PureBasic

About RSA Public/Private Keys

See more RSA Examples

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. PureBasic

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