Paid and free SSL certificates perform the same task — they protect the connection between the site (server) and the browser of the site visitor (client), making it impossible for third parties to intercept sensitive data entered by the client on the site. The presence of an SSL certificate on a site is a mandatory criterion for site administration, and starting in 2020, Google search gives preference to sites with SSL, and Chromium-based browsers highlight warnings for visitors visiting unsecured sites.
The main and most important criterion — At the end of September 2021, the DST Root CA X3 certificate from the Let's Encrypt certificate authority, which provides free SSL certificates, ceased to be valid, as a result of which millions of sites stopped opening for users using outdated operating systems and gadgets that have long been have not received security updates. When talking about outdated, it is customary to mention such operating systems as Windows XP, 7, 8, but Windows 10 is also currently considered outdated, and it can also have problems accessing sites that use free SSL. There are much more gadgets and OSes that are included in the list of obsolete, we recommend that you read this and this article on this topic to understand the details. Having a paid SSL certificate on the site completely solves this difficulty.
A paid SSL certificate has a higher level of trust, security and guarantees, and paid SSL certificates with DV verification type can be issued only after manual verification of the site and business by the certificate authority. Information about the SSL used on the site is public, any visitor can see which certificate is used on the site by simply clicking on the lock icon in the address bar of the browser and decide whether to trust the site with their data or not.
There is a concept of wildcard SSL certificates, after purchasing such a certificate, its effect will be extended not only to the main domain, but also to all its subdomains, which is convenient for sites with a large number of subdomains, since it is enough to purchase one certificate that will work for all subdomains at the same time, without the need control the SSL status and expiration of each individual subdomain. Free SSL certificate does not support wildcard.
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