The issue you're encountering is related to trying to decode raw bytes as a UTF-8 string, but the bytes you have might not correspond to valid UTF-8 data (possibly due to binary data, padding, or other characters that don't map directly to printable text).

To address this:

### 1. **Check the Raw Bytes Format:**
   The data you're dealing with might not be a simple UTF-8 string. It could be some kind of binary data, like a file or encrypted content. Since the original input was a text file, the raw data after decryption might not directly convert into a valid UTF-8 string.

### 2. **Hex Dump:**
   You can first print the bytes as a hex dump to inspect their content. This way, you can identify if there's any padding or non-printable characters.

### 3. **Try Different Decoding Methods:**
   If the data is supposed to be a text file, but you suspect that some part of it is corrupted or includes special characters, you could attempt other encoding types (e.g., `latin-1`, `ascii`, etc.) or try ignoring errors.

### Steps to Fix:

1. **Hex Dump**: Let’s start by printing the raw data as a hex dump to inspect its structure:

```python
# Print the raw bytes as hex to inspect
print("Raw decrypted data (hex):", m_bytes.hex())
```

2. **Try Different Decoding Methods**: If it still seems to be a string but contains non-UTF-8 characters, you can attempt decoding with the `errors='ignore'` option, which skips invalid characters:

```python
# Attempt to decode with error handling to skip invalid characters
try:
    print("Decrypted message:", m_bytes.decode('utf-8', errors='ignore'))
except Exception as e:
    print(f"Error decoding: {e}")
```

3. **Save to File**: If it seems like the decrypted data is binary (like a file or structured data), you can save it to a file and manually inspect it.

```python
# Save the decrypted bytes to a file to inspect later
with open("decrypted_output.bin", "wb") as f:
    f.write(m_bytes)
```

### Next Steps Based on Output:
- If the `hex()` output looks like valid binary data (e.g., it starts with a recognizable header), you may need to treat the output as a binary file instead of text.
- If you still get unreadable characters, there may be additional padding or other encryption-related artifacts in the output. Depending on the encryption method and how the original file was padded, you might need to strip or process those characters.

Let me know what you find from the hex dump, and we can refine the next steps based on that!