Bob takes Alice's public result and raises it to the power of his private number resulting in the same shared secret. For example, instead of the first letter of the alphabet (“A”) Bob and Alice will use the third letter (“C”), instead of the second (“B”) – the fourth one (“D”), and so on. { _ } Kab means symmetric key encryption A Simple Protoco l Since Alice encrypts the message using Bob's public key, Bob is the only one who can decrypt it as only Bob has the private key. Then, instead of Bob using Alice’s public key to encrypt the message directly, Bob uses Alice’s Public Key to encrypt the Symmetric Secret Key. Notice they did the same calculation, though it may not look like it at first. Alice and Bob may use this secret number as their key to a Vigenere cipher, or as their key to some other cipher. Since only Alice and Bob know their private numbers, this is a good way of sending secure information if the numbers are very big and the calculations are difficult. For example 3%2 is 3/2, where the remainder is 1). Bob sends Alice his public key. For example: Suppose Alice wants to send a message to Bob and uses an encryption method. [That’s not very interesting. One of the earliest techniques for this, called the Caesar Cipher, operates as follows. The amazing thing is that, using prime numbers and modular arithmetic, Alice and Bob can share their secret, right under Eve's nose! Similarly, Alice has a key pair. For example, Alice may be writing a will that she wants to keep hidden in her lifetime. We assume that the message \(m\) that Alice encrypts and sends to Bob is an integer. What does this have to do with Alice, Eve and Bob – a security blog? Meanwhile Bob has also chosen a secret number x = 15, performed the DH algorithm: g x modulo p = (5 15 modulo 23) = 19 (Y) and sent the new number 19 (Y) to Alice. Let’s understand this, as you rightly guessed, with the example of Alice and Bob once again. Bob wants to encrypt and send Alice his age – 42. For example, if Alice and Bob agree to use a secret key X for exchanging their messages, the same key X cannot be used to exchange messages between Alice and Jane. Network and Communications Security (IN3210/IN4210) Diffie Hellman Key exchange Alice and Bob agree on (public parameters): − Large prime number p − Generator g (i.e. Computers represent text as long numbers (01 for \A", 02 for \B" and so on), so an email message is just a very big number. Suppose Alice wants to send a message to Bob and in an encrypted way. Figure 15-1 provides an overview of this asymmetric encryption, which works as follows: Figure 15-1. Alice and Bob: - Because Bob knows k, he can efficiently recover m from F(k,m). I did the example on the nRF51 with SDK 12.3. In 1978, Alice and Bob were introduced in the paper “A Method for Obtaining Digital Signatures and Public-key Cryptosystems,” which described a way to encrypt and authenticate data. Public Key Cryptography is a form of asymmetric encryption; For Bob to send Alice a message, ... Notice that Bob's first instruction (shown at right), for example, is to wait until he hears Alice announce something. Bob starts by randomly generating a Symmetric Secret Key. An Example of Asymmetric Encryption in Action. The following shows the grouping after adding a bogus character (z) at the end to make the last group the same size as the others. E(A) → B : “I’m Alice” “I’m Alice” Elvis A Simple Protoco l Alice Bob {“I’m Alice”} Kab A → B : {“I’m Alice”} Kab If Alice and Bob share a key “Kab”, then Alice an encrypt her message. A is 0, B is 1, C is 2, etc, Z is 25. If Eve gets the key, then she'll be able to read all of Alice and Bob's correspondence effortlessly. To give an example: I plan to encrypt a piece of data under the AES algorithm, which allows for a particular type of (symmetric) encryption. Encryption in transit: ... A simple example: Alice and Bob. Of course, the RSA algorithm deals with sending numbers, but seeing as any text can be converted to digits … It's kind of clear at this point that we need to use some kind of encryption to make sure that the message is readable for Alice and Bob, but complete gibberish for Charlie. Alice and Bob in the Quantum Wonderland Two Easy Sums 7873 x 6761 = ? The example that you have stated provides confidentiality. Alice and Bob are not considerably developed characters, but over the years, the convention of using these names has become an effective narrative device. But Bob had the decryption key, so he could recover the plaintext. They have written lots of papers that use Alice and Bob as examples (Alice / Bob fanfic, if you will). Alice encrypts her message with Bob's public key and sends it to Bob. Alice encrypted message with Bob’s Public Key . Using Alice's public key and his secret key, Bob can compute the exact same shared secret key. Let’s describe how that works by continuing to use Alice and Bob from above as an example. The receiver of the message (Alice) sends his public key to a sender (Bob). Example 16.2 Alice needs to send the message “ Enemy attacks tonight ” to Bob. For example, one may wish to encrypt files on a hard disk to prevent an intruder from reading them. Map every letter to the letter that is three higher (modulo 26). As we mentioned earlier in the symmetric encryption example, Bob is an undercover spy agent who’s on a secret mission in a foreign country and Alice is his case manager. ElGamal Encryption System by Matt Farmer and Stephen Steward. Notice that this protocol does not require any prior arrangements (such as agreeing on a key) for Alice and Bob to communicate securely. We will look further at this in the next section. Some additional viewing Simon Singh's video gives a good explanation of key distribution. AES128 Encryption / Decryption. 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