or tables 25 control signals supplied over a line 74 by CPU 36 when of KEKs are stored in encrypted form in a random it is appropriate to allow bi-directional data transfer to access memory (RAM) 42. One set is called a "public" and from external I/O bus 32 through lines 52, and set, since each EDU that will be sold will include this therefore to and from an external device, set. The other set is a "private" set of KEKs, which To provide additional security, CPU 36 operates in optionally may be randomly generated by a user for 30 an encrypted mode. The encrypted mode is deactivated distribution to and storage in those EDUs comprising a prior to the initial loading of program steps and data private network of stations. The significance of the into the embedded RAM of CPU 36. Before the initial KEKs will be apparent from the description that fol- loading of program code and data begins during manulows. Any attempt to expose the internal circuitry of facture of the EDU, a 40-bit encryption mode key is module 30 by use of a chemical, solvent, or mechanical 35 selected for use by CPU 36 in the encrypted mode. A means in order to access RAM 42 electronically or data encryptor circuit and an address encryptor circuit physically so as to access these data will cause loss of (neither separately shown) within CPU 36 respectively the KEKs that are stored therein. RAM 42 preferably control the form in which the program code is stored in comprises a Dallas Semiconductor Tm type DS 1213 the embedded RAM of the CPU and the addresses at smart socket in which is installed a memory integrated 40 which it is stored. As the initial loading of program circuit (not separately shown) comprising 128Kx8bits steps is performed, the data encryptor circuit uses the of storage, i.e., yielding 1,048,576 bits of non-volatile 40-bit encryption mode key to transform or encrypt static RAM organized as 131,072 words by 8 bits. This opcodes, operands, and data bytes at each memory memory integrated circuit is a dual in-line package location defined by the software. Similarly, the address (DIP) package configuration of generally conventional 45 encryptor circuit uses the encryption mode key in a design, but the smart socket contains an internal battery different encryption algorithm to translate or encrypt a supply (not separately shown) sufficient to maintain logical address of each data byte location into an endata integrity in the absence of externally applied power crypted address at which the data are actually stored, for a period in excess of 10 years. Dallas Semiconductor The contents of the embedded RAM are then verified, also supplies an integrated circuit non-volatile memory 50 and the encrypted mode is enabled by setting a security device that includes an integral internal battery supply, lock bit. After the security lock bit is set to enable operand this type of device can be used in place of the smart ation in the encrypted mode, the contents of the CPU's socket and more conventional memory device combina- embedded RAM is unintelligible to an observer that tions. In the event a chemical solution is used to dissolve might attempt to tap into its circuitry to discover the potting compound 34 in an attempt to discover the 55 program code and other data stored therein. The adKEKs stored in RAM 142, the material comprising dress and data encryptor circuitry provides real time RAM 142 (smart socket or memory device that includes translation or decryption of program code and address the integral internal battery supply) will also be dis- locations to CPU 36 during subsequent operation of the solved, thereby disconnecting the internal battery sup- EDU. Only program code and data stored in the CPU's ply and erasing the KEKs stored therein. 60 embedded RAM that does NOT affect secure operation

Operation of module 30 to establish and conduct of the EDU can be changed after the security lock bit is

secure communications is controlled by a CPU 36, set. Any attempt to externally interrogate the CPU to

which includes 32K of embedded RAM (not separately discover the 40-bit encryption key causes its erasure,

shown). In the preferred embodiment, a Dallas Semi- rendering the contents of the embedded RAM useless,

conductor Tm type DS 5000 microchip integrated cir- 65 Even if the encrypted program code and data are there

cuit is used for CPU 36. The DS 5000 integrated circuit after read back from the embedded RAM in CPU 36,

includes non-volatile embedded RAM (not separately they can not be decrypted without the 40-bit encryption

shown) and all information and programming stored mode key, which is lost.