How do I perform SHA-2 family hashing in my PHP application? (more…)

• X.509 Wikipedia page
• RFC2459 - Internet X.509 Public Key Infrastructure (Obsolete)
  
• RFC3280 - Internet X.509 Public Key Infrastructure
  
• Introduction to x.509 certificates
• openssl x.509 manual page
• Creating a self-signed X509 server certificate and a PKCS12 client certificate with openssl
• php OpenSSL functions
• Web Services Security for dummies (Powerpoint Presentation)
• Trust Yourself: Become your own Certification Authority

Add encryption to Gmail - Lifehacker
Lifehacker wrote

Greasemonkey script Gmail Encrypt adds encryption to the emails you write in Gmail.

http://lifehacker.com/software/email/add-encryption-to-gmail-177061.php