Encryption standards: AES, RSA and elliptic curves, and when each is used

Almost every secure system you use, from the padlock in your browser to an encrypted disk, relies on a small set of standardized algorithms. You do not need the mathematics to use them well, but you do need to know which job each one does. This lesson covers the three names you will meet most often: AES, RSA and elliptic-curve cryptography (ECC). For the underlying ideas of keys and one-way functions, the course Cryptography you can rely on goes further.

Symmetric encryption: AES does the heavy lifting

In symmetric encryption, the same secret key locks and unlocks the data. The standard today is the Advanced Encryption Standard (AES), published by the US National Institute of Standards and Technology (NIST) as FIPS 197. AES encrypts data in 128-bit blocks, with keys of 128, 192 or 256 bits. It is fast, it is built into the processors of modern computers and phones, and it is what actually scrambles the bulk of your data: the files on an encrypted disk, the body of a web page sent over HTTPS, a message in a secure messenger.

The weakness of symmetric encryption is not the algorithm. It is the key. Both sides need the same secret, and sending that secret across an open network would expose it to anyone listening. If you encrypt a file with a strong key and then email the key alongside it, the encryption protects nothing. That distribution problem is what public-key cryptography solves.

Asymmetric encryption: RSA and ECC solve key exchange

Public-key (asymmetric) cryptography uses a pair of linked keys. The public key can be shared with anyone; the private key never leaves its owner. Data encrypted with the public key can only be decrypted with the private key, and a signature made with the private key can be checked by anyone holding the public key. This is how two computers that have never met can agree on a shared secret over the internet, and how your Mac can verify that a software update really comes from its developer.

RSA, named after its inventors Rivest, Shamir and Adleman, has been used since the late 1970s. Its security rests on how hard it is to factor very large numbers, which is why RSA keys are long: 2,048 bits is the usual minimum today. Elliptic-curve cryptography reaches comparable strength with much shorter keys. NIST’s key-management guidance (SP 800-57) lists a 256-bit elliptic-curve key as roughly equivalent to a 3,072-bit RSA key. Shorter keys mean faster handshakes and less computing power, which is why ECC now dominates in TLS, messaging apps and hardware security chips. NIST’s Digital Signature Standard, FIPS 186-5, covers both RSA and elliptic-curve signatures.

Why real systems use both

Public-key operations are slow compared with AES, so no one encrypts a whole video with RSA. Instead, systems use a hybrid design. When your browser connects to a website, the two sides use public-key cryptography (usually an elliptic-curve key exchange) to agree on a fresh symmetric key, then switch to AES or a similar cipher for everything that follows. The same pattern appears in encrypted email, in disk encryption that protects its key with your password, and in messaging apps. Public-key cryptography sets up trust and shares the key; symmetric cryptography protects the data.

Choosing well: what “strong” means in practice

• Use standard, widely reviewed algorithms. A home-made cipher, or a product that boasts about its “proprietary encryption”, is a warning sign, not a feature.
• Key length matters, but only within the same family. A 256-bit AES key and a 256-bit elliptic-curve key are not directly comparable, and neither is directly comparable to a 2,048-bit RSA key.
• Most failures are implementation failures: keys stored next to the data they protect, weak random numbers, outdated protocol versions, or encryption that is simply switched off.
• Plan for change. Quantum computers large enough to break RSA and ECC do not exist today, but NIST has already standardized replacement algorithms; the module on emerging threats covers them.