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The authentication tag is defined as an output parameter in GCM (see section 7, step 7 of NIST SP 800-38D). In all the API's I've encountered it's appended to the ciphertext. Where it is actually placed is up to the protocol designer. The protocol designer may well consider the place behind the ciphertext as ad hoc default though. The name "tag" of course ...


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When we have regions of the packet that we only authenticate but not encrypt, that happens because we have data that we want to bind to the encrypted region, but we don't need to include within the encrypted region. Examples of this are: For IPsec, we include the sequence number (as a part of the ESP header). We include that within the authentication ...


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If you want to encrypt a long message with authenticated encryption, you should split it into many small segments (e.g. 4KiB each), with each fragment having its own tag. That way you only release plaintext to the application after verifying its tag. (As usual there are some pitfalls with designing such a construction). Such a construction works with any ...


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The usefulness of online AE (locally): Assume you wrote a program that encrypt arbitrary files. Now further assume the user wants to view a movie, encrypted with this tool. The tool can now use the online-property to stream the movie in real-time as it uses online-encryption. The usefulness of online AE (programatically): Assume you want to process ...



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