(C# UWP/WinRT) Sign PDF: Choosing RSASSA-PSS or PKCS1-v1_5 and Hash Algorithm

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This example demonstrates how to choose between RSASSA-PSS vs. PKCS1-v1_5 and the hash algorithm when signing a PDF.

Chilkat Universal Windows Platform (UWP) / WinRT Downloads Chilkat for the Universal Windows Platform (UWP)

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

Chilkat.Pdf pdf = new Chilkat.Pdf();

// Load a PDF to be signed.
// The "hello.pdf" is available at https://chilkatsoft.com/hello.pdf
bool success = pdf.LoadFile("qa_data/pdf/hello.pdf");
if (success == false) {
    Debug.WriteLine(pdf.LastErrorText);
    return;
}

// Options for signing are specified in JSON.
Chilkat.JsonObject json = new Chilkat.JsonObject();

// In most cases, the signingCertificateV2 attribute is needed.
json.UpdateInt("signingCertificateV2",1);

// ---------------------------------------------------------------------------------------------------------------------------
// This example is the same as the example at Sign a PDF (Simplest Example Possible)
// but with the following lines added to specify the signature scheme and hash algorithm.

// The signature schemes PKCS-v1_5 and RSASSA-PSS have differences. 
// PKCSV1_5 is deterministic. The same message and key will produce an identical signature value each time. 
// PSS is randomized and will produce a different signature value each time.

// To choose the hash algorithm:
// (The typical choices are sha1, sha256, sha384, and sha512.)
json.UpdateString("hashAlgorithm","sha256");

// To choose PKCSV1_5:
json.UpdateString("signingAlgorithm","pkcs");
// Alternatively, to choose RSASSA-PSS:
json.UpdateString("signingAlgorithm","pss");

// If not specified, the default is SHA256 with PKCS-v1_5.

// The remainder of this example is the same as the example at Sign a PDF (Simplest Example Possible)
// ---------------------------------------------------------------------------------------------------------------------------

// Put the signature on page 1, top left
json.UpdateInt("page",1);
json.UpdateString("appearance.y","top");
json.UpdateString("appearance.x","left");

// Use a font scale of 10.0
json.UpdateString("appearance.fontScale","10.0");

// In this example, the appearance of the digital signature will contain three lines:
// 1) The signing certificate's common name
// 2) The current date/time
// 3) Some arbitrary text.
// The keyword "cert_cn" is replaced with the Certificate's Subject Common Name.
// The keyword "current_dt" is replaced with the current date/time.
// Any number of appearance text lines can be added.
json.UpdateString("appearance.text[0]","Digitally signed by: cert_cn");
json.UpdateString("appearance.text[1]","current_dt");
json.UpdateString("appearance.text[2]","The crazy brown fox jumps over the lazy dog.");

// Load the signing certificate. (Use your own certificate.)
Chilkat.Cert cert = new Chilkat.Cert();
success = cert.LoadPfxFile("qa_data/pfx/myPdfSigningCert.pfx","secret");
if (success == false) {
    Debug.WriteLine(cert.LastErrorText);
    return;
}

// Tell the pdf object to use the certificate for signing.
success = pdf.SetSigningCert(cert);
if (success == false) {
    Debug.WriteLine(pdf.LastErrorText);
    return;
}

success = await pdf.SignPdfAsync(json,"qa_output/hello_signed.pdf");
if (success == false) {
    Debug.WriteLine(pdf.LastErrorText);
    return;
}

Debug.WriteLine("The PDF has been successfully cryptographically signed.");

// The appearance of the signature appears in Adobe Acrobat as shown here: