AES vs RSA — Symmetric vs Asymmetric Encryption
AES and RSA are both widely used encryption algorithms, but they serve completely different purposes and are almost always used together in practice. AES is a symmetric cipher optimized for encrypting large amounts of data quickly. RSA is an asymmetric algorithm used to securely exchange keys or authenticate identities. TLS (HTTPS) uses RSA (or ECDH) for the key exchange and then AES for the bulk data encryption.
Comparison Table
| Aspect | AES | RSA |
|---|---|---|
| Algorithm type | Symmetric — same key for encrypt and decrypt | Asymmetric — public key encrypts, private key decrypts (or vice versa) |
| Key sharing problem | Requires secure key exchange; both parties need the secret | Public key can be shared freely; private key stays secret |
| Speed | Very fast; hardware-accelerated on modern CPUs (AES-NI) | Slow for large data; 1000x slower than AES for bulk data |
| Key sizes | 128, 192, or 256 bits | 2048, 4096 bits (recommended) |
| Use case | Bulk data encryption; files, database columns, TLS bulk data | Key exchange, digital signatures, authentication |
| Quantum resistance | AES-256 is considered quantum-resistant | Broken by quantum computers (Shor's algorithm) |
When to Use AES
AES is the right choice for encrypting data — files, database fields, backups, message payloads. Its speed makes it practical for encrypting gigabytes of data. Use AES-256-GCM for authenticated encryption that simultaneously encrypts and detects tampering. The challenge with symmetric encryption is key distribution — you need a secure way to share the key with the recipient, which is where RSA comes in.
When to Use RSA
RSA is used for key exchange and signatures, not for bulk data encryption. In practice, you use RSA to securely send an AES key to a recipient (hybrid encryption), or to sign a document/certificate with your private key so others can verify it with your public key. RSA is also the basis of TLS certificate authentication and SSH key authentication.
Convert Between AES and RSA
Encrypt and decrypt text with AES-GCM using a password. Runs entirely in your browser.
Generate RSA key pairs (2048 or 4096-bit) and export as PEM in your browser.
Paste a PEM private or public key to detect its type, algorithm, and key size.
Paste a PEM certificate and decode its subject, issuer, validity dates, SANs, and more.
FAQ
- Why does TLS use both RSA and AES?
- RSA (or more commonly ECDH today) solves the key exchange problem: the client and server negotiate a shared AES key over the asymmetric channel. Then AES encrypts the actual HTTP traffic. This hybrid approach gets the best of both: secure key exchange from asymmetric crypto, high performance from symmetric crypto.
- Is AES-128 or AES-256 better?
- Both are considered secure for current threats. AES-256 provides a larger security margin against future attacks (including hypothetical quantum attacks on symmetric keys requiring 2^128 vs 2^64 operations). AES-128 is faster. For high-security applications and long-term data, AES-256 is preferred.
- What will replace RSA as quantum computers advance?
- NIST standardized post-quantum cryptography algorithms in 2024: CRYSTALS-Kyber (ML-KEM) for key encapsulation and CRYSTALS-Dilithium (ML-DSA) for digital signatures. These lattice-based algorithms are secure against quantum computers and will eventually replace RSA and ECDSA in TLS and other protocols.