Adding working doc explaining the encryption handling process (#3533)

ENCRYPTION_HANDLING.md

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+# Netmiko Encryption Handling
+
+This document describes the encryption mechanisms available in Netmiko for handling sensitive data in configuration files. These mechanisms are generally intended for use with `~/.netmiko.yml` and Netmiko Tools.
+
+## Overview
+
+Netmiko provides built-in encryption capabilities to secure sensitive data (like passwords) in your Netmiko Tools YAML configuration files. The encryption system is flexible and supports multiple encryption types.
+
+## Configuration
+
+### Basic Setup
+
+Encryption is configured in the `~/.netmiko.yml` file using the `__meta__` field:
+
+```yaml
+__meta__:
+  encryption: true
+  encryption_type: fernet  # or aes128
+```
+
+The two supported encryption types are:
+- `fernet` (recommended)
+- `aes128`
+
+### Encryption Key
+
+The encryption key is read from the environment variable `NETMIKO_TOOLS_KEY`. This should be a secure, randomly-generated key appropriate for the chosen encryption type.
+
+```bash
+# Example of setting the encryption key
+export NETMIKO_TOOLS_KEY="your-secure-key-here"
+```
+
+## Using Encryption
+
+### Encrypted Values in YAML
+
+When encryption is enabled, Netmiko looks for fields that start with `__encrypt__`. For example:
+
+```yaml
+arista1:
+  device_type: arista_eos
+  host: arista1.domain.com
+  username: pyclass
+  password: >
+    __encrypt__ifcs7SWOUER4m1K3ZEZYlw==:Z0FBQUFBQm5CQ9lrdV9BVS0xOWxYelF1Yml
+    zV3hBcnF4am1SWjRYNnVSRGdBb1FPVmJ2Q2EzX1RjTWxYMVVMdlBZSXVqYWVqUVNASXNRO
+    FBpR1MxRTkxN2J0NWxVeZNKT0E9PQ==
+```
+
+### Encryption Functions
+
+#### Encrypting Values
+
+To encrypt a value, use the `encrypt_value` function:
+
+```python
+def encrypt_value(value: str, key: bytes, encryption_type: str) -> str:
+    """
+    Encrypt a value using the specified encryption type.
+    
+    Args:
+        value: The string to encrypt
+        key: Encryption key as bytes
+        encryption_type: Either 'fernet' or 'aes128'
+    
+    Returns:
+        Encrypted string with '__encrypt__' prefix
+    """
+```
+
+#### Decrypting Values
+
+To decrypt a value, use the `decrypt_value` function:
+
+```python
+def decrypt_value(encrypted_value: str, key: bytes, encryption_type: str) -> str:
+    """
+    Decrypt a value using the specified encryption type.
+    
+    Args:
+        encrypted_value: The encrypted string (including '__encrypt__' prefix)
+        key: Encryption key as bytes
+        encryption_type: Either 'fernet' or 'aes128'
+    
+    Returns:
+        Decrypted string
+    """
+```
+
+#### Getting the Encryption Key
+
+To retrieve the encryption key from the environment:
+
+```python
+def get_encryption_key() -> bytes:
+    """
+    Retrieve the encryption key from NETMIKO_TOOLS_KEY environment variable.
+    
+    Returns:
+        Encryption key as bytes
+    """
+```
+
+## Example Usage
+
+Here's a complete example of how to use encryption in your code:
+
+```python
+from netmiko.encryption_handling import encrypt_value, get_encryption_key
+from netmiko.encryption_handling import decrypt_value
+
+# Get the encryption key from environment
+key = get_encryption_key()
+
+# Encrypt a password
+password = "my_secure_password"
+encrypted_password = encrypt_value(password, key, "fernet")
+
+# The encrypted password can now be stored in your YAML file
+# It will automatically be decrypted when Netmiko Tools reads the 
+# file (assuming you have properly set the '__meta__' fields
+```
+
+Alternatively, you can decrypt the value by calling
+
+```python
+clear_value = decrypt_value(encrypted_value, key, encryption_type="fernet)
+```
+
+Or you can create a simple function to decrypt all of the fields in the YAML
+file dynamically (by looking for any fields that start with `__encrypt__`).
+
+Netmiko's 'encryption_handling.py' implements this using the 'decrypt_config'
+function, but this function is a specific to Netmiko Tools' .netmiko.yml format 
+(i.e. it will need modified if you want to use it in a more generic context).
+
+## Implementation Notes
+
+1. Encryption is processed transparently when Netmiko Tools reads the YAML file
+2. Only fields prefixed with `__encrypt__` are processed for decryption
+3. The encryption type is determined by the `__meta__` section
+
+## Security Considerations
+
+1. Store the `NETMIKO_TOOLS_KEY` securely and never commit it to version control
+2. Fernet encryption is recommended over AES128 as it includes additional security features
+3. Encrypted values in YAML files should still be treated as sensitive data