21 April 2025

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RSA for Encryption and Signature

RSA

RSA is a cryptographic algorithm mainly used for two purposes.

• Encryption/Decryption
• Digital Signatures

Historically, HTTPS (TLS) used RSA for both key exchange and authentication. When you visit a website, server sends a digital certificate (often RSA-based) to prove its identity. Today, RSA is still used, but sometimes replaced by newer methods like ECDHE (Elliptic Curve Diffie-Hellman Ephemeral) for better security

Padding Schemes in RSA

Plain RSA encryption without padding is insecure. Padding adds randomness and structure to the message before encryption. It prevents attacks like replay attacks, chosen-plaintext attacks, and makes RSA safer. Padding in RSA = add randomness and structure to protect security

Padding is like adding a secret envelope around your letter before you seal and mail it — Without it, someone might guess what's inside just by the shape!

• PKCS#1 v1.5 for Encryption- Traditional padding for encryption/signature. Deprecated (insecure if not careful).
• RSAES-OAEP for Encryption - Adds randomness. Much stronger security. Standard for RSA encryption now
• PSS (Probabilistic Signature Scheme) - Padding for digital signatures. Very secure. Adds randomness. Standard for RSA signatures today
• No Padding (NOT recommended) - Raw RSA. Very dangerous — totally insecure

RSA Current Situation

• RSA Algorithm (basic idea) - Still in use
• Small RSA key sizes (less than 2048 bits) - Deprecated. Minimum 2048 bits recommended
• RSA-2048 / RSA-3072- Still commonly used
• RSA-4096 - Very strong, but slower
• Using RSA correctly (with OAEP or PSS and strong key sizes) is still safe and standard
• Old RSA methods (like PKCS#1 v1.5 without careful handling) are deprecated