ios sha256加密

+ (NSString *)sha256HashFor:(NSString *)input{
    const char* str = [input UTF8String];
    unsigned char result[CC_SHA256_DIGEST_LENGTH];
    CC_SHA256(str, (CC_LONG)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]];
    }
    ret = (NSMutableString *)[ret uppercaseString];
    return ret;
}