Lecture 3.1: Handling Remote Access: RADIUS Motivation

Attack to shared secret based on
User-Password
User Password attribute
XOR (password)
Arbitrary User-ID
Arbitrary Password (< 16 bytes)
Giuseppe Bianchi
NAS
Access-Request
User-Password attribute (16 bytes)
Request Authenticator (16 bytes)
MD5(secret, RequestAuth)
RADIUS server
Attack to the shared secret based on the
User-Password attribute
Exhaustive search attack
But pre-computation of MD5 state not possible here, as
secret is first
Offline dictionary attacks
Their “effectiveness” depends on chosen
secret
(Often ignored) advice from RFC 2865:
“The secret (password shared between the client and the
RADIUS server) SHOULD be at least as large and
unguessable as a well-chosen password. It is preferred that the
secret be at least 16 octets. This is to ensure a sufficiently
large range for the secret to provide protection against
exhaustive search attacks. The secret MUST NOT be empty
(length 0) since this would allow packets to be trivially
forged.”
Many implementations only allow shared-secrets that are
ASCII characters, and less than 16 characters;
resulting RADIUS shared secrets are low entropy!
Giuseppe Bianchi
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