# How SSL Certificates Work
What is SSL? well Secure Sockets Layer (SSL) is a security protocol that provides privacy, authentication, and integrity to Internet communications.
If you notice When you visit a website, you may notice a lock icon or a certificate in the browser. Every secure website has its own digital certificate.
### Basic Communication Over the Internet
When we communicate with a website over the Internet, there are mainly two components:
* **Client** (User/Browser)
* **Server** (Website/Application)
Normally, if we ignore security for understanding, the client simply sends a request (like a GET, POST... request) to the server, and the server sends back a response.
In this architecture, communication is not secure. WHY? HOW? WELL
When the client communicates with the server, an attacker can **sniff** (capture) the traffic and perform a **Man-in-the-Middle (MITM) attack**.
The attacker can read both:
* Requests
* Responses
*
So the first thing we want is **secure communication** right.
When we talk about security, the first thing that comes to mind is **encryption**.
Now The Questions arise is how we encrypt? well
There are two types of encryption:
1. **Symmetric Encryption**
2. **Asymmetric Encryption**
now we cannot use symettric encryption here bcz example:
### Why Not Use Symmetric Encryption Directly?
Now we cannot use Symmetric Encryption here reason example:
In symmetric encryption, both the client and server use the **same key** for encryption and decryption.
* Imagine the client wants to send sensitive data securely.
* The client encrypts the data using a secret key.
* Now the server also needs the same key to decrypt the data.
* So the client must somehow send the key to the server.
* But if the key is transmitted over the Internet, a hacker can capture it and use it to decrypt the data.
So the real problem becomes:
> **How do we securely transfer the secret key? that cannot leak**
### Asymmetric Encryption
In asymmetric encryption, we use two keys:
* Public Key
* Private Key
* The **Public Key** is used to encrypt data.
* The **Private Key** is used to decrypt data.
The public key can be shared with anyone, but the private key must remain secret.
### Secure Key Exchange Using Asymmetric Encryption
Now let's take an scenario where client have some data that data should transmitted to server securely:
The server has:
* A Public Key
* A Private Key
* The server sends its **public key** to the client.
* Even if a hacker captures this key, it is fine because it is meant to be public
* Now the client generates its own **symmetric key** (secret key).
* The client encrypts this secret key using the server’s public key.
* The encrypted secret key is sent to the server.
* Now suppose a hacker captures this encrypted key during transmission.
Can the hacker decrypt it?
NOOOOOOO
Because decrypting it requires the server’s **private key**, and only the server has that key.
* Server Decrypts the Secret Key: The server uses its private key to decrypt the received secret key.
Now:
* The client has the secret key
* The server also has the same secret key
At this point, both sides share the same symmetric key securely.
* Now the client and server use this shared symmetric key to encrypt and decrypt all communication.
* Even if someone sniffs the traffic, they cannot read the data because they do not have the secret key.
* So Questions is have we finally created secure communication?
### WELL NO, There Is Still a Problem
Hackers are very smart persons. A hacker can still perform a **Man-in-the-Middle (MITM) attack**.
HOW?
When the server sends its public key to the client, the hacker intercepts the communication and sends **their own public key** instead.
Now the client (user) thinks:
> “This is the server’s public key.”
So the client encrypts the secret key using the hacker’s public key and sends it.
The hacker can now:
1. Decrypt the secret key using their private key
2. Read all communication
3. Re-encrypt the data using the real server’s public key
4. Forward it to the server
Now the hacker sits between the client and server like a proxy.
#### Example
* Client sends data
* Hacker intercepts and decrypts it
* Hacker reads or modifies the data
* Hacker forwards it to the server
The same thing happens for responses from the server back to the client.
### So How Do We Prevent This?
Do we have way to prevent this? because user don't know server don't know when he requested the key from server user had no idea that someone is their sniffing and exchanged the key how could user or server know who's key is this
At this point This is where **SSL/TLS certificates** and **Certificate Authorities (CAs)** come into the picture.
The certificate proves that:
* The public key actually belongs to the real server
* The client can trust the server
Now server has
* Private Key
* Public Key
Using that server creates a SSL certificate to verify
We do not create trusted SSL certificates completely by ourselves, Instead, the certificate is signed by a trusted Certificate Authority (CA).
For example:
* Let’s Encrypt
The server asks Let’s Encrypt:
> Can you create an SSL certificate for our website?
Let’s Encrypt replies:
> Sure, but I need your public key.
* The server sends its public key to Let’s Encrypt.
* Now Let’s Encrypt becomes the **issuer** of the certificate.
Let’s Encrypt also has its own:
* Private Key
* Public Key
### Certificate Creation Process
Now Let’s Encrypt creates the certificate.
The certificate contains information such as:
* The domain name
* Example: `l1nuxkid.gitbook.io`
* The server’s public key
* Certificate validity details
* Information about the issuer (Let’s Encrypt)
* Letsencrypt will make a signature
* `signtature = server public key + letsencrypt public`
After that, Let’s Encrypt creates a **digital signature** for the certificate using its **private key**.
> The signature is not simply “server public key + Let’s Encrypt public key.”\
> Instead, the certificate data is mathematically signed using Let’s Encrypt’s private key.
This signature proves that:
> This certificate was verified and issued by Let’s Encrypt.
Finally, the completed and signed certificate is sent back to the server.
Now serevr has the SSL cert signed by the [LetsEncrypt](https://letsencrypt.org/).
### Secure Communication With SSL
The server sends its public key to the client.
Now you might think:
> What if the public key actually belongs to a hacker sitting in the middle and sniffing the traffic?
That is exactly the problem SSL certificates solve.
The certificate essentially says:
> Please verify that my public key actually belongs to this server.
How does user verify it? who builds the cert ([letsencrypt](https://letsencrypt.org/))
* Now user goes to letsencrypt and says can i get your public key?
* let's encrypt sends it
At this point client has 2 public keys
* Server public key
* Letsencrypt certificate issuer's public key
* now client takes the letencrypt public + server public and see the signatures is making matching the certificate.
* if the public key from hacker then signature will be different and not valid and we got to know and certificate is invalid
* And this is how it verify with SSL certificate
#### Secure Communication Begins
Once the certificate is verified:
1. The client trusts the server’s public key
2. The client generates a symmetric session key
3. The client encrypts this key using the server’s public key
4. The encrypted key is sent to the server
5. The server decrypts it using its private key
Now both the client and server share the same symmetric key.
From this point onward:
* All communication is encrypted using symmetric encryption
* Even if traffic is intercepted, it cannot be read
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