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]);