[Exim] hmac comment from the exim manual

i was going through the manual and ran across the following in the
description of ${hmac...}

This function uses cryptographic hashing (either MD5 or SHA-1) to
convert a shared secret and some text into a message authentication
code, as specified in RFC 2104. You could produce a similar effect
using ${md5:secret_text...}, but allegedly HMAC provides better
defence against deducing the secret.

i'd like to propose a more accurate phrasing for the last sentence:

This differs from ${md5:secret_text...} or ${sha1:secret_text...} in that
the hmac step adds a signature to the crypographic hash, allowing for
authentication that is not possible with md5 or sha1 alone.

richard
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Richard Welty                                         rwelty@???
Averill Park Networking                                         518-573-7592
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