fig1 illustrates three agents involved in the assignment of an a 13 key to a cellular telephone , under the invention . one is the cellular telephone 3 itself . the second is an authorization center , or ac , 6 . the third is a trusted industry agent , tia , 9 . the authorization center 6 is equipped with a table 11 , which contains pairs of numbers . each pair associates an index , i , with an electronic serial number , esn . the tia 9 is also equipped with a table 12 , which contains other pairs of numbers . each pair associates the index , i , with a number x . the number x is preferably 64 bits in length . the x , i , and the esn appear in tables 11 and 12 . the table 12 was created by the manufacturer of the telephone , and delivered to the tia 9 . as will be seen , it is important that this table 12 be maintained in confidence . the other table 11 is also generated by the manufacturer , and is used by the authorization center to verify the identity of callers who wish to obtain an a 13 key , as will be explained immediately below . against this background , the assignment of the a 13 key to the cellular telephone 3 can be viewed as requiring eight steps , which are shown in fig2 and 3 . the agents which are actively involved in a given step are drawn in solid outline , for emphasis , while those which are not actively involved are drawn in phantom outline . in fig2 a , the telephone 3 transmits its index , i2 , together with its esn , to the ac 6 , as indicated by the arrow . this transmission can take the form of a normal cellular telephone call , but made to a specific telephone number , which the ac 6 answers . preferably , the data is transmitted using the &# 34 ; cellular signalling message protocol ,&# 34 ; which is an industry - standard protocol . alternately , the data can be carried by a sequence of dual - tone , multi - frequency ( dtmf ) signals . as another alternate , the cellular telephone can be equipped with a simple cellular modem 20 in fig1 which transmits , and receives , the data . after this transmission , the ac 6 is in possession of i2 and esn , as indicated . the ac 6 consults table 11 , and ascertains whether the two codes , i2 and esn , belong to each other . if so , the ac 6 proceeds in the process of assigning an a 13 key to the telephone 3 . if not , the procedure terminates . next , as indicated in fig2 b , the ac 6 transmits the index i2 to the tia 9 . this transmission can be undertaken using network messaging , known in the art , and known security measures can be used by the tia 9 to assure that the calling party is the genuine ac 6 . the tia 9 uses the index i2 , as indicated in fig2 c , to locate the x in table 12 which corresponds to i2 . the tia 9 is now in possession of both i2 and its associated x2 , as indicated . as shown in fig2 d , the tia 9 then performs two computations . first , the tia generates a random number , rand . then , using both rand and x2 , the tia computes a number e , by using a non - reversible algorithm such as ( 1 ) sha , secure hash algorithm or ( 2 ) the algorithm known as md - 5 , both of which are known in the art , or ( 3 ) cave , which is commercially available . the tia transmits both e and rand to the ac , as indicated in fig3 a , thereby placing the ac 6 in possession of the four numbers i2 , esn , e , and rand . then , as indicated in fig3 b , the ac selects an a 13 key . the ac masks the a 13 key , by using e as a mask . the masking process indicated in fig3 b relies on the ex - or function , as indicated by the cross - within - a - circle , but other masking operations are possible . the masking operation produces a number z . as indicated in fig3 c , the ac transmits both z and rand to the telephone 3 . then , as indicated in fig3 d , the telephone 3 first recovers e , using the cave function , based on rand and x2 . finally , the telephone 3 unmasks z , by ex - oring z against e , to obtain the a 13 key . the telephone 3 stores the a 13 key . when the telephone 3 later makes an authorization request , as by contacting the ac , the telephone 3 transmits a code derived from the a 13 key , together with its esn . the ac ascertains whether the a 13 key matches the esn and , if so , allows the telephone 3 to continue with the call . fig4 is a flow chart illustrating logic which implements one form of the invention . in block 50 , the telephone 3 transmits i2 and its esn to the ac , and requests an a 13 key . in block 55 , the ac , using table 11 in fig1 determines whether the esn belongs to i2 , and , if so , proceeds . in block 60 , the ac transmits i2 to the tia . in block 70 , the tia locates the x , which is x2 in this example , assigned by the manufacturer to i2 . in block 75 , the tia generates rand and , based on rand and the x2 located in table 12 , generates e . in block 80 , the tia transmits e and rand to the ac . in block 85 , the ac selects an a 13 key , and masks it with e , to produce z . in block 90 , the ac transmits z and rand to the telephone . in block 95 , the telephone derives e , based on rand and x2 . then , in block 100 , the telephone un - masks z , using e , to obtain the a 13 key . a significant feature of the steps of fig4 is that the process is fully automated . that is , no human observers are involved in , or witness , the computations . since the process is full automated , and no humans witness the variables chosen , such as rand , z , and e , the only possible avenue of obtaining variables is to intercept transmissions between the ac and the telephone . but , as explained above , these interceptions do not produce the a 13 key . 1 . one characterization of the invention is the following . as background regarding terminology , &# 34 ; plain text &# 34 ; refers to a message which is not encrypted . &# 34 ; cipher text &# 34 ; refers to a message in encrypted form . an agent , ac , selects an a 13 key , and encrypts the a 13 key , as in fig3 b , to produce cipher text , in the form of number z . the encryption requires x2 for de - cryption . this is illustrated in fig3 d , wherein x2 is required to obtain e , which is used to un - mask z , to obtain the a 13 key . however , the agent ac does not know x2 . restated , the agent ac produces cipher text , z , which requires x2 for recovery of the plain text , a 13 key , but the ac never uses x2 , and has no access to x2 . 2 . as indicated in fig3 d , three numbers , namely , rand , x2 , and z , are required to obtain the a 13 key . of these , x2 is never available for interception by a hacker . that is , x2 is never transmitted between agents , not even on the ordinary telephone channel connecting the ac 6 with the tia 9 . the only source of x2 is the telephone 3 itself in fig1 . however , it is assumed that the numbers , including x2 , which are stored in the telephone 3 at the time of manufacture , are secure against discovery . that is , it is assumed that an excessive amount of reverse - engineering would be required to ascertain these numbers . approaches to making these numbers secure are known in the art . it is also assumed , as stated above , that table 12 is not available to a hacker . therefore , it is assumed that x2 is not available to a hacker , and , thus , that the a 13 key assigned to the telephone 3 cannot be deduced . from another point of view , if , to make a call , the telephone 3 transmits its a 13 key and its esn to the ac 6 for authorization , a hacker must know these two numbers , in order to impersonate the telephone 3 . however , the only party who knows which a 13 key is associated with the telephone &# 39 ; s esn is the ac 6 . since the ac 6 sells cellular telephone service , it is assumed that the ac 6 will impose tight security on the knowledge of the a 13 key / esn assignments . 3 . transmissions between the ac 6 and the tia 9 are considered secure . they are undertaken , for example , using network messaging approaches , perhaps using encryption . consequently , only the data traffic running between the phone 3 and the ac , and occurring in fig2 a and 3c , are subject to interception . however , this traffic provides no information which can lead a hacker to the a 13 key . for example , assume the best case for the hacker : that he intercepts all of this traffic , and thereby obtains i2 , esn , rand , and z . but he needs the number e to unmask z , to obtain a 13 key , as indicated by the bottom line in fig3 d . to obtain e , he needs number x2 , as indicated by the second - to - last line . but x2 is secure within the telephone 3 , as stated above . thus , to repeat , if a hacker intercepts all interceptable transmissions , the hacker cannot deduce the a 13 key . 4 . point number 3 illustrated the impossibility of a hacker obtaining a key through interception . an alternate possibility is that the hacker would act as an imposter , by representing himself as a telephone 3 . in this scenario , the hacker finds no greater success . for example , assume that the hacker fabricates an index i , and successfully presents it to the ac , as indicated in fig2 a . this i leads to a corresponding x , taken from table 12 of fig2 c , which , in turn , leads to the mask e in fig2 d . if the hacker could obtain mask e , the hacker could recover the a 13 key , using the last step indicated in fig3 d . however , the hacker never receives mask e , only z and rand . the hacker cannot obtain a 13 key without mask e , and , for this , the hacker needs the x obtained from table 12 in fig2 c . this x is available only to ( 1 ) the telephone 3 , ( 2 ) the tia 9 , and ( 3 ) the manufacturer of phone 3 , all of which are considered secure . 5 . a given telephone 3 may request a new a 13 key . this can occur , for example , when the owner of telephone 3 sells the telephone . when the new a 13 key is requested , a new mask e is generated in fig3 d , because the rand of fig2 d will be different , because random . the previous mask e becomes worthless , and the new a 13 key cannot be deduced , using previous variables , such as rand or z , used to derive the previous a 13 key . 6 . the process indicated in fig2 and 3 places a strict limit on the parties who learn the a 13 key assigned to telephone 3 . neither the manufacturer , nor the tia , know the a key assigned to the telephone 3 , because the ac selects the a 13 key , and informs neither of these parties of the a 13 key selected . nor do either the manufacturer , nor the tia , know which telephone 3 is involved in the a 13 key assignment process of fig2 and 3 . the manufacturer , of course , is completely uninvolved , and does not know of the call made in fig2 a . even though the tia knows of the call , the tia merely locates an x , based on an index i , in fig2 c , and performs the two computations of fig2 d . but the tia has no knowledge of which &# 34 ; esn &# 34 ; is associated with the &# 34 ; i ,&# 34 ; and thus does not know which telephone is making the call . 7 . the invention imposes another strict limit , which is on the parties who are able to hack the a 13 key assigned to telephone 3 . as the discussion above indicated , knowledge of the x contained in the telephone 3 is a requirement for obtaining the a 13 key . but , as also explained above , that knowledge cannot be obtained from the telephone 3 itself , because the telephone 3 has been made tamper - proof . the only source of the x is table 12 , in fig2 c , which is currently held by the tia . however , the tia is , by stipulation , considered an honest party who will not cooperate with hackers . the manufacturer is also a potential source of table 12 , but is eliminated , on the basis of assumed honesty . in addition , the manufacturer will probably stand as a potential source of table 12 for only a very brief period of time , thereby significantly reducing the time - frame over which access to table 12 is even possible . that is , the manufacturer produces tables 11 and 12 . table 11 is delivered to the ac ; table 12 is delivered to the tia . after the deliveries , the manufacturer has no further interest in the tables and , in fact , will probably destroy its own copies , in order to eliminate the trouble and overhead involved with storing them . thus , the manufacturer will be in possession of the tables for only a brief period of time , namely , from their creation to their delivery . this time can be as short as a few hours , or even minutes . 8 . the manufacturer of the telephone 3 in fig1 programs the numbers x , i , and esn into the telephone . however , there is no relation between these numbers and the a 13 key . restated , there is no way to deduce the a 13 key from these three numbers . further , every telephone is not required to own a unique x : different telephones can be assigned the same x , provided that the x &# 39 ; s are un - correlated , in the statistical sense . 9 . the e transmitted to the ac 6 in fig3 a will be different , for different transactions , partly because e depends on a random number generated in fig2 d , and partly because e depends on x . both the random number and x will , in general , be different , for different telephones . 10 . preferably , the events of fig2 and 3 occur during a single telephone call made by the telephone 3 to the ac . within the cellular telephone 3 , data reception , transmission , and processing is performed by apparatus known in the art , and indicated by block 120 in fig1 . the telephone 3 is linked to the ac by a cellular channel 125 , and the ac is linked with the tia by any suitable communication link 130 . numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention . what is desired to be secured by letters patent is the invention as defined in the following claims .