What is RSA?

RSA (Rivest–Shamir–Adleman) is a public-key cryptographic algorithm that uses a mathematically linked public and private key pair to encrypt data, exchange keys, and sign digital certificates. Its security rests on the difficulty of factoring the product of two large prime numbers, a problem that remains computationally infeasible at practical key sizes.

Key takeaways

• RSA encrypts data and signs certificates with paired public and private keys.
• Security relies on the difficulty of factoring large prime-number products.
• RSA was published in 1977 by Rivest, Shamir, and Adleman at MIT.
• Versa’s Secure Access Service offers RSA as a TLS key-exchange option.
• Larger key sizes (2048 bits and up) trade speed for stronger security.

On this page: Why it matters · How it works · Types · RSA & Versa · FAQ

Why is RSA important?

RSA was the first practical public-key cryptosystem, published in 1977 by Ron Rivest, Adi Shamir, and Leonard Adleman at MIT. Before it, every scheme required both parties to share a secret key in advance – impractical for internet-style communication at scale. RSA solved that by splitting the key into a public half anyone can use to encrypt or verify, and a private half only the owner holds to decrypt or sign. HTTPS certificates, SSH host keys, code signing, and email encryption trace back to that idea.

RSA still matters even as elliptic-curve algorithms take over new deployments, because the installed base is enormous. Certificate authorities still issue RSA certificates, older clients often support nothing else, and RSA signatures remain a default option in TLS and PKI tooling.

What is RSA?

RSA generates two keys from one mathematical construction. Key generation picks two large, random primes, multiplies them into a modulus, and derives a public and matching private exponent using modular arithmetic. The public key can be shared openly; the private key must stay secret.

Encryption and signing use the keys in opposite directions. To send a confidential message, the sender encrypts it with the recipient’s public key; only the matching private key can decrypt it. To prove authorship, the signer encrypts a hash of the message with their own private key; anyone can verify it using the signer’s public key, confirming identity and integrity.

Security depends on integer factorization: deriving the private key requires factoring the modulus into its two prime factors, and no efficient classical algorithm does that at practical key sizes – typically 2048 bits or larger.

Types of RSA usage

RSA appears in a network in three distinct roles. RSA key transport – loosely called RSA key exchange – lets a TLS client encrypt a premaster secret with the server’s RSA public key, which only the server’s private key can recover. This mode is static, with no forward secrecy, which is why TLS 1.3 dropped it for Diffie-Hellman variants, though it remains available in earlier versions. RSA digital signatures authenticate certificates and handshakes: a certificate authority signs a server’s public key with its own RSA private key, and clients verify the signature to trust it. RSA encryption of bulk data is rare in practice, so RSA typically wraps a symmetric key instead.

Deployments also vary by key size and padding: 2048 to 4096 bits, since smaller sizes are too weak against factoring attacks, and OAEP or PSS padding rather than the older, oracle-vulnerable PKCS#1 v1.5 scheme.

RSA & Versa

Versa’s Secure Access Service – the portal-and-gateway service that connects the Versa SASE client – lets administrators select RSA as a TLS key-exchange algorithm, alongside ECDHE, for connections negotiating TLS versions earlier than 1.3, with a configurable minimum key length. When configuring the secure access gateway, administrators choose TLS/DTLS cipher suites, including RSA key-transport suites such as TLS-RSA-WITH-AES-256-GCM-SHA384 and RSA-signed ECDHE suites such as TLS-ECDHE-RSA-WITH-AES-256-GCM-SHA384.

RSA also underpins certificate-based authentication in the same service: the secure access portal and gateway support Certificate, EAP, and Preshared client authentication types, and device authentication profiles rely on RSA-signed certificates to verify SASE client devices. The Secure SD-WAN and NGFW editions expose the same controls.

Frequently asked questions

What does RSA stand for?

RSA stands for Rivest–Shamir–Adleman, the surnames of Ron Rivest, Adi Shamir, and Leonard Adleman, who published the algorithm in 1977 at MIT – the first practical public-key cryptosystem, and still one of the most widely deployed algorithms in TLS and PKI.

How does RSA encryption work?

RSA uses two mathematically linked keys derived from two large prime numbers. Data encrypted with the public key can only be decrypted with the matching private key, and data signed with the private key can be verified with the public key – letting strangers exchange secrets safely.

Is RSA still secure today?

Yes, at sufficient key sizes. A 2048-bit RSA key is still considered secure against classical computers, though guidance increasingly favors 3072 bits for longer-term protection. A future quantum computer could break RSA via Shor’s algorithm, which is driving research into post-quantum alternatives.

What is the difference between RSA and ECDHE?

RSA key transport lets a client encrypt a secret with the server’s static public key, while ECDHE (Elliptic-Curve Diffie-Hellman Ephemeral) has both sides generate temporary key pairs and derive a shared secret. ECDHE provides forward secrecy, which static RSA key transport cannot.

What RSA key size should be used?

2048 bits is the practical minimum for new deployments; 3072 or 4096 bits adds a stronger security margin at the cost of slower operations and larger certificates. Keys below 2048 bits are considered breakable and should be retired wherever still in use.

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