Sha256 in Objective-C for iPhone

By | January 12, 2018
Questions:

When I use this code to create a sha256 of a string

unsigned char hashedChars[32];
NSString *inputString;
inputString = [NSString stringWithFormat:@"hello"];
NSData * inputData = [inputString dataUsingEncoding:NSUTF8StringEncoding];
CC_SHA256(inputData.bytes, inputData.length, hashedChars);

It returns the hash correctly, but I need to insert a string like this \x00\x25\x53 and in this case, the function returns a sha256 of empty string because the specified encoding cannot be used to convert the receiver.

Now, my question is:How to insert this special characters for generate a correct hash? Thanks

Answers:

Try this, it worked for me

1) To get a hash for plain text input

-(NSString*)sha256HashFor:(NSString*)input
{   
    const char* str = [input UTF8String];
    unsigned char result[CC_SHA256_DIGEST_LENGTH];
    CC_SHA256(str, strlen(str), result);

    NSMutableString *ret = [NSMutableString stringWithCapacity:CC_SHA256_DIGEST_LENGTH*2];
    for(int i = 0; i<CC_SHA256_DIGEST_LENGTH; i++)
    {
        [ret appendFormat:@"%02x",result[i]];
    }
    return ret;
}

2) To get hash for NSData as input

Note:- I have used NSData category, so the code is as follow

- (NSString *)SHA256_HASH {
//if (!self) return nil;

unsigned char hash[CC_SHA256_DIGEST_LENGTH];
if ( CC_SHA256([(NSData*)self bytes], [(NSData*)self length], hash) ) {
    NSData *sha2 = [NSData dataWithBytes:hash length:CC_SHA256_DIGEST_LENGTH]; 

    // description converts to hex but puts <> around it and spaces every 4 bytes
    NSString *hash = [sha2 description];
    hash = [hash stringByReplacingOccurrencesOfString:@" " withString:@""];
    hash = [hash stringByReplacingOccurrencesOfString:@"<" withString:@""];
    hash = [hash stringByReplacingOccurrencesOfString:@">" withString:@""];
    // hash is now a string with just the 40char hash value in it
    //NSLog(@"hash = %@",hash);

    // Format SHA256 fingerprint like
    // 00:00:00:00:00:00:00:00:00
    int keyLength=[hash length];
    NSString *formattedKey = @"";
    for (int i=0; i<keyLength; i+=2) {
        NSString *substr=[hash substringWithRange:NSMakeRange(i, 2)];
        if (i!=keyLength-2) 
            substr=[substr stringByAppendingString:@":"];
        formattedKey = [formattedKey stringByAppendingString:substr];
    }

    return formattedKey;
}
return nil;

}

Questions:
Answers:

It’s important to know that you need to import:

#import <CommonCrypto/CommonDigest.h>

Hope this help!

Questions:
Answers:

You probably should use NSData instead of NSString then. Where do you get that string from?

Questions:
Answers:

Some one looking the solution in Swift 3.0. here is

extension String {

// MARK: - SHA256
func get_sha256_String() -> String {
    guard let data = self.data(using: .utf8) else {
        print("Data not available")
        return ""
    }
    return getHexString(fromData: digest(input: data as NSData))
}

private func digest(input : NSData) -> NSData {
    let digestLength = Int(CC_SHA256_DIGEST_LENGTH)
    var hashValue = [UInt8](repeating: 0, count: digestLength)
    CC_SHA256(input.bytes, UInt32(input.length), &hashValue)
    return NSData(bytes: hashValue, length: digestLength)
}

private  func getHexString(fromData data: NSData) -> String {
    var bytes = [UInt8](repeating: 0, count: data.length)
    data.getBytes(&bytes, length: data.length)

    var hexString = ""
    for byte in bytes {
        hexString += String(format:"%02x", UInt8(byte))
    }
    return hexString
}}

How to Use it

let signatures = "yourStringToBeConverted".get_sha256_String()

also don’t forgot to import #import <CommonCrypto/CommonHMAC.h> in your Bridging-Header.h

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