What is RSA?
RSA (Rivest–Shamir–Adleman) is an asymmetric encryption algorithm that uses two keys: a public key that you share with everyone, and a private key that you keep secret.
Unlike symmetric encryption (where both parties have the same key), RSA solves the "key distribution problem" by letting anyone encrypt data using your public key, but only you can decrypt it with your private key.
How RSA Key Pairs Work
Imagine you have a special mailbox (public key) that anyone can drop letters into, but only you have the key to open it (private key).
- Generate a key pair – Create a public key (shareable) and private key (secret)
- Share your public key – Post it on your website, email it, or tell people directly
- Someone encrypts data with your public key – Anyone can do this
- Only you can decrypt it – Using your private key
- You can also sign messages – Prove you wrote something using your private key
Why is RSA Secure?
RSA relies on the mathematical difficulty of factoring large numbers. Here's the simplified version:
Easy problem: Multiply two large prime numbers together: 61 × 53 = 3,233
Hard problem: Given 3,233, find the original two prime numbers.
For a computer to figure out your private key, it would need to factor your public key. With a 2048-bit key, this would take longer than the age of the universe with current technology.
Key Sizes: 2048 vs 4096
2048-bit Keys
Currently considered secure and widely used. Recommended minimum for most applications.
- Fast generation and encryption/decryption
- Sufficient security for the next 10+ years
- Standard across most systems
4096-bit Keys
Even more secure, but slightly slower. Use for highly sensitive data with long-term requirements.
- Takes longer to generate and process
- Better protection against future threats
- Recommended for government/military use
How to Generate an RSA Key Pair
Generating an RSA key pair involves:
- Choose two large random prime numbers (p and q)
- Multiply them together: n = p × q
- Calculate the totient: φ(n) = (p-1) × (q-1)
- Choose a public exponent (e) – usually 65,537
- Calculate the private exponent (d) using the formula: d × e ≡ 1 (mod φ(n))
- Your public key = (n, e)
- Your private key = (n, d)
Don't worry about doing this manually! Cipherly's RSA generator does all this for you.
Best Practices for RSA Keys
- Keep your private key private – Never share it, ever. Not with friends, not with support.
- Back up your private key – If you lose it, all encrypted data becomes inaccessible.
- Use 2048-bit minimum – 1024-bit is considered broken and should not be used.
- Verify public keys – When receiving someone's public key, verify its authenticity.
- Rotate keys periodically – Generate new key pairs every few years for maximum security.
- Use a secure random generator – Keys must be generated with cryptographically secure randomness.
Common Mistakes to Avoid
❌ Using weak keys
Always generate keys with adequate length (2048-bit minimum).
❌ Storing private keys insecurely
Protect your private key like you'd protect a password to your bank account.
❌ Assuming one key pair works for everything
Use different key pairs for different purposes or systems when possible.
❌ Sharing your private key "just once"
Never share your private key, not even temporarily. Generate a new pair instead.
Generate Your First RSA Key Pair
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