Script for only updating shh-agent on Windows. Based on: https://github.com/PowerShell/openssh-portable/blob/latestw_all/contrib/win32/openssh/install-sshd.ps1

install-sshd.ps1

# @manojampalam - authored initial script
# @friism - Fixed issue with invalid SDDL on Set-Acl
# @manojampalam - removed ntrights.exe dependency
# @bingbing8 - removed secedit.exe dependency

$scriptpath = $MyInvocation.MyCommand.Path
$scriptdir = Split-Path $scriptpath

$sshdpath = Join-Path $scriptdir "sshd.exe"
$sshagentpath = Join-Path $scriptdir "ssh-agent.exe"
$etwman = Join-Path $scriptdir "openssh-events.man"

if (-not (Test-Path $sshdpath)) {
    throw "sshd.exe is not present in script path"
}

if (Get-Service sshd -ErrorAction SilentlyContinue) 
{
   Stop-Service sshd
   sc.exe delete sshd 1>$null
}

if (Get-Service ssh-agent -ErrorAction SilentlyContinue) 
{
   Stop-Service ssh-agent
   sc.exe delete ssh-agent 1>$null
}

# unregister etw provider
wevtutil um `"$etwman`"

# adjust provider resource path in instrumentation manifest
[XML]$xml = Get-Content $etwman
$xml.instrumentationManifest.instrumentation.events.provider.resourceFileName = $sshagentpath.ToString()
$xml.instrumentationManifest.instrumentation.events.provider.messageFileName = $sshagentpath.ToString()

$streamWriter = $null
$xmlWriter = $null
try {
    $streamWriter = new-object System.IO.StreamWriter($etwman)
    $xmlWriter = [System.Xml.XmlWriter]::Create($streamWriter)    
    $xml.Save($xmlWriter)
}
finally {
    if($streamWriter) {
        $streamWriter.Close()
    }
}

#register etw provider
wevtutil im `"$etwman`"

$agentDesc = "Agent to hold private keys used for public key authentication."
New-Service -Name ssh-agent -DisplayName "OpenSSH Authentication Agent" -BinaryPathName `"$sshagentpath`" -Description $agentDesc -StartupType Manual | Out-Null
sc.exe sdset ssh-agent "D:(A;;CCLCSWRPWPDTLOCRRC;;;SY)(A;;CCDCLCSWRPWPDTLOCRSDRCWDWO;;;BA)(A;;CCLCSWLOCRRC;;;IU)(A;;CCLCSWLOCRRC;;;SU)(A;;RP;;;AU)"
sc.exe privs ssh-agent SeImpersonatePrivilege

## We comment out these lines in order to only install ssh-agent
# $sshdDesc = "SSH protocol based service to provide secure encrypted communications between two untrusted hosts over an insecure network."
# New-Service -Name sshd -DisplayName "OpenSSH SSH Server" -BinaryPathName `"$sshdpath`" -Description $sshdDesc -StartupType Manual | Out-Null
# sc.exe privs sshd SeAssignPrimaryTokenPrivilege/SeTcbPrivilege/SeBackupPrivilege/SeRestorePrivilege/SeImpersonatePrivilege

Write-Host -ForegroundColor Green "ssh-agent service successfully installed"