powershell asymmetric encrypt/decrypt functions

Question

I wasn't able to find good powershell functions to utilize asymmetric encryption so I created the following. Would like any feedback in terms of improvement as I'm a crypto noob. With the caveat that these functions are very basic. There isn't error checking and the write-host after a decrypt is hardly necessary. Just want to establish the core functionality before adding things like protected memory and such.

This has been successfully tested on two systems: Win8 w/Powershell v3 & Win2008R2 w/Powershell v2.

Function Encrypt-Asymmetric([string]$Encrypt,[string]$CertPath,[string]$XmlExportPath)
{
    # Encrypts a string with a public key
    $pubcer = New-Object System.Security.Cryptography.X509Certificates.X509Certificate2($CertPath)
    $byteval = [System.Text.Encoding]::UTF8.GetBytes($Encrypt)
    $pubcer.PublicKey.Key.Encrypt($byteval,$true) | Export-Clixml -Path $XmlExportPath    
}

Function Decrypt-Asymmetric([string]$XmlPath,[string]$CertThumbprint)
{
    # Decrypts cipher text using the private key
    # Assumes the certificate is in the LocalMachine store
    $store = new-object System.Security.Cryptography.X509Certificates.X509Store([System.Security.Cryptography.X509Certificates.StoreLocation]::LocalMachine)
    $store.open([System.Security.Cryptography.X509Certificates.OpenFlags]::ReadOnly)
    $cer = $store.Certificates | %{if($_.thumbprint -eq $CertThumbprint){$_}}
    $ciphertext = Import-Clixml -Path $XmlPath
    $decryptedBytes = $cer.PrivateKey.Decrypt($ciphertext,$true)
    $ClearText = [System.Text.Encoding]::UTF8.GetString($decryptedBytes)
    Write-Host $ClearText
}

Answer 1

I know this is old. I took the starting point you had here and added some items. I tried to clean where appropriate and use variable names that might help make things a little more easier for someonelse that reads this to understand.

Encryption:

Function Encrypt-Asymmetric {
[CmdletBinding()]
[OutputType([System.String])]
param(
    [Parameter(Position=0, Mandatory=$true)][ValidateNotNullOrEmpty()][System.String]
    $ClearText,
    [Parameter(Position=1, Mandatory=$true)][ValidateNotNullOrEmpty()][ValidateScript({Test-Path $_ -PathType Leaf})][System.String]
    $PublicCertFilePath
)
# Encrypts a string with a public key
$PublicCert = New-Object System.Security.Cryptography.X509Certificates.X509Certificate2($PublicCertFilePath)
$ByteArray = [System.Text.Encoding]::UTF8.GetBytes($ClearText)
$EncryptedByteArray = $PublicCert.PublicKey.Key.Encrypt($ByteArray,$true)
$EncryptedBase64String = [Convert]::ToBase64String($EncryptedByteArray)

Return $EncryptedBase64String 
}

Decryption:

Function Decrypt-Asymmetric
{
[CmdletBinding()]
[OutputType([System.String])]
param(
    [Parameter(Position=0, Mandatory=$true)][ValidateNotNullOrEmpty()][System.String]
    $EncryptedBase64String,
    [Parameter(Position=1, Mandatory=$true)][ValidateNotNullOrEmpty()][System.String]
    $CertThumbprint
)
# Decrypts text using the private key
# Assumes the certificate is in the LocalMachine\My (Personal) Store
$Cert = Get-ChildItem cert:\LocalMachine\My | where { $_.Thumbprint -eq $CertThumbprint }
if($Cert) {
    $EncryptedByteArray = [Convert]::FromBase64String($EncryptedBase64String)
    $ClearText = [System.Text.Encoding]::UTF8.GetString($Cert.PrivateKey.Decrypt($EncryptedByteArray,$true))
}
Else {Write-Error "Certificate with thumbprint: $CertThumbprint not found!"}

Return $ClearText
}

http://grokgarble.com/blog/?p=228