Unicode C

Create a DKIM-Signature for a MIME Message

Demonstrates the Chilkat Dkim.DkimSign method, which creates a DKIM-Signature header and prepends it to a complete MIME message held in a BinData, modifying it in place. The DkimAlg, DkimCanon, DkimDomain, DkimSelector, DkimHeaders, and DkimBodyLengthCount properties configure the generated 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: The signature is computed over a hash of the body plus a chosen set of headers, so the message must be completely built — every header, encoding, and boundary — before signing; any later change invalidates it. The d= (domain) and s= (selector) tags tell a verifier where to find the public key: at selector._domainkey.domain in DNS. relaxed/relaxed canonicalization tolerates the minor whitespace changes mail systems make in transit, which is why it is the common choice.

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Unicode C

#include <C_CkDkimW.h>
#include <C_CkPrivateKeyW.h>
#include <C_CkBinDataW.h>

void ChilkatSample(void)
    {
    BOOL success;
    HCkDkimW dkim;
    HCkPrivateKeyW privKey;
    HCkBinDataW mimeData;

    success = FALSE;

    //  Demonstrates the Dkim.DkimSign method, which creates a DKIM-Signature header and prepends it to
    //  a complete MIME message.  The only argument is a BinData holding the MIME, which is modified in
    //  place.
    //  
    //  The DkimAlg, DkimCanon, DkimDomain, DkimSelector, DkimHeaders, and DkimBodyLengthCount
    //  properties configure the signature that is generated.

    dkim = CkDkimW_Create();

    //  Configure the DKIM signature.
    CkDkimW_putDkimDomain(dkim,L"example.com");
    CkDkimW_putDkimSelector(dkim,L"myselector");

    //  Signing algorithm written to the a= tag.
    CkDkimW_putDkimAlg(dkim,L"rsa-sha256");

    //  Canonicalization for headers and body, written to the c= tag.
    CkDkimW_putDkimCanon(dkim,L"relaxed/relaxed");

    //  Colon-separated list of header fields to sign, written to the h= tag.
    CkDkimW_putDkimHeaders(dkim,L"From:To:Subject:Date:Message-ID");

    //  Maximum number of canonicalized body bytes to hash.  0 means the entire body.
    CkDkimW_putDkimBodyLengthCount(dkim,0);

    //  Load the RSA private key and give it to the Dkim object.
    privKey = CkPrivateKeyW_Create();
    success = CkPrivateKeyW_LoadPemFile(privKey,L"qa_data/dkim_private.pem");
    if (success == FALSE) {
        wprintf(L"%s\n",CkPrivateKeyW_lastErrorText(privKey));
        CkDkimW_Dispose(dkim);
        CkPrivateKeyW_Dispose(privKey);
        return;
    }

    success = CkDkimW_SetDkimPrivateKey(dkim,privKey);
    if (success == FALSE) {
        wprintf(L"%s\n",CkDkimW_lastErrorText(dkim));
        CkDkimW_Dispose(dkim);
        CkPrivateKeyW_Dispose(privKey);
        return;
    }

    //  Load the complete MIME message to be signed.  It must already contain all headers, transfer
    //  encodings, MIME boundaries, and body bytes.
    mimeData = CkBinDataW_Create();
    success = CkBinDataW_LoadFile(mimeData,L"qa_data/message.eml");
    if (success == FALSE) {
        wprintf(L"%s\n",CkBinDataW_lastErrorText(mimeData));
        CkDkimW_Dispose(dkim);
        CkPrivateKeyW_Dispose(privKey);
        CkBinDataW_Dispose(mimeData);
        return;
    }

    //  Sign.  The DKIM-Signature header is prepended to the MIME in place.
    success = CkDkimW_DkimSign(dkim,mimeData);
    if (success == FALSE) {
        wprintf(L"%s\n",CkDkimW_lastErrorText(dkim));
        CkDkimW_Dispose(dkim);
        CkPrivateKeyW_Dispose(privKey);
        CkBinDataW_Dispose(mimeData);
        return;
    }

    //  Save the signed message.
    success = CkBinDataW_WriteFile(mimeData,L"qa_output/signed.eml");
    if (success == FALSE) {
        wprintf(L"%s\n",CkBinDataW_lastErrorText(mimeData));
        CkDkimW_Dispose(dkim);
        CkPrivateKeyW_Dispose(privKey);
        CkBinDataW_Dispose(mimeData);
        return;
    }

    wprintf(L"Message signed.\n");


    CkDkimW_Dispose(dkim);
    CkPrivateKeyW_Dispose(privKey);
    CkBinDataW_Dispose(mimeData);

    }