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# alice and bob encryption example ###### by

Since computers can use very complicated math to encrypt things, this stops people from trying a brute force attack to guess the numbers until it … If Eve gets the key, then she'll be able to read all of Alice and Bob's correspondence effortlessly. The receiver of the message (Alice) sends his public key to a sender (Bob). Systems like this are call symmetric encryption, because Alice and Bob both need an identical copy of the key. The message that Alice wants to send Bob is the number 1275. So, what are Alice and Bob to do? The receiver (Alice) decrypts the sender’s message (Bob) using her private key. Alice and Bob agree on a public key algorithm. For example: Bob and Alice agree on two numbers, a large prime, p = 29, and base g = 5; Now Bob picks a secret number, x (x = 4) and does the following: X = g^x % p (in this case % indicates the remainder. 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. AES128 Encryption / Decryption. For example: Suppose Alice wants to send a message to Bob and uses an encryption method. 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 now computes Y x modulo p = (19 6 modulo 23) = 2. The RSA Encryption Scheme Suppose Alice wants her friends to encrypt email messages before sending them to her. Bob sends Alice his public key. The example that you have stated provides confidentiality. 6. I did the example on the nRF51 with SDK 12.3. Alice and Bob in the Quantum Wonderland Two Easy Sums 7873 x 6761 = ? Figure 16.3.1. Only Bob can then decrypt the encrypted session key, because he is the only one who knows the corresponding private key. 4) A worked example of RSA public key encryption Let’s suppose that Alice and Bob want to communicate, using RSA technology (It’s always Alice and Bob in the computer science literature.) So her calculation was the same as 3 to the power 13 to the power 15 mod 17. Using Alice's public key and his secret key, Bob can compute the exact same shared secret key. The breakthrough was the realisation that you could make a system that used different keys for encoding and decoding. So, the the last three letters shift to the ﬁrst three. Let’s describe how that works by continuing to use Alice and Bob from above as an example. We give an introduction to the ElGamal Encryption System and an example in the video in Figure 16.3.1. So A goes to D 1. This encrypted symmetric key is sent across the wire to Alice. Using Bob's public key, Alice can compute a shared secret key. That is, Alice and Bob have exchanged a key, xab, that can now be used in a conventional cryptosystem to encrypt any messages between Alice and Bob. - Alice and Bob agree on a random, large key k, and both agree to keep it secret. - Alice wants to send message m; she computes F(k,m) and sends it over the public network to Bob. Bob decrypts Alice's message with his private key. Bob has a pair of keys — public and private. Encryption. 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. The RSA Encryption Scheme is often used to encrypt and then decrypt electronic communications. For example 3%2 is 3/2, where the remainder is 1). Alice B “The Attacker” can pretend to be anyone. In Chapter 12 we saw how a message can be encoded into integers. X = 5 ^4 % 29 = 625 % 29 = 16 This diagram shows the basic setup of computers and who says what. 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. 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. 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. 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. Decoding Alice and Bob. Bob now computes Y x modulo p = (8 6 modulo 23) = 2. Alice encrypts her message with Bob's public key and sends it to Bob. One of the earliest techniques for this, called the Caesar Cipher, operates as follows. Notice they did the same calculation, though it may not look like it at first. ... for example, Alice and Bob don’t know each other’s private keys) The public key can be distributed – the idea is that if someone does know the public key, they still can’t decipher the message, so it can be considered as being available to anyone and it doesn’t matter if enemies know it or not . Bob wants to encrypt and send Alice his age – 42. { _ } Kab means symmetric key encryption A Simple Protoco l = 26 292 671 Superposition The mystery of How can a particle be a wave? ? Before sending a message to Bob, Alice would encrypt it with a secret key, turning plaintext into ciphertext; even if Eve intercepted the ciphertext, she could make no sense of it. For example, Alice may be writing a will that she wants to keep hidden in her lifetime. [That’s not very interesting. Some additional viewing Simon Singh's video gives a good explanation of key distribution. For example, take two users Alice and Bob. The general scenario is as follows: Alice wishes to send a message to Bob so that no one else besides Bob can read it. The sender (Bob) encrypts his message with the public key of the receiver (Alice). Eve obtains F(k,m), but since she doesn't know k, she cannot efficiently recover m (she can at best perform a brute-force attack). Encryption in transit: ... A simple example: Alice and Bob. Bob takes Alice's public result and raises it to the power of his private number resulting in the same shared secret. x ? 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. By using both private key and public key, the shared secret key would be generated. Alice and Bob have agreed to divide the text into groups of five characters and then permute the characters in each group. 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. 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. Alice encrypted message with Bob’s Public Key . Well, last week, Dark Reading, ... or how it works, as it’s the security of the keys that matters. Suppose Alice wants to send a message to Bob and in an encrypted way. 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. By encrypting it using personal secrets shared with Bob, only he can read it after her death but he does not need to be made aware of it by an explicit key transfer. 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. But Bob had the decryption key, so he could recover the plaintext. The message receiver (Alice) generates a private key and a public key. On the next page is the public key crypto widget. Using public-key authenticated encryption, Bob can encrypt a confidential message specifically for Alice, using Alice's public key. We will look further at this in the next section. 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. Asymmetric ciphers are quite slow when compared with the symmetric ones, which is why asymmetric ciphers are used only to securely distribute the key. sent for future decryption by Bob. Public and private keys are two extremely large numbers, chosen such that there's a mathematical relation between them, and yet it's extremely hard (i.e. Similarly, Alice has a key pair. First imagine all letters as numbers. would take many billions of years) to derive the private key from the public key. A is 0, B is 1, C is 2, etc, Z is 25. Then, Alice and Bob can use symmetric cipher and … Asymmetric encryption, often called "public key" encryption, allows Alice to send Bob an encrypted message without a shared secret key; there is a secret key, but only Bob knows what it is, and he does not share it with anyone, including Alice. Let’s understand this, as you rightly guessed, with the example of Alice and Bob once again. For some cryptosystems, Alice and Bob must each hold a copy of the same key, which both encrypts and decrypts. Farmer and Stephen Steward 7873 x 6761 =, take two users Alice Bob. 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