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Cryptology Academy · Lesson

RSA Real-World Use Cases & Key Sizes

Discover how RSA protects HTTPS, SSH, and code signing today.

RSA Real-World Use Cases & Key Sizes is a free Cryptology Academy lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Cryptology Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Welcome

RSA protects communications worldwide. In this lesson we map RSA to real systems: HTTPS certificates, SSH authentication, code signing, and encrypted email.

RSA in TLS/HTTPS

TLS certificates contain the server's RSA or ECC public key. During the TLS handshake, the client uses this public key to verify the server's identity and establish a shared session key.

RSA Key Exchange in TLS 1.2

In TLS 1.2 RSA mode: client generates a random pre-master secret, encrypts it with the server's RSA public key, sends it. Server decrypts with private key. Both derive session keys.

Why TLS 1.3 Dropped RSA Key Exchange

TLS 1.3 eliminated RSA key exchange because it lacks forward secrecy — if the server's private key is later compromised, past sessions can be decrypted. ECDHE replaced it.

SSH Public Key Authentication

SSH key auth flow: 1. Server has user's public key in ~/.ssh/authorized_keys 2. Server sends a challenge 3. Client signs challenge with private key 4. Server verifies with stored public key No password sent over the wire.

Code Signing with RSA

Developer signs binary: signature = RSA-Sign(hash(binary), private_key). OS verifies: RSA-Verify(signature, hash(binary), public_cert). Mismatch = reject installation.

Email Signing: PGP/GPG

GPG uses RSA to sign emails: gpg --sign message.txt The recipient verifies with the sender's public key from a keyserver. Combined with encryption: gpg --encrypt --sign.

RSA Key Size Recommendations 2025

RSA-1024: do not use (factored in 2010). RSA-2048: acceptable minimum to 2030. RSA-3072: recommended for new deployments. RSA-4096: high security, 4× slower. Consider ECC as alternative.

ECDSA vs RSA for Certificates

ECDSA-256 ≈ RSA-3072 in security. ECDSA keys are 10× smaller. ECDSA handshakes are faster. Most modern TLS deployments now use ECDSA certificates over RSA.

RSA vs Quantum Computers

A cryptographically relevant quantum computer running Shor's algorithm could break RSA in polynomial time. NIST is standardizing post-quantum alternatives (Kyber, Dilithium) to replace RSA.

Key Lifecycle Management

RSA keys must be: generated with a CSPRNG, protected by hardware (HSM/secure enclave), rotated periodically, revoked when compromised, destroyed securely when retired.

Quick Check

Why was RSA key exchange removed from TLS 1.3?

Recap

Excellent! You've mapped RSA to real systems. Next we study digital signatures — how RSA and other algorithms provide authentication and non-repudiation.

Frequently asked questions

Is the “RSA Real-World Use Cases & Key Sizes” lesson free?

Yes — the full text of “RSA Real-World Use Cases & Key Sizes” is free to read here on the web, and the Cryptology Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Cryptology Academy course, upgrade to CoddyKit PRO.

What will I learn in “RSA Real-World Use Cases & Key Sizes”?

Discover how RSA protects HTTPS, SSH, and code signing today. You practise Cryptology Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Cryptology Academy?

No prior experience is required. Cryptology Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “RSA Real-World Use Cases & Key Sizes” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Cryptology Academy lesson?

Yes. Every Cryptology Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Public-Key Cryptography Concepts
  2. RSA Key Generation Step by Step
  3. RSA Encryption & Decryption Math
  4. RSA Real-World Use Cases & Key Sizes
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