Context

The following questions deal with the theory of modern ciphers and modern cryptography. The topics are:

$($pseudo$)$random numbers

$$ security

$$ exchange of keys

$$ modern block and flow symmetric ciphers

$$ asymmetric cryptography

$$ applications of cryptography

Claims

$1.\backslash$geoquad It is possible to generate a truly random sequence of numbers on a fully deterministic computer.

$2.\backslash$geoquad Operating systems enable the generation of real random numbers.

$3.\backslash$geoquad It is possible to obtain a cryptographically secure source of pseudorandom numbers using block ciphers.

$4.\backslash$geoquad Most random number generators are cryptographically secure in practice.

$5.\backslash$geoquad A code can have a security higher than the number of bits of the key.

$6.\backslash$geoquad A one$-$time notebook is the only perfectly secure code.

$7.\backslash$geoquad Exchanging keys for a disposable notebook is easy.

$8.\backslash$geoquad With a disposable notebook, the cipher is obtained from the plain text by applying an operation to the bits

XAND $($exclusive I$).$

$9.\backslash$geoquad RC$4$ is a usable stream cipher today.

$10.\backslash$geoquad Feistel's network is a stream code in itself.

$11.\backslash$geoquad The substitution and permutation network $($SPN$)$ and the Feistel network are the basic architectures of modern block

codes.

$12.\backslash$geoquad Substitution and permutation network $($SPN$)$ and Feistel network are synonyms.

$13.\backslash$geoquad We call a code a block code if it encrypts bytes of different lengths.

$14.\backslash$geoquad DES has an $8-$byte key of $56$ bits.

$15.\backslash$geoquad $3$DES is a cipher based on the Feistel network.

$16.\backslash$geoquad AES has a key length of $64$ bits.

$17.\backslash$geoquad AES is a usable cipher today.

$18.\backslash$geoquad AES has keys of length $128$ and more bits.

$19.\backslash$geoquad ECB mode is a monoalphabetic code.

$20.\backslash$geoquad CBC $($cipher$-$block chaining$)$ mode of block ciphers is useful for encrypting files.

$21.\backslash$geoquad CFB $($cipher$-$feedback$)$ mode of block ciphers uses an initialization vector that is communicated independently of

password.

$22.\backslash$geoquad CTR $($counter$)$ mode of block ciphers is a good random number generator $($CSPRNG$).$

$23.\backslash$geoquad The cryptographic hash function must be resistant to finding multiple patterns that give the same hash.

$24.\backslash$geoquad MD$5$ is today a cryptographically secure hash.

$25.\backslash$geoquad There are very good hash functions $($for some applications$)$ that are unusable cryptographically.

$26.\backslash$geoquad HMAC is the only form of Message Acknowledgment Code $($MAC$)$ used in practice.

$27.\backslash$geoquad The Merkle puzzle roughly consists of sending a "bunch" of problems to the other side, which

then it chooses which problem to solve and the solution of that problem contains the shared secret key.

$28.\backslash$geoquad The Diffie$-$Hellman$-$Merkle protocol provides security close to the number of bits of the prime number on which it is based.

$29.\backslash$geoquad The Diffie$-$Hellman$-$Merkle protocol uses huge prime numbers in practice.

$30.\backslash$geoquad Asymmetric cryptography is based on the idea that some mathematical operations have a large difference in efficiency

between the operation itself and its inverse.

$31.\backslash$geoquad The factorization problem of large integers is a cryptographically difficult problem.

$32.\backslash$geoquad With asymmetric cryptography, the public key should be handled carefully.

$33.\backslash$geoquad In order to make a message secret with asymmetric cryptography, we encrypt it publicly and decrypt it secretly

key.

$34.\backslash$geoquad A digital signature is generally impossible to forge.

$35.\backslash$geoquad A digital signature essentially consists of encrypting the message hash with a secret key.

$36.\backslash$geoquad IPv$6$ has a built$-$in form of security $($IPSEC$)$ based on cryptography.

$37.\backslash$geoquad Smart cards $($cards with chips$)$ are based on symmetric cryptography.

$38.\backslash$geoquad SSL is a quality protocol at the application level.