Decode-AsusRouterConfig.ps1

<# 
.SYNOPSIS

 Decodes the .cfg file resulted from backing up the configuration of an Asus Router

.NOTES
 Author: Vlad Drumea (VladDBA)
 Website: https://vladdba.com/

 Based on this Bash script:
 https://github.com/billchaison/asus-router-decoder

.LINK
 https://github.com/VladDBA/Asus-Router-Config-Decoder

.EXAMPLE
 PS>.\Decode-AsusRouterConfig.ps1 '.\Settings_RT-AX86U Pro.CFG'

.EXAMPLE
 PS>.\Decode-AsusRouterConfig.ps1 'C:\Users\SomeUser\Documents\Settings_RT-AX86U Pro.CFG'

.EXAMPLE
 PS>.\Decode-AsusRouterConfig.ps1 'C:\Users\SomeUser\Documents\Settings_RT-AX86U Pro.CFG' -SkipHeaderCheck 

#>

[cmdletbinding()]
param (
    [Parameter(Position = 0, Mandatory = $true)]
    [string]$File,
    [Parameter(Position = 1, Mandatory = $false)]
    [switch]$SkipHeaderCheck = $false
)

if (-not (Test-Path $File)) {
    Write-Host " Provide the ASUS router config file name." -Fore Red
    exit
}
# Resolve file path in case of relative path
$File = Get-Item -Path $File | Select-Object -ExpandProperty FullName -ErrorAction Stop

$Size = (Get-Item $File).length

if ($Size -lt 10) {
    Write-Host " File size is too small." -Fore Red
    exit
}
try {
    $FileData = [System.IO.File]::ReadAllBytes($File) | ForEach-Object { "{0:x2}" -f $_ }
} catch {
    Write-Host " Cannot read file." -Fore Red
    Write-Host " Try providing the full file path."
    exit
}

if ($FileData.Count -ne $Size) {
    Write-Host " File read error." -Fore Red
    exit
} elseif ((($FileData[0] -ne "48") -or ($FileData[1] -ne "44") -or ($FileData[2] -ne "52") -or ($FileData[3] -ne "32")) -and ($SkipHeaderCheck -eq $false)) {
    Write-Host " File header check failed." -Fore Red
    exit
} else {

    $DataLength = "$($FileData[6])$($FileData[5])$($FileData[4])"
    $DataLength = [convert]::ToInt32($DataLength, 16)

    if ($DataLength -ne ($Size - 8)) {
        Write-Host " Data length check failed."
        exit
    } else {
        Write-Host " Configuration file appears to be valid."
    }
}

$Rand = [convert]::ToInt32($FileData[7], 16)
$i = 8
$DecodedBytes = New-Object System.Collections.Generic.List[byte]

Write-Host " Decoding configuration file..."
while ($i -lt $Size) {
    $CurrentByte = [convert]::ToInt32($FileData[$i], 16)
    if ($CurrentByte -gt 252) {
        if ($i -gt 8 -and $FileData[$i - 1] -ne "00") {
            $DecodedBytes.Add(0x00)
        }
    } else {
        $B = 0xff + $Rand - $CurrentByte
        # Ensure $B is within the valid range for a byte
        if ($B -lt 0) {
            $B = 0
        } elseif ($B -gt 255) {
            $B = 255
        }
        $DecodedBytes.Add([byte]$B)
    }
    $i++
}

# Convert decoded bytes to a string and replace the null character with a new line
$DecodedString = -join ($DecodedBytes | ForEach-Object { if ($_ -eq 0) { "`n" } else { [char]$_ } })

# Write the decoded string to the output file
$FileNameNoExt = [System.IO.Path]::GetFileNameWithoutExtension("$File")
$FilePath = [System.IO.Path]::GetDirectoryName("$File")
$DestName = $FileNameNoExt + "_Decoded.txt"
$OutputFile = Join-Path -Path $FilePath -ChildPath $DestName
$DecodedString | Out-File -FilePath "$OutputFile" -Encoding ascii -Force
Write-Host " ->Decoded configuration file has been saved to:"
Write-Host "   $OutputFile" -Fore Green
# Export dhcp_staticlist
$FoundInfo = Select-String -Path $OutputFile -Pattern 'dhcp_staticlist=.+'
$FoundInfo = $FoundInfo -replace ".+Decoded\.txt:[0-9]+:dhcp_staticlist=", ""
if ($FoundInfo.Length -gt 0) {
    Write-Host " Found DHCP client list"
    $Header = "        MAC       |      IP       |   HostName "
    $FoundInfo = $FoundInfo -replace "<", "`n" -replace ">>", " | " -replace ">", " | "
    $FoundInfo = "$Header$FoundInfo"
    $DestName = $FileNameNoExt + "_DHCP.txt"
    $DHCPFile = Join-Path -Path $FilePath -ChildPath $DestName
    
    $FoundInfo  | Out-File -FilePath "$DHCPFile" -Encoding ascii -Force
    Write-Host " ->DHCP client list has been saved to:"
    Write-Host "   $DHCPFile" -Fore Green    
}
# Retrieve admin username & password, and any configured SSID and password
Write-Host " ->Attempting to identify:`n    HTTP (admin) username & password`n    PPPOE credentials`n    SSIDs (Wi-Fi names)`n    WPA PSKs (Wi-Fi passwords)"
$FoundInfo = Select-String -Path $OutputFile -Pattern '_wpa_psk=.+|wl.*_ssid=.+|http_passwd=.+|http_username=.+|pppoe_passwd=.+|pppoe_username=.+' 

# Cleanup output for PS versions older than 7
Write-Host $("=" * 60) -Fore Green
$FoundInfo -replace ".+Decoded\.txt:[0-9]+:", ""
Write-Host $("=" * 60) -Fore Green