The most comprehensive information on Salesforce security can be found on their Developer website: https://developer.salesforce.com/page/Security.
Also attached are 3 whitepapers from Salesforce providing greater details on platform security. The whitepapers include:
- Overview on Security, Privacy & Trust: higher level overview on Salesforce's platform architecture, security and privacy policies
- Platform Encryption Architecture: detailed discussion on encryption and key management
- Salesforce Security Guide: provides an overview, best practices and admin steps for implementing security in a Salesforce org
- Salesforce Platform Encryption Implementation Guide: provides admin steps for implementing platform encryption