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Duplicate openssl dgst -md5 -sign myKey.pem something.txt | openssl enc -base64 -A

See more RSA Examples

Demonstrates how to duplicate the creation of an RSA signature produced by this OpenSSL command:
openssl dgst -md5 -sign myKey.pem something.txt | openssl enc -base64 -A

Chilkat PowerShell Downloads

PowerShell
Add-Type -Path "C:\chilkat\ChilkatDotNet47-x64\ChilkatDotNet47.dll"

$success = $false

# This example assumes the Chilkat API to have been previously unlocked.
# See Global Unlock Sample for sample code.

$pkey = New-Object Chilkat.PrivateKey

# Load the private key from an RSA PEM file:
$success = $pkey.LoadPemFile("myKey.pem")

$rsa = New-Object Chilkat.Rsa

# Import the private key into the RSA component:
$success = $rsa.UsePrivateKey($pkey)
if ($success -eq $false) {
    $($rsa.LastErrorText)
    exit
}

# OpenSSL uses BigEndian byte ordering:
$rsa.LittleEndian = $false

# The resulting signature will be a Base64 string:
$rsa.EncodingMode = "base64"

# For simplicity, we're not loading
# the data to be signed from a file.  We are instead simply
# using a literal string value.  
$strData = "This is the text to be signed."

# Hash the input using MD5, and then sign the hash:
# Other valid hash algorithm choices are "md2" and "sha-1".
$base64Sig = $rsa.SignStringENC($strData,"md5")

$($base64Sig)

$("Success!")