Objective-C

ECDSA Sign Data and Get Raw R and S Values

See more ECC Examples

Demonstrates getting the raw R and S value of an ECDSA signature. Objective-C

#import <CkoCrypt2.h>
#import <NSString.h>
#import <CkoPrivateKey.h>
#import <CkoPrng.h>
#import <CkoEcc.h>
#import <CkoAsn.h>
#import <CkoXml.h>
#import <CkoBinData.h>

BOOL success = NO;

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

//  To create an ECDSA signature, the data first needs to be hashed.  Then the hash
//  is signed.

CkoCrypt2 *crypt = [[CkoCrypt2 alloc] init];
crypt.HashAlgorithm = @"SHA256";
crypt.Charset = @"utf-8";
crypt.EncodingMode = @"base64";

//  Hash a string.
NSString *hash1 = [crypt HashStringENC: @"The quick brown fox jumps over the lazy dog"];
NSLog(@"%@%@",@"hash1 = ",hash1);

//  -----------------------------------------------------------
//  An ECDSA private key is used for signing.  The public key is for signature verification.
//  Load our ECC private key.
//  Our private key file contains this:

//  	// -----BEGIN PRIVATE KEY-----
//  	MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQg3J8q/24D1sEKGdP9
//  	72MGYElLGpw/a56Y3t6pfON3uhShRANCAATlSmoizyhAwoYZAOuFBATl07/1RR54
//  	a1Dzfm16grxJe666AGKR+bSs24hk7TEpaeCTvT8YOOM3l+xKFg7zq6Q9
//  	-----END PRIVATE KEY-----

CkoPrivateKey *privKey = [[CkoPrivateKey alloc] init];
success = [privKey LoadPemFile: @"qa_data/ecc/secp256r1-key-pkcs8.pem"];
if (success != YES) {
    NSLog(@"%@",privKey.LastErrorText);
    return;
}

//  Sign the hash..
CkoPrng *prng = [[CkoPrng alloc] init];
CkoEcc *ecdsa = [[CkoEcc alloc] init];
NSString *ecdsaSigBase64 = [ecdsa SignHashENC: hash1 encoding: @"base64" privkey: privKey prng: prng];
if (ecdsa.LastMethodSuccess != YES) {
    NSLog(@"%@",ecdsa.LastErrorText);
    return;
}

//  The ECDSA signature is ASN.1 that contains a sequence of 2 large integers (r and s)
//  For example:
//  SEQUENCE (2 elem)
//    INTEGER (255 bit) 792134D9B4AD82D5431ED03835A88E2596EB35E5B13054BD9B05A0069281ACC9
//    INTEGER (255 bit) 481E758CC1E3CBF825537EC3D9A2CA627E5FAD1137BBEA65DF38658DCB0A9ED5

NSLog(@"%@%@",@"Base64 ECDSA signature = ",ecdsaSigBase64);

//  If the raw R and S values are needed, here's how to get them:
CkoAsn *asn = [[CkoAsn alloc] init];
success = [asn LoadEncoded: ecdsaSigBase64 encoding: @"base64"];
if (success == NO) {
    NSLog(@"%@",asn.LastErrorText);
    return;
}

//  The R and X will be in hexidecimal in the XML.
CkoXml *xml = [[CkoXml alloc] init];
[xml LoadXml: [asn AsnToXml]];
NSLog(@"%@",[xml GetXml]);

//  The XML looks like this:
//  <sequence>
//  <int>792134D9B4AD82D5431ED03835A88E2596EB35E5B13054BD9B05A0069281ACC9</int>
//  <int>481E758CC1E3CBF825537EC3D9A2CA627E5FAD1137BBEA65DF38658DCB0A9ED5</int>
//  </sequence>

//  Copy raw R and S hex values into a Chilkat BinData object.
CkoBinData *bd = [[CkoBinData alloc] init];
NSString *r = [xml GetChildContent: @"int[0]"];
NSString *s = [xml GetChildContent: @"int[1]"];
[bd AppendEncoded: r encoding: @"hex"];
[bd AppendEncoded: s encoding: @"hex"];

NSLog(@"%@%d",@"Number of bytes in bd: ",[bd.NumBytes intValue]);