# SSL/TLS Certificates

X.509 is an ITU standard defining the format of public key certificates. X.509 are used in TLS/SSL, which is the basis for HTTPS. An X.509 certificate binds an identity to a public key using a digital signature. A certificate contains an identity (hostname, organization, etc.) and a public key (RSA, DSA, ECDSA, ed25519, etc.), and is either signed by a Certificate Authority or is Self-Signed.

### Self-Signed Certificates

#### Generate CA

1. Generate RSA

```bash
openssl genrsa -aes256 -out ca-key.pem 4096
```

2. Generate a public CA Cert

```bash
openssl req -new -x509 -sha256 -days 3650 -key ca-key.pem -out ca.pem
```

#### Optional Stage: View Certificate's Content

```bash
openssl x509 -in ca.pem -text
openssl x509 -in ca.pem -purpose -noout -text
```

#### Generate Certificate

1. Create a RSA key

```bash
openssl genrsa -out cert-key.pem 4096
```

2. Create a Certificate Signing Request (CSR)

```bash
openssl req -new -sha256 -subj "/CN=exploit.se" -key cert-key.pem -out cert.csr
```

3. Create a `extfile` with all the alternative names

```bash
echo "subjectAltName=DNS:exploit.se,IP:10.10.10.10" >> extfile.cnf
```

```bash
# optional
echo extendedKeyUsage = serverAuth >> extfile.cnf
```

4. Create the certificate

```bash
openssl x509 -req -sha256 -days 3650 -in cert.csr -CA ca.pem -CAkey ca-key.pem -out cert.pem -extfile extfile.cnf -CAcreateserial
```

### Certificate Formats

X.509 Certificates exist in Base64 Formats **PEM (.pem, .crt, .ca-bundle)**, **PKCS#7 (.p7b, p7s)** and Binary Formats **DER (.der, .cer)**, **PKCS#12 (.pfx, p12)**.

#### Convert Certs

| COMMAND                                                | CONVERSION |
| ------------------------------------------------------ | ---------- |
| `openssl x509 -outform der -in cert.pem -out cert.der` | PEM to DER |
| `openssl x509 -inform der -in cert.der -out cert.pem`  | DER to PEM |
| `openssl pkcs12 -in cert.pfx -out cert.pem -nodes`     | PFX to PEM |

### Verify Certificates

`openssl verify -CAfile ca.pem -verbose cert.pem`

### Install the CA Cert as a trusted root CA

#### On Debian & Derivatives

* Move the CA certificate (`ca.pem`) into `/usr/local/share/ca-certificates/ca.crt`.
* Update the Cert Store with:

```bash
sudo update-ca-certificates
```

Refer the documentation [here](https://wiki.debian.org/Self-Signed_Certificate) and [here.](https://manpages.debian.org/buster/ca-certificates/update-ca-certificates.8.en.html)

#### On Windows

Assuming the path to your generated CA certificate as `C:\ca.pem`, run:

```powershell
Import-Certificate -FilePath "C:\ca.pem" -CertStoreLocation Cert:\LocalMachine\Root
```

* Set `-CertStoreLocation` to `Cert:\CurrentUser\Root` in case you want to trust certificates only for the logged in user.

OR

In Command Prompt, run:

```sh
certutil.exe -addstore root C:\ca.pem
```

* `certutil.exe` is a built-in tool (classic `System32` one) and adds a system-wide trust anchor.

#### On Android

The exact steps vary device-to-device, but here is a generalized guide:

1. Open Phone Settings
2. Locate `Encryption and Credentials` section. It is generally found under `Settings > Security > Encryption and Credentials`
3. Choose `Install a certificate`
4. Choose `CA Certificate`
5. Locate the certificate file `ca.pem` on your SD Card/Internal Storage using the file manager.
6. Select to load it.
7. Done!