Cryptography 1 Flashcards
the security of the encryption scheme must depend only on the secrecy of the key, and not on the secrecy of the algorithm.
*** significant idea behind public-key cryptography—the key to encrypt a message is different from the key to decrypt that message.
Kerckhoffs’s principle Etc …
Like encryption, authentication uses a secret key that Alice and Bob both know. When a message is sent from Alice she computes the message authentication code (MAC) and sends both to Bob. When bob receives the message and MAC he recomputes what the message should have been using the key and checks to see if messages is correct. Eve can listen to message exchange and try to record messages and their MACs and replay them by sending them to Bob later. Bob can check sequence number of old message sent from Eve which has correct MAC and discard the message.
Encryption / Secret Keys Etc …
The best way to look at it is to consider the case where Alice sends a sequence of messages m1,m2,m3,…Bob only accepts messages with a proper MAC and whose message number is strictly greater than the message number of the last message he accepted. So Bob receives a sequence of messages that is a subsequence of the sequence that Alice sent. A subsequence is simply the same sequence with zero or more messages deleted.
Cryptography Info …
When Alice wants to send a message to Bob, she first obtains Bob’s public key. She might obtain the public key from a public directory, or perhaps she obtains the public key from someone else she trusts. Alice encrypts the message m with the public key PBob to get the ciphertext c and sends c to Bob, Bob uses his secret key and decryption algorithm to decrypt message and get message m.
Cryptography Info 2 …
PKI : The main idea is to have a central authority called the certificate authority, or CA. Each user takes his public key to the CA and identifies himself to the CA. The CA then signs the user’s public key using a digital signature. The signed message, or certificate, states: “I, the CA, have verified that public key PBob belongs to Bob.” The certificate will often include an expiration date and other useful information.
PKI …
In a PKI, each participant only has to have the CA certify his public key, and know the CA’s public key so that he can verify the certificates of other participants. This is far less work than exchanging keys with every party he communicates with. That’s the great advantage of a PKI: register once, use everywhere.
PKI #2 …
For practical reasons, a PKI is often set up with multiple levels of CAs. There is a top-level CA, called the root, which issues certificates on the keys of lower-level CAs, which in turn certify the user keys. The system still behaves in the same way, but now Alice has to check two certificates to verify Bob’s key.
PKI #3 …
This is the situation in which Alice and Bob are encrypting their data, and all you as the attacker get to see is the ciphertext. Trying to decrypt a message if you only know the ciphertext is called a ciphertext-only attack. This is the most difficult type of attack, because you have the least amount of information.
Ciphertext-Only
??? : attacker can choose random plaintexts to be encrypted and obtain the corresponding ciphertexts.
??? : Attacker knows both plaintext and ciphertext. You can choose any number of plaintexts and get the corresponding ciphertexts.
Chosen plaintext / Known Plaintext …
In a chosen-ciphertext attack, you get to choose both plaintext values and ciphertext values. For every plaintext that you choose, you get the corresponding ciphertext, and for any ciphertext you choose, you get the corresponding plaintext.
Chosen-Ciphertext
