如何通过 Powershell 从在线 ZIP 存档中仅下载单个文件？

Question

I want to download only a single file from an online ZIP archive via Powershell. For this I created a demo-code which is already working, but I am still struggling to get the correct parsing logic on the ZIP-directory. Here is the code I have so far:

# demo code downloading a single DLL file from an online ZIP archive
# and extracting the DLL into memory for mounting it to the main process.

cls
Remove-Variable * -ea 0

# definition for the ZIP archive, the file to be extracted and the checksum:
$url = 'https://github.com/sshnet/SSH.NET/releases/download/2020.0.1/SSH.NET-2020.0.1-bin.zip'
$sub = 'net40/Renci.SshNet.dll'
$md5 = '5B1AF51340F333CD8A49376B13AFCF9C'

# prepare HTTP client:
Add-Type -AssemblyName System.Net.Http
$handler = [System.Net.Http.HttpClientHandler]::new()
$client  = [System.Net.Http.HttpClient]::new($handler)

# get the length of the ZIP archive:
$req = [System.Net.HttpWebRequest]::Create($url)
$req.Method = 'HEAD'
$length = $req.GetResponse().ContentLength
$zip = [byte[]]::new($length)

# get the last 10k:
# how to get the correct length of the central ZIP directory here?
$start = $length-10kb
$end   = $length-1
$client.DefaultRequestHeaders.Add('Range', "bytes=$start-$end")
$result = $client.GetAsync($url).Result
$last10kb = $result.content.ReadAsByteArrayAsync().Result
$last10kb.CopyTo($zip, $start)

# get the block containing the DLL file:
# how to get the exact file-offset from the ZIP directory?
$start = $length-3537kb
$end   = $length-3201kb
$client.DefaultRequestHeaders.Clear()
$client.DefaultRequestHeaders.Add('Range', "bytes=$start-$end")
$result = $client.GetAsync($url).Result
$block = $result.content.ReadAsByteArrayAsync().Result
$block.CopyTo($zip, $start)

# extract the DLL file from archive:
Add-Type -AssemblyName System.IO.Compression
$stream = [System.IO.Memorystream]::new()
$stream.Write($zip,0,$zip.Length)
$archive = [System.IO.Compression.ZipArchive]::new($stream)
$entry = $archive.GetEntry($sub)
$bytes = [byte[]]::new($entry.Length)
[void]$entry.Open().Read($bytes, 0, $bytes.Length)

# check MD5:
$prov = [Security.Cryptography.MD5CryptoServiceProvider]::new().ComputeHash($bytes)
$hash = [string]::Concat($prov.foreach{$_.ToString("x2")})
if ($hash -ne $md5) {write-host 'dll has wrong checksum.' -f y ;break}

# load the DLL:
[void][System.Reflection.Assembly]::Load($bytes)

# use the single demo-call from the DLL:
$test = [Renci.SshNet.NoneAuthenticationMethod]::new('test')
'done.'

Only open point in this code is the correct method to identify the length of the central directory at the end of the ZIP archive and how to get the correct file-offset for the single file to be extracted (in my code I just found the ranges by pure try&error).

I already checked this wiki https://en.wikipedia.org/wiki/ZIP_(file_format)#Structure and also the PKWARE definitions https://gist.github.com/steakknife/820b73ebf25146180198febdb6f0e183 but beside the block definitions I could not find a programmatical approach to get the offset for ethe EOCD and the individual file. Can someone help here, please?

Answer 1

After a couple of additional tests I came to this solution:

# demo code downloading a single DLL file from an online ZIP archive
# and extracting the DLL into memory to mount it finally to the main process.

cls
Remove-Variable * -ea 0

# definition for the ZIP archive, the file to be extracted and the checksum:
$url = 'https://github.com/sshnet/SSH.NET/releases/download/2020.0.1/SSH.NET-2020.0.1-bin.zip'
$sub = 'net40/Renci.SshNet.dll'
$md5 = '5B1AF51340F333CD8A49376B13AFCF9C'

'prepare HTTP client:'
Add-Type -AssemblyName System.Net.Http
$handler = [System.Net.Http.HttpClientHandler]::new()
$client  = [System.Net.Http.HttpClient]::new($handler)

'get the length of the ZIP archive:'
# dont use System.Web.HttpRequest, it is frequently hanging:
$req = [System.Net.Http.HttpRequestMessage]::new('HEAD', $url)
$result = $client.SendAsync($req).Result
$zipLength = $result.Content.Headers.ContentLength
$zip = [byte[]]::new($zipLength)
$req.Dispose()

'get the last 10k:'
$start = $zipLength-10kb
$end   = $zipLength-1
$client.DefaultRequestHeaders.Add('Range', "bytes=$start-$end")
$result = $client.GetAsync($url).Result
$last10kb = $result.content.ReadAsByteArrayAsync().Result
$last10kb.CopyTo($zip, $start)

"get the 'End of CD' block:"
$enc = [System.Text.Encoding]::GetEncoding(28591)
$end = $enc.GetString($last10kb, $last10kb.Length-256, 256)
$eocd = [regex]::Match($end, 'PK\x05\x06.*').value
$eocd = $enc.GetBytes($eocd)

'get the central directory:'
$cdLength = [bitconverter]::ToUInt32($eocd, 12)
$cdStart  = [bitconverter]::ToUInt32($eocd, 16)
$cd = [byte[]]::new($cdLength)
[array]::Copy($zip, $cdStart, $cd, 0, $cdLength)

'search all file headers for correct file name:'
$fileHeaders = [regex]::Split($enc.GetString($cd),'PK\x01\x02')
foreach ($header in $fileHeaders) {
    $len = $header.Length
    if ($len -ge 42) {
        $bytes = $enc.GetBytes($header)
        $nameLength = [bitconverter]::ToUInt16($bytes, 24)
        if ($nameLength -eq $sub.length -and ($nameLength + 42) -le $len) { 
            $name = $header.Substring(42, $nameLength)
            if ($name -eq $sub) {
                $size   = [bitconverter]::ToUInt32($bytes, 16) + 256
                $start  = [bitconverter]::ToUInt32($bytes, 38)
                break
            }
        }
    }
}
if (!$start) {write-host 'we could not find file in the ZIP archive' -f y ;break}

'get the block containing the file:'
$end   = $start+$size
$client.DefaultRequestHeaders.Clear()
$client.DefaultRequestHeaders.Add('Range', "bytes=$start-$end")
$result = $client.GetAsync($url).Result
$block = $result.content.ReadAsByteArrayAsync().Result
$block.CopyTo($zip, $start)
$client.dispose()

'extract the DLL file from archive:'
Add-Type -AssemblyName System.IO.Compression
$stream = [System.IO.Memorystream]::new()
$stream.Write($zip,0,$zip.Length)
$archive = [System.IO.Compression.ZipArchive]::new($stream)
$entry = $archive.GetEntry($sub)
$bytes = [byte[]]::new($entry.Length)
[void]$entry.Open().Read($bytes, 0, $bytes.Length)

'check MD5:'
$prov = [Security.Cryptography.MD5CryptoServiceProvider]::new().ComputeHash($bytes)
$hash = [string]::Concat($prov.foreach{$_.ToString("x2")})
if ($hash -ne $md5) {write-host 'dll has wrong checksum.' -f y ;break}

'load the DLL:'
[void][System.Reflection.Assembly]::Load($bytes)

'use the single demo-call from the DLL:'
$test = [Renci.SshNet.NoneAuthenticationMethod]::new('test')
'done.'