CA1102453A

CA1102453A - Computer accessing system

Info

Publication number: CA1102453A
Application number: CA322,630A
Authority: CA
Inventors: Frank T. Check, Jr.
Original assignee: Pitney Bowes Inc
Current assignee: Pitney Bowes Inc
Priority date: 1978-03-31
Filing date: 1979-03-02
Publication date: 1981-06-02
Also published as: JPS54136205A; FR2421426A1; US4310720A; GB2019060A; GB2076615A; GB2076615B; FR2421426B1; DE2912696A1; GB2019060B; DE2912696C2

Abstract

ABSTRACT OF THE DISCLOSURE

A communications link between a computer and a remote user is effected by a portable access unit which generates a non-recurring access code which is a function of a password and a pseudorandomly generated number. She access code is transmitted and compared with an access code generated by a computer access controller. In addition to the access code, the pseudorandomly generated number provides an encryption key which ? not trans-mitted but corresponds to a mating encryption key generated at the controller. The mating encryption keys are utilized to set up data transmission enciphering/deciphering networks successive links are established by incremented pseudorandomly generated access codes.

Description

~,-'`- ' ` , ., ` ~

. ' .

E3ACKGRCUND OF TED3 INVFNTIO~J
~'''`'"'~ ' .
-~ _1. Summar~ of the Invention .~ : This invention relates generally to securi.ng communica tion links between a computer and a remote terminal and more par-j :~ ticularly to a system for accessing a computer and keying an encryption network for data transmission through pseudorandom ¦
: number generation.

: . . ` . .

,-~ . . . .
`: , ~ ' ' ' . ':
:` ~ ` ', ,: - ' ~ ' `` ' `:
. , . : ... ,. , ~ `, ` .
.: . ` - . ` ~ ::

.. - -` ~ Zg~
. -! l -: :; , I . .
,.. , I .
Society's multifaceted dependency upon computer systems ¦has increased the possible effe~ts of unauthorized computer access ¦to untold level3. Computer networking,such as time sharing ¦systems, has led to inherent security problemsisince programs ¦e~ecuted by ~uch sy3tems shared access and cooperated wlth each ¦other. The capabilities of recent computer systems included not ¦only the utilization o remote terminals but extensive resource ¦sharing and batch processing of different projects. The use of ¦such systems has necessarily increased the likelihood of deliberat a or even accidental acquisition and/or alteration of data.
¦ computer systems have been sub~ect to a variety of ¦ security risks ranging from misappropriation of confidential data ¦ through unauthorized use of computing time. Access controls have ¦ been used in virtually all time sharing and most other computer ¦ syst2ms. Differences in the nature of the information being ¦ processed have given rise to various security measures and pro- `
¦ cedures commensurate with the value attached to such data and the ¦ consequences of unauthorized access and/or appropriation thereof.
For example, the data proces`sed in computer systems utilized for financial transactions has a substantial value due to the monetar~
losses which could be sustained as a result of system penetration.¦
A user has been traditionally identified by at least one of the following:

la) through somethirg he knew or had memorized, e.g. a password;
(b) through somethiny he carried with him, e.g. a card or badge: or '~' ' . . - ' , , . '.
. - , . . .

.-: - .
.
. . .- : ' . -: .. , ~ ` 115~Z453 ¦ (c) through a personal physical characteristic, e.g.
¦recognition by a guard.
I Passwords have been widely employed to authenticate a ¦remote terminal user. The use oE passwords has been augmented by ¦secondary security measures in many instances. For example, a ¦typical consumer banXing terminal accesses its computer by using la combination of a magnetically encoded card and the user's ` ¦memorized password commonly referred to as a personal identi~ica~
¦tion number.
¦ While such systems might have been satisfactory from a ¦cost/loss risk standpoint, there were subject to penetration with ¦ or without access to the individual user's card. Various pass-¦ word selection procedures and their susceptibility to penetration I have been discussed in a U. S Department of Commerce publication ¦ entitled The Use of Passwords for Controlled Access to Computer ¦ Resources, NBS Special Publication 500-9 dated May 1977.
In addition to penetration of the user's password, ¦ passwords themselves were ineffective, for example, against the ¦ penetration threat of between-lines entry and piggy-~ack infiltra-I tion. Unauthoxized interception of communication links between ¦ the computer and a remote user has been a further security problen ¦ and resulted in obtalning not only the data transmitted but the ¦ user's password Eor subsequent acces~.
.. . ' .' ' . , . , : : '. , : :
' ` ': .

"~ , llQZ'.'53 i ¦ SUMMARY OF ~HE INVENTION
,.,." I .
A remote computer i9 a~cessed by a nonrecurring acces~
code generated as a function of a pseuaorandom number and a user password. The access code is transmitted over telephone lines through the`use of a digital ~ignal to audio tone modem and decoded at the computer location. An access controller receives the decoded access code and compares it to an expected access cod which has baen congruently generated. Upon recognition of an equality between the controller generated access code and the transmitted access code, the control~er grants access to the computer. The generated pseudorandom number can be divided into two segments, one of which is used to furnish the access code and the other an encryption key utilized for subsequent data transmis sion between the user and the computer or, in the olternative, tw pseudorarldom numbersare generated, one of which is utili~ed for access code generation and the o~her for the encryption Xey.
From the above compendium it will be appreciated that it is an object of the present invention to provide a remote accessing system of the general character described Eor establish ing a communications link between a computer and a remote user which is not subject to the disadvantages aforementioned.
A further object of the present invention is to provld , a remote accessing system of the general character described for establishing a communications link between a computer and a user employing an accessing password which is not transmitted over communlcations lines. -_4_ .~ , . . , , . , .
~, . : .. ., , .. ~
.:
.
- '. ' ' . : , -. . : . : . :
~: ' : , :
-. , -:. ~ ' ' . ' ' ' ':

A further object of th~ pre~nt invention is to pro-vide a remote accessihg syste~ cf the general character descrlbed for establishing a communications link bPtween a computer and a user which utilizes p~e~dorandom number yeneration to provide nonrecurring access code~.
Another object of the pre~ent invention is to provide a remote accesqing system of the general character described for e~tablishing communication links between a computer and a user which employs data encryption established in conjunction with a nonrecurring ancryption key.
A further object of the present invention is to pro-vide a remote accessing system of the general character described for establishing communications links between a computer and a user which utili~es pseudorandom number generation for the pur-pose of both acces~ing the computer and providing a data en-cryption key.
Another object of the prese~t invention is to provide i a remote accessing syste~ of the general character described for es~tablish1ng communlcations links between a computer and a user wher~in interception of user-computer communications will not provide sufficient data for independently accessing the computer.
Other objects of the invention in part will be ob-vious and in part will be pointed out hereinafter.

' ' ' ' ' ' ' ' - ' . ... .
:~.
5 _ 1 ...-., ..., 1 '~''~'; . .

., . :: : ' ' : .. , -'', . ~, : .. '.: . , :, , .
- . ~ : : . .

According to one aspect o~ the present inven-tion there is provided a system for accessing a computer by a user having an assigned password to establish a communications link for data transmission between a computer terminal and a computer, the system comprising means at the terminal sequentially generating nonrecurring pseudorandom numbers in a reproducible fashion means receiving the pseudorandom number and the user assigned password and i.n response thereto modifying the pseudorandom number as a predetermined function of the password to provide an access code signal, means transmitting the access code signal from the terminalt controller means at the computer, the controller means receiving the transmitted access code signal, the controller means including means for sequentially generating congruent pseudorandom numbers and ~or modifying a congruent pseudorandom number with the user assigned password to provide a congruent access code signal and in response to the equality thereof providing an access signal, switch means receiving the access signal and in response thereto establishing a data transmisision path between the terminal and the computer, whereby computer access is secured through a sequential change of access codes in a pseudorandom fashion without direct transmission of the userls password.
According to a second aspect there is provided a . .
. method of accessing a computer at one location by a user positioned at a second location and having an assigned password, .` the method comprising the steps of : (a) sequentially generating a nonrecurri.ng pseudo-random number at the second location, , '' ~

. ~it ~
0-- - 5a -. . .
csm/ ~ ~
: . . . .
'' ', ' " ' '' ' ''` .'' .. - . : :

~l~Z~L53 ~...... ' ~

(b) modifying the pseudorandom number as a function of the user assigned password to provide an access code, ~ c) transmitting the access code Erom the second location to the first location, (d) receiving the transmitted access code at the first location, (e) sequentially generating a congruent nonrecurring - pseudorandom numbe.r, (f) modifying the congruent pseudorandom number with the user assigned password to provide a congruent access code, (g) comparing the received access code with the ~ congruent access code, and .-~ (h) granting access to the:computer upon recognition . -- of an equality between the received access code and the congruent -. access code, whereby computer access is secured through a change of access codes in a pseudorandom fashion without direct transmission of the user's password.
:,. . .
.. ~

.; .
' ,, . ' - 5b -"
;,, . ~ cs~

' .:
,. . .. . . :
:: . . .
':

2~S;~
.. . .
.

With these ends in view the invention finds embodiment . - in certain combir~ations of elements, arrangements of parts .: and serie~ of ~teps by whi~h the object3 aforementioned and cer~
. : tain other o~jects are hereinafter attained, all as ~ul~y de-: scribed with reference to the accompanying drawings and the scope of which is more particularly pointed out and indicated - in the appended claims.

` - ~ .RIEE DES~RIPTION O~ TKE DR~ I~GS

}n the accompanying drawings in which is shown one of the var~ous possible exemplary embodiments of the inventio~, FIG. 1 is a schematized block diagram of a remote accessing system constructed in accordance with and embodying : the invention and illustrating an access unit at a remote terminal establishing a communications link to a main computer;
FIG. 2 is a flow diagram illustrating an access con-troller routine for establishing access to the computer: and : FIG. 3 is a flow diagram illustrating a continuation ~ of tne controller routine originating in ~IG. 2.
; DESCRIPTIO~ OF THE PREFERRED EMBODIME~T
Referring now in detail to the drawings, and specif-. ~ ically to FIG. 1 wherein a block diagram illustrating a typical ~ communications channel linking a remote user which a computer :.~ is shown, the reference numeral 10 denotes generally a remote - terminal from which a user communicates with a computer 12.
: ~ In accordance with the present invention, a communicatiOnS lin~

14 is established through the use of conventional data trans-mission facilities such as, for e~ample, a telephone line or carrier wave transmission.

.r~.~ . . .
.. . .
~'`' , ' ' ' ` ' ' '' , ''" ` `

. ' , ` ' ' , :
'':, ~: .

;"`` ~ Si3 , ~
.
The present invention prevents unauthorized aeees~
to the eomputer 12 through the use of a user assigned password which funetions only in eonjunetion with a user assigned aeees~
unit 16. The aeeess unit 16 is preferably lightweight and hand-carried and typieally ineludes a mieroproeessor 18 sueh as MOS Teehnology 6502 programmed to generate a series of pseudo-random numbers in a reproducible ashion from an initial group-ing of seed numbers.
The aeeess unit 16 further includes a data eneoder 20 for the purpose of enerypting data for transmission over the link 14 to render intercepted data communications ~nintelligibl .
The encoder 20 may comprise data encryption hardware compatible with the Data Eneryption Standard algorithm promulgated by the National Bureau of Standards. Among the devices presently available for data encryption utilizing the Data Encryption Standard algorithm are the Rockwell-collins CR-100 Network En-cryptor, the Hansco Data Processing -Federal 007 and the Mo~oro a "In~oGuard" system. Use of the Data Eneryption Standard algo-rithm is by way of example only, and any other encryption algo-rithms and encryptors such as the DatoteX DEI-26 security device ean be employed.
In addition to the microprocessor lS and t~e encoder 20, the access unit also includes an input~output device 22, typically comprisin~ a keyboard and a transient display for .
indicating keyboard entries which,as will be described herein, comprise the user assigned pas~word.

_7_ ..' ' .' ,,~r-' ''' ' , '.
- - . ~ - .
. .
:, :' .
;
, ~ `

- -~ ~ 453 ~, . .
~-:,' .
Preferably, an authorized user carries his access unit 16 to the remote terminal 10 and interconnects the access unit with remote terminal computer peripherals such as an input output device ~4 Which may comprise A keyboard and/or CRT or TT
Communication from the input/output device 24 to the computer 1 is first encrypted by the encoder 20 and then encoded into audi _ tones at a tone encoder/decoder modem 26 which may comprise, by way of example, a Bell System 103A Data Set. The ton~ en-coded data is transmitted to the computer 12 ~ia the communica-tions link 14.

Pursuant to the in~ention,the microprocessor 18 is actuated to generate a chain of nonrecurring numbers from a group of seed numbers originally implanted. A typical pro-cessor sub-routine fo~ repeatable pseudorandom number generatio is disclosed in U. S. Patent ~o. 3,792,446 issued February 12, 1974 to McFiggins et al and assigned to the assignee of the present invention. It should be understood that such pseudo-random number generation sub-routine i5 exemplary of manifold random nu;nber generatiOn algorithms which may be employed.
User password integrity is maintained in accordance with the present invention by not directly transmitting the pas _ word over the communications link 14. Further, pursuant to the instant invention, possession of the password witho~t the ac-cess unit will not permit access to the computer, nor will possession of the access unit without the user's assigned password.
''.-~' . ,, , . .
~ -8-'',''' , " .

.
' ' , ~ /

~. ~ } d4b3 ._ `:

Generation of an acceptable access code signal by the acces9 unLt 16 entails the generation of a predicted pseudo-random numbar by the microprocessor 18 and modiEication of th~
pseudorandom numbes as a function of the password which is en-tered on the keyboard ~input/output device 22). For example, the access code may comprise pseudorandomly generated numbers multiplied by the user's password which also comprises a number.
The access code thus generated is fed to the modem 26 along with a constant access unit preface number for transmission via the communications link 14 to the computer 12. Interception of the access code transmission will not reveal the user's password but only the access unit preface number and the access code for the intercepted communication. As will be later described, such access code i3 not valid for subsequent communication links and cannot form the basis for deriving subsequent valid codes.
The tone encoded transmissions through the communica-tions link 14 are received at a companion modem 28 for conver-sion to digital waveEorms. Access to the main computer 12 is controlled by a controller unit 30 which monitors the communica-tions link 14. The controller may comprise a minicomputer such as the Data General NOVA series or even a microprocessor such as the microprocessor used in conjunction with random number generation at the access unit. -_9_ '' ~ ., ' . . , ,.
. .
''', '', , , , ' . : "
.~ .
: .
-:
.

' ' : l I . .
¦ It ~hould bo appreciated that the controller 30 ls : ¦ programmed with a congruent random number generation algorithm - 1 and initial seed numbers compatible with those assigned to all ... ¦ authori2ed access units. Further, the controller 30 may be set.
. . ¦ up such that the initial access code generated by each author-- I ized access unit and its associated assigned password is stored in a self-contained memory for ready reference.
: ¦ Referring now to the flow diagram of FIGS. 2 and 3 ... ¦ where the accessin~ sub-routine of the controller unit 30 is 1~ 1 depicted, .it will be appreciated that.after answering an incom-; ¦ ing telephone signal, the controller receives, through the modem .
. 1 30, the unit preface numb~r followed by the transmitted access .. I code as shown at an input block 32.
.~ ¦ As shown in a decision block 34,inquiry is made for verification of the incoming access un.it preface number with . . I access unit preface numbers stored in the controller memory to 1 determine whether the attempt to gain computer access is being ~ I . made through the use of a validly assigned access unit 16 which .,. ¦ has not been reported to be in the possession of anyone other . . than its authorized user. If the access unit preface.number is . ¦ not valid or if the preface number has been generated by a unit reported as stolen or mi.ssing, the program exits the deci-¦ sion block.34 on a branch 36 and the controller 30 logs the security violation and terminates the telephone link 14. ' ~ ' ' , -10- '. ~ ~
''.;"~ ~ . . ' .
' .' I . .
I ~ , ...
. r -: - ~ ' ., ~, , ^`~ .~

~2~L~i3 .,'. . , .
Upon acknowledgment of an incoming signal from a valid acces~ unit, the program exits the decision block 34 on a branch 38 and the controller retrieves the expected access code to be receivad from such access unit from a memory and compares the expected acce4s code witll the~code received over the com-munications link as depicted in a further decision block 40.
In order to prevent access code discrepancies due to signal interference from terminating a valid accees atteMpt, the controller 30 will permit a valid access unit to transmit its.access code a predetermined number of times. If the trans-mitted access code does not match the access code assigned to .
that unit number and stored in the controller memory, the program exits on a branch 42, the access code attempt is logged .
and the ccntroller determines how many attempts have been made to transmit the access code as shown in a decision block 44. If there have been less than the allotted number of attempts, a message is returned to indicate that the transmitted acce~
code has been re~ected and is to be retransmitted and the pro-gram reverts to the input block 32. After the allotted number of attempts has been reached, the program exits the decision block 44 on a branch 46 to break the communications link 14.
Upon receiving a transmitted access code which co- ¦
incides with the stored access code anticipated, the program exits the deci~ion block 40 on a branch 48 and the controller ' 30 thereafter sends a message to the access unit microprocessor 18 indicating acceptance of the access code such that the micro-processor will generate the next sequential pseudorandom number . . .
.,. , ' -11- ' ' . .

. ~
- ' :'" ~', . ~"' ' ' ` ~, . ':

.

::
~`

,; upon ~ubsequent actuation rather than repeat its last pseudo-random number.
~ he controller 30 then searches its nonvolatile memory and retrie~es a stored encryption key for the assigned access unit 16. The encryption key can comprise either a segment - of a single random number generation or a separate random number as generated by both the access unit microprocessor 1~ and the controller 30. The encryption key retrieved by the controller 30 is used to key an encoder/decoder 50 associated with the controller. It should be appreciated that the encryption key is not transmitted over the communications link 14 and i5 in-dependently generated at both the access unit 16 and the con-troller 30 on opposite ends of the link 14. Thus, anyone inter-cepting and recording the data transmission over the link 14 will not obtain the encryption key used to set up the enciphering/ , deciphering network.
- As shown in a block 52 of FIG. 3, the controller 30 : thereafter generates the next expected access code for the access ; unit 16 through the pseudorandom number generation algorithm with'. I!
; '! seed numbers which have been generated, e.g. as described in U.S.
Patent 3,79Z,446, and modifies the pseudorandom number by the I password. Additionally generated is the successive encryption key which may comprise a segment of the random number. The con-troller 30 thereafter rcplaces the previous access code and en-cryption Xeys ~or the aFcess unlt 6 in its nonvolatile memory.

'' . . - , ~ : ~

- I ~Z~5;3 ..; l .
',''; I .
¦ The program thereafter exits the block 52 and advi3es the computer 12 of the access unit preface number,as shown in a block 54, and connects a data path from the remote terminal 10 , to the computer 12 through the encoder 50 and a computer access ¦switch 56.
¦ At the termination of the communication between the ¦input/output device 24 at the terminal 10 and the computer 12, ¦the controller logs the communication with reference to the acce~s ¦unit preface number,the time and the date and thereafter di~-¦connects the communications iink 14.
¦ It should be appreciated that the controller 30 may ¦readily be adapted to simultaneously control a plurality of com-¦munication links 14 established by various access units. Further , ¦in lieu of generating the access code and encryption key in ad-¦ vance, the controller may generate these numbers during accessing ¦procedure. Similarly, the acces~ unit microprocessor 18 may generate its pseudorandom numbers upon each actuation or generate ¦ the numbers once and store them in a memory for recall when an access code i5 to be generated.
The access unit 16, upon actuation, will generate its ¦next sequential pseudorandom number modified by whatever password ¦is attempted. If the user is not authorized, he will not know ¦that the access code generated will not grant access unless an ¦attempt is made to gain access. Such attempts are logged by the controller 30, and security personnel will be alerted upon the ¦occurrence of any unusual pattern of access attempts by any given ¦access unit as identified by its preface number.
., I . ''. .' '.

' ' : , .. :
- : . .
- - ' ~ ,~ ,'' ' " ,' ' ~' .
.

` Il~Z4~3 _ ;''` . .
' '~ .

Thus, it will be seen that there i8 provided a co~-. puter accessing ~yst~m which achieve~ the, various object,~ of the .~ invention and i3 well suit7d to meet the co~ditions of practical usage.

~ ' - . .

~'' 'I . . .
,,,, . ' ` .

'''' ' ' .
' . .
., . . .

. .~., ` ' , ' . .
~ ~ .

' :

Claims

(B-484) PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A system for accessing a computer by a user having an assigned password to establish a communications link for data transmission between a computer terminal and a computer, the system comprising means at the terminal sequentially gener-ating nonrecurring pseudorandom numbers in a reproducible fashion means receiving said pseudorandom number and said user assigned password and in response thereto modifying the pseudorandom number as a predetermined function of the password to provide an access code signal, means transmitting the access code signal from the terminal, controller means at said computer, the controller means receiving the transmitted access code signal, the controller means including means for sequentially generating congruent pseudorandom numbers and for modifying a congruent pseu-dorandom number with the user assigned password to provide a congruent access code signal and in response to the equality thereof providing an access signal, switch means receiving the access signal and in response thereto establishing a data transmission path between the terminal and the computer, whereby computer access is secured through a sequential change of access codes in a pseudorandom fashion without direct transmission of said user's password.

2. A system fox accessing a computer to establish a communications link for data transmission constructed in accordance with claim 1 wherein the computer terminal is positioned remote from the computer.

3. A system for accessing a computer to establish a communications link for data transmission from a remote terminal as constructed in accordance with claim 2 wherein the means transmitting the access code comprises a telephone communi-cations link.

4. A system for accessing a computer to establish a communications link for data transmission constructed in accordance with claim 1 wherein the means generating the pseu-dorandom number comprises a portable unit.

5. A system for accessing a computer to establish a communications link for data transmission constructed in accordance with claim 1 wherein a plurality of remote terminals are provided, the system further including a plurality of means generating pseudorandom numbers in a reproducible fashion.

6. A system for accessing a computer to establish communication links for data transmission constructed in accor-dance with claim 1 wherein the means generating the pseudorandom number comprises an access unit, the access unit further including encryption means adapted to establish an enciphering/deciphering network in response to an encryption key signal, the controller means including a congruent encryption means, the pair of encryption means being adapted to encrypt data transmission between the terminal and the computer.

7. A system for securing a communications link for data transmission constructed in accordance with claim 6 wherein the access unit includes means adapted to separate the pseudo-random number into segments, one of the segments comprising an encryption key signal, the controller means including means to separate the congruent pseudorandom number into segments, a congruent segment comprising the encryption key for the con-gruent encryption means, whereby data transmission is secured through a sequential change of encryption keys in a pseudorandom fashion without direct transmission of the encryption keys.

8. A system for securing a communications link for data transmission constructed in accordance with claim 6 wherein the means adapted to generate the pseudorandom number is adapted to generate a pair of pseudorandom numbers, one of which is modi-fied by the password and the other of which comprises the encryp-tion key signal, the means for sequentially generating congruent pseudorandom numbers being adapted to generate a pair of congruent pseudorandom numbers one of which is modified by the password to provide the congruent access code signal and the other of which comprises the congruent encryption key signal, whereby data transmission is secured through a sequential change of encryption keys in a pseudorandom fashion without direct transmission of the encryption keys.

9. A system for securing data transmission between a computer and a computer terminal, the system comprising means at the terminal for sequentially generating nonrecurring pseudoran-dom numbers in a reproducible fashion, encryption means receiving a pseudorandom number as an encryption key and in response thereto establishing an encryption pattern, controller means associated with the computer, the controller means including means for sequentially generating congruent pseudorandom numbers in synchronization with the pseudorandom numbers generated at the terminal, congruent encryption means, the congruent encryption means receiving a congruent pseudorandom number as an encryption key and in response thereto establishing a congruent encryption pattern for data transmission, whereby data transmission is secured through a sequential change of encryption keys in a pseudorandom fashion without direct transmission of the encryp-tion keys.

10. A system for securing data transmission con-structed in accordance with claim 9 wherein the computer terminal is positioned remote from the computer.

11. A system for securing data transmission communi-cations between a computer and a computer terminal constructed in accordance with claim 9 wherein the means sequentially generating the pseudorandom numbers comprises a portable unit.

12. A system for securing data transmission communi-cations between a computer and a computer terminal constructed in accordance with claim 9 wherein a plurality of remote terminals are provided, the system further including a plurality of means sequentially generating pseudorandom numbers.

13. A method of accessing a computer at one location by a user positioned at a second location and having an assigned password, the method comprising the steps of (a) sequentially generating a nonrecurring pseudo-random number at the second location, (b) modifying the pseudorandom number as a function of the user assigned password to provide an access code, (c) transmitting the access code from the second location to the first location, (d) receiving the transmitted access code at the first location, (e) sequentially generating a congruent nonrecurring pseudorandom number, (f) modifying the congruent pseudorandom number with the user assigned password to provide a congruent access code, (g) comparing the received access code with the congruent access code, and (h) granting access to the computer upon recognition of an equality between the received access code and the congruent access code, whereby computer access is secured through a change of access codes in a pseudorandom fashion without direct transmission of the user's password.

14. A method of accessing a computer as set forth in claim 13 wherein the next sequential congruent access code is generated prior to receipt of the next transmitted access code, the method further including the steps of storing the next congruent access code and recalling said next congruent access code upon receipt of the next transmitted access code.

15. A method of accessing a computer as set forth in claim 13 wherein the pseudorandom number generated at the second location comprises two segments, one of which is used to generate the access code, the method further including the step of keying an encryptor at the second location with a further segment of the pseudorandom number, the congruent pseudorandom number comprising two segments, one of which is used to generate the congruent access code, the method further including the step of keying a congruent encryptor at the first location with a further segment of the congruent pseudorandom number, whereby data transmission is secured through sequential change of encryption keys in a pseudorandom fashion without transmission of the encryption key from one location to the other.

16. A method of establishing a data encryption network between a computer at one location and a terminal at a remote location, said method comprising the steps of (a) generating a nonrecurring pseudorandom number at the remote location, (b) keying an encryptor at the remote location with the pseudorandom number, (c) generating a congruent nonrecurring pseudorandom number at the one location and keying a congruent encryptor at the one location with the congruent pseudorandom number, whereby data transmission is secured through a sequential change of encryption keys in a pseudorandom fashion without transmission of encryption keys between the one location and the remote location.

17. A system for accessing a computer by a user having an assigned password to establish a communication link for data transmission between a computer terminal and a computer via a data transmission path, said system comprising a portable access unit connectable as a peripheral to said terminal and comprising:
(a) means sequentially generating nonrecurring pseudorandom numbers in a reproducible fashion, and (b) means modifying said pseudorandom numbers as a predetermined function of said password to provide a first access code signal, and means applying said access code signal to said data transmission path prior to data from said computer terminal; said system further comprising, at said computer;

(c) means for sequentially generating pseudorandom numbers congruent with those generated by said access unit, (d) means for modifying said congruent pseudorandom numbers with said user assigned password to provide a second access code signal, (e) means for receiving said first access code signal and comparing it with said second access code signal and in response to the equality thereof providing an access signal, (f) switch means receiving the access signal and in response thereto establishing a data transmission path between the terminal and the computer, whereby computer access is secured through a sequential change of access codes in a pseudorandom fashion without direct transmission of the user's password.

18. The system of claim 17, wherein said terminal comprises a I/O unit fox providing data signals, and said access unit is connectable to said I/O unit.

19. The system of claim 18, wherein said access unit includes keyboard means for entering said password.

20. The system of claim 18, wherein said access unit further comprises means for preceeding said access code signal with a constant access unit code signal individual to said access unit.

21. The system of claim 18, wherein said access unit comprises means for encrypting data from said I/O unit prior to application to said data transmission path.

22. A portable access unit for insertion in a data transmission path between a digital computer and a remote terminal, to enable access of said terminal to said computer, said unit having a communication path adapted to be connected in series in said transmission path and including encrypting means for encrypting data from said terminal, a source of pseudorandom number signals, a keyboard for receiving a password unique to said unit coproducing password signals, means modifying said random number signals by said password signals and applying the modified random number signals to said communication path, and means applying a constant identification signal unique to said unit to said communication path.

23. The access unit of claim 22 further comprising a display for displaying entries in said keyboard.