Disabling Rosetta
When using the OS X keychain, if you encounter decryption errors in Safari or any other browser, please make sure the browser is not configured to run under Rosetta. Rosetta is a special technology made to support old PowerPC applications on Intel Macs and is not needed to run modern, Universal applications.
To make sure that Safari is not running under Rosetta, select Safari in the /Applications folder and click File > Get Info (or press ⌘I). Disable the Open using Rosetta option: