rfc7298v3.txt   rfc7298.txt 
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Many practical applications of HMAC for authentication of datagram- Many practical applications of HMAC for authentication of datagram-
based network protocols (including routing protocols) involve the based network protocols (including routing protocols) involve the
padding procedure, a design-specific conditioning of the message that padding procedure, a design-specific conditioning of the message that
both the sender and the receiver perform before the HMAC computation. both the sender and the receiver perform before the HMAC computation.
The specific padding procedure of this mechanism addresses the The specific padding procedure of this mechanism addresses the
following needs: following needs:
o Data Initialization o Data Initialization
A design that places the HMAC result(s) computed for a message A design that places the HMAC result(s) computed for a message
inside the same message after the computation has to allocate in inside that same message after the computation has to have
the message some data unit(s) purposed for the result(s) (in this previously (i.e., before the computation) allocated in that
mechanism, it is the HMAC TLV(s); see Section 4.3). The padding message some data unit(s) purposed specifically for those HMAC
procedure sets the respective octets of the data unit(s), in the result(s) (in this mechanism, it is the HMAC TLV(s); see
simplest case to a fixed value known as the padding constant. Section 4.3). The padding procedure sets the respective octets of
the data unit(s), in the simplest case to a fixed value known as
the padding constant.
The particular value of the constant is specific to each design. The particular value of the constant is specific to each design.
For instance, in [RIP2-AUTH] as well as works derived from it For instance, in [RIP2-AUTH] as well as works derived from it
([ISIS-AUTH-B], [OSPF2-AUTH], and [OSPF3-AUTH-BIS]), the value is ([ISIS-AUTH-B], [OSPF2-AUTH], and [OSPF3-AUTH-BIS]), the value is
0x878FE1F3. In many other designs (for instance, [RFC3315], 0x878FE1F3. In many other designs (for instance, [RFC3315],
[RFC3931], [RFC4030], [RFC4302], [RFC5176], and [ISIS-AUTH-A]), [RFC3931], [RFC4030], [RFC4302], [RFC5176], and [ISIS-AUTH-A]),
the value is 0x00. the value is 0x00.
However, the HMAC construction is defined on the basis of a However, the HMAC construction is defined on the basis of a
cryptographic hash algorithm, that is, an algorithm meeting a cryptographic hash algorithm, that is, an algorithm meeting a
particular set of requirements made for any input message. Thus, particular set of requirements made for any input message. Thus,
any padding constant values, whether single- or multiple-octet, as any padding constant values, whether single- or multiple-octet, as
well as any other message-conditioning methods, don't affect well as any other message-conditioning methods, don't affect
cryptographic characteristics of the hash algorithm and the HMAC cryptographic characteristics of the hash algorithm and the HMAC
construction, respectively. construction, respectively.
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