Monday, 20 November 2017

No, your web site being broken is not OK!

Those of us who write software know that we make mistakes. Developers don't consider specific scenarios, Testers miss certain tests and with the best will in the world, even the simplest applications can have bugs.

BUT

The main happy paths should work largely fine. If something goes seriously wrong for a large-scale public-facing web site, one of a number of things absolutely must happen:

  1. Ideally, the company will already know because they will get an error message emailed/displayed on a big screen/whatever
  2. If it is more subtle, maybe a user will contact the company and if this happens, it is embarrassing, so you act immediately, especially when the bug relates to a happy path that you ABSOLUTELY should have tested
  3. If it is something with a non-obvious workaround (or none at all) the Development Team make it number 1 priority and work flat out, 24/7 if required, until it is fixed. Why? Because it was a screw up that something so serious got out the door and it is a matter of quality and corporate pride that it gets fixed and quickly.
  4. The Test Manager gets a serious talking to along the lines of, if this happens again, you're fired.
  5. The Technical Team has a serious review about how this was allowed to happen and puts in place real measures to prevent a repeat the next time. This is fed back to the Management Team so that people can be accountable where they need to be - the Management Team need to ensure they are getting the whole truth, not just what someone might say to cover their own back.
What isn't OK is:

  1. Not putting any kind of banner on the web site to say that you are experiencing problems
  2. Not working with whoever found the problem to quickly work out exactly what has happened and why
  3. Telling users to delete their cookies to make it work
  4. Telling users that only some users are having the problem (as if that makes it better that it's broken for me)
  5. Not properly testing updates to consider not just the new site in a clean happy place, but what happens when a user with existing cookies and a number of different browsers comes back to a new site.
  6. Acting like a serious bug report from an end user is just business-as-usual rather than, "I'm really sorry, I'm just going to call the Software Manager to tell them" or even, "We know of a problem and the Team are still trying to find exactly what causes it".

What do we assume when we contract people to write software for us?

Many of you have been there. You need something done, you find someone, they give you a quote for the work, you agree, they do stuff then send it to you. It's just not really good enough. It's probably not terrible (but it might be) it's probably OK but there are enough things wrong with it that you can't just shake hands and pay them their money.

On the other hand, they have put in the hours, so it's probably not right just to not pay them anything but if you are going to have to mostly rewrite it - which defeats the point of getting the work done - what can you do short of legal action?

One of the most obvious things that we don't always do is write a good contract/requirements. Like a good Job Description, if you write it well, it should simply be a case of "is the person doing this?" if so, great, if not, they don't get paid.

Let us take an example. We want someone to write a plugin for Magento that handles an OAuth2 handshake for a web site. Sounds simple and it sounds like something that a Contractor would say, yeah, OK, I'll do that! But there are many things missing from such a simple requirement. One of those might be a simple question: "Have you ever written a Magento plugin before?" Why? Because although the PHP might be easy, the architecture and philosophy of Magento is not something you can simply learn from a book in a few days and I certainly don't want to pay a Contractor to watch videos and try and learn it. What if they produce something that seems to work but they did it really badly? You might not know until later.

Secondly, assuming that they have some experience that you are comfortable with, there is then a question of quality and speed. Most of the time, we are not contracting to crazy deadlines but there is still a large difference between fast and slow, especially when you are paying a day-rate of money and to make it worse, speed and quality are proportional so fast is not always good. How can you tell what their quality and speed are like before taking the plunge and committing to large amounts of money?

You can do two simple things up-front. Ask them to send you an example of some of their code from another project (or even something they have contributed to Github or whatever) does it read well? Does it look like a professional or someone who might have made something work by luck rather than skill! Secondly, set them a test - or rather the first part of the work. For example, in our example above, ask them for the basics of a plugin that doesn't do much (maybe does a browser redirect) using some hard-coded values, some basic UI - anything that should be quick and easy, the hard stuff is always in the details. You can pay them for that work if they have done OK up until now and then review what they've done and decide whether it is good and whether it matches the expectations they set. You should be honest with these people - if they don't convince you that they are producing good enough code in reasonable time, you are not going to continue to use them. Paying 2 days up-front for a project that might be 2 months long is good business! Best to lose a small amount early on and find someone else than to have to fix everything later.

Another important point is to document and communicate your expectations. If they need code to look neat, it needs to be said. Not all Developers care and if they don't know you need that, you can't complain to them when they don't deliver it. What about Unit Tests? Design sign-off? Acceptable libraries? Browser testing? If there is something complex that your project involves, can you separate that into another package and get them to prove they can do that? If not, let them do the easy stuff and pay someone else to do the hard bit.

Hopefully, you eventually find some good Contractors who you trust, whose code you know is quality, who are responsive to the work you are giving them and who are not charging crazy amounts of money in the process. This will be ongoing if your business is growing but so many of us have to use Contractors that it is a skill that your company needs to have.

Wednesday, 15 November 2017

How to interview for a Senior Developer

This is based on my experiences in the UK, trying to recruit quality people into Senior positions. My conclusion, there is a very big difference between how people view themselves and how I view the role of Senior Developer. The average salary request for a Senior Dev in the UK (outside of London) is about £50K+ ($65K+), which for many companies is a lot of money to pay out in addition to the recruiters fees, which can be anything up to about 25% of that yearly salary and all of this before you even know whether the person is any good.

I am an employer and I get nervous when I interview someone. They are usually polite and of course they can do the job that you need them to do but the simple truth is that the recruiter and the potential recruit have a virtually zero-risk opportunity to talk themselves up to convince me they can do the job. If I take the risk on them and they are not very good, I either have to let them go at 3 months, losing several thousand in recruiters fees and potentially wasting a lot of time on a person who takes more than they are bringing to the company.

If you are that person who is applying for a role at my company, what am I going to ask you?

Firstly and hopefully this shouldn't be a shock, I am going to ask you about your experience in the areas of the job description. Example: This position requires a strong interest or experience in web application security. "Tell me about your experience in web application security", "I haven't done much". "Then why are you here wasting my time just on the hope that somehow you will convince me that I should still take you on?"

We even had a guy apply for a Development Manager position and all of my questions about, "What will you need to do as a Manager that you don't currently do as a Developer?" basically caused responses along the lines of "erm...", "hmmm.." as if the person hadn't even asked himself what a Development Manager actually does.

Secondly, I will ask what it is that makes you Senior (even if you are not a Senior, I would still ask you what separates you from the crowd) and I am fed up with the number of times that the answer is basically, "It means I have more experience", "What experience do you have that a Junior doesn't?". "Ermm...."

What do you know about Dependency Injection? IoC Containers? Test-Driven-Development? Deployment? The cloud? Node js? Angular?. These are all things that I would expect a Senior Developer to understand. Not to be super-experienced: we don't all get to do these at work - but anyone with any decent interest in web design meets these subjects all across the web. Even if you don't know exactly what it is, do you not even know the basics of why an IoC container might be useful? If not, why not?

Thirdly, I will ask why you are special. So you know some stuff about .Net and you have been programming for 15 years? Top tip: I don't care about anything before the last 5 years because we don't use Web Forms, VBA or FoxPro here! We are a startup and it takes commitment, interest and passion. Don't have a blog? Why not? Your own web site? Involved in any clubs outside of work? Developer hangout events? Member of an Institution?

The simple reality is that for most of the people we have interviewed, the sum total of their CV is: I have been writing code for average companies for X years and there is nothing that demonstrates that I am anything other than a sheep who will do what I'm told but I never think of the bigger picture and my job is largely just to pay the bills.

Even though the market for Developers in the UK is massive and the supply is terrible, I will not take any person on who is asking for £50K just because they have 15 years in the business. If you want that Senior Developer job, you should love coding. You should love it so much that you can easily demonstrate how much you love it. How you owned stuff in your previous job, you were the go-to person, you built stuff, fixed stuff, upgraded it, especially when you weren't asked to do it!

.Net MVC Controller Action 404 only on one system!

We have an existing web app that is running successfully in production. We deployed it to a new machine and any action we try and access on a certain controller returns a 404 (an IIS one, not the custom 404 page that we use in the app). The Home controller and another controller seemed to work as expected.

Logging proved fruitless, perhaps because the underlying problem was masking the logs.

Long story short: The controller that wasn't working had a default constructor (that was creating some services). One of these services was failing (as it happens due to the format of a connection string being incorrect), which caused the constructor to fail and rather than a 500, IIS/.Net was producing a 404 instead!

Because of the lack of usable logs, I had to keep commenting things in and out to work out what was actually going wrong (as well as adding an empty 'test' action to remove any other possible variables).

Tuesday, 24 October 2017

Address labels in PHP without special software

This sounded like a simple job. Print out address labels from a database in a certain format (8 x 2 labels on a page) in a way that can then be printed directly onto the label sheets. Easy right?

Not so much. HTML and CSS3 is supposed to add a load of print functionality and physical sizes but they don't work well at all. Browser treat them all differently, Chrome applies margins in addition to what you set in CSS so everything gets squashed and whatever I tried, it didn't seem to make sense. On top of that, the Developer tools allow you to render using the print css but this does not really allow a real print preview while tweaking the styles.

Fortunately, I chanced upon a suggestion to use FPDF, a PHP library to generate PDFs in code. It looked easy enough although unfortunately, you cannot simply create a fixed size "cell" and wrap text in it. A Cell is one line of text and multi-cell will simply create more cells for each new line of text. Not quite right but fortunately, using the position functions setX, setY etc. the maths is relatively simple to keep track of column number, row number and then work out where to add a new page.

Use the following code as a reference - note it is from Yii 2 framework and so some of this won't be relevant to you. Then check out the notes below for additional help.

public function actionAddresslabels()
{
    $request = \Yii::$app->request;

    // Set defaults for layout
    $cols = $request->get('cols', 2);           // Number of columns
    $rows = $request->get('rows', 8);           // Number of rows
    $top = $request->get('top', 8);             // Top margin in mm
    $left = $request->get('left', 5);           // Left margin in mm
    $vspacing = $request->get('vspacing', 0);   // Spacing vertically between each label in mm (excludes outside margins)
    $hspacing = $request->get('hspacing', 2.5);   // Spacing horizontally between columns in mm (excludes outside margins)
    $padding = $request->get('padding', 3);

    // Compute some numbers
    $pageSize = $rows * $cols;
    $colSpacing = (210.0 - (2*$left) + $hspacing) / ($cols);
    $rowSpacing = (297.0 - (2*$top) + $vspacing) / ($rows);

    $dataProvider = new ActiveDataProvider([
        'query' => User::find()
            ->joinWith(['applications'])
            ->where(['year' => Date('Y')]),
        'pagination' => false,
    ]);

    // Load data into local variables for loop
    $models = $dataProvider->getModels();
    $modelCount = $dataProvider->getCount();
    $currentModel = 0;
    $currentY = 0;
    $currentX = 0;

    // Basic setup of PDF
    $pdf = new FPDF();
    $pdf->SetLeftMargin($left + $padding);
    $pdf->SetTopMargin($top + $padding);
    $pdf->SetFont('Arial','',11);
    $pdf->SetAutoPageBreak(false);

    // For each cols x rows of addresses, add a page and render them correctly
    while ( $currentModel < $modelCount )
    {
        if ( $currentModel % $pageSize === 0)
        {
            $pdf->AddPage();
            $currentX = $left + $padding;
            $currentY = $top + $padding;
        }
        $pdf->SetXY($currentX,$currentY);
        $pdf->SetLeftMargin($currentX);
        $model = $models[$currentModel];
        $this->writeAddressLabel($pdf, $model);
        $currentY += $rowSpacing;
        if ( $currentY > (297 - 20) )
        {
            $currentY = $top + $padding;
            $currentX += $colSpacing;
        }
        $currentModel++;
    }

    $this->layout = false;
    \Yii::$app->response->format = \yii\web\Response::FORMAT_RAW;
    $pdf->Output();
    \Yii::$app->end();
}


  • The first section allows you to pass different values from the defaults into the query string for this action.
  • $padding allows all the text to be in from the top-left corner of each label and needs to be included in various calculations
  • The second section does some calculations for page size (number of labels total per page), column and row spacing are the pitch values so include the gutters between the labels.
  • The ActiveDataProvider is simply how I am querying the people to produce labels for. What I end up with is an array of objects ($models) that I will pull the individual address parts from.
  • $modelCount is simply used to control how long the loop below will continue for
  • The next section sets some static values for the PDF instance. The margins will shift all of the setXY stuff in from the edges of the page.
  • The main loop goes through all of the "users" in my models array 1 by one. 
  • The first section inside the loop uses mod arithmetic to see whether the current item is the first on a page, in which case a new page is created, and the X and Y positions are reset (they are relative to the current page, not the entire document).
  • The cursor is then positioned with SetXY
  • SetLeftMargin is called to ensure the current column has a hard left edge, otherwise the text becomes indented.
  • The method WriteAddressLabel is a helper method in my class that simply contains a number of calls to $pdf->Write(5,$model->town.PHP_EOL); With some wrapped in if ( $model->address3 !== "") so that they are not printed if blank. In your code, they might equate to null but in my code, they are blank strings if not set.
  • After the address is written, the Y position is moved down by a label pitch and then if this goes below the bottom of the page (hard-coded for A4 paper size 297mm minus a margin), then the column is incremented, Y is reset back to the top. We do not need to check for the column overflowing the page, since the mod arithmetic at the top of the loop will automatically create a new page when we have written the total number of items on the page.
  • The 4 lines below the loop are partly methods to tell Yii to output the correct format and not render a HTML layout and the call to $pdf->Output() closes the document and sends it to the standard output, which in this case is the response object.


Monday, 16 October 2017

JWT, JWE, JWS in .Net - Pt 3 - JWE

JWE is the encrypted version of a JWT. Encryption provides a way to ensure privacy of the data from an attacker and if using a pre-shared key, a very strong way of transmitting private data.

The .Net version of the JWT libraries does not also require a signature to be applied, you could assume that the data has integrity if you use an AEAD algorithm for encryption - which you should. However, it appears that you cannot validate the token if it does not have a signature - I'm not sure if there is a way to do that or whether it does not make sense to validate a token with no signature?

Fortunately, to produce a JWE in .Net is very similar to producing a JWS, although you need to generate a cryptographically secure symmetrical key as well as using a certificate to sign it. Naturally, all of this has overhead so although encryption-by-default can be useful, it does come at a price, especially for high-volume systems.

To create a key (the Content Encryption Key - CEK) , you can either just use RNGCryptoServiceProvider from the System.Security.Cryptography namespace like this:

var crng = new RNGCryptoServiceProvider();
var keyBytes = new byte[32];   // 256 bits for AES256
crng.GetBytes(keyBytes);

Or you can hash some other piece of data using SHA256 to stretch it. Be careful with this method since you need the input to the SHA function to already be cryptographically secure random or an attacker could discover a pattern and work out how you generate your keys! For instance, do not stretch a numeric userid or guid. In my case, I was stretching a 32 character randomly generated "secret" from an oauth credential to create my pre-shared key.

var keyBytes = SHA256.Create().ComputeHash(Encoding.UTF8.GetBytes("some data to stretch"));

Be careful with SHA256 and other cryptography classes for thread safety. It might be quicker to pre-create certain types like SHA256 but if ComputeHash is not thread safe, you might break something when used by multiple threads. I believe some forms of the .Net cryptography classes are thread safe and others are not.

Once you have your CEK, the only extra step is to create EncryptingCredentials as well as SigningCredentials:

var symSecurityKey = new SymmetricSecurityKey(keyBytes);
var creds = new EncryptingCredentials(symSecurityKey, SecurityAlgorithms.Aes256KW, SecurityAlgorithms.Aes256CbcHmacSha512);

Note that you need to use algorithms that are supported in .Net (I can't guarantee that the SecurityAlgorithms enum equates to what is supported), that the selected algorithms match the length of the key provided (i.e. 32 bytes for AES256) and that the second algorithm, which is used to encrypt the actual data is a type that includes authenticated data - i.e. a signature for the encrypted data to verify it was not tampered with before decrypting (such as GCM or HMACSHA). If you choose the wrong length of key, the call to CreateSecurityToken will throw a ArgumentOutOfRangeException. The first algorithm is the one that will be used to encrypt the key itself before it is added to the JWE token.

You can use RSA-OAEP for the first parameter but this is not the same as when it is used for the JWE. Firstly, it will only use a 256 bit key for RSA to match the second algorithm (the size of the key) but also, it will need a public key to encrypt and the recipient of the token will need the related private key to decrypt the CEK.

By providing the SigningCredentials and EncryptingCredentials to the call to CreateSecurityToken(), the library will create the token, sign it and then encrypt this as the payload in an outer JWE. This means that the header for the JWT will only contain data about the encrypting parameters (alg, enc etc) and only after it is decrypted, will the signing parameters be visible.

As mentioned before, you do not have to set a SigningCredential but when I tried this, the call to ValidateToken failed which sounds like it cannot validate data that is only encrypted, which might be possible to bypass (since the encrypted data already requires the use of an authenticated algorithm),

Validating is otherwise the same as it is for JWS, except for also setting the value of the TokenDecryptionKey in the TokenValidationParameters in the same way as it was set when it was created.

JWT, JWE, JWS in .Net - Pt 2 - Validating JWS

Fortunately, validating a JWS (and for that matter, a JWE) is very straight-forward thanks to JwtSecurityTokenHandler.ValidateToken().

Quite simply, you take the serialized string, create a TokenValidationParameters object with the relevant fields filled in to validate and then call ValidateToken, it looks like the following. Note that the same code is used for JWS and JWE tokens, the only difference is whether you fill in the TokenDecryptionKey property. This shows both:

 private ClaimsPrincipal ValidateToken(string tokenString, byte[] keybytes)  
 {  
   var signingkey = new X509SecurityKey(new X509Certificate2(certFilePath,certPassword));  
   var jwt = new System.IdentityModel.Tokens.Jwt.JwtSecurityToken(tokenString);  
   // Verification  
   var tokenValidationParameters = new TokenValidationParameters()  
   {  
     ValidAudiences = new string[]  
     {  
       "123456"  // Needs to match what was set in aud property of token
     },  
     ValidIssuers = new string[]  
     {  
       "https://logindev.pixelpin.co.uk"  // Needs to match iss property of token
     },  
     IssuerSigningKey = signingkey,  
     TokenDecryptionKey = keybytes == null ? null : new SymmetricSecurityKey(keybytes)  
   };  
   SecurityToken validatedToken;  
   var handler = new System.IdentityModel.Tokens.Jwt.JwtSecurityTokenHandler();  
   return handler.ValidateToken(tokenString, tokenValidationParameters, out validatedToken);  
 }  

In my method (in a Unit Test), I simply return the ClaimsPrincipal that is returned from ValidateToken() but you could also get the validated and decrypted token that is returned as an out parameter if you wanted to continue to use it.

Also note that I am simply loading the same pfx I used to sign the token to validate it, whereas in real like, you are likely to visit the url of the issuer and perhaps https://theurl.com/.well-known/jwks and find the public key for the signed data using the kid property from the token.

This method allows the caller to pass null for the keybytes if only validating a signed JWS or real key bytes matching the encryption key used for the JWE. This is for pre-shared keys only. In a later post, I will talk about extracting the encryption key, which is actually embedded in a JWE and does not need to be pre-shared.

In part 3, we'll look at JWE (encrypted JWT)