What is RSA?
RSA is the world's best-known asymmetric encryption algorithm, named after its inventors Ron Rivest, Adi Shamir and Leonard Adleman, who published it in 1977. It was the first practically usable algorithm that could both encrypt and sign with the same key pair, and it has carried a large part of the internet's security ever since.
The mathematics in one sentence
RSA's security rests on an asymmetry of effort: It is trivial to multiply two enormous prime numbers together, but practically impossible to run the process backwards and factor the result back into the two primes. The public key is built on the product; the private key on the primes. As long as factoring remains infeasible, the private key is safe. This also explains the key sizes: RSA keys must be large (today 2048 bits is the minimum and 3072/4096 bits are common), whereas modern elliptic curve cryptography achieves the same security level with keys of a few hundred bits.
What is RSA used for?
Two things, both everywhere in practice:
- Encryption/key transport: Data (typically a symmetric session key) is encrypted with the recipient's public key. In modern TLS, however, this role has been taken over by Diffie-Hellman-based key exchange, which provides forward secrecy.
- Digital signatures: The owner signs with their private key; anyone can verify with the public one. It is still RSA that sits behind a very large share of the world's TLS certificates, code signing and document signatures.
Because RSA is slow, it never encrypts the bulk data itself. It handles keys and signatures, while AES does the heavy lifting.
RSA's future: a phase-out in two stages
RSA is under pressure from two sides. In the short term from elliptic curves (ECC), which are faster and produce shorter keys and certificates. Many new systems choose ECC by default. In the longer term from quantum computers: Shor's algorithm can factor efficiently on a large quantum machine, and with that RSA falls entirely, regardless of key size. The answer is the quantum-resistant NIST standards (ML-KEM and ML-DSA), which are already being rolled out in hybrid configurations; see quantum cryptography and the threat of harvest now, decrypt later. The EU's roadmap sets 2030 as the deadline for critical infrastructure, so the phase-out of RSA is no longer theoretical. It is a planning question.
How MI Support IT can help
We map where RSA lives in your environment (certificates, VPN, signatures and integrations) through PKI management, and we plan the replacement in step with standards and vendors, as part of your IT security. You can also download our e-book PKI and HSM in practice, or contact us directly.