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mechanism to overcome the key distribution problem in multivendor environments. Many also
use it because it is the simpler approach. Password-derived symmetric keys are no more secure
than the passwords they are derived from.
4.6.3 The SNMP Key Updates do not Provide for Perfect Forward Secrecy.
SNMPv3’s key update capability does not provide for perfect forward secrecy (PFS). PFS is
defined in section 3.3 of RFC 2409 as:
“When used in the memo Perfect Forward Secrecy (PFS) refers to the notion that
compromise of a single key will permit access to only data protected by a single
key. For PFS to exist the key used to protect transmission of data MUST NOT be
used to derive any additional keys, and if the key used to protect transmission of
data was derived from some other keying material, that material MUST NOT be
used to derive any more keys.”
Specifically, in SNMPv3, replacement keys can be used to derive previous keys. As a result, if
an attacker recovers a SNMPv3 authentication or privacy key, then he can decrypt all (recorded)
traffic in the past even from previous key sets—assuming that he also captured the key change
operation packets.
4.6.4 The SNMP Symmetric Key Distribution Problems.
Key distribution is a very serious implementation problem for symmetric key-based systems like
SNMPv3. This problem has not been addressed by the IETF in general, and certainly has not
been addressed within the IETF’s SNMPv3 working group, in particular. Thus, there are no
common or standard mechanisms used by SNMP implementations to perform initial symmetric
key distribution. That is why so many systems rely upon password-based distributions.
Solutions do exist within the IETF milieu for securely exchanging symmetric keys. However,
none of them are standards, nor are they commonly deployed by SNMP products. For example,
Kerberos has provided a mechanism for distributing symmetric keys, and RFC 2786 provides a
Diffie-Hellman-like key exchange mechanism that is available for SNMP systems to use.
However, the latter is an experimental RFC (i.e., it is not a standard SNMP mechanism) that is
only implemented in a subset of SNMP products.
It is well known that many otherwise perfectly good security systems have been rendered
ineffectual through improper or inadequate key distribution implementations. In regard to a
deployment’s use of SNMPv3, it is obvious that both the SNMP agent and the administrator’s
management system need to share a consistent, coherent, and interoperable approach to securely
distribute these keys if the symmetric keys are to be securely distributed between them.
Unfortunately, because this essential requirement is implemented on an ad hoc and idiosyncratic
manner by different SNMP implementations, there is a strong basis for questioning whether
these keys are being securely conveyed in multivendor environments, unless the deployment
itself has used its own resources to create an out-of-band mechanism to do so.
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