ADB Key
Google ADB uses RSA keys for authentication. It supports multiple keys, and the client will try each key in order until one is accepted by the device.
Private key
The private key is in PEM format, which means the content is a RSA private key in PKCS#8 format.
Public key
The public key is a mincrypt RSA public key, encoded in Base64, followed by a comment (usually the email address of the key owner).
mincrypt was a minimal cryptography library used before Android 7. From Android 7, it's replaced by OpenSSL, but the old public key format is still used for compatibility.
Google ADB never reads the public key file. It always generates the public key from the private key. The key name sent to devices is always username@hostname, not the comment in the public key file.
User key
User key is stored at ~/.android/adbkey (private key) and ~/.android/adbkey.pub (public key). If it doesn't exist, ADB will generate a new key pair when it starts.
Vendor keys
The ADB_VENDOR_KEYS environment variable can be set to list of paths to additional private key files. The list is separated by the platform-specific path delimiter (: on Unix-like systems, ; on Windows).
Each item in ADB_VENDOR_KEYS can be either a file or a directory. If it's a directory, all *.adb_key files in the directory will be treated as private key files.