Generating public key from modulus and exponent

Question

I've Modulus

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

and Exponent

65537

Please help me on generating public key using modulus and exponent using objective-C.

score 1 · Accepted Answer · edited May 23 '17 at 11:59

As written by zaph in this answer, the following code should do what you want :

NSData* bytesFromHexString(NSString * aString) {
    NSString *theString = [[aString componentsSeparatedByCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] componentsJoinedByString:nil];

    NSMutableData* data = [NSMutableData data];
    int idx;
    for (idx = 0; idx+2 <= theString.length; idx+=2) {
        NSRange range = NSMakeRange(idx, 2);
        NSString* hexStr = [theString substringWithRange:range];
        NSScanner* scanner = [NSScanner scannerWithString:hexStr];
        unsigned int intValue;
        if ([scanner scanHexInt:&intValue])
            [data appendBytes:&intValue length:1];
    }
    return data;
}

NSString *modulusString =  @"...";
NSString *exponentString = @"65537";

NSData *pubKeyModData = bytesFromHexString(modulusString);
NSData *pubKeyExpData = bytesFromHexString(exponentString);
NSArray *keyArray = @[pubKeyModData, pubKeyExpData];

//Given that you are using SCZ-BasicEncodingRules-iOS:
NSData *berData = [keyArray berData];
NSLog(@"berData:\n%@", berData);

NSString *berBase64 = [berData base64EncodedStringWithOptions:0];
NSString *preamble = @"-----BEGIN CERTIFICATE REQUEST-----";
NSString *postamble = @"-----END CERTIFICATE REQUEST-----";
NSString *pem = [NSString stringWithFormat:@"%@\n%@\n%@", preamble, berBase64, postamble];
NSLog(@"pem:\n%@", pem);

I've tried this, but generated public key is failing to encrypt the data. — Manjunath, Nov 18 '16 at 04:27
It's actually generating the content of a pem file. Could you show how you are encrypting the data ? — Sunder, Nov 18 '16 at 08:51
iOS doesnt provide machanism of generating public key as we get it from java or android to be very specific. solove the issue by adding an API and sending mod and exp to server and get the public key back. Thanks for help — Manjunath, Nov 25 '16 at 07:51

score 0 · Answer 2 · answered Nov 22 '16 at 00:52

To do this make sure you have the OpenSSL library linked (instructions here http://code.google.com/p/ios-static-libraries/)

Once linked you'll have access to several BIGNUM converters. I turned the modulus into hex using the BN_hex2bn method saving the hex string into 'exponent'

Then create the BIGNUM struct and encrypt using RSA_public_encrypt

RSA *rsa = NULL;
rsa->n = BN_new();
BN_copy(rsa->n,modulus);   
rsa->e = BN_new();
BN_copy(rsa->e,exponent);     
rsa->iqmp=NULL;
rsa->d=NULL;
rsa->p=NULL;
rsa->q=NULL;

Generating public key from modulus and exponent

2 Answers2

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