- (NSString *)convertDataToHexStr:(NSData *)data{
    if (!data || [data length] == 0) {
        return @"";
    }
    NSMutableString *string = [[NSMutableString alloc] initWithCapacity:[data length]];
    
    [data enumerateByteRangesUsingBlock:^(const void *bytes, NSRange byteRange, BOOL *stop) {
        unsigned char *dataBytes = (unsigned char*)bytes;
        for (NSInteger i = 0; i < byteRange.length; i++) {
            NSString *hexStr = [NSString stringWithFormat:@"%x", (dataBytes[i]) & 0xff];
            if ([hexStr length] == 2) {
                [string appendString:hexStr];
            } else {
                [string appendFormat:@"0%@", hexStr];
            }
        }
    }];
    
    return string;
}

簡潔版

- (NSString *)getHexStringForData:(NSData *)data{
    NSUInteger len = [data length];
    char *chars = (char *)[data bytes];
    NSMutableString *hexString = [[NSMutableString alloc] init];
    for (NSUInteger i = 0; i < len; i++) {
        [hexString appendString:[NSString stringWithFormat:@"%0.2hhx", chars[i]]];
    }
    return hexString;
}