Chapter 2 — Computing systems
Lesson map
Lesson 02 published. Map, example, EX004.
flowchart TD
N0["Learning outcomes"]
N1["2.1 Layers of a system"]
N2["2.2 Three states of data"]
N3["2.3 Matching protections"]
N4["Synthesis"]
N0 --> N1 --> N2 --> N3 --> N4
N4 --> EX["Worked example"]
EX --> Q["Exercise EX004"]
Logical drawing
flowchart LR
N0["Learning outcomes"]
N1["2.1 Layers of a system"]
N2["2.2 Three states of data"]
N3["2.3 Matching protections"]
N4["Synthesis"]
N0 --> N1 --> N2 --> N3 --> N4 --> Q["EX004"]
[Learning outcomes]
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v
[2.1 Layers of a system]
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v
[2.2 Three states of data]
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v
[2.3 Matching protections]
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v
[Synthesis]
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v
[Worked example] --> [Exercise EX004]
Learning outcomes
You will (1) describe where cryptography sits in a computing system; (2) explain data at rest, in transit and in use; (3) identify which component protects each.
2.1 Layers of a system
Priya maps Tidewell's systems in layers: hardware in branch servers and patron laptops, operating systems, applications such as the catalogue and patron portal, networks connecting branches, and cloud services hosting e-books. Cryptography appears at several layers: disk encryption in the operating system, TLS in network libraries, password hashing in the portal application and signatures on software updates. She learns that a protection at one layer does not cover the others; encrypting a disk does nothing for data travelling between branches, and TLS does nothing for a stolen laptop's disk.
2.2 Three states of data
Data exists in three states. At rest, it sits on disks and backups; full-disk encryption and database encryption protect it against theft of media. In transit, it moves across networks; TLS protects it against eavesdropping and tampering. In use, it is being processed in memory, where it is usually decrypted and protected mainly by access control and operating-system isolation. Priya classifies Tidewell's patron records in each state and notes that the consortium encrypts laptops and uses TLS for the portal, but backup tapes sent to an off-site store are not encrypted.
2.3 Matching protections
Priya builds a table matching each data flow to its protection and owner. Patron logins travel over TLS and are stored as salted password hashes; loan history sits in an encrypted database; backups travel by courier unencrypted. She recommends encrypting backups with keys held by the consortium, not by the courier company, and records that key loss would make backups unreadable, so the keys need their own protected backup. Grace Okafor accepts the recommendation and asks Priya to estimate the cost of the backup software change.
Worked example
Priya's layer map and data-state table show unencrypted backup tapes as a gap, and she recommends consortium-held backup encryption with protected key backup.
Synthetic case. Tidewell's systems are invented.
Errors and limits
Encryption at rest does not protect data in use by a compromised application. Losing encryption keys destroys data as surely as losing the media.
Exercises
- EX004 — Layer limits. Why does disk encryption not protect data moving between branches? Rubric: 3 = it protects data at rest only; transit needs TLS;; 2 = layers differ;; 1 = it does;; 0 = none.
- EX005 — Data in use. What mainly protects data in use? Rubric: 3 = access control and operating-system isolation, since data is decrypted in memory;; 2 = access control;; 1 = encryption;; 0 = none.
- EX006 — Backup gap. What gap did Priya find and what did she recommend? Rubric: 3 = unencrypted backup tapes; encrypt with consortium-held keys and back up keys;; 2 = gap;; 1 = vague;; 0 = none.
Synthesis
Cryptography protects specific layers and data states; mapping them reveals gaps such as unencrypted backups.
Continue studying — chapter 02 of 40
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