This blog contains experience gained over the years of implementing (and de-implementing) large scale IT applications/software.

Powershell PSCredential – Exception calling .ctor with 2 arguments

Scenario: You are using Powershell with “System.Management.Automation.PSCredential” to capture credentials to an encrypted file using “ConvertFrom-SecureString”.
You are then trying to read those credentials as a different Windows account under which they were originally captured, using ConvertTo-SecureString.

You get an error like “Exception calling .ctor with 2 arguments”.

As per David Lee’s blog page (here

…PowerShell is using the native Windows Data Protection API (DAPI) functionality to encrypt the password from the ‘secure string’ into a text string. This string can be written to a plain text file, but the way that DAPI works is that the encryption is such that only the original user on the original machine the encryption was performed on can decrypt the string back into a ‘Secure string’ to be reused“.

Therefore, following David’s blog, simply use the “-key” parameter when calling “ConvertFrom-SecureString” or “ConvertTo-SecureString” along with a pre-determined AES key.

What I actually did was store the Key file securely with ACL permissions along with the credential file.
Then when reading them back in using “ConvertFrom-SecureString” from a different Windows account on the same machine, I was able to successfully decrypt and use the credential.

Example using credential to map a target UNC to the H: drive on the current machine:

# Some variable declarations.
$keyFile = “C:somefile.key”
$credFile = “C:somefile.cred”
$myUser = “someuser”
$targetPath = “H”
$sourcePath = “//some/unc/path”

# Generate AES key and save to keyFile:
$AESKey = New-Object Byte[] 32
Set-Content $keyFile $AESKey

# Prompt for a user/password and save the password to a credential file.
$credential = Get-Credential “$myUser”
$credential.Password | ConvertFrom-SecureString -Key (Get-Content $keyFile) | Set-Content $credFile

# As other user account.
# We can now reference the key file (imagine we are running as another user account in another script or something at this point).
# Read in the credential from the file and use it.
$encrypted = Get-Content $credFile | ConvertTo-SecureString -key (Get-Content $keyFile)
$credential = New-Object -TypeName System.Management.Automation.PsCredential -ArgumentList “$myUser”, $encrypted

# Map a drive using the credential.
New-PsDrive -Persist -Name $targetPath -PsProvider “FileSystem” -Scope “Global” -Root $sourcePath -Credential $credential -ErrorAction Stop

Using the separate AES key got around the error.

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