Crypto & Security · Ch. 4
RSA for Beginners: Which Key Encrypts and Which Key Signs?
WebTool Team · Published 2026-09-08 · RSA / Asymmetric Encryption / Digital Signatures / Security
RSA uses a mathematically linked key pair: the public key is shared, the private key is kept secret. What the public key encrypts, only the private key can decrypt (confidentiality); what the private key signs, anyone can verify with the public key (identity and non-repudiation). The phrase "encrypt with the private key, decrypt with the public key" is inaccurate — that's called signing. This site offers an online RSA tool; key generation and all computation happen locally in your browser.
Don't mix up the two paths
| Goal | Operation | Who can do it | What it proves |
|---|---|---|---|
| Confidential transfer | Public-key encrypt, private-key decrypt | Anyone can encrypt | Only the private-key holder can read it |
| Identity / signing | Private-key sign, public-key verify | Only the private-key holder | It genuinely came from the key holder |
Signing actually applies the private-key operation to the message's hash, not the full text. So JWT's RS256 is essentially SHA-256 hashing + RSA private-key signing.
Why real systems use hybrid encryption
RSA has two hard limits: it's slow (orders of magnitude slower than AES), and a single operation can encrypt less data than the key length (a 2048-bit key with OAEP padding encrypts only about 190 bytes). So real systems (HTTPS, PGP) all do this:
- Generate a random AES symmetric key;
- Encrypt the message body with AES (fast, no length limit);
- Encrypt that AES key with the RSA public key (short enough for one operation);
- The recipient uses their private key to recover the AES key, then decrypts the body.
Engineering choices
- Key length: at least 2048 bits for new systems; 1024-bit is no longer recommended.
- Padding: use OAEP for encryption and PSS for signatures; textbook "raw RSA" and the older PKCS#1 v1.5 encryption padding both have known attacks.
- When to use ECC: at equivalent security, ECC (e.g. P-256) has shorter keys and faster operations — worth preferring in new systems; RSA wins on compatibility and ecosystem.
Last updated: 2026-09-08