- Example 1: Bob and Alice. Alice wants to send a confidential message to Bob; She should use Bob's public key to encrypt the message. Because Bob is the only one holding the private decryption key, he is also the only person that can decrypt the message. Example 2: Bob and Alice Redux
- For example, let E 1 and E 2 be two encryption functions and let "M" be the message so if Alice encrypts it using E 1 and sends E 1 (M) to Bob. Bob then again encrypts the message as E 2 (E 1 (M)) and sends it to Alice.
- a.Encrypt the plaintext sendmoremoney with the key stream 9 0 1 7 23 15 21 14 11 11 2 8 9. b.Using the ciphertext produced in part a, find a key so that the ciphertext decrypts to the plaintext cashnotneeded. 6.How Alice and Bob authenticate each other by using secret key cryptography is shown as follows.
- Just to add, in the public-key example above, the mailman could take Bob's unlocked padlock that he sends to Alice, and impersonate Alice by replying to Bob with it, or the mailman could send his own unlocked padlock to Alice (claiming to be Bob), get her response and then forward her response to Bob with Bob's unlocked padlock, allowing him to read the transactions between Alice and Bob, while they think they're secure.
- Bob would have to manually confirm with Alice that the public key he thinks is Alice's public key is indeed hers (by calling her and reading it out load and confirming by her voice that it is Alice). This problem is normally solved with a trusted third party (a Certificate Authority for example, like VeriSign) that issues certificates stating ...
- To generate a key shared with Bob whose public key is K = [k]B, Alice proceeds as follows: 1.Generate random r < p; 2.Generate expanded encryption key k S [r]K = (k S[0];k S[1]) 2F2 q: 1The S-box type and the number of rounds depend on q. For example the S-box x5 is recommended for being the prime subgroup size of curve BN254. 1

In this example Alice and Bob each generate an ECDH key pair, then exchange public keys. They then use deriveKey() to derive a shared AES key, that they could use to encrypt messages. See the complete code on GitHub. – For example, if p and q are 512-bit numbers, then b is either 1022 or 1023 • Suppose Alice wants to send a message to Bob – She partitions the message into a sequence of b-bit blocks (padding the last block with zeros if necessary) – Encryption and decryption is done on a per block basis – Later we’ll discuss some variations of ...

Figure 1: Alice and Bob use a private-key encryption scheme. Sometimes, in a private key system, an encryption scheme is associated with a message space M and some distribution on M. For example, M may be the set of all strings for a given length. The ciphertext space is the set of all possible plaintext messages m ∈M 4-1 3.1.1 Encryption Encryption is used to communicate securely over an insecure communication channel. Consider Alice communicating with Bob. Any message from Alice to Bob is also received by Eve. To prevent Eve from understanding the message an encryption function E (K enc, m) is used to transform the so called Plaintext, m,

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