PureBasic

Sign PDF: Choosing RSASSA-PSS or PKCS1-v1_5 and Hash Algorithm

See more PDF Signatures Examples

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

IncludeFile "CkCert.pb"
IncludeFile "CkJsonObject.pb"
IncludeFile "CkPdf.pb"

Procedure ChilkatExample()

    success.i = 0

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

    pdf.i = CkPdf::ckCreate()
    If pdf.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    ; Load a PDF to be signed.
    ; The "hello.pdf" is available at https://chilkatsoft.com/hello.pdf
    success = CkPdf::ckLoadFile(pdf,"qa_data/pdf/hello.pdf")
    If success = 0
        Debug CkPdf::ckLastErrorText(pdf)
        CkPdf::ckDispose(pdf)
        ProcedureReturn
    EndIf

    ; Options for signing are specified in JSON.
    json.i = CkJsonObject::ckCreate()
    If json.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    ; In most cases, the signingCertificateV2 attribute is needed.
    CkJsonObject::ckUpdateInt(json,"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.)
    CkJsonObject::ckUpdateString(json,"hashAlgorithm","sha256")

    ; To choose PKCSV1_5:
    CkJsonObject::ckUpdateString(json,"signingAlgorithm","pkcs")
    ; Alternatively, to choose RSASSA-PSS:
    CkJsonObject::ckUpdateString(json,"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
    CkJsonObject::ckUpdateInt(json,"page",1)
    CkJsonObject::ckUpdateString(json,"appearance.y","top")
    CkJsonObject::ckUpdateString(json,"appearance.x","left")

    ; Use a font scale of 10.0
    CkJsonObject::ckUpdateString(json,"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.
    CkJsonObject::ckUpdateString(json,"appearance.text[0]","Digitally signed by: cert_cn")
    CkJsonObject::ckUpdateString(json,"appearance.text[1]","current_dt")
    CkJsonObject::ckUpdateString(json,"appearance.text[2]","The crazy brown fox jumps over the lazy dog.")

    ; Load the signing certificate. (Use your own certificate.)
    cert.i = CkCert::ckCreate()
    If cert.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    success = CkCert::ckLoadPfxFile(cert,"qa_data/pfx/myPdfSigningCert.pfx","secret")
    If success = 0
        Debug CkCert::ckLastErrorText(cert)
        CkPdf::ckDispose(pdf)
        CkJsonObject::ckDispose(json)
        CkCert::ckDispose(cert)
        ProcedureReturn
    EndIf

    ; Tell the pdf object to use the certificate for signing.
    success = CkPdf::ckSetSigningCert(pdf,cert)
    If success = 0
        Debug CkPdf::ckLastErrorText(pdf)
        CkPdf::ckDispose(pdf)
        CkJsonObject::ckDispose(json)
        CkCert::ckDispose(cert)
        ProcedureReturn
    EndIf

    success = CkPdf::ckSignPdf(pdf,json,"qa_output/hello_signed.pdf")
    If success = 0
        Debug CkPdf::ckLastErrorText(pdf)
        CkPdf::ckDispose(pdf)
        CkJsonObject::ckDispose(json)
        CkCert::ckDispose(cert)
        ProcedureReturn
    EndIf

    Debug "The PDF has been successfully cryptographically signed."

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

    ; (image:https://example-code.com/images/signature1.jpg/endImage)


    CkPdf::ckDispose(pdf)
    CkJsonObject::ckDispose(json)
    CkCert::ckDispose(cert)


    ProcedureReturn
EndProcedure