Objective-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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Objective-C

#import <CkoDkim.h>
#import <CkoPrivateKey.h>
#import <CkoBinData.h>

BOOL success = NO;

//  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.

CkoDkim *dkim = [[CkoDkim alloc] init];

//  Configure the DKIM signature.
dkim.DkimDomain = @"example.com";
dkim.DkimSelector = @"myselector";

//  Signing algorithm written to the a= tag.
dkim.DkimAlg = @"rsa-sha256";

//  Canonicalization for headers and body, written to the c= tag.
dkim.DkimCanon = @"relaxed/relaxed";

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

//  Maximum number of canonicalized body bytes to hash.  0 means the entire body.
dkim.DkimBodyLengthCount = [NSNumber numberWithInt:0];

//  Load the RSA private key and give it to the Dkim object.
CkoPrivateKey *privKey = [[CkoPrivateKey alloc] init];
success = [privKey LoadPemFile: @"qa_data/dkim_private.pem"];
if (success == NO) {
    NSLog(@"%@",privKey.LastErrorText);
    return;
}

success = [dkim SetDkimPrivateKey: privKey];
if (success == NO) {
    NSLog(@"%@",dkim.LastErrorText);
    return;
}

//  Load the complete MIME message to be signed.  It must already contain all headers, transfer
//  encodings, MIME boundaries, and body bytes.
CkoBinData *mimeData = [[CkoBinData alloc] init];
success = [mimeData LoadFile: @"qa_data/message.eml"];
if (success == NO) {
    NSLog(@"%@",mimeData.LastErrorText);
    return;
}

//  Sign.  The DKIM-Signature header is prepended to the MIME in place.
success = [dkim DkimSign: mimeData];
if (success == NO) {
    NSLog(@"%@",dkim.LastErrorText);
    return;
}

//  Save the signed message.
success = [mimeData WriteFile: @"qa_output/signed.eml"];
if (success == NO) {
    NSLog(@"%@",mimeData.LastErrorText);
    return;
}

NSLog(@"%@",@"Message signed.");