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Must Bob and Alice establish a 'connection' before a symmetric or asymmetric encryption scheme is secure? If so, is there ever some expected plaintext whose decryption authenticates the users?

In addition, could anyone recommend a good introductory book or paper on cryptography for a beginner like me?

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    $\begingroup$ It is not clear what you are asking about. You have to define what it means to be "secure" - suppose that Bob hands Alice a flash drive with some files on it, then there's no need for any connection (or cryptography) for authenticity to be assured. You have to specify the context before you can define "secure" as well as what steps are actually being performed. $\endgroup$
    – Ella Rose
    Jun 24, 2018 at 1:01
  • $\begingroup$ @ Ella Rose You are right, and it is partly my fault because I edited the article. I knew it was ambiguous, but I let it stand. Perhaps the questioner might step back in and a shed some light on his or her original intent. $\endgroup$
    – Patriot
    Jun 24, 2018 at 13:29

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A start would be the Wikipedia article about Authenticated Encryption, and the GCM mode is an AE-mode for block ciphers.

Additionally to the key, IV and cypher text you need the authentication tag, which is generated during the encryption process.

Decryption reconstructs the tag and compares it two the original. If they do not match, the decrypted text is invalid.

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  • $\begingroup$ IMO the question has been significantly changed by the edit. My answer now looks somewhat inappropriate. $\endgroup$
    – gammatester
    Jun 23, 2018 at 20:49
  • $\begingroup$ I tried to make the original question(s) coherent. More to the point, I really learned something: if the questions are incoherent, then don't try to guess, fill in the blanks, be polite, give shape. Like in Frozen, "Let it go." $\endgroup$
    – Patriot
    Jun 24, 2018 at 13:50
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In modern cryptography, asymmetric encryption schemes are used to exchange a shared key used by the symmetric encryption which is used for actual data encryption. This is because asymmetric encryption schemes require more processor resources and longer computational time compared with symmetric encryption. In another word, asymmetric encryption encrypts symmetric encryption key, symmetric encryption encrypts data.

You are right, there are some plain text while the parties try to establish a 'connection'. That is where the magic of the mathematics comes in, asymmetric encryption is based on the well-known mathematical difficult problems(ex. RSA based on big integer factorisation, Diffie-Hellman Key Exchange based on discrete logarithm problem, ECC is based on elliptic curves over finite fields problem). The properties of mathematical difficult problems have made the shared symmetric key safe even in the form of plain text. I suggest you start from Diffie-Hellman Key Exchange, Wikipedia has a fairly easy article to read(https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange).

If you want to learn cryptography from scratch, I suggest the book called "Introduction to Modern Cryptography, Second Edition By Jonathan Katz" for you, it involves most modern cryptography technologies and easy to understand even for a beginner. enter image description here

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