Create an Opaque (Attached) S/MIME Signature

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Demonstrates the Chilkat Mime.ConvertToSigned method, which creates an opaque (attached) S/MIME signature. The only argument is the signing Cert. On success the entity is replaced by a CMS/PKCS #7 signed-data structure that wraps the content inside the signature.

Note: The file paths are relative to the application's current working directory. Absolute paths may also be used. Supply the paths appropriate to your own environment.

Background: An opaque signature bundles the content inside the CMS structure, so the message is a single application/pkcs7-mime blob that only S/MIME-aware software can read. It is more tamper-resistant in transit — nothing can alter the content without breaking the wrapper — at the cost of the backward compatibility a detached signature offers. Use it when every recipient is known to support S/MIME.

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#include <C_CkMime.h>
#include <C_CkCert.h>

void ChilkatSample(void)
    {
    BOOL success;
    HCkMime mime;
    const char *pfxPassword;
    HCkCert cert;

    success = FALSE;

    mime = CkMime_Create();
    success = CkMime_SetBodyFromPlainText(mime,"This message will be signed.");
    if (success == FALSE) {
        printf("%s\n",CkMime_lastErrorText(mime));
        CkMime_Dispose(mime);
        return;
    }

    //  Load the signing certificate (which must have access to its private key) from a PFX file.
    //  The PFX password should come from a secure source rather than being hard-coded.
    pfxPassword = "myPfxPassword";
    cert = CkCert_Create();
    success = CkCert_LoadPfxFile(cert,"qa_data/signer.pfx",pfxPassword);
    if (success == FALSE) {
        printf("%s\n",CkCert_lastErrorText(cert));
        CkMime_Dispose(mime);
        CkCert_Dispose(cert);
        return;
    }

    //  Create an OPAQUE (attached) S/MIME signature.  The entity is replaced by a CMS/PKCS #7
    //  signed-data structure that wraps the content inside the signature.
    success = CkMime_ConvertToSigned(mime,cert);
    if (success == FALSE) {
        printf("%s\n",CkMime_lastErrorText(mime));
        CkMime_Dispose(mime);
        CkCert_Dispose(cert);
        return;
    }

    success = CkMime_SaveMime(mime,"qa_output/signed_opaque.eml");
    if (success == FALSE) {
        printf("%s\n",CkMime_lastErrorText(mime));
        CkMime_Dispose(mime);
        CkCert_Dispose(cert);
        return;
    }

    printf("Message converted to an opaque signed message.\n");


    CkMime_Dispose(mime);
    CkCert_Dispose(cert);

    }