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Thursday, 9 January 2020

SPKI/SDSI证书

SPKI, SDSI and the other certificate mechanisms from 1996 onward started out to address apparent overcomplication in the X.509 world. However, by starting with a blank sheet of paper, they ended up addressing a more basic problem than complexity. Earlier certificate mechanisms suffered from the Walton's Mountain Assumption - that if you know a name for someone you know their identity (all their defining characteristics) - and that if you know someone's identity, then you know whether they're authorized to do or have something they request. This assumption is true in a world of small towns where, as the old joke goes, ``you don't have to use your turn signals because everyone knows where you're going''. That world no longer exists, so earlier certificate mechanisms based on it (X.509 in particular) fail miserably. Specifically, the Walton's Mountain Assumption is that those characteristics of a small town apply to the entire world.
This assumption is replaced by one of local knowledge: that knowledge required for security decisions and identity establishment remains local but the world extends beyond any one locality. In SDSI, an identifier is valid only locally to the person who creates it but the underlying raw public key is valid globally. In SPKI, an authorization grant is made only locally. If you need to grant authorization to someone beyond your locality, then you may (must) delegate that grant through a chain of local relationships. The same applies to PolicyMaker, KeyNote and XrML V2 (when properly used).
Along the way, we have learned that what is important in certificate (and related) security systems is not the computer-readable data structures and protocols alone. Rather, these certificates, licenses, grants, ACL entries, ..., are a cyberspace reflection of relationships in the physical world - and the security of these systems rests most heavily on the security of the process by which the physical world relationships are bound to their cyberspace reflections. That security far outweighs the more trivial security of private key protection, key length, choice of algorithm, etc., that people have obsessed about for decades. In pursuing this line of reasoning, we now have the Ceremony work but this work is in its infancy. There is much more to come.
[24 January 2004]

There are five certificate syntax forms referenced from this page:  SPKI/SDSI, X.509, PGP, X9.59 (AADS), PolicyMaker and KeyNote.  This page defines SPKI/SDSI and gives some links to the others but doesn't claim to provide a full set.
Check out John Pritchard's SPKI resource page.
There are frequent questions about the status of SPKI in IETF and in general. I wrote to the cryptography list about this in this message and, as I often do, in a reply to that message.

Table of contents

SPKI/SDSI Documentation

The SPKI/SDSI certificate format is the product of the SPKI Working Group of the IETF.  The IETF SPKI documentation is in four parts:
  1. RFC2692: Requirements giving the requirements gathered by the working group at the start of the process.
  2. RFC2693: Theory giving the theory of authorization certificates, as opposed to name or ID certificates that most people (e.g., X.509) discuss. This document points out some of the flawed assumptions in ID certificate theory and shows how SPKI's certificates (both authorization and ID) attempt to correct those flaws.
  3. structure #5 (old, #6 coming soon) -- giving the detailed structure of certificates that satisfy the theory RFC.
  4. examples #1 (even older) -- giving actual examples of certificates, both for instruction in how to use authorization certificates and for testing implementations for interoperability.
The SPKI mailing list is handled by majordomo@metzdowd.com. You can subscribe by sending a message whose body consists of the one line:
subscribe spki
As with other majordomo mailing lists, you can send the command:
help
to learn of other commands you can issue.
The mailing list archive can be accessed at http://www.sandelman.ottawa.on.ca/spki/. [There is another mailing list archive that appears to be more current. Thanks to the list member who pointed me there...]
The SDSI (Simple Distributed Security Infrastructure) part of SPKI/SDSI was developed separately by Ron Rivest and Butler Lampson.  The early documents on SDSI are available at:

Code and product pages

SPKI/SDSI Papers

  • Ellison, C. M., ``Home Network Security.'', Intel Technology Journal. http://developer.intel.com/technology/itj/2002/volume06issue04/ (November 2002).
  • Dohrmann and Ellison, ``Public-key Support for Collaborative Groups'', 1st Annual PKI Research Workshop, April 2002.
  • The theory behind SPKI certificates has been summarized in a journal paper:  Ellison, ``The nature of a usable PKI'', Computer Networks 31 (1999) pp. 823-830, which includes figures that might help in reading RFC2693, although the RFC is far more detailed and complete than this paper.
  • M. Burnside, D. Clarke, T. Mills, A. Maywah, S. Devadas, and R. Rivest, ``Proxy-Based Security Protocols in Networked Mobile Devices'', Proceedings SAC 2002. [PDF] [PS]
  • J.Y. Halpern and R. van der Meyden, ``A logical reconstruction of SPKI''.
  • Jon Howell's research papers include his thesis, other discussions of SPKI and examples of its use. These include an extension of SPKI to include the formalization of the principal type ``A quoting B''. This is used in implementation of authorized gateway functions (where a gateway has to translate data and therefore invalidate the digital signature of the data signer).
  • A Trusted Execution Platform for Multiparty Computation

    • Sameer Ajmani. Masters thesis, Massachusetts Institute of Technology, July 2000.
    [ PostScript | PDF ]
    [CME: This thesis addresses the distributed certificate chain validation problem when you don't want to release your pool of certificates to the world.]
  • Per Harald Myrvang, An Infrastructure for Authentication, Authorization and Delegation.
  • Ninghui Li, Local Names in SPKI/SDSI.
  • Juha Paajarvi, ``XML Encoding of SPKI Certificates'', March 2000 [cached copy]
  • Dwaine Clarke, SPKI/SDSI HTTP Server / Certificate Chain Discovery in SPKI/SDSI, September 2001. (Master's thesis, reporting on his use of SPKI/SDSI for access control to web pages.)
  • Tuomas Aura, Carl Ellison, Privacy and Accountability in Certificate Systems, Research Report A61, Laboratory for Theoretical Computer Science, Helsinki University of Technology, Espoo, Finland, April 2000. [PS][PDF][abstract][BibTeX].
  • Tuomas Aura, Dieter Gollmann, Software license management with smart cards, in Proc. USENIX Workshop on Smartcard Technology, Chicago, May 1999, pp. 75-85, USENIX Association 1999. [PS][PDF][HTML][abstract/BibTeX].
  • Tuomas Aura, Distributed access-rights management with delegation certificates, Secure Internet Programming: Security Issues for Distributed and Mobile Objects, J. Vitek and C. Jensen (Eds.), LNCS 1603, pp. 211-235, Springer 1999. [PS][PDF][abstract/BibTeX]. (Copyright 1999 Springer)
  • Tuomas Aura, Fast access control decisions from delegation certificate databases, in proceedings of 3rd Australasian Conference on Information Security and Privacy ACISP '98, Brisbane, Australia, July 1998, pp. 284-295, Lecture Notes in Computer Science 1438, Springer 1998. [PS][PDF][abstract/BibTeX].
  • Tuomas Aura, On the structure of delegation networks, in proceedings of 11th IEEE Computer Security Foundations Workshop, Rockport, Massachusetts, June 1998, pp. 14-26, IEEE Computer Society Press 1998. [PS][PDF][abstract/BibTeX].

UPnP Papers

PKI in general

Digital Signature Risks

The idea that digital signatures could enable electronic commerce through what has come to be known as non-repudiation was first proposed by Diffie and Hellman in their seminal paper, ``New Directions in Cryptography''.  The idea has since gained much popularity.
It is generally asserted that one can achieve the desired non-repudiation through the combination of strength of cryptography and security of Certificate Authorities.  However, as Don Davis and others have pointed out, this puts a burden on the individual keyholder that that person may not be equipped to handle.  The result, when this is applied to normal consumers with home computers, is a potential victimization of that consumer.
One of the problems here is the change in computer cost and therefore ubiquity. In 1976, when Diffie and Hellman were writing and proposed non-repudiation (under a different term), computers were in guarded glass rooms. Now they're in the family room where the neighbor's teenage son has easy, unguarded access. A guarded computer might well serve as a check writing machine, but an unguarded one is too dangerous to empower that way.

Miscellaneous Papers

Other certificate formats

  • X.509, PGP and SPKI:  Summary comparison table
  • X9.59 (also known as AADS) is a mechanism that uses ACLs only, instead of certificates.  Specifically, the ACL is a bank's account database that has had a public-key field added, so that the bank can look up the account-holder's public key.  This makes the X9.59 ACL an authorization instrument, in SPKI terms, similar to the ACL implemented by SSH's file: .ssh/authorized_keys.  For more information on X9.59, see: http://www.garlic.com/~lynn
  • PGP - an identity certificate format, the first to gain widespread usage and the form supporting the well known Web of Trust
  • X.509 - the identity certificate from the X.500 effort, with an attribute certificate added recently (perhaps borrowed from X9)
  • Blaze, Feigenbaum and Lacy trust management forms (computing authorization with certified code)

Misc. articles and how-to

This is not intended to be a complete list at all. These are just some links as people refer me to articles on PKI.

Glossary

  • asymmetric cryptography: defined originally by Diffie and Hellman, a cryptographic system using different keys for encipherment and decipherment such that one of the keys (private key) can not be derived efficiently from the other (public key).
  • certificate: a digitally signed data record communicating some information from the signer (issuer) of the certificate to the verifier of the certificate.  A certificate differs from a general signed message usually in that:
    1. the data structure is well defined so that a computer can interpret the structure, and
    2. the certificate's ``message'' is of the form ``to whom it may concern'', rather than addressed to a specific party
  • Certificate Authority: in the X.509 world, a special certificate issuing entity, usually part of a hierarchy, responsible for issuing all certificates to end entity keyholders.
  • digital signature: a computation with a private key and typically the hash of a document or data record such that any entity in possession of the matching public key can verify computationally that the computation was performed by the associated private key and that the signed document has not changed since the signature was computed.
  • hash: a cryptographic computation over a message yielding a fixed length quantity (the hash value) such that it is computationally difficult to find any two different input messages yielding the same hash value.  The ``strength of a hash'' is a reference to the difficulty of finding such message pairs.
  • keyholder: the holder of the private key.
  • non-repudiation: the notion that the keyholder is legally liable for any statement digitally signed by that keyholder's signature key.
  • PGP: Pretty Good Privacy -- an early public key application, defining the first public key infrastructure to be widely deployed.
  • PKI: Public Key Infrastructure -- a mechanism to permit the distributed use of public keys, involving certificates that bind information of interest to public keys
  • private key: a key in an asymmetric cryptosystem that is kept secret and held by one entity, called the keyholder.
  • public key:  a key in an asymmetric cryptosystem that need not be kept secret and is often
  • SDSI: Simple Distributed Security Infrastructure
  • secret key: a key in a symmetric cryptosystem.
  • speaks for: the notion that a private digital signature key speaks digitally signed statements in cyberspace on behalf of the keyholder
  • SPKI: Simple Public Key Infrastructure
  • symmetric cryptography: the original kind of cryptography, in which the same key is used for both encipherment and decipherment.
  • trust: a term so frequently misused that it has become almost devoid of meaning. It means whatever you want it to mean, but vendors keep using it because it evokes warm and fuzzy feelings and that helps sales.
  • Web of Trust: a mechanism for fault tolerance of certificate signature, associated with PGP.
  • X.509: a data structure defined as part of the X.500 global directory effort, designed to bind an X.500 distinguished name to a public key.  The presumption in X.509 is that the named entity is the keyholder of the associated public key.  In some cases, it is assumed that the public key speaks for the named entity.

Carl M. Ellison; mailto:cme@acm.org?subject=spki.html

frm  http://world.std.com/~cme/html/spki.html
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