computer science

Privacy-Enhanced Mail

Privacy-Enhanced Mail is a computer science 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 Privacy-Enhanced Mail rather than just read about it. In short: Privacy-enhanced mail (PEM) is a de facto file format for storing and sending cryptographic keys, certificates, and other data, based on a set of 1993 IETF standards defining "privacy-enhanced mail". While the original standards were never broadly adopted and were supplanted by PGP and S/MIME, the textual encoding they defined became very popular.

Key takeaways

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

Reference excerpt

Privacy-enhanced mail (PEM) is a de facto file format for storing and sending cryptographic keys, certificates, and other data, based on a set of 1993 IETF standards defining "privacy-enhanced mail". While the original standards were never broadly adopted and were supplanted by PGP and S/MIME, the textual encoding they defined became very popular. The PEM format was eventually formalized by the IETF in RFC 7468.

Format Many cryptography standards use ASN.1 to define their data structures, and Distinguished Encoding Rules (DER) to serialize those structures. Because DER produces binary output, it can be challenging to transmit the resulting files through systems, like electronic mail, that only support ASCII. The PEM format solves this problem by encoding the binary data using base64. PEM also defines a one-line header, consisting of -----BEGIN, a label, and -----, and a one-line footer, consisting of -----END, a label, and -----. The label determines the type of message encoded. Common labels include CERTIFICATE, CERTIFICATE REQUEST, PRIVATE KEY and X509 CRL.

PEM data is commonly stored in files with a ".pem" suffix, a ".cer" or ".crt" suffix (for certificates), or a ".key" suffix (for public or private keys). The label inside a PEM file represents the type of the data more accurately than the file suffix, since many different types of data can be saved in a ".pem" file. In particular PEM refers to the header and base64 wrapper for a binary format contained within, but does not specify any type or format for the binary data, so that a PEM file may contain "almost anything base64 encoded and wrapped with BEGIN and END lines".

Examples An operating system might provide a PEM file containing a list of trusted CA certificates, each of which in its own BEGIN/END sections; A web server might be configured with a "chain" file containing an end-entity certificate plus a list of intermediate certificates, each of which in its own BEGIN/END sections.

Development The PEM format was first developed in the privacy-enhanced mail series of Request for Comments (RFCs): RFC 1421, RFC 1422, RFC 1423, and RFC 1424. These standards assumed prior deployment of a hierarchical public key infrastructure (PKI) with a single root. Such a PKI was never deployed, due to operational cost and legal liability concerns. These standards were eventually obsoleted by PGP and S/MIME, competing e-mail encryption standards.

History The initiative to develop Privacy-Enhanced Mail began in 1985 on behalf of the PSRG (Privacy and Security Research Group) also known as the Internet Research Task Force. This task force is a subsidiary of the Internet Architecture Board (IAB) and their efforts have resulted in the Requests for Comment (RFCs) which are suggested Internet guidelines.

References

Worked examples

Example 1 — a first encounter with Privacy-Enhanced Mail

Start with the simplest possible case. Write down what Privacy-Enhanced Mail claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, 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 Privacy-Enhanced Mail 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 Privacy-Enhanced Mail 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 Privacy-Enhanced Mail

In research: Privacy-Enhanced Mail appears in computer science 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 Privacy-Enhanced Mail 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: Privacy-Enhanced Mail is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer file formats, Cryptographic protocols, so understanding it makes those chapters shorter.
In everyday life: Look for Privacy-Enhanced Mail 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 Privacy-Enhanced Mail in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Privacy-Enhanced Mail 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 Privacy-Enhanced Mail out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Privacy-Enhanced Mail in simple terms?

Why does Privacy-Enhanced Mail matter?

Because it connects several computer science 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 Privacy-Enhanced Mail?

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 Privacy-Enhanced Mail.