in the following description , a preferred embodiment of the invention is described with regard to preferred process steps and data structures . however , those skilled in the art would recognize , after perusal of this application , that embodiments of the invention may be implemented using one or more general purpose processors ( or special purpose processors adapted to the particular process steps and data structures ) operating under program control , or other special purpose circuits , and that implementation of the preferred process steps and data structures described herein using such equipment would not require undue experimentation or further invention . [ 0016 ] fig1 shows a block diagram of a preferred embodiment of a system for secure data downloading and upgrading . a client device 10 has a processor 12 connected to a permanent memory 14 and writeable memory 16 by a data bus 18 . both permanent memory 14 and writeable memory 16 are non - volatile or other persistent memory . permanent memory 14 is a non - writeable memory , such as a rom , while the writeable memory 16 is a persistent memory , such as a nvram , flash memory or disk . client device 10 also incorporates other memory , such as ram into which code is loaded for execution . stored within permanent memory 14 is boot code 22 , which controls the initialization boot process for client device 10 after a reset or power cycle . boot code 22 is further subdivided into host boot code 30 and navio boot code 32 . the host boot code 30 is a vendor provided , client - resident code that if present initializes the system after power - on or a reset and then transfers control to the navio boot code 32 . the navio boot code 32 is the code that is responsible the selection and execution of either the downloader code 24 or the application code 26 . the downloader code 24 is stored within permanent memory 14 and is a secure set of code which controls the connection of client device 10 via i / o port 40 to a communication channel 50 . the downloader code 24 is the only copy of code that can be trusted after random or intentional corruption as it resides in permanent read - only storage . i / o port 40 is connected to data bus 18 , and permits data such as updater code 70 from a remote server 60 to be downloaded under the control of downloader 24 via communication channel 50 to writeable memory 16 ( ram ). updater code 70 is code that is stored on the remote server 60 and downloaded to the client device 10 to upgrade or recover the application code 26 . the application code 26 is stored in writeable memory 16 and is the application seen by the user ( i . e . the browser along with the operating system ). in order to minimize the size of permanent memory 14 and to maximize flexibility in upgrading application code 26 for client device 10 , it is desirable for boot code 22 to be as streamlined as possible . additionally , downloader 24 can be compressed in permanent memory 14 to save memory . later it can be decompressed before being loaded into ram for execution . communication channel 50 comprises a telephone line , isdn line , cable , fiberoptic , or any other data transfer line connected to a modem or any other network interface device connected to , or contained within , i / o port 40 . such a connection links client device 10 through a network , either a private intranet or the public internet to any of a plurality of remote servers 60 . client device 10 includes a forced update feature 20 which when depressed reboots the client device 10 . this forced update feature 20 does not need to be a physical switch . it may for example , consist of a combination of switches which when toggled in a specific sequence achieve a reboot and force a download . alternatively , depressing a reset switch for a predetermined extended period of time , such as for five seconds , could activate the forced update feature 20 . [ 0023 ] fig2 shows a process flow diagram for booting client device 10 and determining whether there is a need for downloading and upgrading application code 26 . initially , boot code 22 , which is further subdivided into its host boot code and navio boot code portions , takes control of client device 10 . the host boot code 30 , a vendor provided , client - resident code initializes the system after power - on or a reset and then transfers control to the navio boot code 32 . after control has been passed to the navio boot code 32 the decision to run the application code 26 or the downloader 24 is made at step 110 . the navio boot code performs an integrity check on the application code 26 using a verification method such as an md5 digest because at this part of the process only corruption is being checked as opposed to authenticity . the writeable memory 16 stores two binary values as indicators of system integrity . the first is rundownloader which if set causes the downloader 24 to run an upgrade of the application code 26 . the second is trustdata which if not set creates the assumption that the data in writeable memory 16 is corrupt despite any appearance to the contrary . this is a validity check to determine whether the writeable memory 16 which contains information like the local isp phone number and client - id / password as well as application code 26 may be trusted . when either of these two indicators have these assigned values , a reset flag 28 is set in the writeable memory and the navio boot code 32 forces a system reboot and update at step 120 . similarly , the user may also press the forced update feature and force a system reboot and update at step 120 . in either case once the reboot and update has commenced , the downloader 24 initializes and connects to the remote server 60 and passes the information regarding client - id / password and synchronizes with the remote server 60 at which point the client device 10 may be given a new isp phone number to call . at step 130 it is determined whether the application code 26 is intact . if application code 26 is intact , control is transferred to application code 26 at 140 and client device 10 proceeds with its normal function . this is the usual result of the boot sequence . if application code 26 is corrupt , control of client device 10 is transferred to the update downloading sequence 150 . [ 0029 ] fig2 illustrates the update downloading sequence of the updater 70 from remote server 60 at step 150 . fig3 illustrates the updating and downloading sequence in greater detail . starting at step 210 where the decision has been made to load the downloader , the loading process continues at step 220 with the client device 10 establishing contact with the remote server 60 . the client device 10 transmits at step 222 identification information regarding itself , e . g ., version number , memory size , and requests transmission of an updater code package , a digitally - signed manifest which in some embodiments may contain additional program code for executing the update . when the correct updater is located at step 224 , a chunk of updater code 70 will be downloaded to the client device from the remote server 60 . next , client device 10 receives the transmitted updater code 70 and checks it at step 226 to determine whether it is valid . in the illustrated embodiment , downloader 24 compares a digital signature or digest contained within the updater package with known signature data saved in downloader code 24 . because downloader 24 is stored in permanent memory 14 , it is secure and trustworthy , and hence a match between the digital signature of the updater code package and the stored digital signature validates the identity of the updater . this allows client device 10 to update application code 26 irrespective of the relative security of either communication channel 50 or remote server 60 . next client device 10 reviews the results of the comparison at step 230 . if the signature does not match , the process returns to step 220 , preferably with connection to a different remote server 60 , and a new updater is received . if the signatures match , the update process begins with an initialization process at step 232 . the updater contains a table or manifest , which logically divides application code 26 into smaller code segments , representing portions of or complete logical segments of application code 26 . when a valid updater is received the updater is given control of client device 10 . it sets the “ in update ” flag , and loads a new update of code segment by segment . the updater checks each segment of application code 26 for corruption by determining whether the contents of a specific code segment match a hash or digest . in a preferred embodiment this is implemented by including a manifest or table of secure hashes such as sha1 in the updater 70 . each hash in the table should correspond to an image of the most recent segment of application code 26 . each hash in the manifest is accompanied by a location identifier , such as an internet url , for a corresponding replacement code segment . this allows different replacement code segments to be stored in separate locations , such as different remote servers 60 . this method allows individual components of application code 26 to be loaded separately . after the initialization process at step 232 , a segment of code from application 26 is verified at step 240 using the hash checklist . as shown at step 242 , if that block of code is valid , client device 10 checks at step 249 to see if there are remaining code segments to be verified . if there are more code segments , the process repeats starting at step 240 for the next code segment . if there aren &# 39 ; t any more code segments to check , the application is loaded at step 250 and the update is complete . if the block of code is invalid at step 242 , a substitute segment of code is downloaded from the corresponding url specified by the updater . the downloaded code segment may be encrypted to provide additional security or compressed for transmission to speed download time . after the code segment is downloaded at step 244 and any required decryption or decompression is performed , the new code segment is checked against its respective hash at step 246 and if it matches at step 247 it is then written to writeable memory 16 at step 248 to replace the corrupted segment of application code 26 . when a hash does not correspond to an image of the most recent chunk of application code 26 , the updater 70 downloads a file that is designated in the table . at step 246 a check is made to see if there are any more code segments of application code 26 to be checked . after all of the code segments of application code 26 have been checked and validated at steps 242 through 247 and any necessary replacement code segments written to memory at step 248 , the application code 26 is completely loaded and ready to execute at step 250 . in a preferred embodiment each file downloaded by the updater 70 is compressed to minimize bandwidth and speed transmission through communication channel 50 . in addition , each segment of code downloaded preferably has a default size of no greater than 64 kilobytes to correspond with the size of a sector of flash memory . additionally , the segments of code to be downloaded should be in ascending order of memory address , non - overlapping and they should not cross a flash sector boundary . once a given segment of code is downloaded and decompressed , it is authenticated and validated by using the same hash that was used to check the segment of application code 26 which it is replacing and which originally failed the hash check . if the hash matches that in the table of updater 70 then the downloaded segment of code is authentic and is allowed to overvrite its respective segment of application code 26 in writeable memory 16 . this is a means of linking security from the digital signature of the updater to the downloaded segments of code because the authentication happens in the process of downloading . thus , the digital signature is on the downloaded datastream as well as the updater image in memory . in a preferred embodiment the table of secure hashes , sha1 or other equivalents thereof , has a dual purpose . because the table of secure hashes can check the digest of segments of application code from the updater for validity without checking the whole code segment , a means of compression is achieved in addition to a validity check . for example , a twenty - byte long sha1 hash can be used to verify a large code segment like a 128 - kilobyte segment by skipping the segments that are valid . if only a few segments of code need to be replaced , the savings in transmission time and bandwidth will be substantial . this time savings is important not only to the user of client device 10 , but also to the remote server 60 when a large number of client devices 10 need to be updated , such as when a new version of a segment of application code 26 is released or when the number of client devices in service is very large . using the process described above , the present invention ensures that a client device 10 can restore a corrupted application code 26 using minimal program code stored in permanent memory 14 . because the validation process occurs each time client device 10 is initialized , the integrity of application code 26 can be maintained on an ongoing basis . further , because of the use of digitally signed hashes in the updater , secure transmission of only those segments of application code 26 which need to be updated can be achieved using unsecured remote servers 60 and communication channels 50 . although preferred embodiments are disclosed herein , many variations are possible which remain within the concept , scope , and spirit of the invention , and these variations would become clear to those skilled in the art after perusal of this application .