engineering

Simple public-key infrastructure

Simple public-key infrastructure is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Simple public-key infrastructure rather than just read about it. In short: Simple public key infrastructure (SPKI, pronounced spoo-key) was an attempt to overcome the complexity of traditional X.509 public key infrastructure. It was specified in two Internet Engineering Task Force (IETF) Request for Comments (RFC) specifications – RFC 2692 and RFC 2693 – from the IETF SPKI working group.

Key takeaways

Simple public-key infrastructure belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Simple public-key infrastructure to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Simple public-key infrastructure from memory before moving on to harder problems.

Reference excerpt

Simple public key infrastructure (SPKI, pronounced spoo-key) was an attempt to overcome the complexity of traditional X.509 public key infrastructure. It was specified in two Internet Engineering Task Force (IETF) Request for Comments (RFC) specifications – RFC 2692 and RFC 2693 – from the IETF SPKI working group. These two RFCs never passed the "experimental" maturity level of the IETF's RFC status. The SPKI specification defined an authorization certificate format, providing for the delineation of privileges, rights or other such attributes (called authorizations) and binding them to a public key. In 1996, SPKI was merged with Simple Distributed Security Infrastructure (SDSI, pronounced sudsy) by Ron Rivest and Butler Lampson.

History and overview The original SPKI had identified principals only as public keys but allowed binding authorizations to those keys and delegation of authorization from one key to another. The encoding used was attribute:value pairing, similar to RFC 822 headers. The original SDSI bound local names (of individuals or groups) to public keys (or other names), but carried authorization only in Access Control Lists (ACLs) and did not allow for delegation of subsets of a principal's authorization. The encoding used was standard S-expression. Sample RSA public key in SPKI in "advanced transport format" (for actual transport the structure would be Base64-encoded):

The combined SPKI/SDSI allows the naming of principals, creation of named groups of principals and the delegation of rights or other attributes from one principal to another. It includes a language for expression of authorization – a language that includes a definition of "intersection" of authorizations. It also includes the notion of threshold subject – a construct granting authorizations (or delegations) only when K of N of the listed subjects concur (in a request for access or a delegation of rights). SPKI/SDSI uses S-expression encoding, but specifies a binary form that is extremely easy to parse – an LR(0) grammar – called Canonical S-expressions. SPKI/SDSI does not define a role for a commercial certificate authority (CA). In fact, one premise behind SPKI is that a commercial CA serves no useful purpose. As a result of that, SPKI/SDSI is deployed primarily in closed solutions and in demonstration projects of academic interest. Another side-effect of this design element is that it is difficult to monetize SPKI/SDSI by itself. It can be a component of some other product, but there is no business case for developing SPKI/SDSI tools and services except as part of some other product. The most prominent general deployments of SPKI/SDSI are E-speak, a middleware product from HP that used SPKI/SDSI for access control of web methods, and UPnP Security, that uses an XML dialect of SPKI/SDSI for access control of web methods, delegation of rights among network participants, etc.

Worked examples

Example 1 — a first encounter with Simple public-key infrastructure

Start with the simplest possible case. Write down what Simple public-key infrastructure claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Simple public-key infrastructure before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Simple public-key infrastructure ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Simple public-key infrastructure

In research: Simple public-key infrastructure appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Simple public-key infrastructure in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Simple public-key infrastructure is common in secondary-school and first-year university syllabi. It links to neighbouring topics Key management, so understanding it makes those chapters shorter.
In everyday life: Look for Simple public-key infrastructure outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Simple public-key infrastructure in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Simple public-key infrastructure means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Simple public-key infrastructure out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Simple public-key infrastructure in simple terms?

Why does Simple public-key infrastructure matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Simple public-key infrastructure?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Simple public-key infrastructure.