KEYCHAIN, Mac OS X Keychain File (.keychain)
The KEYCHAIN Data file format
Files with the KEYCHAIN extension are exclusively assigned to files that are created using the Apple Keychain program which is part of the Mac OS X operating system. KEYCHAIN files serve the purpose of storing authentication data. The files automatically authenticate a Mac OS X user, but only if the option to remember authentication information has been turned on prior. Websites, protected files and secured SSH accounts can use KEYCHAIN files as well.
Technical details of KEYCHAIN files
If the option to remember authentication information on a Mac computer has been checked, a KEYCHAIN file is used to automatically authenticate the Mac user. The file itself contains the authentication data as an encrypted data collection. They include passwords that are linked to specific user names, private keys and certificates used by the OS X operating system. The most common KEYCHAIN file is named "login.keychain" and can be found on almost any MAC computer. It contains passwords and login information for websites, developer certificates and more.