Chapter 17 — Cryptography concepts

Chapter 17 — Cryptography concepts

Lesson map

Lesson 17 published. Map, example, EX049.

flowchart TD
  N0["Learning outcomes"]
  N1["17.1 Encryption and signatures"]
  N2["17.2 Usability problems"]
  N3["17.3 Invisible cryptography"]
  N4["Synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4
  N4 --> EX["Worked example"]
  EX --> Q["Exercise EX049"]

Logical drawing

flowchart LR
  N0["Learning outcomes"]
  N1["17.1 Encryption and signatures"]
  N2["17.2 Usability problems"]
  N3["17.3 Invisible cryptography"]
  N4["Synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4 --> Q["EX049"]

[Learning outcomes]
    |
    v
[17.1 Encryption and signatures]
    |
    v
[17.2 Usability problems]
    |
    v
[17.3 Invisible cryptography]
    |
    v
[Synthesis]
    |
    v
[Worked example] --> [Exercise EX049]

Learning outcomes

You will (1) explain encryption and signatures in plain terms; (2) describe usability problems with cryptographic tools; (3) recommend where cryptography should be invisible.

17.1 Encryption and signatures

Encryption scrambles data so only someone with the right key can read it. Digital signatures let a recipient check who sent a message and that it was not changed. Nadia learns these ideas in plain terms, enough to see their roles. Web connections to the library catalogue are encrypted automatically, so members do not need to do anything. Email signatures could help staff trust internal messages, but only if checking them were simple. The value of cryptography depends heavily on how much it asks of people.

17.2 Usability problems

Whitten and Tygar (1999) studied an email encryption program and found that most participants could not use it correctly within ninety minutes, and some exposed secrets they meant to protect. The problems lay in confusing concepts, such as public and private keys, and an interface that did not guide users. Nadia notes that two decades later, the most successful encryption, such as in web browsers and messaging apps, is the kind users barely notice, because it works automatically without decisions. That lesson shapes her advice.

17.3 Invisible cryptography

Nadia recommends that Ashcombe rely on cryptography that works without staff decisions: encrypted connections, encrypted laptop drives enabled centrally and security keys that use cryptography inside a simple tap. She advises against asking staff to encrypt individual email attachments manually, which would be error-prone. Where staff must make a choice, such as when sending member data externally, the system should offer one obvious secure option. Making cryptography invisible applies the lessons of usable security to some of its hardest tools. IT agrees to the plan.

Worked example

Nadia explains encryption and signatures, recalls Whitten and Tygar's findings and recommends invisible cryptography.

Synthetic case. Recommendations are synthetic; Whitten and Tygar (1999) is real.

Errors and limits

Invisible tools may hide failures. Central encryption needs key recovery.

Exercises

• EX049 — Signature purpose. What does a digital signature show? Rubric: 3 = who sent a message and that it was unchanged;; 2 = origin;; 1 = secrecy;; 0 = none.
• EX050 — Study finding. What did Whitten and Tygar find? Rubric: 3 = most could not use the encryption correctly in ninety minutes;; 2 = usability;; 1 = vague;; 0 = none.
• EX051 — Manual encryption. Why avoid manual attachment encryption? Rubric: 3 = it is error-prone for staff;; 2 = errors;; 1 = vague;; 0 = none.

Synthesis

Cryptography protects best when it asks people for few decisions.

Continue studying — chapter 17 of 40

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