Let’s encrypt: the silent revolution of SSL certificates
If have ever bought a SSL certificate —in fact that is the old name, because now everything is TLS  — you will know that they have a cost and that cost is because one “trusted” organization places its “digital sign” in our certificate so that browsers, in turn, through this “trust chain”, accept this certificate.
And that is all about this SSL/TLS technology: trusting chain.
Asymmetric Cryptography: the shortest description ever
To understand why SSL/TLS is so important for today’s internet security and that characteristic “green” we see when we write https:// to access our favorite site, we have to understand what is Asymmetric Cryptography  and how it relates with what we mentioned before: “trusting chain”.
Shortly, asymmetric cryptography allows to generate a public certificate and a private key so that everything that is cyphered with the public certificate will only be available for decoding with the private key (which is the one installed at the server and never will get out of there, unless security breach).
On top of this mathematical cryptography pillar lays the TLS protocols  (evolved version of SSL), which provides a set of information exchange between the connecting client and the server so both parties can exchange information in a secure manner.
However, there is a “but” and it located in that part that talks about “exchanging information in a secure manner”.
The missing part to complete SSL/TLS: the trusting chain
The only thing that ensures SSL/TLS is that both parties, once completed the handshake, will be able to exchange messages without having to worry about a third party will have access to them as they transit.
However, the big problem to solve follows: how to ensure that we are talking with the server we want to and not another intercepting this communication?
Here is where the trust chain and Certificate Authorities that we all know enter, to name some: GeoTrust, Thawte, Verisign, Comodo…
What extra mile Certificate Authorities provides
With all these technical items identified, the missing piece to complete the puzzle are those companies and organizations that have reputation and due to agreements, they have managed to include their certificates –simplifying the process for the shake of clarify— into browsers so most of them recognizes them by default.
Because browsers accept and trust these certificates, everything that is signed by them will be also recognized and accepted without error.
What provides Let’s encrypt?
The foundational aim of the project is: free and secure certificates for all. But, without having to pay anything to legacy certificate authorities?
Yes. Then, where is the trick? There is no catch.
However, we have to understand its origin to better understand project’s purpose.
Let’s encrypt is an initiative backed by big companies in the tech filed that need their devices, intranets and management portals, etc, to have a certificate recognized by most of all browsers.
After all, what stop these companies to reach similar agreements with browsers’ vendors so their certificates are also supported?
Mixing a protocol to validate and deploy certificates, let’s encrypt not only provides certificates that are totally recognized and without costs: it also automates requesting and configuring certificate, freeing from this burden to system administrators.
Then, will certificate authorities disappear?
In our opinion, no. They will have to specialize to issue certificates that requires a new extra mile. At the same time they will keep issuing certificates for companies, entities and organizations. That is where Let’s Encrypt “do not want to go” (but they could).