You don't generally need a certificate in addition to perform key exchange over email, though that is an option, because you either know and trust who you are manually exchanging keys with or the remote user is already trusted because you are both on the same server that is automating the exchange. On the web everything is anonymous so there is no trust without that certificate. If you really want one though modern email clients will allow you to create a personal certificate and digital signature to establish identity.
Key exchange isn't meant to provide identity. That is what signed certificates are for. The keys are only there to provide distributed encryption. Most modern email clients will allow you to generate a certificate for your emails with a digital signature.
Any website key retention is out of convention rather to ensure data access, and a convention Let's Encrypt is fast upending:
The certificate is valid for 90 days, during which renewal can take place at any time.
https://en.wikipedia.org/wiki/Let%27s_Encrypt
Key exchange is a component of PKI. You either need to know, or be able to request, a given key to validate it, or transmissions based on it.
TLS (as SSL before it) relies on certificate authorities (CAs), whose keys 1) are distributed to all major browsers and/or operating systems, and 2) which then vouch for website keys. Other mechanisms (e.g., key pinning) are also used. This model has many critics.
PGP (or Gnupg, and alternate implementation) rely, also to various degrees of general criticism, on a web of trust model, in which keys are signed by various other keys, possibly but not necessarily the user's own. Trust is ideally personally verified. It's been observed that this poses scaling issues.
Other PKI models have their own approaches to key trust. S/MIME uses CAs similarly to TLS/SSL, for key authentication, but keys themselves must still be retained, and secured, to assure future data access and integrity. SSH effectively relies on TOFU (trust on first use) or OOB (out of band) key distribution -- both of which effectively punt the process.
TLS, SSL, and SSH are all session-based (data-in-flight) protocols. PGP and S/MIME are storage-oriented, data-at-rest protocols. My initial comments apply to each.
https://en.wikipedia.org/wiki/Transport_Layer_Security#Algor...
Yes, the Let's Encrypt certificates are valid for only 90 days.
> whose keys 1) are distributed to all major browsers and/or operating systems
Not exactly. Root certificates are widely distributed to user-agent applications but the actual website certificates are available upon request from visiting the sites. A CA is an issuing authority for certificates. Certificates are not directly a component of encryption, but serve to validate the encryption is trusted.
* https://en.wikipedia.org/wiki/Root_certificate
* https://en.wikipedia.org/wiki/Certificate_authority
* https://en.wikipedia.org/wiki/Public_key_certificate
* certificate distribution schemes - https://www.ics.uci.edu/~keldefra/teaching/fall2016/uci_comp...
https://tools.ietf.org/html/rfc8446#section-4.4.2.2
If you have information otherwise please do us both the favour of quoting the relevant language from an authoritative source directly in your reply.
You're also creating a dispute based on a claim I've not made. CA keys being "distributed to all major browsers and/or operating systems", my claim, which you quote, does not preclude other distribution mechanisms. That discussion is unnecessary here.
Your understanding, generally, seems in error.
Again: TLS sever keys have no minimum duration requirement, as the Let's Encrypt description previously cited makes explicitly clear: "renewal can take place at any time".
I never said anything like that. Web certificates are around for as long as both the website and CA are willing to service them.
I am thinking you reading something more into what I wrote than the words on the screen.
Which was my point in the first place.
A distinct difference from the case of data-at-rest.
I am thinking you reading something more into what I wrote than the words on the screen.
I disagree, but suggest the same for yourself.
Encrypted email, and all data-at-rest encryption require long-term key retention and confidentiality. Loss of a key means loss of data. Disclosure of a key means loss of confidentiality.
TLS / data-in-flight is a tremendously simpler problem than email / data-at-rest.