Objective-C

Verify an RSA Signature Against the Signed String

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Demonstrates how to validate an RSA Signature against the string that was signed. Objective-C

#import <CkoStringBuilder.h>
#import <NSString.h>
#import <CkoPublicKey.h>
#import <CkoRsa.h>

BOOL success = NO;

//  Create the same string we previously signed in this example: 
//  RSA Sign utf-8 Bytes of String to get Base64 RSA Signature

CkoStringBuilder *sb = [[CkoStringBuilder alloc] init];
BOOL crlfLineEnding = YES;
int i;
for (i = 0; i <= 10; i++) {
    [sb AppendLine: @"This is a test." crlf: crlfLineEnding];
}

NSString *base64_rsa_sig = @"gWgpEXQqvXN6wh0MuFXPiw2xCb8cnmdizBT1TD1Tpm2GlJ8gnD59DGSj35GGXk1tM+mCOvMa2uW/9gmz8p6A90JLbn918i/2wKGvcde4wXnzPO7JADSNn2UwYzf3rp4/q/JrHv1GZETRJhABSFqTxOgdmdbEJQE/zijOSctCtQAM2CTb6t2BO8uHKSSUAwPH2cbeWxqsaVRLT4ruk5nZOxhoLgLMSjupiSzbE8zSTkkFOEHD+Mbb2xwg85AI0E4DsUnp+hLwwtS0blesgSyvtBY8oJODJeFcJsR7JDOTPdzHQHgJRl/2A4ij5xT91TpXy7ok43jhT6O9j7Q3qrafIQ==";

//  Get the public key to be used for signature verification.
CkoPublicKey *pubKey = [[CkoPublicKey alloc] init];
success = [pubKey LoadFromFile: @"rsaKeys/chilkat-rsa-2048.pem"];
if (success == NO) {
    NSLog(@"%@",pubKey.LastErrorText);
    return;
}

CkoRsa *rsa = [[CkoRsa alloc] init];
success = [rsa UsePublicKey: pubKey];
if (success == NO) {
    NSLog(@"%@",rsa.LastErrorText);
    return;
}

//  Verify the string against the signature.
//  We signed the utf-8 byte representation of the string, so we must indicate
//  to also validate against the utf-8 byte representation.
rsa.Charset = @"utf-8";
rsa.EncodingMode = @"base64";
NSString *signedString = [sb GetAsString];
success = [rsa VerifyStringENC: signedString hashAlg: @"sha256" sig: base64_rsa_sig];

if (success == NO) {
    NSLog(@"%@",rsa.LastErrorText);
    NSLog(@"%@",@"Signature invalid.");
}
else {
    NSLog(@"%@",@"Signature valid.");
}