Windows Script to brute force a GnuPG secret key (basic, slow, single-threaded) (a-z0-9 only)

brute-gpg.ps1

# Download GnuPG through GPG4WIN: https://www.gpg4win.org
# Add "C:\Program Files (x86)\GnuPG\bin" to System PATH variable
# Before running this script, do gpg-agent --allow-loopback-pinentry
# If it says "gpg-agent running and available" do "gpgconf --kill gpg-agent" and try again

param (
    [string]$KeyID,        # GPG Key ID to brute force
    [int]$Digits = 8,      # Number of digits in passphrase (default 3)
    [long]$StartIndex = 0  # Resume point (default 0)
)

$chars = @((0..9) | ForEach-Object { [string]$_ }) + @([char[]]"abcdefghijklmnopqrstuvwxyz")
function Convert-ToBase36Lower {
    param($num, [int]$length)
    $res = ""
    for ($i = 0; $i -lt $length; $i++) {
        $digit = [int]($num % 36)
        $res = $chars[$digit] + $res
        $num = [math]::Floor($num / 36)
    }
    return $res
}

$max = 1
for ($i = 0; $i -lt $Digits; $i++) { $max *= 36 }
$max -= 1

if ($StartIndex -lt 0 -or $StartIndex -gt $max) {
    Write-Error "StartIndex out of range (0..$max)"
    exit 1
}

$ret = 1
for ($idx = $StartIndex; $idx -le $max; $idx++) {
    $pass = Convert-ToBase36Lower $idx $Digits
    $gpg_test = & gpg --default-key $KeyID -n --passphrase "$pass" --pinentry-mode loopback --clearsign NUL 2>&1
    $ret = $LASTEXITCODE

    if ($ret -eq 0) {
        Write-Host "Found! Passphrase: $pass"
        break
    } elseif ($gpg_test -match "Bad passphrase") {
        Write-Host -NoNewline "."
        if (($idx % 1000) -eq 0) {
            Write-Host ""
            Write-Host "Summary: $StartIndex-$idx/$max ($pass)"
        }
    } else {
        Write-Host "Else " + $ret + " " + $gpg_test
        exit $ret
    }
}

if ($ret -ne 0) {
    Write-Host "Not found :("
    exit -1
}