Month: December 2009

Rewriting for SEO-Friendly URLs: .htaccess or PHP?

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Modern database driven web sites implement SEO-friendly URLs emulating static directories and files. Switching to such “clean” URLs enables good indexing by search engines, makes URLs more user-friendly and hides the server-side language. For example, this clean URL may refer to the page in some product directory:

http://somesite.com/products/network/router.html

In fact, there is no /products/network folder on the server and no router.html file at all. The page is generated by server script using database query for “network” product category and “router” product. But who calls the script and where it gets the query parameter values?

This technique is usually referred as “URL rewriting”. It allows web server to recognize what information was requested by parsing the URL string. Apache and PHP allow multiple options to implement URL rewriting. So which one is the best?

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PHP regular expressions and UTF-8

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Perl-compatible regular expression functions in PHP can properly work with Unicode strings. Just add /u modifier to turn on UTF-8 support in preg_replace, preg_match, preg_match_all, preg_split and other PCRE (preg) functions. This way you can parse strings with national characters. For example:

$clean = preg_replace('/\s\s+/u', ' ', $dirty);

If used without /u modifier this code damages UTF-8 encoded strings by replacing national character bytes improperly interpreted as whitespace characters. This and many other problems are caused by improper interpretation of every byte as ASCII character which is not always true for UTF-8.

The modifier is available from PHP 4.1.0 or greater on Unix and from PHP 4.2.3 on win32. UTF-8 validity of the pattern is checked since PHP 4.3.5.
I found this tip as well as many other useful info on regular-expressions.info. It’s not easy to find it in the PHP documentation but it’s actually hidden here.

SEO-friendly URLs and relative links

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The Web community is going crazy about SEO-friendly URLs like http://somesite.com/products/network/router/. Well, it looks much better than a script URL http://somesite.com/products.php?c=network&p=router which may actually serve the page behind the scenes. There are a lot of good articles on how to implement SEO-friendly URLs, for example this one or my own post. But they do not warn the reader about one usual problem: once you have updated your site to handle virtual paths you will probably get a bad surprise:

CSS, image and internal page links are totally broken!

Why? Because those links are usually relative to the page location. The browser has no idea about virtual folders and tries to get files from locations relative to the page URL context. For example, if there is a usual CSS link in the page header:

<link rel="stylesheet" href="style.css" type="text/css" media="screen" />

Then the browser will try to download non-existing file http://somesite.com/products/network/router/style.css and fail silently. No CSS style will be applied.

It’s incredible how many words were spoken about SEO-friendly URLs with almost no word about this relative link problem.
So, what you have to do? Don’t worry, there are multiple solutions available and I’ll try to explain them all.

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Reading, writing and converting RSA keys in PEM, DER, PUBLICKEYBLOB and PRIVATEKEYBLOB formats

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This post finishes my epic about the implementation of RSA encryption. See the part I and part II of my post about RSA encryption for C++/Delphi (CryptoAPI) and PHP (OpenSSL) applications.

The main problem we faced was incompatibility of key formats. CryptoAPI uses PRIVATEKEYBLOB and PUBLICKEYBLOB formats to export and import RSA keys while OpenSSL extension for PHP uses PEM format. In order to use both libraries in communicating applications we needed some tool to convert keys from one format to another. The only tool we found for this was OpenSSL 1.0.x beta. Notice that earlier versions of OpenSSL do not support CryptoAPI BLOBs.

Update: It was found later that CryptoAPI has native functions for key conversion. See “Update” section at the bottom of the post.

Below is a command line syntax example for conversion of private key from PEM to PRIVATEKEYBLOB format:

openssl rsa -inform PEM -in private.pem -outform MS\ PRIVATEKEYBLOB -out private.blob



And this example converts PUBLICKEYBLOB to PEM format:


openssl rsa -pubin -inform MS\ PUBLICKEYBLOB -in public.blob -outform PEM -out public.pem



Notice that backslash (\) in format names. You need to type it as it actually escapes the space character.


However, we found some drawbacks in usage of OpenSSL 1.0.x beta:


    

  • There was no Windows build of it available at the time of the post but we wanted to convert keys on Windows.
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  • We also wanted to convert keys directly in our code w/o any need for external application.
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As far as PRIVATEKEYBLOB, PUBLICKEYBLOB and PEM format structures are known, we decided to develop code that will read and write them using low-level functions. It actually took 1-2 days for me to develop that code so I don’t think it’s a really hard task.


Later we faced another problem: PHP versions prior to 5.2 don’t support openssl_pkey_get_details function. Once again, handling key formats directly helped us to resolve the issue by providing a replacement for the function.


So, let me explain how you can implement reading/writing PEM, DER, PRIVATEKEYBLOB and PUBLICKEYBLOB formats with some code examples in PHP for PEM and DER formats and in C++/VCL for CryptoAPI BLOBs. As the task was a part of a commercial project I cannot post a complete working example here. But I will do my best helping you to assemble such code on your own. You can also request our service at Pumka.net.


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RSA encryption for C++/Delphi (CryptoAPI) and PHP (OpenSSL) [part 2]

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In my previous post I explained that we needed to encrypt a communication messages between Windows C++/VCL client and PHP based web service. We cannot use SSL and decided to use RSA encryption with the help of low-level functions provided by CryptoAPI at the client side and OpenSSL PHP extension at the server.

We also faced and resolved the key incompatibility problem. See my post about this.

In this post I will describe implementation or RSA encryption/decryption and digital signing.

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