main.m

#import <Foundation/Foundation.h>

@interface NSData (MD5)
- (NSString *)MD5String;
@end
#import <CommonCrypto/CommonDigest.h>
@implementation NSData (MD5)
- (NSString *)MD5String {
    unsigned char result[16];
    CC_MD5(self.bytes, self.length, result);
    
    return [NSString stringWithFormat:
            @"%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X",
            result[0], result[1], result[2], result[3],
            result[4], result[5], result[6], result[7],
            result[8], result[9], result[10], result[11],
            result[12], result[13], result[14], result[15]
            ];  
}
@end

int main(int argc, const char * argv[])
{
    @autoreleasepool {
        // get the public key
        NSString *pubkey = [[NSString alloc] initWithData:[NSData dataWithContentsOfFile:[@"~/.ssh/id_rsa.pub" stringByExpandingTildeInPath]] encoding:NSASCIIStringEncoding];
        NSLog(@"pubkey = %@", pubkey);
        
        // extract the middle portion
        NSString *dataEncoded = [pubkey componentsSeparatedByString:@" "][1];
        
        // base64 decode it
        NSData *dataDecoded = [[NSData alloc] initWithBase64EncodedString:dataEncoded options:0];
        
        // md5 it
        NSString *md5 = [dataDecoded MD5String];
        
        // add colons
        NSMutableString *_fp = [NSMutableString new];
        for (int i = 0; i < md5.length; i++) {
            [_fp appendString:[md5 substringWithRange:NSMakeRange(i, 1)]];
            if (i % 2 && i < md5.length - 1)
                [_fp appendString:@":"];
        }
        
        // make everything lowercase
        NSString *fp = [_fp lowercaseString];
        
        NSLog(@"fingerprint = %@", fp);
    }
    return 0;
}