Objective-C

Create an Opaque (Attached) S/MIME Signature

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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Objective-C

#import <CkoMime.h>
#import <NSString.h>
#import <CkoCert.h>

BOOL success = NO;

CkoMime *mime = [[CkoMime alloc] init];
success = [mime SetBodyFromPlainText: @"This message will be signed."];
if (success == NO) {
    NSLog(@"%@",mime.LastErrorText);
    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.
NSString *pfxPassword = @"myPfxPassword";
CkoCert *cert = [[CkoCert alloc] init];
success = [cert LoadPfxFile: @"qa_data/signer.pfx" password: pfxPassword];
if (success == NO) {
    NSLog(@"%@",cert.LastErrorText);
    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 = [mime ConvertToSigned: cert];
if (success == NO) {
    NSLog(@"%@",mime.LastErrorText);
    return;
}

success = [mime SaveMime: @"qa_output/signed_opaque.eml"];
if (success == NO) {
    NSLog(@"%@",mime.LastErrorText);
    return;
}

NSLog(@"%@",@"Message converted to an opaque signed message.");