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| --- This message came from the IEEE 802.11 Working Group Reflector ---
Hello,
I may not be at the 11mc session where 11-14/1104r1 is presented so I
want to get discussion on it started on the mailing list.
There are some issues with the proposed description of various designations
of the Secure Hash Algorithm family of hash functions. In an effort to obtain
what the author views as "consistency" he has introduced ambiguity and,
in my opinion, incorrectness.
There are 3 families of the Secure Hash Algorithm. There's the 1st family of
SHA, (which is just designated SHA1) and there's the 2nd family of SHA (known
as SHA2) which is SHA256 and SHA512, including truncated versions of each,
SHA224 is a truncated version of SHA256 and SHA384 is a truncated version of
SHA512. The 3rd family of SHA, SHA3 is a weird beast that we don't need to
trouble ourselves with since we're not using it.
It is important to understand, though, that SHA1, SHA256, and SHA512 are
different. They are not just truncated versions of the same function. And while
SHA256 does actually produce 256-bits of digest output, SHA1 does not
produce 1-bit of digest output, it produces 160-bits. But we (802.11) do
truncate the output of SHA family hash functions. For instance, we sometimes
want only the first 128-bits of SHA1. Just to make things more complicated,
we also use the HMAC construct with a SHA family algorithm.
So how to deal with identifying the appropriate algorithm in the appropriate
family of SHA, whether it is alone or as HMAC, and how to identify the particular
output bit length that we are concerned with? 11-14/1104r1 does a poor job.
Let me propose a better way:
* the particular algorithm of the family is used without any hyphens— for
example, it's SHA256 not SHA-256, and it's SHA1 always.
* when we want to truncate we add a hyphen and indicate the bit length we
want to use— for example, it's SHA256-128 (not sure why we'd do this but it's
an example) or SHA1-128.
* if we don't add a hyphen and number indicating truncation then the entire
length of the algorithm output is used.
* if we want to use the HMAC construct we prepend "HMAC-" to the SHA family
algorithm indicated, including any possible indication of truncation— for
example, HMAC-SHA256 or HMAC-SHA1-128.
* if we don't prepend "HMAC-" then we are using the algorithm directly, and
not as a keyed hash function.
While 11-14/1104r1 does this correct in some cases— it proposes to
change "HMAC-SHA-256" to "HMAC-SHA256"-- it does it wrong in other
cases— it proposes to change "SHA1" to "SHA-1".
Also, if we adopt this more correct way of referring to these various
incarnations of hashing we do not need to say something like
"Truncate-128(HMAC-SHA1(xxx))" we just say HMAC-SHA1-128(xxx)".
Minor grammatical gripe: there is no such algorithm called
"HMAC-SHA1-64", or even "HMAC-SHA-1-64" so we should not be using
the definite article when describing the use of SHA1 in an HMAC
construct while truncating the output to 64 bits.
Please modify 11-14/1104r1 to adopt this more correct terminology
before it gets adopted.
regards,
Dan.
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