Electrical computers and digital processing systems: support – Multiple computer communication using cryptography – Particular communication authentication technique
Reexamination Certificate
1998-09-29
2001-03-27
Barron, Jr., Gilberto (Department: 2767)
Electrical computers and digital processing systems: support
Multiple computer communication using cryptography
Particular communication authentication technique
C713S156000, C713S158000, C713S178000, C705S076000
Reexamination Certificate
active
06209091
ABSTRACT:
BACKGROUND
Public key certificates are electronic documents signed by a trusted issuer and used to attest to the binding of a user's name to a public key and other related data. Certificates provide assurance to the public that the public key identified in the certificate is owned by the user whose name is in the certificate. Major standards which describe public key certificate systems include ITU-T X.509 The Directory—Authentication Framework, and American Bankers Association ANSI X9.30-Part 3: Certificate Management for DSA (draft). Many implementations impose a hierarchical structure in which each trusted issuer, referred to as a Certification Authority (CA) certifies keys for entities that are subordinate to it. The CA affixes a digital signature to the electronic document in a way that is verifiable (one can prove that the CA signed the document) and cannot be forged (one can be assured to a high level of confidence that no one other than the CA signed the document). For example, at the top of the CA hierarchy there may be relatively few “root” CAs, perhaps one per country which certify subordinate CAs. Below the root CAs in the hierarchy, high level CAs (perhaps banks) certify lower level CAs beneath them (e.g., companies), which in turn sign individual user certificates.
A CA's signature becomes more valuable as it creates a large hierarchy of users beneath it and uses its signature key to sign the certificates of both high-value users and subordinate CAs. The CA's signature key then also becomes a more likely target for terrorists, criminals bent on economic gain, and foreign military and espionage services bent on economic spying or de-stabilizing the economy via information warfare. All these issues also apply with equal force to keys used to sign electronic representations of money.
Thus far, the need for security of a CA's private signature key has been addressed by providing a “certificate signing unit” (CSU), which is a tamper-proof secure module satisfying standards set forth in Federal Information Processing Standard (FIPS) PUB 140-1, level 3 or 4 as issued by the U.S. Dept. of Commerce, National Institute of Standards and Technology (NIST). Such a CSU generates its public/private signature key pair internally, “confines” the private signature key securely and permanently inside an area of the device that cannot be read externally, and outputs only the corresponding public key, which will be used to verify its signatures. One CSU available from Bolt, Baranek, and Newman of Boston, Mass. (BBN) is configured to allow a back-up version of its private signature key to be created using a “K-of-N threshold” scheme, in which the private key is split into N shares and placed on small plastic data-keys, each of which contains a memory chip. The data-keys are a patented product of Datakey, Inc. of Burnsville, Minn. Then, should the CSU device be destroyed, a quorum of at least K data-keys can reconstruct the private key.
At least one major security standards body, the American Bankers Association ANSI X9.F1 committee on cryptographic security in wholesale banking applications has recommended that CSU's should be designed to forbid any export of the private key from the device in any form in order to prevent any possible unauthorized theft and use of the key. This approach would require an elaborate procedure for disaster recovery, involving the use of several key pairs simultaneously. Because a single key would exist only in a single CSU at a single site, the loss of a CSU or of a site would force the CA to use another key pair in order to continue business. This would require the CA to publicize and/or securely distribute several (at least two or three) public keys, each identified by a distinct code number (e.g., BT01, BT02, BT03), so that users could continue to verify the signatures that the CA would issue after one CSU (possibly containing the private key for BT01) had been destroyed. See X9.30-Part 3 concerning procedures for disaster recovery.
SUMMARY
An object of the present invention is to provide a digital signing system (“signing system”) for certificates and other high value documents (including contracts, electronic representations of currency, negotiable documents, etc.) with improved security and flexibility.
A further object of the present invention is to provide a signing system in which a digital signature veriiably relates to a signature key, and in which no single signing device needs to contain the signature key during the document signing operation.
A further object of the present invention is to provide a signing system which permits loss or compromise of one or more signing devices while maintaining available, un-compromised signing services.
A further object of the present invention is to provide a signing system in which multiple signing devices each create, modify, or combine one or more partial signatures, and the result of operations by multiple signing devices produces a single digital signature.
A further object of the present invention is to provide a signing system in which multiple authorizing agents directly or indirectly authorize each individual signing device to affix or modify a partial signature.
A further object of the present invention is to provide a robust and easy-to-use mechanism in which authorizing agents can temporarily delegate their authorizing capability.
The multi-step signing system described here uses a public key cryptosystem approach to sign an electronic document such that a recipient of the document can verify the signature using a public verification key of the signer. The private signature key which corresponds to the public verification key is not permitted to exist in whole, available form in one place at any time during normal signing operations. Instead, a private signature key consists of “operational shares” which can be used to affix or modify a partial signature, and sequential operation of multiple shares produces a signature that can be verified using the public verification key. The full signature is not completed until all, or some quorum, of the signing devices have signed. Each signing device in turn requires authorization from all, or some quorum, of its associated authorizing agents before participating in the signature process.
If, during the initial generation of operational shares, a whole signature key is generated, the whole signature key is destroyed after shares are distributed. Because the risk of loss from the theft or compromise of any one device is now greatly reduced, the information content of each signing device can be now duplicated (e.g., for remote backup or for a plug-in replacement or “hot” standby) so that if any device fails, it can be replaced (or reconstituted) and service can resume quickly. The consequence of subversion of any individual signing device is lowered, because the signing operation cannot be completed with a single device.
A multi-layered authorization management system is established, such that each signing device has registered within it a number of individuals (or external smart cards used by designated individuals), and the signing device participates in the signing operation only upon authorization from a quorum of registered individuals. A quorum of these individuals (called authorizing agents) are also required to authorize changes to the system, such as registering additional authorizing agents, deleting authorizing agents, altering the quorum requirements for any of the various actions that the signing devices can perform, or generating and distributing additional or substitute key sets.
In this way, a signature can be applied that can be verified using a public verification key, but no private signature key exists at a single location where it may be subject to compromise or catastrophe. Multiple sites must fail or be compromised before interrupting signing services or before an adversary acquires sufficient information to forge signatures. Individual signing devices need not be as be as highly secure for a CSU using a sin
Freund Peter C.
Huang Stuart T. F.
Sudia Frank W.
Barron Jr. Gilberto
CertCo Inc.
Steptoe & Johnson LLP
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