(PureBasic) About RSA Public/Private Keys

This example provides some additional information for understanding public/private key pairs. In demonstrates how a private key is a superset of the public key. A public key contains the modulus and exponent. The matching private key also contains the modulus and exponent, but also contains the additional private key parts.

Note: This example requires Chilkat v11.0.0 or greater.

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IncludeFile "CkCert.pb"
IncludeFile "CkPrivateKey.pb"
IncludeFile "CkPublicKey.pb"

Procedure ChilkatExample()

    success.i = 0

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

    ; Load a digital certificate.
    success = CkCert::ckLoadFromFile(cert,"digitalCert.cer")
    If success = 0
        Debug CkCert::ckLastErrorText(cert)
        CkCert::ckDispose(cert)
        ProcedureReturn
    EndIf

    ; A .cer file does not contain the private key.  It should contain
    ; the public key...
    pubKey.i = CkPublicKey::ckCreate()
    If pubKey.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    CkCert::ckGetPublicKey(cert,pubKey)

    ; Let's have a look at it (in XML format).
    Debug "Public Key from Certificate:"
    Debug CkPublicKey::ckGetXml(pubKey)

    ; An RSA public key consists of a modulus and exponent.

    ; An RSA private key includes both the modulus and exponent,
    ; as well as other "big" numbers: P, Q, D, etc.

    ; Let's load an RSA private key from a DER-encoded file:
    privKey.i = CkPrivateKey::ckCreate()
    If privKey.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    success = CkPrivateKey::ckLoadAnyFormatFile(privKey,"PrivateKey.key","")
    If success <> 1
        Debug CkPrivateKey::ckLastErrorText(privKey)
        CkCert::ckDispose(cert)
        CkPublicKey::ckDispose(pubKey)
        CkPrivateKey::ckDispose(privKey)
        ProcedureReturn
    EndIf

    ; If this private key is the matching half to the public key from
    ; the certificate, then the modulus and exponent should
    ; be identical.  (Thus, a "private key" really contains both the public part as well as the private parts...).
    Debug "Private Key from DER:"
    Debug CkPrivateKey::ckGetXml(privKey)


    CkCert::ckDispose(cert)
    CkPublicKey::ckDispose(pubKey)
    CkPrivateKey::ckDispose(privKey)


    ProcedureReturn
EndProcedure