# Patent

## Title
Mechanism for efficient private bulk messaging

## Abstract

Secure bulk messaging mechanism in which, roughly described, a sender first encrypts a message once. The message can be decrypted with a message decryption key. These can be symmetric or asymmetric keys. For each recipient, the sender then encrypts the message decryption key with the recipient's public key. The sender then sends the encrypted message and the encrypted message decryption keys to a store-and-forward server. Subsequently, one or more recipients connect to the server and retrieve the encrypted message and the message encryption key that has been encrypted with the recipient's public key. Alternatively, the server can forward these items to each individual recipient. The recipient then decrypts the encrypted message decryption key with the recipient's private key, resulting in an un-encrypted message decryption key. The recipient then decrypts the message using the un-encrypted message decryption key.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation of and claims priority from U.S. patent application Ser. No. 09/792,949, entitled “Mechanism for Efficient Private Bulk Messaging” filed Feb. 26, 2001, now U.S. Pat. No. 6,912,285, which is incorporated herein by reference, and claims priority to U.S. provisional patent application No. 60/184,785 filed Feb. 24, 2000, which is incorporated herein by reference.


BACKGROUND
1. Field of the Invention
The invention relates to secure transmission of documents, and more particularly, to transmission of documents to a large number of recipients, securely and efficiently.
2. Description of Related Art
The Internet and corporate networks have made the transmission of documents and messages via e-mail commonplace. Bulk messaging has also become commonplace, such as for advertising and promotional purposes. For bulk messaging, typically a user on one computer composes a message and addresses it to an e-mail group. The message is transmitted to a server, which substitutes the individual addresses of all the target recipients in the group, which may number in the thousands, and transmits the message individually to each target recipient.
Unlike advertising and promotional uses, many businesses require that their communications take place securely. When messages are to be transmitted across an insecure network, such as the Internet, security is typically accomplished by encrypting the message in a manner that can be decrypted only with knowledge of a decryption key. Since only the intended recipient is expected to have the decryption key, only that recipient will be able to open the message and view its contents. Encryption may be performed using a symmetrical encryption algorithm, in which the encryption key matches the decryption key, or by an asymmetric algorithm, in which the encryption key is different from the decryption key. One popular form of asymmetric encryption is public/private key encryption, described in “Public-key Cryptography Standards,” RSA Data Security, Inc. (1991), and in Rivest U.S. Pat. No. 4,405,829, both incorporated by reference herein.
According to the public/private key crypto system, each target recipient has both a private key that only the recipient knows, and a public key that is publicly available. When a sender desires to send a message securely to one of the target recipients, the sender encrypts the message using the target recipient's public key. Only the target recipient then is able to open the message and view its contents.
Secure messaging becomes problematical when the sender desires to send the message to a large number of target recipients. If a public/private key cryptosystem is to be used, then the sender must encrypt the message N times, once using the public key of each of the N target recipients, and then send the message separately to each of the target recipients. If the document to be transmitted is large, and/or if N is in the thousands, this can be a formidable task. The encryption part of the task can be minimized if all of the target recipients share a single decryption key, because then the sender need encrypt the message only once. But the need for all recipients to have the decryption key poses risks both in the transmission and in the storage of the key. This solution also does not overcome the need for the sender to transmit the message separately, once to each of the N target recipients.
Accordingly, there is a need for a more efficient mechanism for secure bulk transmission of messages.
SUMMARY OF THE INVENTION
According to the invention, roughly described, a sender first encrypts the message once. The message can be decrypted with a message decryption key. These can be symmetric or asymmetric keys. For each recipient, the sender then encrypts the message decryption key with the recipient's public key. The sender then sends the encrypted message and the encrypted message decryption keys to a store-and-forward server. Subsequently, one or more recipients connect to the server and retrieve the encrypted message and the message encryption key that has been encrypted with the recipient's public key. Alternatively, the server can forward these items to each individual recipient. The recipient then decrypts the encrypted message decryption key with the recipient's private key, resulting in an un-encrypted message decryption key. The recipient then decrypts the message using the un-encrypted message decryption key.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described with respect to particular embodiments thereof, and reference will be made to the drawings, in which:
FIG. 1 is a block diagram of a system incorporating the invention.
FIG. 2 is a flowchart of basic steps undertaken by a sender in transmitting a secure bulk message using the arrangement of FIG. 1.
FIG. 3 is a flowchart illustrating the process undertaken by a recipient to retrieve and open the message.
FIG. 4 illustrates a format by which an encrypted message and the encrypted decryption keys are stored on the server of FIG. 1.



DETAILED DESCRIPTION
FIG. 1 is a block diagram of a system incorporating the invention. It comprises a sender 110, which sends the encrypted message and encrypted message decryption keys to a server 112, which can then be accessed by each of N target recipients 114-1, 114-2, 114-3, . . . 114-N (collectively, target recipients 114). One or more of the transmission paths from the sender 110 to the server 112 or from the server 112 to the recipients 114 are potentially insecure. As used herein, the term “message” is intended to be read broadly to include all kinds of information that might be transmitted, such as e-mail messages, documents, financial transactions, and so on. Also as used herein, the server 112 need not be limited to a single computer. It can include multiple computers which need not even be located physically together.
FIG. 2 is a flowchart of the basic steps undertaken by the sender in transmitting a secure bulk message using the arrangement of FIG. 1. In step 210, the sender first creates the message to be sent. In step 212, the sender encrypts the message. As mentioned, encryption at this stage can be either by a symmetric or an asymmetric encryption algorithm. Although there are many examples of acceptable encryption algorithms, one common symmetric algorithm is that described in National Institutes of Standards and Technology, “Data Encryption Standard”, FIPS Publication No. 46-1 (January 1988) (hereinafter “DES”), incorporated by reference herein. The encryption process in step 212 can be reversed using a message decryption key known by the sender.
In step 214, the sender encrypts the message decryption key N times—once using the public key of each of the N target recipients. This yields N encrypted message decryption keys. In step 216, the sender sends the encrypted message, the addresses of the target recipients, and the list of encrypted message decryption keys to the server 112. It will be appreciated that one of the target recipients could be a third-party monitor, such as a government agency that is permitted to view the message if required by law.
Optionally, the sender can also send to the server 112 (or the server itself generate) a digital signature protecting all of the encrypted decryption keys associated with a particular encrypted message. The list of encrypted decryption keys thereafter cannot be tampered with without being detectable by reference to the digital signature. A digital signature is created by digesting the list, or significant portions of the list, using a well-known digesting algorithm, and then encrypting the digest with the sender's (or server's) private key of a public/private pair. In order to check for tampering, an auditor repeats the digesting of the list of encrypted decryption keys, to form a new digest, and then decrypts the digital signature using the sender's (or the server's) public key, to recover the original digest, and then compares the two for equality. A satisfactory digesting algorithm is that describe in R. Rivest, “MD5 Message-Digest Algorithm”, Internet Engineering Task Force RFC No. 1321 (April 1992), incorporated by reference herein.
On the server 112, the encrypted message and the encrypted decryption keys are stored as illustrated in FIG. 4. The encrypted message is stored at 410. In conjunction with the encrypted message 410, the server stores each of the encrypted decryption keys 412-1, 412-2, . . . , 412-N. One of the encrypted decryption keys can, as mentioned above, optionally be a monitor's decryption key 414. Optionally also stored in conjunction with the encrypted message 410, is a digital signature 416 protecting the list of encrypted decryption keys. The elements illustrated in FIG. 4 may be stored all in one contiguous region of computer-readable memory, or across discontiguous regions, or across discontiguous regions of multiple computer-readable media.
In one embodiment, the server maintains a document management system which not only stores multiple encrypted messages and their associated encrypted decryption keys, but also provides logical and structured restricted access to the various items by individual senders and individual recipients. For example, one such document management system allows senders to change the message stored on the server 112, while not allowing other senders to do so and while not allowing any recipient to do so. Another such document management system allows senders to add, delete or change entries in the list of encrypted decryption keys for messages that were transmitted by the sender, while not allowing such modifications by other senders or by any recipient. Yet another such document management system, when accessed by a particular recipient, shows the recipient only those messages on which the particular recipient is identified as a target recipient, hiding any messages for which there is no encrypted decryption key for the particular recipient.
FIG. 3 is a flowchart illustrating the process undertaken by a recipient to retrieve and open the message. In step 310, the recipient accesses the server 112, and in step 312, the recipient downloads the encrypted message and at least the particular recipient's encrypted message decryption key 412. Alternatively, the server 112 can forward these items to the recipient without awaiting action from the recipient. In step 314, the particular recipient decrypts the recipient's encrypted message decryption key, yielding an unencrypted message decryption key. In step 316, the recipient decrypts and views the encrypted message using the now-unencrypted message decryption key.
It will be appreciated that the above-described mechanism is capable of many variations. As one example, in step 216, the sending of the encrypted message and list of encrypted message decryption keys need not take place in a single transmission. Some of all of the encrypted message decryption keys can be sent earlier or later than the encrypted message.
As another example, encrypted decryption keys could be bundled into the message and the single message with the encrypted decryption keys could be broadcast to all recipients without compromising the security of the mechanism.
As yet another example, public and private keys for encrypting the decryption keys could be replaced with symmetric private keys without affecting the security or efficiency of the mechanism.
As still another example, server 112 could be eliminated and the message with the encrypted decryption keys could be broadcast to all recipients and any other listeners, and only the target recipients will be able to decrypt the message and the security of the mechanism is not compromised.
As yet another example, for one or more of the target recipients, the sender can multiply encrypt the recipient's message decryption key, thereby requiring multiple entities to be involved in the decryption of the message decryption key. For example, the sender may first encrypt the message decryption key with the target recipient's public key, yielding a “partially-encrypted” message decryption key. The sender may then re-encrypt the partially-encrypted message decryption key, using the public key of an authorizer, thus yielding the final encrypted message decryption key. Upon receipt of the message, the recipient first has the encrypted decryption key decrypted by the authorizer, using the authorizer's private key. This recovers the partially-encrypted message decryption key. The recipient then decrypts the partially-encrypted message decryption key, using the recipient's private key, thus yielding the un-encrypted message decryption key. Alternatively, the order of encryption for the multiple parties can be reversed, as long as the decryption sequence takes place in the same order as the encryption sequence.
The foregoing description of preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. In particular, and without limitation, any and all variations described, suggested or incorporated by reference in the Background section of this patent application are specifically incorporated by reference into the description herein of embodiments of the invention. The embodiments described herein were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.

## Claims

1. A method comprising the steps of:
encrypting a message to develop an encrypted message, said encrypted message being decryptable using a first decryption key;
encrypting said first decryption key with encryption keys of a plurality of target recipients, to develop a plurality of recipient associated encrypted decryption keys; and
transmitting said encrypted message;
sending said recipient associated encrypted decryption keys to a server for inclusion in a list of authorized encrypted decryption keys; and
thereafter changing at least one but less than all of said recipient associated encrypted decryption keys on said list, but not said encrypted message itself which remains unchanged, said changing thereby impeding access to the message by at least one target recipient for which the at least one recipient associated encrypted decryption key was changed, but preserving access, by way of unchanged ones of the recipient associated encrypted decryption keys, to said unchanged encrypted message.

2. A method according to claim 1, wherein said step of transmitting comprises the steps of:
sending said encrypted message to said server; and
said server forwarding said encrypted message to each of said target recipients as the unchanged encrypted message.

3. A method according to claim 2, wherein said step of transmitting further comprises the step of sending said encrypted decryption keys to one of said target recipients bypassing said server.

4. A method according to claim 1, wherein said step of transmitting comprises sending said encrypted message to said server.

5. A method according to claim 1, wherein said step of changing one of said recipient associated encrypted decryption keys comprises deleting said one of said recipient associated encrypted decryption keys on said server after said step of transmitting said encrypted message.

6. A method according to claim 1, wherein one of said target recipients is a monitor.

7. A method according to claim 1, further comprising using a digital signature to protect at least a portion of said recipient associated encrypted decryption keys.

8. A method comprising the steps of:
receiving an encrypted message, said encrypted message being decryptable using a first decryption key;
retrieving, by a target recipient, from a server at least a particular one of a plurality of recipient associated encrypted decryption keys included in a list thereof stored on the server, said retrieved from list having been changed by a message sender after the receiving step to delete or change at least one other one of the recipient associated encrypted decryption keys;
decrypting the retrieved particular one of said plurality of recipient associated encrypted decryption keys to recover said first decryption key; and
decrypting said received encrypted message using said first decryption key, wherein said received encrypted message remains unchanged even after said deletion or change of at least one other one of the recipient associated encrypted decryption keys.

9. A method according to claim 8, wherein said step of decrypting a particular encrypted decryption key comprises the steps of:
decrypting said particular recipient associated encrypted decryption key with a key of a first party, to develop a partially decrypted decryption key; and
decrypting said partially decrypted decryption key with a key of a second party.

10. A method according to claim 8, wherein said step of receiving said encrypted message comprises the step of receiving said encrypted message from a server.

11. A method according to claim 10, wherein said step of receiving a plurality of recipient associated encrypted decryption keys comprises the step of a user accessing said server, said plurality of recipient associated encrypted decryption keys being stored on said server in conjunction with said encrypted message, said server permitting access to only those messages stored thereon for which said user is a target recipient based on inclusion of said particular one of said plurality of recipient associated encrypted decryption keys in said list of encrypted decryption keys.

12. A method according to claim 8, further comprising the step of downloading said encrypted message from said server prior to said step of decrypting said encrypted message using said first decryption key.