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CISSP 7.1 - Asymmetric Cryptography (Part 1 of 2)
This episode of the ISC2 Certified Information Systems Security Professional (CISSP) exam prep series opens the world of public key cryptography in Domain 3, building a working picture of how asymmetric systems let strangers communicate securely. It walks through the math that makes the approach trustworthy and the practical judgment calls that keep those protections sound for years after you make them.
What this episode covers
- The key distribution problem — why pre-shared secrets fail across thousands of strangers, and how key pairs fix it.
- Public and private keys — publish one openly, guard the other, and encrypt to the recipient’s public half.
- Longer keys, slower math — the compute cost that reserves asymmetric crypto for small jobs, not bulk data.
- The RSA algorithm — security built on the brutal difficulty of factoring the product of two large primes.
- Choosing key length — weighing data criticality, rising attacker compute, and long-lived sensitivity.
- Strength vs bit counts — a short elliptic curve key can match a much longer RSA key.
Watch the full episode above for the worked examples and detailed explanations of each concept.
Frequently Asked Questions
What problem does public key cryptography actually solve?
Symmetric systems are fast, but both sides must already share one secret key, which falls apart the moment you have thousands of users who have never met. Asymmetric cryptography removes that hurdle entirely. Each user holds a pair of related keys, so you can set up secure channels without ever hand-delivering a shared secret.
How does a key pair protect a message?
Every user gets two matched keys: the public key you publish openly to the whole world, and the private key you guard and never share apart from formal escrow or recovery arrangements. To send you a message, I encrypt it with your public key — once I do that, not even I can read it back, because only your private key can unlock it. The keys are mathematically linked, so possessing the public half gives an attacker no way in.
Why are asymmetric systems slower and their keys longer?
Public key math is far more computationally demanding than symmetric math, so to reach the same strength an asymmetric key must be much longer than a symmetric one. That extra work adds up quickly on anything beyond a short message. In practice, architects lean on symmetric cryptography for bulk data and reserve the asymmetric approach for the parts where its unique power is worth the cost.
How does the RSA algorithm work under the hood?
RSA’s security rests on one hard problem: multiplying two very large prime numbers is easy, but factoring the result back into those primes is brutally difficult. To build a key pair, you pick two large primes and multiply them, deriving a public value everyone gets and a private value you keep secret. A sender raises the message to the public exponent and takes the remainder against that product to get ciphertext, and you reverse it with your private value.
How do you choose a key length that stays safe over time?
The more critical the data, the stronger the key it deserves, and you must factor in time — computing power keeps climbing, and attackers rent scalable cloud power and cheap graphics processors by the hour. If your data will still be sensitive years from now, over-provision the key length now. Remember that equivalent strength varies by system: a short elliptic curve key can match a much longer RSA key, so compare strength, not raw bit counts.
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Reference: This article is based on concepts discussed in CISSP 7.1 - Asymmetric Cryptography (Part 1 of 2).