(Objective-C) RSASSA-PSS Sign String to Create Base64 PCKS7 Signature

Signs a string to create a PKCS7 signature in the base64 encoding. The signature algorithm is RSASSA-PSS with SHA256.

Note: This example requires Chilkat v9.5.0.67 or greater.

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#import <CkoCrypt2.h>
#import <CkoPfx.h>
#import <CkoCert.h>
#import <NSString.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];

// Get a digital certificate with private key from a .pfx
// (Chilkat has many different ways to provide a cert + private key for siging.
// Using a PFX is just one possible option.)
CkoPfx *pfx = [[CkoPfx alloc] init];
BOOL success = [pfx LoadPfxFile: @"qa_data/rsassa-pss/privatekey.pfx" password: @"PFX_PASSWORD"];
if (success != YES) {
    NSLog(@"%@",pfx.LastErrorText);
    return;
}

// Get the certificate to be used for signing.
// (The typical case for a PFX is that it contains a cert with an associated private key,
// as well as other certificates in the chain of authentication.  The cert with the private
// key should be in the first position at index 0.)
CkoCert *cert = [pfx GetCert: [NSNumber numberWithInt: 0]];
if (pfx.LastMethodSuccess != YES) {
    NSLog(@"%@",pfx.LastErrorText);
    return;
}

[crypt SetSigningCert: cert];

// Indicate that RSASSA-PSS with SHA256 should be used.
crypt.SigningAlg = @"pss";
crypt.HashAlgorithm = @"sha256";

crypt.EncodingMode = @"base64";

// Sign a string and return the base64 PKCS7 detached signature
NSString *originalText = @"This is a test";
NSString *pkcs7sig = [crypt SignStringENC: originalText];
NSLog(@"%@",@"Detached Signature:");
NSLog(@"%@",pkcs7sig);

// This signature looks like this:
// MIIG5wYJKoZIhvcNAQcCoIIG2DCCBtQCAQExDzANBgl .. YToLqEwTdU87ox5g7rvw==

// The ASN.1 of the signature can be examined by browsing to https://lapo.it/asn1js/ ,
// then copy-and-paste the Base64 signature into the form and decode..

// The signature can be verified against the original data like this:
success = [crypt VerifyStringENC: originalText encodedSig: pkcs7sig];
NSLog(@"%@%d",@"Signature verified: ",success);
success = [crypt VerifyStringENC: @"Not the original text" encodedSig: pkcs7sig];
NSLog(@"%@%d",@"Signature verified: ",success);

// Now we'll create an opaque signature (the opposite of a detached signature). 
// An opaque signature is a PKCS7 message that contains both the original data and
// the signature.  The verification process extracts the original data.
NSString *opaqueSig = [crypt OpaqueSignStringENC: originalText];
NSLog(@"%@",@"Opaque Signature:");
NSLog(@"%@",opaqueSig);

// The ASN.1 of the signature can be examined by browsing to https://lapo.it/asn1js/ ,
// then copy-and-paste the Base64 signature into the form and decode..

// We can verify and extract the original data:
NSString *origTxt = [crypt OpaqueVerifyStringENC: opaqueSig];
if (crypt.LastMethodSuccess != YES) {
    NSLog(@"%@",@"Signature verification failed.");
    NSLog(@"%@",crypt.LastErrorText);

    return;
}

NSLog(@"%@",@"Signature verified.");
NSLog(@"%@%@",@"Extracted text:",origTxt);