C2 Key parity and key check values
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Odd parity on DES keys, and the key check values (KCV) every key exchange is verified with.
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What this module covers
You will be able to
- Check and fix odd parity
- Explain why parity is not security
- Compute a DES KCV and an AES KCV
- Use KCVs to find a mistyped component
The lesson
- Why it matters
- The concept
- Step by step
- In the real world
- Common mistakes
- What to test
In the module
- The lesson, with diagrams, common mistakes and what to test
- A worked example you can change: Parity, then the key check value
- Practice: 4 kinds of generated exercise, each with a step-by-step solution
- A quiz of 9 questions
Key terms
- Key check value (KCV)
- A short value computed from a key, usually the first 3 bytes of encrypting a zero block (for AES often a CMAC of zeros). It confirms the right key was loaded without revealing it.
- Odd parity
- The DES key convention: each byte has an odd number of one bits. A byte with even parity suggests a typing or transfer error.
- Parity adjustment
- Setting the lowest bit of each byte so the key has odd parity, for example after deriving a key.
- Parity bit
- The lowest bit of each DES key byte, not used by the cipher. It is set so each byte has an odd number of one bits.
The lesson, worked example, practice and quiz open with a supporter pass, which you can try free for 7 days.