RSA keysize limited fo 4096 with certbot for any reason?


Just out of curiosity - why is the RSA keysize limited to only 4096 bits? Is this a legal requirement? I tried to go a 8191 and was rejected. Why the limitation? Just out of curiosity.


It’s not a limitation of Certbot but of the Let’s Encrypt CA itself. It won’t sign any certificate with a RSA key length greater than 4096 bits.

I think the general consensus is that there’s no substantial security benefit over 4096 bits, and it makes the TLS handshake much slower.

There’s a number of CAs that do allow RSA keys that large, there’s no specific rule against it:


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