CRC is a Cyclic Redundancy Check - it is a fast, reasonably reliable way to ensure that a message was not damaged in transit.

GrandFather makes an excellent point about uniquness. Digests are very good for integrity checking, because the odds are very low that a substantially similar message will result in the same digest. However, you are reducing a message of arbitrary size down to a limited-size digest (16 bytes, for MD5). There will absolutely be collisions, it's a question of when they will occur.

Now, if you need to uniquely identify e-mail messages to use as keys, there are a few ways. One is to use an additional "pretty unique" attribute of the E-Mail message, and append that to the hash string. For example, the Message-ID: SMTP header should be a unique value anyway, but combined with a digest of the entire message, the chance of collision is essentially zero.

For example, if the Message-ID was <907068073421@smtp.yourhost.com>, and the Digest of the message was 5eb63bbbe01eeed093cb22bb8f5acdc3 (the MD5 of "hello world", if you care). Your key might be 907068073421@smtp.yourhost.com||5eb63bbbe01eeed093cb22bb8f5acdc3 -- that's pretty likely to be unique!

You could also use something like Data::UUID to associate the message with a truly unique identifier. I don't know if this would work for your application, because I don't know your requirements. I'm guessing you wish to be able to derive the key given the message? If so, than Data::UUID won't work for you.

<-radiant.matrix->
A collection of thoughts and links from the minds of geeks
The Code that can be seen is not the true Code
I haven't found a problem yet that can't be solved by a well-placed trebuchet

In reply to Re: Simple Digests? by radiantmatrix
in thread Simple Digests? by pileofrogs

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.