Cryptography – Key management – Key distribution
Reexamination Certificate
1998-02-10
2001-05-29
Swann, Tod (Department: 2132)
Cryptography
Key management
Key distribution
C380S285000, C380S030000, C705S071000
Reexamination Certificate
active
06240187
ABSTRACT:
COPYRIGHT NOTICE
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the xerographic reproduction by anyone of the patent document or the patent disclosure in exactly the form it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyrights whatsoever.
1. Field of the Invention
The present invention relates to the field of secure transaction processing, more specifically to the field of public key encryption of transaction data.
2. Background Art
A cryptographic system is a system for sending a message from a sender to a receiver over a medium so that the message is “secure”, that is, so that only the intended receiver can recover the message. A cryptographic system converts a message, referred to as “plaintext” into an encrypted format, known as “ciphertext.” The encryption is accomplished by manipulating or transforming the message using a “cipher key” or keys. The receiver “decrypts” the message, that is, converts it from ciphertext to plaintext, by reversing the manipulation or transformation process using the cipher key or keys. So long as only the sender and receiver have knowledge of the cipher key, such an encrypted transmission is secure.
A “classical” cryptosystem is a cryptosystem in which the enciphering information can be used to determine the deciphering information. To provide security, a classical cryptosystem requires that the enciphering key be kept secret and provided to users of the system over secure channels. Secure channels, such as secret couriers, secure telephone transmission lines, or the like, are often impractical and expensive.
A system that eliminates the difficulties of exchanging a secure enciphering key is known as “public key encryption.” U.S. Pat. No. 4,405,829 and Diffie and Hellman, “New Directions in Cryptography,” IEEE Trans. Inform. Theory, vol. IT-22, pp. 644654, November 1976, teach public key encryption. With public key encryption, two keys are used, a private key and a public key. The keys are symmetrical, i.e., either key can be the public key or the private key—the labels “public” and “private” simply identify which key is made available to the public, and which key is kept private by the “owner” of the key pair. Public key encryption is applied to a “message”. A message is text, graphics, data, or other digitized information, and public key encryption is used to either encrypt the message making it unreadable by anyone unless they have the private key or to create a readable message with a digital signature. A digital signature is created for a specific message using the private key. Only a person with knowledge of the private key is able to create a valid digital signature for a given message, so this prevents others from generating or altering messages and creating forged signatures.
To keep a message to the key owner private, the sender of the message will obtain the recipient's public key and use that key to encrypt the message. Before encryption, the message is said to be a “plain text” message (although the message might not be text at all) and following encryption, the message is said to be a “cipher text” message. The cipher text message can only be converted back to the original plain text message by a decryptor knowing the recipient's private key (the other key in the recipient's key pair). Of course, with enough computing power and a poorly chosen encryption scheme or key pair, a decryptor might be able to extract the plain text message without knowing the key. It is assumed here that a robust encryption scheme is selected such that the private key is indeed required.
A message is digitally “signed” by the key owner by applying a key and the message to a digital authenticator, which outputs a digital signature to be attached to the message. The recipient of the message can then apply the message, the digital signature and the key used to generate the signature to an authenticator which will indicate whether or not the digital signature was generated from that exact message and the key. With public key signatures, the private key is used to generate the digital signature and the public key is used to verify the signature.
In a transaction processing system, such as with the use of smart cards or terminals, a transaction is formed into a message and encrypted using the secret key of the operator of the transaction processing system. The term “smart card” refers to a card such as a bank card which contains data storage and computing ability, as opposed to a more conventional card, which contains only data storage, typically in the form of data stored on a magnetic stripe. A terminal might be an automatic teller machine (ATM), a terminal in a bank, a home personal computer, or other means for a user to send and receive data.
U.S. Pat. No. 4,972,472 issued to Brown et al. shows a method and apparatus for changing a master key in a cryptographic system. That system provides storage locations for three keys: a pending key, an active key and a retired key, When a key is to be replaced, the new key is stored in pending key location When a key update command is given, the existing active key is shifted to the retired key location and the pending key is shifted into the active key location. The retired key is used for applications which have not yet been made aware of the key change. Over time, applications are made aware of the change and shift over from using the retired key to using the active key.
One disadvantage of the Brown et al. system is that a replacement key could be sent by someone with unauthorized access to the channel used to transmit the keys. Thus, the key replacement apparatus is only useful where the channel in which the replacement keys are sent out is secure.
As should be apparent, anyone knowing the key owner's secret key can pose as the key owner, read the key owner's messages and create or alter messages sent in the name of the key owner. In an insecure system, unauthorized persons have the ability to view the traffic between the key server and the key users, whether or not such eavesdroppers know the secret keys being used. Once a secret key is compromised, it can no longer serve its purposes of making messages private.
One problem with a distributed system of smart cards or terminals is that they are widely distributed and when a secret key is compromised, it is impractical for all the holders of the smart cards or users of terminals to return to the central key authority to exchange keys or otherwise establish a clear channel to transmit the replacement key.
Another problem is the rapid and continual increase in computing power available. The impending obsolescence of DES (Data Encryption Standard—a secret key algorithm) is in part due to the subsequent developments in computing. At one time, a noted cryptologist calculated that a message encoded with DES could be decrypted without knowing the secret key in a month using $20 million in computer hardware. Recently, a group of noted cryptographers estimated that a $10 million investment in hardware would recover a DES key in 6 minutes (see “Minimal Key Lengths for Symmetric Ciphers to Provide Adequate Commercial Security” Blaze et al., A Report by an Ad Hoc Group of Cryptographers and Computer Scientists, January 1996, published at http://www.bsa.org/bsa/cryptologist.html). Thus, what is needed is a capability to increase security of keys as large amounts of raw computing power becomes more accessible to potential attackers.
SUMMARY OF THE INVENTION
Improved key management is provided by virtue of the present invention. The present invention provides an active public key and a “masked” replacement public key to nodes of a network. Herein “masked” or “the mask of” refers to any manner of securing the replacement key so that it is computationally difficult to determine the replacement key from its masked version. In one embodiment of the invention the masking of the replacement key is accom
Beyer Weaver & Thomas LLP
Kabakoff Steve
Swann Tod
Visa International
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