What Is RSA Encryption?

RSA is the asymmetric algorithm that made practical public-key cryptography possible: a key pair where one key encrypts and only the other decrypts, removing the need to share a secret in advance. It underpins a great deal of the certificate infrastructure still in use.

It is also slow, increasingly displaced by elliptic-curve alternatives, and the primary casualty of quantum computing.

What it is actually used for

Almost never for bulk data. RSA operations are orders of magnitude slower than symmetric encryption and are limited to messages smaller than the key, so it is used for two things: establishing a symmetric key that then does the real work, and producing digital signatures that prove origin and integrity.

Signing is arguably the more important role today. Certificate chains, code signing and document signing all rest on the asymmetric property that only the holder of the private key could have produced a given signature.

Where implementations go wrong

Key length is the visible parameter and the least interesting failure. 1024-bit keys are no longer acceptable and 2048 is the practical minimum, but weak keys are not where most incidents originate.

Padding is. Textbook RSA — the raw operation with no padding — is insecure, and several historically important attacks target flawed or legacy padding schemes rather than the algorithm. Modern padding constructions exist precisely to close these, and using a library default from a current version is the correct approach.

Key generation is the other recurring failure. Keys produced with insufficient entropy — notably on embedded devices generating keys at first boot before the system has gathered randomness — have been found to share factors across devices, which breaks them entirely regardless of length.

Elliptic curve and the direction of travel

Elliptic-curve cryptography achieves comparable security with far smaller keys and faster operations, which matters for mobile, embedded and high-volume systems. New deployments generally default to it, and RSA persists largely through existing certificate estates and compatibility requirements.

Both are equally exposed to quantum computing. A sufficiently capable machine breaks RSA and elliptic curve alike, which is why post-quantum migration concerns key exchange and signatures specifically — and why data encrypted today under either can be captured now for decryption later. See quantum computing threats.

Signatures are where it matters most now

As key exchange shifts toward elliptic curve and eventually post-quantum alternatives, the durable role of asymmetric cryptography in most estates is signing — certificate chains, code signing, software update verification and document signatures.

That makes private key protection the operational priority. A signing key that can be extracted allows an attacker to produce artefacts your systems will trust implicitly, and software update mechanisms are a particularly valuable target because they distribute to every endpoint automatically.

Signing keys belong in hardware security modules or equivalent protected storage, with signing performed as a service rather than by handing the key to a build pipeline. A signing key sitting in a CI system is a key held by everyone who can modify that pipeline.

Where to start

Inventory your certificates and their key sizes, and confirm renewal is automated. Anything still on 1024-bit keys needs replacing, and anything renewed manually will eventually be renewed late.

See encryption in practice. GuardsArm runs security risk assessments.