AES vs RSA Encryption: Which Should You Use?

BY TOOLS.FUN · MARCH 28, 2026 · 6 min read

Encryption is fundamental to modern security, but the two dominant algorithms — AES and RSA — solve different problems and are used in different contexts. Understanding the distinction between symmetric and asymmetric encryption, and how protocols like TLS use both together, makes you a more security-literate developer and helps you make better architectural decisions.

Related tools: use our DES Encryptor to understand legacy encryption, Hash Generator to see the difference between hashing and encryption, Base64 Encoder to encode ciphertext for safe transport, and Password Generator to create strong encryption keys.

Symmetric vs Asymmetric Encryption

Symmetric encryption uses the same key for both encryption and decryption. The key is a shared secret that both parties must possess before they can communicate. It's fast and efficient — suitable for encrypting large amounts of data. The challenge is key distribution: how do you securely share the key in the first place?

Asymmetric encryption uses a key pair: a public key and a private key. Data encrypted with the public key can only be decrypted with the corresponding private key. The public key can be freely distributed; the private key is kept secret. This elegantly solves the key distribution problem — but asymmetric encryption is much slower than symmetric encryption.

Key point: In practice, asymmetric encryption is rarely used to encrypt data directly. It's used to securely exchange a symmetric key, which then encrypts the actual data. This "hybrid" approach is how TLS, PGP, and most other secure protocols work.

How AES Works

AES (Advanced Encryption Standard) is a symmetric block cipher standardised by NIST in 2001, replacing the aging DES. It encrypts data in 128-bit blocks using a key of 128, 192, or 256 bits. AES-256 (256-bit key) is considered secure against all known attacks, including quantum computing attacks with Grover's algorithm (which effectively halves key strength, leaving AES-256 with ~128 bits of post-quantum security).

AES alone is a block cipher — it encrypts exactly one 128-bit block. To encrypt arbitrary-length data, it's combined with a mode of operation:

AES-CBC (Cipher Block Chaining) — each block is XOR'd with the previous ciphertext block before encryption. Requires an Initialisation Vector (IV). Vulnerable to padding oracle attacks if not implemented carefully.
AES-GCM (Galois/Counter Mode) — a modern authenticated encryption mode. Provides both confidentiality and integrity (detects tampering). The recommended default for new systems.
AES-CTR (Counter Mode) — turns AES into a stream cipher. Fast and parallelisable, but requires a unique nonce per message.

# AES-256-GCM in Python (using cryptography library)
from cryptography.hazmat.primitives.ciphers.aead import AESGCM
import os

key = os.urandom(32)          # 256-bit key
nonce = os.urandom(12)        # 96-bit nonce for GCM
aesgcm = AESGCM(key)
ciphertext = aesgcm.encrypt(nonce, b"plaintext data", None)
plaintext = aesgcm.decrypt(nonce, ciphertext, None)

Key point: Never reuse a nonce/IV with the same key in AES-GCM. Nonce reuse catastrophically breaks the confidentiality of the cipher. Generate a fresh random nonce for every encryption operation.

How RSA Works

RSA (Rivest–Shamir–Adleman) is an asymmetric encryption algorithm based on the mathematical difficulty of factoring the product of two large prime numbers. The public key consists of the modulus n (the product of two large primes) and a public exponent e. The private key includes the factored primes, allowing computation of the modular inverse.

RSA key sizes are measured in bits and must be much larger than AES keys to achieve equivalent security, because the underlying mathematical problem (integer factorisation) is easier to attack than the brute-force exhaustion that threatens AES:

RSA-1024 — broken/deprecated. Do not use.
RSA-2048 — currently acceptable minimum for most uses.
RSA-4096 — recommended for long-lived keys (certificate authorities).

RSA encryption is typically used with OAEP (Optimal Asymmetric Encryption Padding) for confidentiality, or PSS (Probabilistic Signature Scheme) for digital signatures.

AES vs RSA: Performance

The performance difference between AES and RSA is dramatic. On modern hardware:

AES-256 (AES-NI hardware acceleration) — 1–4 GB/s throughput on a single core
RSA-2048 encryption — roughly 10,000–50,000 operations per second per core — adequate for key exchange, completely impractical for bulk data

This is why nobody encrypts large files with RSA directly. RSA is used to encrypt a small symmetric key (32 bytes for AES-256), not the data itself.

How TLS Uses Both

TLS (Transport Layer Security — the protocol behind HTTPS) demonstrates the optimal use of both algorithms:

The server presents its RSA (or ECDSA) certificate containing its public key.
The client and server perform a key exchange (RSA key transport, or more commonly today, Diffie-Hellman key agreement) to establish a shared symmetric key without transmitting it over the network.
All subsequent communication is encrypted with AES-GCM using the negotiated symmetric key.

Modern TLS 1.3 mandates forward secrecy via ephemeral Diffie-Hellman key exchange, meaning that even if the server's private key is later compromised, past sessions can't be decrypted.

Choosing the Right Algorithm

Encrypting data at rest (files, database fields, backups) → AES-256-GCM
Encrypting data in transit → TLS 1.3 (which uses both AES and DH under the hood)
Encrypting a small secret for a specific recipient → RSA-OAEP with RSA-2048+ or elliptic curve equivalent (ECIES)
Signing a document or code → RSA-PSS or ECDSA with SHA-256

Key point: Elliptic Curve Cryptography (ECC) provides equivalent security to RSA at much smaller key sizes — a 256-bit ECC key is roughly equivalent to a 3072-bit RSA key. ECDSA and ECDH are increasingly preferred over RSA for new systems.

Key Size Recommendations

NIST's current recommendations for security through 2030 and beyond: AES-128 minimum (AES-256 preferred), RSA-2048 minimum (RSA-3072 preferred for new systems), and 256-bit elliptic curves (P-256 or higher).

Test Encryption Online

Experiment with AES encryption and RSA encryption at Tools.Fun — encrypt and decrypt text in your browser to understand how these algorithms work in practice. Both tools run entirely client-side, so your data never leaves your machine.

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