(Objective-C) Encrypt a file to a PKCS7 encrypted message using multiple certificates from different users

Demonstrates how to encrypt a file to a PKCS7 encrypted message using multiple certificates from different users. Any one of the users can decrypt using his/her own certificate + private key.

Note: When doing public key encryption, it is the public key that is used to encrypt. The private key is required for decryption.

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#import <CkoCrypt2.h>
#import <CkoCert.h>
#import <CkoBinData.h>

// This example requires the Chilkat Crypt API to have been previously unlocked.
// See Unlock Chilkat Crypt for sample code.

CkoCrypt2 *crypt = [[CkoCrypt2 alloc] init];

// Tell the crypt object to use 3 certificates.
// Do this by calling AddEncryptCert for each certificate.

// Load a digital certificate. 
// We don't need the private key for encryption.
// Only the public key is needed (which is included in a certificate).
CkoCert *cert1 = [[CkoCert alloc] init];
BOOL success = [cert1 LoadFromFile: @"qa_data/user1/cert_user1.pem"];
// Assume success for the example, but make sure your application checks for success/failure...
[crypt AddEncryptCert: cert1];

CkoCert *cert2 = [[CkoCert alloc] init];
success = [cert2 LoadFromFile: @"qa_data/user2/cert_user2.pem"];
[crypt AddEncryptCert: cert2];

CkoCert *cert3 = [[CkoCert alloc] init];
success = [cert3 LoadFromFile: @"qa_data/user3/cert_user3.pem"];
[crypt AddEncryptCert: cert3];

// Indicate that we want PKI encryption (i.e. public-key infrastructure)
// to produce a CMS message (Cryptographic Message Syntax/PKCS7),
// that is be created with RSAES-OAEP padding, SHA256, and AES-128 for the
// bulk encryption.
crypt.CryptAlgorithm = @"pki";
crypt.Pkcs7CryptAlg = @"aes";
crypt.KeyLength = [NSNumber numberWithInt:128];
crypt.OaepHash = @"sha256";
crypt.OaepPadding = YES;

// Load the file to be encrypted...
CkoBinData *fileData = [[CkoBinData alloc] init];
success = [fileData LoadFile: @"qa_data/jpg/penguins.jpg"];
// Your app should check for success/failure..

// Encrypt the data.  The contents of the fileData object are replaced with the PKCS7 encrypted message.
success = [crypt EncryptBd: fileData];
if (success != YES) {
    NSLog(@"%@",crypt.LastErrorText);
    return;
}

// Save the PKCS7 encrypted message to a file..
success = [fileData WriteFile: @"qa_output/pkcs7_encrypted.p7"];

// Now indicate that the PKCS7 output is to be returned in the base64 encoding.
crypt.EncodingMode = @"base64";

NSLog(@"%@",@"OK.");