Definition
RSA is a public-key cryptosystem published in 1977 by Ron Rivest, Adi Shamir, and Leonard Adleman, building on the Diffie-Hellman framework. It bases its security on the difficulty of factoring the product of two large prime numbers. RSA can be used both for encryption (encrypting data with a public key, decrypting with the matching private key) and for digital signatures (signing data with the private key, verifying with the public key).
RSA in VPNs
RSA's primary use in VPNs is signing and certificate-issuing. The VPN provider's Certificate Authority signs each server's certificate with an RSA private key; clients verify that signature with the matching public key embedded in the app. RSA was also the original key-exchange method in TLS 1.0 through 1.2, but this 'RSA key transport' lacked Forward Secrecy and has been removed in TLS 1.3 — replaced by ephemeral Diffie-Hellman, which RSA is still used to authenticate.
Migration to ECC
RSA's security depends on key size: 2048-bit RSA is roughly equivalent to 112-bit symmetric strength, 3072-bit RSA to 128-bit. Equivalent strength in ECC requires only 256-bit keys. The size difference matters for handshake overhead, certificate chains, and performance. Most new VPN deployments use ECC certificates (typically NIST P-256 or P-384) and reserve RSA for backward compatibility.