Cryptography Reference
In-Depth Information
Non-repudiation
is the assurance that an entity cannot deny a previous com-
mitment or action. Most commonly, non-repudiation is the assurance that
the original source of some data cannot deny
to a third party
that this is the
case. Note that this is a stronger requirement than data origin authentication,
since data origin authentication only requires this assurance to be provided to
the receiver of the data. Non-repudiation is a property that is most desirable in
situations where there is the potential for a dispute to arise over the exchange
of data.
Entity authentication
is the assurance that a given entity is involved
and currently
active
in a communication session. In other words, if a technique provides
entity authentication of Alice then this means that by applying the technique
we can be sure that Alice is really engaging with us
now
, in 'real time'. If we
fail to establish this temporal aspect of entity authentication (which requires
the adoption of a
freshness
mechanism, see Section 8.2) then we have failed
to achieve entity authentication. In certain contexts, entity authentication is
referred to as
identification
because it is concerned with determining
who am
I communicating with now, in real time
?
1.3.2
Relationships between security services
It is important to recognise that these basic security services are all essentially
different
, even though on first encounter they may seem similar. The following
statements further illustrate this.
DATA ORIGIN AUTHENTICATION IS A STRONGER NOTION
THAN DATA INTEGRITY
In other words, if we have data origin authentication then we also have data
integrity (but most certainly not the other way around).
To see that data origin authentication would be meaningless without data
integrity, suppose that Alice has sent us some data. If we have no data integrity
then we cannot be sure that the data received has not been changed by an attacker
in transit. The actual data that we received might therefore have come from the
attacker and not from Alice. How could we possibly claim to have data origin
authentication from Alice in this case? We have thus tied ourselves in a logical
knot. Therefore data origin authentication can only be provided if data integrity
is also provided. It can be helpful to think of data origin authentication as a
stronger version of data integrity. More precisely, data origin authentication is
data integrity with the extra property of assurance of the identity of the original
source of the data.
A commonly offered attempt at a counter-example to this relationship is
recognition of the source of a broken voice message over a noisy channel (such
as a telephone call). Since the voice message is audibly broken, we clearly do not
have data integrity. However, because the voice is recognisable it could be argued
