McDATA 316095-B21 - StorageWorks Edge Switch 2/24 Manual page 505

length, the length of both the random and delta components is that
fixed length; if P allows the length of K to be variable up to a
particular maximum length, the length of the random component is
that maximum length and the length of the delta component is any
length less than or equal to that maximum length.
For example, usmHMACMD5AuthProtocol requires K to be a fixed
length of 16 octets and L - of 16 octets. usmHMACSHAAuthProtocol
requires K to be a fixed length of 20 octets and L - of 20 octets. Other
protocols may define other sizes, as deemed appropriate.
When a requester wants to change the old key K to a new key
keyNew on a remote entity, the 'random' component is obtained from
either a true random generator, or from a pseudorandom generator,
and the 'delta' component is computed as follows:
• a temporary variable is initialized to the existing value of K; if the
length of the keyNew is greater than L octets, then:
• - the random component is appended to the value of the
temporary variable, and the result is input to the the hash
algorithm H to produce a digest value, and the temporary
variable is set to this digest value;
• - the value of the temporary variable is XOR-ed with the first
(next) L-octets (16 octets in case of MD5) of the keyNew to
produce the first (next) L-octets (16 octets in case of MD5) of the
'delta' component.
The above two steps are repeated until the unused portion of the
keyNew component is L octets or less,the random component is
appended to the value of the temporary variable, and the result is
input to the hash algorithm H to produce a digest value. This digest
value, truncated if necessary to be the same length as the unused
portion of the keyNew, is XOR-ed with the unused portion of the
keyNew to produce the (final portion of the) 'delta' component.
For example, using MD5 as the hash algorithm H:
iterations = (lenOfDelta - 1)/16; /* integer division */
temp = keyOld;
for (i = 0; i < iterations; i++) {
temp = MD5 (temp || random);
delta[i*16 .. (i*16)+15] =
temp XOR keyNew[i*16 .. (i*16)+15]; }
SNMPv3 MIB
SNMPv3 MIB
G-5
G