KEYRING-INTRO(2) KEYRING-INTRO(2)
NAME
Keyring intro - introduction to the Keyring module
SYNOPSIS
include "keyring.m";
keyring := load Keyring Keyring->PATH;
SigAlg: adt
{
name: string;
};
PK: adt
{
sa: ref SigAlg;
owner: string;
};
SK: adt
{
sa: ref SigAlg;
owner: string;
};
Certificate: adt
{
sa: ref SigAlg;
ha: string;
signer: string;
exp: int;
};
DigestState: adt
{
# hidden
};
Authinfo: adt
{
mysk: ref SK;
mypk: ref PK;
cert: ref Certificate;
spk: ref PK;
alpha: ref IPint;
p: ref IPint;
};
DESCRIPTION
This module contains a mixed set of functions that vari-
ously:
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+o perform infinite precision modular arithmetic; see
keyring-ipint(2)
+o form cryptographically secure digests; see keyring-
sha(2)
+o generate public/private key pairs and transform them to
and from textual form; see keyring-gensk(2) and
keyring-certtostr(2)
+o create and verify cryptographic signatures using the
public keys; see keyring-auth(2)
+o authenticate the parties on a connection; see keyring-
auth(2)
+o read and write files containing the information needed
to authenticate the parties on a connection; see
keyring-auth(2)
+o send Limbo byte arrays and strings across a connection;
see keyring-getstring(2)
Each collection is discussed in turn.
Large Precision Arithmetic
The IPint adt is provided to allow some cryptographic func-
tions 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
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people to know only the public keys of certifying authori-
ties (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 reg-
istration procedures described below.
Authentication
Two parties conversing on a network connection can authenti-
cate 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 converstation to guarantee varying levels of security:
• none
• messages cannot be forged
• messages cannot be intercepted
Encoding uses the line formats provided by the Secure Sock-
ets Layer. See security-intro(2) for more detail.
Login and registration
The Inferno authentication procedure requires that both par-
ties 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
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require an out of band channel between the CA and the user.
The login procedures are used by typed commands and by pro-
grams 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 tele-
phone. 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) inter-
face 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 allow-
able values are md5, md4 and sha that specify which
one-way hash function is used to produce a digital sig-
nature 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 par-
tially completed hash functions during processing.
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.
SOURCE
/interp/keyring.c
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/keyring/*.c
/crypt/*.c
SEE ALSO
security-intro(2)
B. Schneier, Applied Cryptography, 1996, J. Wiley & Sons,
Inc.
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