Patent Publication Number: US-7596696-B1

Title: Efficiently managing keys to make data permanently unreadable

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to techniques for encrypting and decrypting data. More specifically, the present invention relates to a method and an apparatus for encrypting data and managing keys to facilitate making data permanently unreadable. 
     2. Related Art 
     To ensure data is not lost prematurely, it is common to create multiple backup copies of the data. However, it is also desirable in some cases to ensure that a file, once deleted, is not recoverable. This can be a rather complicated task if backup copies of the data are created and stored at different locations. 
     This problem can be solved by only storing the data in encrypted form. In this way, destroying the data is a somewhat easier problem because only the key must be deleted. However, long-term user keys can, over time, be obtained by an adversary through compromise or coercion. To remedy this problem, it is possible for keys to be kept in tamper-resistant smart cards, in which case it is not feasible to covertly discover the keys. To delete the data, the user need only destroy the smart card. However, it is expensive to require every user to have a smart card and every computer to have a smart card reader. 
     A more-sophisticated technique for managing secret keys (developed by a company called “Disappearing Inc.”) uses a special server called an “ephemerizer” whose job it is to create and destroy keys. A nice property of this technique is that the ephemerizer can be built so it does not see any data. However, the ephemerizer must create and store a key for every ephemerally-created message. This can involve storing a large amount of data if the system is used to encrypt many messages. (Also see a related system disclosed in U.S. Pat. No. 6,363,480, entitled “Ephemeral Decryptability,” by inventor Radia J. Perlman.) 
     This storage-space problem can be alleviated by modifying the ephemerizer so that it only maintains one key per expiration time, and having that key used across many users and many files. (See U.S. patent application Ser. No. 10/959,928, filed on 5 Oct. 2004, entitled “Method and Apparatus for Using Secret Keys to Make Data Permanently Unreadable,” by inventor Radia J. Perlman. This application is hereby incorporated by reference to disclose how an ephemerizer is designed and operates.) 
     However, this approach does not provide a scalable solution when a file is deleted on demand, since if a key is used for multiple files, that key cannot be deleted without making all other files encrypted with that key unreadable. 
     Hence, what is needed is a method and an apparatus that facilitates making data disappear without the above-described problems. 
     SUMMARY 
     One embodiment of the present invention provides a system that facilitates making the files permanently unreadable. During operation, the system encrypts a file with a key K at a file manager and then stores the encrypted file in non-volatile storage. Next, the system stores the key K in a key database located in volatile storage at the file manager. The system then encrypts the key database, and stores the encrypted key database in non-volatile storage. Additionally, a key that can be used to decrypt the encrypted key database is maintained by a key manager, and is not maintained in non-volatile form by the file manager. In this way, if the file manager crashes, losing the contents of its volatile storage, the file manager must interact with the key manager to decrypt the encrypted key database. 
     In a variation on this embodiment, encrypting the key database involves encrypting the key database with a key X and then encrypting the key X with a key P N  to form an encrypted key. This encrypted key and the encrypted key database are stored in non-volatile storage. 
     In a further variation, P N  is a public key, which is associated with a corresponding private key maintained by the key manager. 
     In a further variation, P N  is a secret key, which is maintained by the key manager. 
     In a variation on this embodiment, the method further comprises making a file permanently unreadable. This involves removing the key K from the key database at the file manager to produce a modified key database. Next, the system encrypts the modified key database with a successive key P N+i  to form an encrypted modified key database. This successive key P N+i  is maintained by the key manager (which involves maintaining a corresponding private key if P N+i  is a public key). The system then stores the encrypted modified key database in non-volatile storage, and eventually causes the key manager to delete the key P N , which involves deleting a corresponding private key if P N  is a public key. 
     In a variation on this embodiment, upon recovering from a crash, the system retrieves the encrypted key database and the encrypted key from non-volatile storage. Next, the system communicates with the key manager to decrypt the encrypted key, thereby restoring the key X. The system then uses the key X to decrypt the encrypted key database at the file manager, thereby enabling the file manager to decrypt encrypted files. 
     In a further variation, communicating with the key manager to decrypt the encrypted key can involve: communicating with the key manager through a secure exchange; or using a blind decryption scheme to decrypt the encrypted key. 
     In a variation on this embodiment, the key that can be used to decrypt the encrypted key database is maintained by multiple key managers. 
     In a variation on this embodiment, the key database is encrypted using a k-out-of-n scheme with n key managers, such that as long as k key managers are available, the encrypted key database can be decrypted. 
     In a variation on this embodiment, storing the encrypted key database in non-volatile storage involves: storing a copy of the encrypted key database in local non-volatile storage associated with the file manager; and storing additional copies of the encrypted key database in remote non-volatile storage. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates both a file manager and associated key managers in accordance with an embodiment of the present invention. 
         FIG. 2  presents a flow chart illustrating how a file and the key database are encrypted and stored in accordance with an embodiment of the present invention. 
         FIG. 3  presents a flow chart illustrating the process of making a file permanently unreadable in accordance with an embodiment of the present invention. 
         FIG. 4  presents a flow chart illustrating the process of restoring a key database upon restarting a file manager that has crashed, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     The data structures and code described in this detailed description are typically stored on a computer-readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. This includes, but is not limited to, magnetic and optical storage devices, such as disk drives, magnetic tape, CDs (compact discs) and DVDs (digital versatile discs or digital video discs), and computer instruction signals embodied in a transmission medium (with or without a carrier wave upon which the signals are modulated). For example, the transmission medium may include a communications network, such as a LAN, a WAN, or the Internet. 
     File Manager and Key Manager 
       FIG. 1  illustrates both a file manager  102  and a number of key managers  104 - 105  in accordance with an embodiment of the present invention. File manager  102  can generally include any type of computer system or device that can manage files. Note that all accesses to a set of files are performed though file manager  102 , which is trusted to enforce access, and which can access everything except deleted data. 
     File manager  102  is coupled to both volatile storage  108  and non-volatile storage  112 . Volatile storage  108  can include any type of semiconductor-based random access memory, such as SRAM or DRAM memory. Non-volatile storage  112  can include any type of non-volatile memory that can hold data when powered down. This includes, but is not limited to, magnetic storage, flash memory, ROM, EPROM, EEPROM, and battery-backed-up RAM. As is illustrated by dashed lines in  FIG. 1 , backups of items in non-volatile storage  112  are periodically propagated to remote non-volatile storage  120 , or alternatively, to a local backup device, such as a tape drive. 
     File manager  102  communicates with a number of key managers  104 - 106 . These key managers  104 - 106  are similar in concept to the “ephemerizer” described in U.S. patent application Ser. No. 10/959,928, filed on 5 Oct. 2004, entitled “Method and Apparatus for Using Secret Keys to Make Data Permanently Unreadable,” by inventor Radia J. Perlman, which is incorporated by reference in this application. Also see a related system disclosed in U.S. Pat. No. 6,363,480, entitled “Ephemeral Decryptability,” by inventor Radia J. Perlman. 
     However, unlike the ephemerizer, which maintains one key per expiration time, the key managers  104 - 106  hold at least two keys per client (file manager). As is illustrated in  FIG. 1 , the two keys are P N  (for time N) and P N+1  (for time N+1). Only after file manager  102  is assured that its database of keys has been safely backed up with key P N+1 , can key managers  104 - 106  be notified that they should delete key P N . Note keys P N  and P N+1  can be replicated across multiple key managers  104 - 106  for fault-tolerance purposes. Also note that although the system illustrated in  FIG. 1  maintains two keys per file manager, in general the system can maintain a larger number of keys for each file manager. 
     Note that although the following discussion is written, for simplicity, as if there is a single key manager, in general multiple key managers could be used, each with different sets of public keys. For example, in a one-out-of-n scheme, a key X would be separately stored, encrypted with a different public key for each key manager (e.g., P N , R N  and Q N ). Or, X could be broken up into n shares for recovery in a k-out-of-n scheme, and each share stored encrypted with one of the key managers&#39; public keys. 
     As is illustrated in  FIG. 1 , file manager  102  maintains copies of files  116  and  118  in encrypted form in non-volatile storage  112 . File manager  102  also maintains a key database  110  in volatile storage  108 , which contains copies of keys for all active (or recoverable) files. File manager  102  additionally maintains a backup copy of key database  110 , which has been encrypted with key P N , in non-volatile storage  112  (see encrypted key database  114  in  FIG. 1 ). 
     If file manager  102  crashes, it interacts with key managers  104 - 106  to decrypt encrypted key database  114 , and to store the decrypted result in volatile storage  108 . At all times, the encrypted copy  114  of the key database in local non-volatile storage and a most-recent backup of the key database in remote non-volatile storage  120  are recoverable, either with key P N  or key P N+1 . 
     Note that interactions with key managers  104 - 106  can be accomplished with blind decryption (as explained in the Sun Microsystems Laboratory technical report no. TR-2005-140, entitled, “The Ephemerizer: Making Data Disappear,” February 2005), or it can be accomplished with any sort of secure exchange with key managers  104 - 106 , such as using the SSL (Secure Socket Layer) protocol or using the IPsec (IP security) protocol, wherein the protocol uses either public keys or shared secret keys. Also, the key database can be encrypted in a k-out-of-n scheme with n key managers, such that as long as k key managers are available (and haven&#39;t prematurely lost the key), the database is decryptable, and as long as n−k+1 of the key managers do indeed delete the key when demanded, the deleted data is unreadable. Note that blind decryption places less trust in the key manager, since the key manager will not be able to retain any values (other than old keys) with which to decrypt old backups. 
     An example key database would be where all the keys of active or recoverable files (files that have been deleted but not “securely deleted”) are kept encrypted with some randomly chosen key K. K is also stored, encrypted with key P N , which is maintained by the key managers. Hence, to decrypt the key database, K must be recovered by interacting with k of the key managers. 
     Note that key P N  can be a public key or a secret key. If it is blindable (through for instance, one of the three functions in the above-referenced Sun Microsystems Laboratory Technical Report), then interaction with the key manager can be accomplished through blind decryption, which will be more secure. 
     If the environment is such that there is no possibility for eavesdroppers, then there can be a simple interaction where the encrypted K is sent to the key manager, which returns K, or where the key manager&#39;s key P N  is sent to the file manager  102 , which retains that key, but only in volatile storage  108 . The most secure way of doing it, though, is to use public keys for the key manager, and blind decryption. 
     Note that the communications between the file manager and the key manager can be accomplished through some secure interaction, such as doing mutual authentication based on shared secrets or public keys, and establishing a security association through a protocol such as IPsec or SSL, and then having the key manager send the secret key, or encrypt a secret key of the file manager&#39;s choice (to be used for encrypting the key database). 
     Alternatively, the key manager can send the file manager a public key. This public key can also be learned by the file manager securely through other means, such as a certificate, signed by someone that file manager trusts, being posted in a place accessible to the file manager. 
     Encrypting a File 
       FIG. 2  presents a flow chart illustrating how a file and the key database are encrypted and stored in accordance with an embodiment of the present invention. First, the system encrypts the file with a randomly generated key K (step  202 ), and stores the encrypted file in nonvolatile storage (step  204 ). The key K is also stored in a key database in volatile storage (step  206 ). Next, the system encrypts the key database with a randomly generated key X (step  208 ). 
     The system also encrypts the key X with a key P N , which is maintained by the key manager (step  210 ). If the encryption process is performed with a public key P N , then it can be done locally at the file manager and the key manager maintains a corresponding private key. On the other hand, if P N  is a secret key, then key manager maintains a copy of the secret key P N  and the file manager has to interact with the key manager to obtain P N  securely, or alternatively to get a locally-generated secret X, encrypted by the key manager, with X stored in volatile storage, and X encrypted with P N  stored in nonvolatile storage. 
     Finally, the system stores the encrypted key and the encrypted key database in the non-volatile storage (step  212 ). 
     Making a File Permanently Unreadable 
       FIG. 3  presents a flow chart illustrating the process of making a file permanently unreadable in accordance with an embodiment of the present invention. First, the file manager removes the key K associated with the encrypted file from the key database to produce a modified key database (step  302 ). Next, the file manager encrypts the modified key database with a successive key P N+i  (step  304 ). The file manger then stores the encrypted modified key database in non-volatile storage (step  306 ). After the encrypted modified key database has been committed to non-volatile storage (or additionally committed to backup storage, or remote non-volatile storage), the file manager eventually instructs the key manager to delete the previous key P N  (step  308 ). This makes the file permanently unreadable if at the last time the key database contained the key K and was encrypted, it was encrypted with the key P N . 
     Note that in general, the key database at the file manager is updated as files get created and deleted. The key database is also periodically written out to nonvolatile storage, at the file manager&#39;s discretion (perhaps every time unit, or perhaps every time the key database is modified). Moreover, there are various stages of nonvolatile storage (like off-site, replicated, versus solely local). At some time the file manager can start using the next version of the key, and at some time the key manager can delete the oldest key, perhaps when informed to do so by the file manager, or perhaps periodically. 
     Restoring the Key Database 
       FIG. 4  presents a flow chart illustrating the process of restoring a key database upon restarting a file manager that has crashed, in accordance with an embodiment of the present invention. During a restart from a crash (step  402 ), the file manager system retrieves the encrypted key database and the associated encrypted key from non-volatile storage (step  404 ). 
     Next, the file manager then causes the encrypted key to be decrypted. If the encrypted key is encrypted with a shared secret, the file manager obtains the shared secret from the key manager through a secure exchange. The file manager then uses the shared secret to decrypt the encrypted key to restore key X. 
     On the other hand, if the encrypted key is encrypted with a public key, the file manager sends the encrypted key using a secure exchange to the key manager (step  406 ). The key manager then uses its corresponding private key to decrypt the encrypted key, thereby restoring the key X. The file manager then receives the decrypted key X from the key manager through a secure exchange (step  408 ). Finally, the file manager uses the key X to decrypt the encrypted database (step  410 ). The file manager is subsequently able to use the key in the key database to decrypt encrypted files. 
     The foregoing descriptions of embodiments of the present invention have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.