Large Precision Arithmetic

The IPint adt is provided to allow some cryptographic functions to be implemented in Limbo. IPint stands for infinite precision integer, though, for space considerations, our implementation limits the maximum integer to 28192-1.

An IPint can be converted into two external formats. The first is an array of bytes in which the first byte is the highest order byte of the integer. This format is useful when communicating with the ssl (3) device. The second is a MIME base 64 format, that allows IPints to be stored in files or transmitted across networks in a human readable form.

Public Key Cryptography

Public key cryptography has many uses. Inferno relies on it only for digital signatures. Each Inferno user may generate a pair of matched keys, one public and one private. The private key may be used to digitally sign data, the public one to verify the signature. Public key algorithms have been chosen to make it difficult to spoof a signature or guess the private key.

For public keys algorithms to work, there must be a way to distribute the public keys: in order to verify that X signed something, we must know X's public key. To simplify the problem, we have instituted a trust hierarchy that requires people to know only the public keys of certifying authorities (CAs). After generating a public key, one can have the concatenation of one's name, expiration date, and key signed by a CA. The information together with the name of the CA and the signature is called a certificate.

At the beginning of a conversation, the parties exchange certificates. They then use the CA's public key to verify each other's public keys. The CA's public key, a system wide Diffie-Hellman base and modulus, one's private key, one's public key and certificate are kept in a Limbo adt called Keyring->Authinfo. An Authinfo adt can be read from from a file using readauthinfo or written to a file using writeauthinfo, both from keyring-auth (2).

Authinfo adts are normally created during the login and registration procedures described below.

Authentication

Two parties conversing on a network connection can authenticate each other's identity using the functions in keyring-auth (2). They use the Keyring->Authinfo information to run the Station to Station (STS) authentication protocol. STS not only authenticates each party's identity to the other but also establishes a random bit string known only to the two parties. This bit string can be used as a key to encrypt or authenticate subsequent messages sent between the two parties.

Secure Communications

After exchanging secrets, communicating parties may encode the conversation to guarantee varying levels of security:

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none

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messages cannot be forged

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messages cannot be intercepted

Encoding uses the line formats provided by the Secure Sockets Layer. See security-intro (2) for more detail.

Login and registration

The Inferno authentication procedure requires that both parties possess an Authinfo adt containing a locally generated public/private key pair, the public key of a commonly trusted CA, and a signed certificate from the CA that links the party's identity and public key. This Authinfo adt is normally kept in a file. At some point, however, it must be created, and later conveyed securely between the user's machine and the CA. There are two ways to do this, the login procedure and the registration procedure. Both require an out of band channel between the CA and the user.

The login procedures are used by typed commands and by programs using Tk. The login procedure relies on the CA and the user having established a common secret or password. This is done securely off line, perhaps by mail or telephone. This secret is then used to provide a secure path between CA and user machine to transfer the certificate and CA public key. See security-intro (2) for more detail.

The registration procedure is built into the mux (1) interface and is intended for the set top box environment. When the set top box is first turned on, it creates a public/private key pair and dials the service provider's CA to get a key signed. The CA returns its public key and a signed certificate, blinded by a random bit string known only to the CA. A hash of the information is then displayed on the user screen. The user must then telephone the CA and compare this hashed foot print with the one at the CA. If they match and the user proves that he is a customer, the CA makes the blinding string publicly known.

Data Types

SigAlg

The SigAlg adt contains a single string that specifies the algorithm used for digital signatures. The allowable values are md5, md4 and sha1 that specify which one-way hash function is used to produce a digital signature or message digest.

PK and SK

The PK adt contains the data necessary to construct a public key; the SK adt contains the data necessary to construct a secret key. Both keys are built from the combination of a specified signature algorithm and a string representing the name of the owner of the key.

Certificate

The Certificate adt contains a digital signature with the certification of the trusted authority (CA).

DigestState

The DigestState adt contains the hidden state of partially completed hash functions during processing. Its copy operation returns a reference to a copy of a given state.

Authinfo

The Authinfo adt contains an individual user's private and public key, the signer's certificate and the signer's public key, and the Diffie-Hellman parameters.