include "keyring.m";
keyring := load Keyring Keyring->PATH;
auth:           fn(fd: ref Sys->FD, info: ref Authinfo, setid: int)
                : (string, array of byte);
readauthinfo:   fn(filename: string): ref Authinfo;
writeauthinfo:  fn(filename: string, info: ref Authinfo): int;

Auth returns a string and a byte array. If the byte array is nil then the authentication has failed and the string is an error message. If the byte array is non-nil, it represents a secret shared by the two communicating parties, and the string names the party at the other end of the connection.

If the authentication is successful and setid is non-zero then auth attempts to write the name of the party at the other end of the connection into /dev/user (see cons(3)); no error is generated if that does not succeed. If the authentication is not successful and setid is non-zero, auth writes the name nobody into /dev/user.

The authentication protocol is based on the Station-to-Station protocol. In the following, the parties are labelled 0 and 1. Sig0(x) is x signed with 0's private key.

0 -> 1  alpha**r0 mod p, CERTu0, PKu0
1 -> 0  alpha**r1 mod p, CERTu1, PKu1
0 -> 1  sig0(alpha**r0 mod p, alpha**r1 mod p)
1 -> 0  sig1(alpha**r0 mod p, alpha**r1 mod p)

At this point both 0 and 1 share the secret alpha**(r0*r1) which is returned in the byte array. Amongst other things, it can be the secret to digest or encrypt a conversation (see security-ssl(2)).

Readauthinfo reads a representation of an Authinfo from a file. It returns nil if there is a read error or a conversion error; it returns a reference to the Authinfo otherwise.

Writeauthinfo writes a representation of info to a file. It returns -1 if the write operation fails, 0 otherwise.

/usr/user/keyring

The conventional directory for storing Authinfo files

/usr/user/keyring/default

The key file normally used by server programs

/usr/user/keyring/net!server

The key file normally used by clients for a given server