There is only one file type that uses the .KEYCHAIN extension:
Mac OS X Keychain File
A KEYCHAIN file is created by Apple Keychain, a program bundled with Mac OS X that stores authentication information. It contains an encrypted collection of resource authentication data, such as passwords, certificates, and private keys.
KEYCHAIN files are used to automatically authenticate users after they have checked the option to remember authentication information. They are used for websites, SSH accounts, FTP files, and wireless networks.
NOTE: KEYCHAIN files are commonly found with the name login.keychain.
login.keychain - The global user keychain containing passwords, website logins, trusted identities, developer certificates, and other keys. It is located in the ~/Library/Keychains/ directory.
|Apple Keychain Access||Bundled|
Updated: February 25, 2016