In the second chapter of Bruce Schneier's book Applied Cryptography, 2nd Edition, 1996, we find this:

  1. Alice encrypts her message to Bob with KA and sends it to Trent.
  2. Trent decrypts the message with KA.
  3. Trent takes the decrypted message and a statement that he has received this message from Alice, and encrypts the whole bundle with KB.
  4. Trent sends the encrypted bundle to Bob.
  5. Bob decrypts the bundle with KB. He can now read both the message and Trent’s certification that Alice sent it.

I don't understand the last step. The "certification" had not been mentioned in any of the previous steps.

Does "certification" mean "a statement that he has received this message from Alice"?


Yes, it means ‘a statement that he has received this message from Alice’.

WARNING: Applied Cryptography is archaic and bad, and these days is useful only for historical research, not for learning how to use, choose, or think about cryptography. Consider the later Cryptography Engineering by the same author instead.

  • $\begingroup$ Does also the '20th Anniversary Edition' archaic and bad? $\endgroup$ – Rex Smith May 25 at 14:43
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    $\begingroup$ @RexSmith As far as I can tell, the only difference is that there's a new introduction. $\endgroup$ – Squeamish Ossifrage May 25 at 15:15
  • $\begingroup$ Back then Schneier said, "The second edition of Applied Cryptography is a major rewrite of the first edition: 50% more words, 7 more chapters, and over 1600 new references. Not only did I make corrections to the first edition and add developments since it was published, but I also included topics left out of the first edition. The second edition has lots of new algorithms (including GOST, Blowfish, RC4, and A5), more information on the Clipper Chip and key escrow, dozens of new protocols, more information on how PGP works, detailed information on key management...", etc. $\endgroup$ – Patriot May 27 at 6:50
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    $\begingroup$ @Patriot The second edition was published over two decades ago. The 20th anniversary edition appears to be essentially a reprint of the second edition, with a new introduction. $\endgroup$ – Squeamish Ossifrage May 27 at 22:20
  • $\begingroup$ @SqueamishOssifrage You are exactly right. I misread Schneier's blog and thought that the 2nd edition and the 20th anniversary edition were exactly the same book. They're not. $\endgroup$ – Patriot May 28 at 2:44

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