RSA is a public-key algorithm, named after Rivest, Shamir and Adleman, that relies on the difficulty of factoring the product of two large primes. It can sign data with a private key so anyone with the public key can verify it, and can encrypt data to a public key.
Why it matters
Most TLS certificates issued over the past two decades contain RSA keys, and many still do. Today the usual minimum is 2048 bits; 3072 and 4096 are common choices when you want more margin, at the cost of slower handshakes. In TLS 1.3, RSA is no longer used to transport the session key, which was the weakness that denied forward secrecy in older setups. It remains in use only for signatures, such as proving a server owns its certificate.
Example
openssl genrsa -out key.pem 2048
openssl x509 -in cert.pem -noout -text | grep "Public-Key"The second command prints something like Public-Key: (2048 bit) so you can confirm what a certificate actually uses.
Common mistakes
- Using 1024-bit keys, which public CAs no longer accept.
- Confusing key size with certificate signature algorithm. A 2048-bit key can still be signed with SHA-256 or something weaker.
- Reusing one private key across many servers, which widens the damage if it leaks.
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