An Introduction to OpenSSL Programming, Part II of II
We saw earlier that rehandshaking could cause SSL_write() to return with SSL_ERROR_WANT_READ. Similarly, SSL_read() can return with SSL_ERROR_WANT_WRITE if rehandshake occurs and the write buffers fill up (this is relatively unusual on modern platforms since the write buffers are generally large enough to accommodate typical SSL handshake records). In either case, the correct procedure is to select for read/write-ability and then reissue the offending call. This leads to some confusing logic since it means that under certain circumstances you respond to a socket being ready to read by calling SSL_write().
Let's take the case of SSL_write() first. If SSL_write() returns SSL_ERROR_WANT_READ we set the state variable write_blocked_on_read. Ordinarily, this would cause us to call SSL_read() but because write blocked on read is set, we skip the read loop and fall through to the write loop where the call to SSL_write() is reissued. Note that since the SSL handshake requires a number of reads and writes, it's quite possible that this call won't complete either, and we'll have to reenter the select() loop.
Now consider the case of SSL_read(). If SSL_read() returns SSL_ERROR_WANT_WRITE we set the state variable read_blocked_on_write and reenter the select() loop waiting for the socket to be writable. When it is we arrange to call SSL_read() again (see the second line of the test right after the call to select(). Note that after we call SSL_read() we fall through the the write section of the loop. However, even though the socket is writeable and so the FD ISSET test passes, SSL_write() will only be called if c2sl is nonzero and there's really data to write.
Taken together, these articles demonstrate most of the essentials of writing SSL clients and servers with OpenSSL. However, OpenSSL offers a number of other powerful features that we don't cover, including:
Underlying OpenSSL's SSL implementation is a crypto toolkit implementing all the major cryptographic primitives, including RSA, DH, DES, 3DES, RC4, AES, SHA and MD5, as well as an ASN.1 encoder. Thus, OpenSSL is useful for people writing other kinds of cryptographic applications, even if they don't involve SSL.
OpenSSL contains the basic software required to write a certificate authority (CA). A number of CAs have been written on top of OpenSSL, including the free OpenCA project (see the references section).
OpenSSL now includes an S/MIME implementation, allowing it to be used to write secure mail clients. This is still somewhat of a hard hat area--the S/MIME support isn't quite complete yet and neither is the documentation.
Parts of this article were adapted from my book SSL and TLS: Designing and Building Secure Systems, Addison-Wesley 2000. SSL and TLS offers a comprehensive guide to the SSL protocol and its applications. See www.rtfm.com/sslbook for more information.
Machine-readable source code for the programs presented here can be downloaded from the author's web site at: www.rtfm.com/openssl-examples. The source code is available under a BSD-style license.
You can get OpenSSL from www.openssl.org. The docs are on-line here as well.
The SSLv2 and SSLv3 specifications are on-line at www.netscape.com/eng/security/SSL_2.html and home.netscape.com/eng/ssl3/index.html The specifications for TLS (RFC 2246) and HTTPS (RFC 2818) are available at www.ietf.org/rfc/rfc2246.txt and www.ietf.org/rfc/rfc2818.txt.
Many thanks to Lutz Jaenicke and Bodo Moeller for help with OpenSSL and catching a number of problems with the example programs. Thanks to Lisa Dusseault for review of this article.
Eric Rescorla has been working in internet security since 1993.
|Using Salt Stack and Vagrant for Drupal Development||May 20, 2013|
|Making Linux and Android Get Along (It's Not as Hard as It Sounds)||May 16, 2013|
|Drupal Is a Framework: Why Everyone Needs to Understand This||May 15, 2013|
|Home, My Backup Data Center||May 13, 2013|
|Non-Linux FOSS: Seashore||May 10, 2013|
|Trying to Tame the Tablet||May 08, 2013|
- Using Salt Stack and Vagrant for Drupal Development
- Making Linux and Android Get Along (It's Not as Hard as It Sounds)
- New Products
- Drupal Is a Framework: Why Everyone Needs to Understand This
- Validate an E-Mail Address with PHP, the Right Way
- A Topic for Discussion - Open Source Feature-Richness?
- New Products
- New Products
- The Pari Package On Linux
- Home, My Backup Data Center
- This is the easiest tutorial
3 hours 36 sec ago
- Ahh, the Koolaid.
8 hours 39 min ago
- git-annex assistant
14 hours 38 min ago
- direct cable connection
15 hours 1 min ago
- Agreed on AirDroid. With my
15 hours 11 min ago
- I just learned this
15 hours 15 min ago
15 hours 45 min ago
- not living upto the mobile revolution
18 hours 37 min ago
- Deceptive Advertising and
19 hours 12 min ago
- Let\'s declare that you have
19 hours 13 min ago
Enter to Win an Adafruit Prototyping Pi Plate Kit for Raspberry Pi
It's Raspberry Pi month at Linux Journal. Each week in May, Adafruit will be giving away a Pi-related prize to a lucky, randomly drawn LJ reader. Winners will be announced weekly.
Fill out the fields below to enter to win this week's prize-- a Prototyping Pi Plate Kit for Raspberry Pi.
Congratulations to our winners so far:
- 5-8-13, Pi Starter Pack: Jack Davis
- 5-15-13, Pi Model B 512MB RAM: Patrick Dunn
- Next winner announced on 5-21-13!
Free Webinar: Linux Backup and Recovery
Most companies incorporate backup procedures for critical data, which can be restored quickly if a loss occurs. However, fewer companies are prepared for catastrophic system failures, in which they lose all data, the entire operating system, applications, settings, patches and more, reducing their system(s) to “bare metal.” After all, before data can be restored to a system, there must be a system to restore it to.
In this one hour webinar, learn how to enhance your existing backup strategies for better disaster recovery preparedness using Storix System Backup Administrator (SBAdmin), a highly flexible bare-metal recovery solution for UNIX and Linux systems.