LOGIN(6) LOGIN(6)
NAME
login - key exchange protocol
DESCRIPTION
The following encrypted key exchange protocol is used
between a client such as login in security-login(2), and a
certificate signing process such as logind(8), to justify
the latter's issuing a certificate that can later be pre-
sented to an Inferno service to establish credentials.
A shared secret must previously be agreed between user and
certifying authority (CA). It is used by the protocol to
establish a secure channel between user and CA.
In the description below:
ivec is an 8 byte random number (`initialisation vector')
chosen for this conversation.
sha is the 20 byte secure hash (SHA-1) of the password
key is an 8 byte secret formed as follows:
key[0] = ivec[0]^sha[0]^sha[8]^sha[16]
key[1] = ivec[1]^sha[1]^sha[9]^sha[17]
...
key[5] = ivec[5]^sha[5]^sha[13];
key[6] = ivec[6]^sha[6]^sha[14];
key[7] = ivec[7]^sha[7]^sha[15];
alpha is a Diffie-Hellman base used system wide
p is a Diffie-Hellman modulus used system wide
key(m) is m encrypted using the RC4 algorithm with key.
Rx is a random number of the same order as p.
secret is the Diffie-Hellman secret alpha**(r0*r1) mod p.
The protocol follows. ``user→CA xxx'' means that the user
sends the message ``xxx'' to the certifying authority. Any
party can send an error instead of a message at any point to
terminate the protocol.
user→CA name
CA→user ACK
user→CA ivec
CA→user key(alpha**r0 mod p), alpha, p
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LOGIN(6) LOGIN(6)
user→CA alpha**r1 mod p
CA→user CA's public key, SHA(CA's public key + secret)
user→CA user's public key, SHA(user's public key + secret)
CA→user user's public key certificate
The complexity of this protocol is intended to shield the
password. To start a clear text attack against the pass-
word, one needs to first attack the Diffie-Hellman exponen-
tial to determine alpha**r0 mod p. A possible weakness is
that the encrypted quantity is base64 encoded, constraining
the possible values of each byte. This could aid a brute
force attack.
Alpha and p are sent unprotected, though the user code does
a few sanity checks on the values it receives. This is
another likely point of attack. We should like to know
about any.
The role of ivec is to foil any replay attacks by someone
spoofing the CA though this is probably overkill.
SEE ALSO
security-intro(2), security-login(2), logind(8), signer(8)
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