Network Working Group C. Ellison
Request for Comments: 2692 Intel
Category: Experimental September 1999
SPKI Requirements
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
The IETF Simple Public Key Infrastructure [SPKI] Working Group is
tasked with producing a certificate structure and operating procedure
to meet the needs of the Internet community for trust management in
as easy, simple and extensible a way as possible.
The SPKI Working Group first established a list of things one might
want to do with certificates (attached at the end of this document),
and then summarized that list of desires into requirements. This
document presents that summary of requirements.
Table of Contents
Charter of the SPKI working group................................2
Background.......................................................2
General Requirements.............................................3
Validity and CRLs................................................4
Implementation of Certificates...................................4
List of Certificate Uses.........................................5
Open Questions..................................................11
References......................................................12
Security Considerations.........................................12
Author's Address................................................13
Full Copyright Statement........................................14
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Charter of the SPKI working group
Many Internet protocols and applications which use the Internet
employ public key technology for security purposes and require a
public key infrastructure to manage public keys.
The task of the working group will be to develop Internet standards
for an IETF sponsored public key certificate format, associated
signature and other formats, and key acquisition protocols. The key
certificate format and associated protocols are to be simple to
understand, implement, and use. For purposes of the working group,
the resulting formats and protocols are to be known as the Simple
Public Key Infrastructure, or SPKI.
The SPKI is intended to provide mechanisms to support security in a
wide range of Internet applications, including IPSEC protocols,
encrypted electronic mail and WWW documents, payment protocols, and
any other application which will require the use of public key
certificates and the ability to access them. It is intended that the
Simple Public Key Infrastructure will support a range of trust
models.
Background
The term certificate traces back to the MIT bachelor's thesis of
Loren M. Kohnfelder [KOHN]. Kohnfelder, in turn, was responding to a
suggestion by Diffie and Hellman in their seminal paper [DH]. Diffie
and Hellman noted that with public key cryptography, one no longer
needs a secure channel over which to transmit secret keys between
communicants. Instead, they suggested, one can publish a modified
telephone book -- one with public keys in place of telephone numbers.
One could then look up his or her desired communication partner in
the directory, find that person's public key and open a secure
channel to that person. Kohnfelder took that suggestion and noted
that an on-line service has the disadvantage of being a performance
bottleneck. To replace it, he proposed creation of digitally signed
directory entries which he called certificates. In the time since
1978, the term certificate has frequently been assumed to mean a
binding between name and key.
The SPKI team directly addressed the issue of <name,key> bindings and
realized that such certificates are of extremely limited use for
trust management. A keyholder's name is one attribute of the
keyholder, but as can be seen in the list of needs in this document,
a person's name is rarely of security interest. A user of a
certificate needs to know whether a given keyholder has been granted
some specific authorization.
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General Requirements
We define the term KEYHOLDER of a public key to refer to the person
or other entity that controls the corresponding private key.
The main purpose of an SPKI certificate is to authorize some action,
give permission, grant a capability, etc. to or for a keyholder.
The keyholder is most directly identified by the public key itself,
although for convenience or other purposes some indirection (delayed
binding) may be employed. That indirection can be via a collision-
free hash of the public key or via a name, later to be resolved into
a key.
The definition of attributes or authorizations in a certificate is up
to the author of code which uses the certificate. The creation of
new authorizations should not require interaction with any other
person or organization but rather be under the total control of the
author of the code using the certificate.
Because SPKI certificates might carry information that the keyholder
might not want to publish, we assume that certificates will be
distributed directly by the keyholder to the verifier. If the
keyholder wishes to use a global repository, such as LDAP, the global
PGP key server or the DNS database, that is up to the keyholder and
not for the SPKI WG to specify.
Because SPKI certificates will carry information that, taken together
over all certificates, might constitute a dossier and therefore a
privacy violation, each SPKI certificate should carry the minimum
information necessary to get a job done. The SPKI certificate is
then to be like a single key rather than a key ring or a single
credit card rather than a whole wallet. The keyholder should be able
to release a minimum of information in order to prove his or her
permission to act.
It is necessary for at least some certificates to be anonymous.
Because one use of SPKI certificates is in secret balloting and
similar applications, an SPKI certificate must be able to assign an
attribute to a blinded signature key.
One attribute of a keyholder is a name. There are names the
keyholder prefers to be called and there are names by which the
keyholder is known to various other keyholders. An SPKI certificate
must be able to bind a key to such names. The SDSI work of Rivest
and Lampson has done an especially good job of defining and using
local name spaces, therefore if possible SPKI should support the SDSI
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name construct. [Note: SPKI and SDSI have merged.]
Validity and CRLs
An SPKI certificate, like any other, should be able to carry a
validity period: dates within which it is valid. It may also be
necessary to have on-line refinement of validity. This is frequently
achieved via a Certificate Revocation List (CRL) in previous
certificate designs.
A minimal CRL contains a list of revoked certificates, identified
uniquely, a sequence number and a signature. Its method of
transmission is not specified. If it encounters some certificate
that it lists, then it annihilates that certificate. If it
encounters a previous CRL, as indicated by sequence number, then it
annihilates that previous CRL. Such a CRL leads to non-deterministic
program behavior. Therefore, we take as a requirement that if SPKI
uses CRLs, then the certificate that uses it must explicitly tell the
verifier where to find the CRL, the CRL must carry explicit validity
dates and the dates of a sequence of CRLs must not overlap. Under
this set of requirements, behavior of certificate validation is
deterministic (aside from the question of clock skew).
A CRL is a negative statement. It is the digital equivalent of the
little paper books of bad checks or bad credit cards that were
distributed to cashiers in the 1970's and before. These have been
replaced in the retail world by positive statements -- on-line
validation of a single check, ATM card or credit card.
SPKI should support both positive and negative on-line validations.
Any CRL or revalidation instrument must have its own lifetime. A
lifetime of 0 is not possible because of communication delays and
clock skews, although one can consider an instrument whose lifetime
is "one use" and which is delivered only as part of a
challenge/response protocol.
Implementation of Certificates
The authorization certificates that are envisioned for SPKI (and
needed to meet the demands of the list given at the end of this
document) should be generated by any keyholder empowered to grant or
delegate the authorization in question. The code to generate
certificates should be written by many different developers,
frequently persons acting alone, operating out of garages or dorm
rooms. This leads to a number of constraints on the structure and
encoding of certificates. In addition, SPKI certificates should be
usable in very constrained environments, such as smart cards or small
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embedded systems. The code to process them and the memory to store
them should both be as small as possible.
An SPKI certificate should be as simple as possible. There should be
a bare minimum of fields necessary to get the job done and there
should be an absolute minimum of optional fields. In particular, the
structure should be specific enough that the creator of a certificate
is constrained by the structure definition, not by complaints (or
error messages) from the reader of a certificate.
An SPKI certificate should be described in as simple a method as
possible, relating directly to the kind of structures a C or PASCAL
programmer would normally write.
No library code should be required for the packing or parsing of SPKI
certificates. In particular, ASN.1 is not to be used.
A certificate should be signed exactly as it is transmitted. There
should be no reformatting called for in the process of checking a
certificate's signature (although one might canonicalize white space
during certificate input, for example, if the format is text).
For efficiency, if possible, an SPKI certificate should be encoded in
an LR(0) grammar. That is, neither packing nor parsing of the
structure should require a scan of the data. Data should be read
into the kind of structure a programmer would want to use without
touching the incoming bytes more than once.
For efficiency, if possible, an SPKI certificate should be packed and
parsed without any recursion.
List of Certificate Uses
The list below is a brainstorming list, accumulated on the SPKI
mailing list, of uses of such certificates.
- I need a certificate to give me permission to write electronic
checks.
- My bank would need a certificate, proving to others that it is
a bank capable of cashing electronic checks and permitted to
give permission to people to write electronic checks.
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- My bank would issue a certificate signing the key of a master
bank certifier -- perhaps NACHA -- so that I could follow a
certificate chain from a key I know (my bank's) to the key of
any other bank in the US and, similarly, to any other bank in
the world.
- I might generate a certificate (a "reputation voucher") for a
friend to introduce him to another friend -- in which
certificate I could testify to my friend's political opinion
(e.g., libertarian cypherpunk) or physical characteristics or
anything else of interest.
- I might have a certificate giving my security clearance, signed
by a governmental issuing authority.
- I want a certificate for some software I have downloaded and am
considering running on my computer -- to make sure it hasn't
changed and that some reputable company or person stands behind
it.
- I need certificates to bind names to public keys:
- [traditional certificate] binding a key to a name, implying
"all the attributes of the real person having this name are
transferred to this key by this certificate". This requires
unique identification of a person (which is difficult in
non-digital space, as it is) and someone trustworthy binding
that unique name to the key in question. In this model, a
key starts out naked and acquires attributes, permissions
and authority from the person bound to it.
- [direct certificate] binding a name to a key, implying "I
(the person who is able to use the associated private key to
make this signature) declare that I go by the name of
XXXXXXX." The unique identification of the key is automatic
-- from the key itself or a cryptographic hash of the key.
The binding is done by the key itself -- in a self-signed
certificate. In this model, a key is loaded with
attributes, permissions and authority directly by other
certificates, not indirectly through some person's name, and
this certificate declares only a name or nickname by which
the key's owner likes to be addressed.
- [personal binding] binding a key to a nickname. This kind
of certificate is signed by me, singing someone else's key
and binding it to a nickname by which I know that person.
It is for my use only -- never given out -- and is a signed
certificate to prevent tampering with my own private
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directory of keys. It says nothing about how I certified
the binding to my own satisfaction between the key and my
friend.
- I might be doing genealogy and be collecting what amounts to
3x5 cards with facts to be linked together. Some of these
links would be from one content to another reference [e.g.,
indexing and cross-referencing]. Others might be links to the
researcher who collected the fact. By rights, the fact should
be signed by that researcher. Viewing only the signature on
the fact and the link to the researcher, this electronic 3x5
card becomes a certificate.
- I want to sign a contract to buy a house. What kind of
certificate do I need?
- I have found someone on the net and she sounds really nice.
Things are leading up to cybersex. How do I make sure she's
not really some 80-year-old man in a nursing home?
- I have met someone on the net and would like a picture of her
and her height, weight and other measurements from a
trustworthy source.
- Can I have a digital marriage license?
- Can I have a digital divorce decree?
- ..a digital Voter Registration Card?
- There are a number of cards one carries in a typical wallet
which could become certificates attached to a public key:
- health insurance card
- prescription drug card
- driver's license (for permission to drive)
- driver's license (for permission to buy alcohol)
- supermarket discount card
- supermarket check-cashing card [I know -- anachronism]
- Blockbuster Video rental card
- ATM card
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- Credit card
- membership card in the ACLU, NRA, Republican party, Operation
Rescue, NARAL, ACM, IEEE, ICAR....
- Red Cross blood donor card
- Starbuck's Coffee buy-10-get-1-free card
- DC Metro fare card
- Phone calling card
- Alumni Association card
- REI Membership card
- Car insurance card
- claim check for a suitcase
- claim check for a pawned radio
- authorization for followup visits to a doctor, after surgery
- Better Business Bureau [BBB] style reputation certificates
[testimonies from satisfied customers]
- BBB-style certificate that no complaints exist against a
business or doctor or dentist, etc.
- LDS Temple Recommend
- Stock certificate
- Stock option
- Car title
- deed to land
- proof of ownership of electronic equipment with an ID number
- time card certificate [activating a digital time clock]
- proof of degree earned [PhD, LLD, MD, ...]
- permission to write digitally signed prescriptions for drugs
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- permission to spend up to $X of a company's money
- permission to issue nuclear launch codes
- I'm a sysadmin, I want to carry a certificate, signed by SAGE,
that says I'm good at the things sysadmins are good at.
- I'm that same sysadmin, I want an ephemeral certificate that
grants me root access to certain systems for the day, or the
week, or...
Certain applications *will* want some form of auditing, but the
audit identity should be in the domain of the particular
application... For instance an "is a system administrator of
this host" certificate would probably want to include an audit
identity, so you can figure out which of your multiple admins
screwed something up.
- I'm an amateur radio operator. I want a signed certificate
that says I'm allowed to engage in amateur radio, issued by the
DOC. [I currently have a paper version of one]. This would be
useful in enforcing access policies to the amateur spectrum;
and in tracking abuse of that same spectrum. Heck! extend
this concept to all licensed spectrum users.
- I'm the a purchasing agent for a large corporation. I want to
posses a certificate that tells our suppliers that I'm
authorized to make purchases up to $15,000. I don't want the
suppliers to know my name, lest their sales people bug me too
much. I don't want to have to share a single "Megacorp
Purchasing Department Certificate" with others doing the same
job [the private key would need to be shared--yuck!].
- "This signed-key should be considered equivalent to the
certifying-key until this certificate expires for the following
purposes ..."
[This is desirable when you wish to reduce the exposure of
long-term keys. One way to do this is to use smart cards,
but those typically have slow processors and are connected
through low-bandwidth links; however, if you only use the
smart card at "login" time to certify a short-term key pair,
you get high performance and low exposure of the long term
key.
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I'll note here that this flies in the face of attempts to
prevent delegation of certain rights. Maybe we need a
"delegation-allowed" bit -- but there's nothing to stop
someone who wishes to delegate against the rules from also
loaning out their private key.].
- "I am the current legitimate owner of a particular chunk of
Internet address space."
[I'd like to see IPSEC eventually become usable, at least
for privacy, without need for prior arrangement between
sites, but I think there's a need for a "I own this
address"/"I own this address range" certificate in order for
IPSEC to coexist with existing ip-address-based firewalls.]
- "I am the current legitimate owner of a this DNS name or
subtree."
- "I am the legitimate receiver of mail sent to this rfc822 email
address. [this might need to be signed by a key which itself
had been certified by the appropriate "DNS name owner"
certificate]."
[This is in case I know someone owns a particular e-mail
address but I don't know their key.]
- Encryption keys for E-mail and file encryption
- Authentication of people or other entities
- Digital signatures (unforgeability)
- Timestamping / notary services
- Host authentication
- Service authentication
Other requirements:
- Trust model must be a web (people want to choose whom they
trust). People must be able to choose whom they trust or
consider reliable roots (maybe with varying reliabilities).
- Some applications (e.g., notary services) require highly
trusted keys; generation complexity is not an issue here.
- Some applications (e.g., host authentication) require
extremely light (or no) bureaucracy. Even communication
with the central administrator may be a problem.
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- Especially in lower-end applications (e.g. host
authentication) the people generating the keys (e.g.,
administrators) will change, and you will no longer want
them to be able to certify. On the other hand, you will
usually also not want all keys they have generated to
expire. This may imply a "certification right expiration"
certificate requirement, probably to be implemented together
with notary services.
- Keys will need to be cached locally to avoid long delays
fetching frequently used keys. Cf. current name servers.
The key infrastructure may in future get used almost as
often as the name server. The caching and performance
requirements are similar.
- Reliable distribution of key revocations and other
certificates (e.g., the ceasing of the right to make new
certificates). May involve goals like "will have spread
everywhere in 24 hours" or something like that. This
interacts with caching.
Open Questions
Given such certificates, there remain some questions, most to do with
proofs of the opposite of what a certificate is designed to do.
These do not have answers provided by certificate definition or
issuing alone.
- Someone digitally signs a threatening e-mail message with my
private key and sends it to president@whitehouse.gov. How do I
prove that I didn't compose and send the message? What kind of
certificate characteristic might help me in this?
This is an issue of (non-)repudiation and therefore a matter of
private key protection. Although this is of interest to the
user of certificates, certificate format, contents or issuing
machinery can not ensure the protection of a user's private key
or prove whether or not a private key has been stolen or
misused.
- Can certificates help do a title scan for purchase of a house?
Certificates might be employed to carry information in a
tamper-proof way, but building the database necessary to record
all house titles and all liens is a project not related to
certificate structure.
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- Can a certificate be issued to guarantee that I am not already
married, so that I can then get a digital marriage license?
The absence of attributes can be determined only if all
relevant records are digitized and all parties have inescapable
IDs. The former is not likely to happen in our lifetimes and
the latter receives political resistance.
A certificate can communicate the 'positive' attribute "not
already married" or "not registered as a voter in any other
district". That assumes that some organization is capable of
determining that fact for a given keyholder. The method of
determining such a negative fact is not part of the certificate
definition.
- The assumption in most certificates is that the proper user will
protect his private key very well, to prevent anyone else from
accessing his funds. However, in some cases the certificate
itself might have monetary value [permission to prescribe drugs,
permission to buy alcohol, ...]. What is to prevent the holder of
such a certificate from loaning out his private key?
This is a potential flaw in any system providing authorization
and an interesting topic for study. What prevents a doctor or
dentist from selling prescriptions for controlled substances to
drug abusers?
References
[DH] Diffie and Hellman, "New Directions in Cryptography", IEEE
Transactions on Information Theory IT-22, 6 (Nov. 1976), 644-
654.
[KOHN] Loren Kohnfelder, "Towards a Practical Public-key
Cryptosystem", Bachelor's thesis, MIT, May, 1978.
Security Considerations
Security issues are discussed throughout this memo.
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Author's Address
Carl M. Ellison
Intel Corporation
2111 NE 25th Ave M/S JF3-212
Hillsboro OR 97124-5961 USA
Phone: +1-503-264-2900
Fax: +1-503-264-6225
EMail: carl.m.ellison@intel.com
cme@alum.mit.edu
Web: http://www.pobox.com/~cme
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