System Administration Guide, Volume 2

Diffie-Hellman Authentication

The Diffie-Hellman method of authenticating a user is non-trivial for an intruder to crack. The client and the server each has its own private key (sometimes called a secret key) which they use together with the public key to devise a common key. They use the common key to communicate with each other, using an agreed-upon encryption/decryption function (such as DES). This method was identified as DES authentication in previous Solaris releases.

Authentication is based on the ability of the sending system to use the common key to encrypt the current time, which the receiving system can decrypt and check against its current time. Make sure you synchronize the time on the client and the server.

The public and private keys are stored in an NIS or NIS+ database. NIS stores the keys in the publickey map, and NIS+ stores the keys in the cred table. These files contain the public key and the private key for all potential users.

The system administrator is responsible for setting up NIS or NIS+ tables and generating a public key and a private key for each user. The private key is stored encrypted with the user's password. This makes the private key known only to the user.

Implementation of Diffie-Hellman Authentication

This section describes the series of transactions in a client-server session using DH authorization (AUTH_DH).

Generating the Public and Secret Keys

Sometime prior to a transaction, the administrator runs either the newkey or nisaddcred commands that generates a public key and a secret key. (Each user has a unique public key and secret key.) The public key is stored in a public database; the secret key is stored in encrypted form in the same database. To change the key pair, use the chkey command.

Running the keylogin Command

Normally, the login password is identical to the secure RPC password. In this case, a keylogin is not required. If the passwords are different, the users have to log in, and then do a keylogin explicitly.

The keylogin program prompts the user for a secure RPC password and uses the password to decrypt the secret key. The keylogin program then passes the decrypted secret key to a program called the keyserver. (The keyserver is an RPC service with a local instance on every computer.) The keyserver saves the decrypted secret key and waits for the user to initiate a secure RPC transaction with a server.

If the passwords are the same, the login process passes the secret key to the keyserver. If the passwords are required to be different and the user must always run keylogin, then the keylogin program may be included in the user's environment configuration file, such as ~/.login, ~/.cshrc, or ~/.profile, so that it runs automatically whenever the user logs in.

Generating the Conversation Key

When the user initiates a transaction with a server:

  1. The keyserver randomly generates a conversation key.

  2. The kernel uses the conversation key to encrypt the client's time stamp (among other things).

  3. The keyserver looks up the server's public key in the public-key database (see the publickey(4) man page).

  4. The keyserver uses the client's secret key and the server's public key to create a common key.

  5. The keyserver encrypts the conversation key with the common key.

First Contact With the Server

The transmission including the encrypted time stamp and the encrypted conversation key is then sent to the server. The transmission includes a credential and a verifier. The credential contains three components:

The window is the difference the client says should be allowed between the server's clock and the client's time stamp. If the difference between the server's clock and the time stamp is greater than the window, the server would reject the client's request. Under normal circumstances this will not happen, because the client first synchronizes with the server before starting the RPC session.

The client's verifier contains:

The window verifier is needed in case somebody wants to impersonate a user and writes a program that, instead of filling in the encrypted fields of the credential and verifier, just stuffs in random bits. The server will decrypt the conversation key into some random key and use it to try to decrypt the window and the time stamp. The result will be random numbers. After a few thousand trials, however, there is a good chance that the random window/time stamp pair will pass the authentication system. The window verifier makes guessing the right credential much more difficult.

Decrypting the Conversation Key

When the server receives the transmission from the client:

  1. The keyserver local to the server looks up the client's public key in the publickey database.

  2. The keyserver uses the client's public key and the server's secret key to deduce the common key--the same common key computed by the client. (Only the server and the client can calculate the common key because doing so requires knowing one secret key or the other.)

  3. The kernel uses the common key to decrypt the conversation key.

  4. The kernel calls the keyserver to decrypt the client's time stamp with the decrypted conversation key.

Storing Information on the Server

After the server decrypts the client's time stamp, it stores four items of information in a credential table:

The server stores the first three items for future use. It stores the time stamp to protect against replays. The server accepts only time stamps that are chronologically greater than the last one seen, so any replayed transactions are guaranteed to be rejected.


Note -

Implicit in these procedures is the name of the caller, who must be authenticated in some manner. The keyserver cannot use DES authentication to do this because it would create a deadlock. To solve this problem, the keyserver stores the secret keys by UID and grants requests only to local root processes.


Verifier Returned to the Client

The server returns a verifier to the client, which includes:

The reason for subtracting 1 from the time stamp is to ensure that the time stamp is invalid and cannot be reused as a client verifier.

Client Authenticates the Server

The client receives the verifier and authenticates the server. The client knows that only the server could have sent the verifier because only the server knows what time stamp the client sent.

Additional Transactions

With every transaction after the first, the client returns the index ID to the server in its second transaction and sends another encrypted time stamp. The server sends back the client's time stamp minus 1, encrypted by the conversation key.