Authenticating AT command usage

nRF9160 AT Commands

The %XSUDO command is used to authenticate AT commands.

Before you start the authentication, perform the following two steps (only once):

Generate private and public keys with OpenSSL:

openssl ecparam -name prime256v1 -genkey -noout -out [private key PEM file]
openssl ec -in [private key PEM file] -out [public key PEM file] -pubout

Write the public key with the AT command:
```
AT%XPMNG=0,"<public key>"
OK
```

To authenticate an AT command, perform the following steps:

Calculate an AT command signature.
1. Create an AT command text file for an authenticated AT command:
  Example:
```
%CMNG=0,1,0,"TEST ROOT CERTIFICATE"
```
2. Create a digest file with OpenSSL from AT command that needs authentication:
```
openssl sha256 -binary [AT command text file] > [digest file]
```
3. Create a signature file with OpenSSL from the digest file:
```
openssl pkeyutl -sign -in [digest file] -out [signature file] -inkey [private key PEM file]
```
4. Convert the signature to Base64 format:
```
base64 < [signature file] > [signature base64 file]
```
Write the authenticated AT command.
The %XSUDO command is used to authenticate the %CMNG command:
Example:
```
AT%XSUDO=35,"<signature base64>";%CMNG=0,1,0,"TEST ROOT CERTIFICATE"
OK
```
For more information on the command, see Authenticated access %XSUDO.