Chapter 3 - Encryption Flashcards
What is CIA?
Confidentiality, Integrity, and Authentication, in the case of cryptography.
What is a cipher?
A cipher is an algorithm that encrypts and decrypts.
How can ciphers be public and still be secure?
Ciphers are only part of the actual encryption; a key is what actually allows the content (plaintext) to be read clearly from the ciphertext.
What is a substitution cipher?
Pretty simple, where you substitute one letter (or bit) for another in each place.
What is a transpostion cipher?
Transposition ciphers are simple as well, where the individual letters or bits are not changed, but change their orders.
How are modern ciphers used?
Modern ciphers tend to use both substitution and transposition ciphers, repeatedly in mixed orders with different keys.
What’s the difference between a cipher and a code?
A cipher is the way of encoding messages, codes are where one thing is substituted for another thing (STOP = 26733). Generally, it means symbols are used in place of whole words.
What is a weakness of a code? How is this avoided?
If a code uses the same substitution for a thing, it becomes easily recognizable. This can be avoided by having multiple things that can be substituted for one plaintext word.
How does the length of key bits change the number of possible keys?
The more key bits, the larger the number of possible keys, exponentially.
What are the three stages of cryptographic systems?
The three stages of cryptographic systems are:
- The two parties agree upon a system to use.
- Handshaking stages go down.
- Encrypted communication happens.
What is a key?
A key is a random string of 40 to 40,000 bits.
How do keys affect ciphers?
Any given cipher will produce different ciphertext from the same plaintext, based on the key used.
What is Kerckhoff’s Law?
Kerckhoff’s Law states that the secret to secure encryption is to keep keys secret, not the cipher.
What is cryptanalysis?
Cryptanalysis is the process of breaking encryption.
What is confidentiality?
Confidentiality is when people who intercept messages cannot understand them.
What is symmetric key encryption?
Symmetric key deciphering is where a cipher is encrypted and decrypted at both ends with the same key.
What is the benefit of symmetric key encryption?
Symmetric key encryption is fast, and takes little in the way of resources, so it is well-suited for small transactions, mobile devices, etc.
What is an exhaustive search?
An exhaustive search is an attempt to learn a key by trying all possible keys until the right one is found.
How does increasing key length affect the time of an exhaustive search?
Each additional bit in a key doubles the time it takes to find the result.
Why is the word symmetric used in symmetric key encryption?
It is called “symmetric” key encryption because each side uses the same key to encrypt and decrypt.
When two parties communicate with each other using symmetric key encryption, how many keys are used in total?
Symmetric key encryption uses a total of one key.
What type of encryption cipher is almost always used in encryption for confidentiality?
Symmetric encryption is used in all but a tiny fraction of ciphers.
What is the best way to thwart exhaustive searches by cryptanalysis?
The best way to thwart exhaustive (brute-force) searches is by increasing the key length of the key.
If a key is 43 bits long, how much longer will it take to crack it by exhaustive search if it is extended to 45 bits?
It will take four times as long.
If a key is 43 bits long, how much longer will it take to crack by exhaustive search if it is extended to 50 bits?
2^7th times as long.
If a key is 40 bits long, how many keys must be tried, on average, to crack it?
2^40 attempts will be necessary (barring luck).
How long must a symmetric encryption key be to be considered strong today?
Keys must be at least 100 bits long to be considered strong, and that is growing longer constantly as computers grow stronger.
Why is cryptography not an automatic protection?
Cryptography can fail if one of the two parties isn’t vigilant, and cannot keep the key secure.
What are the two advantages of RC4?
RC4 is extremely fast, and uses a small amount of RAM. It also can accept a wide array of key lengths.
Why is a common RC4 key length 40 bits?
RC4’s most common key length is 40 bits due to national export restrictions held by countries, requiring that encryption to not be so strong as that the nation importing the system couldn’t break it if needed.
How long is a DES key? Is this a strong length?
A DES key is actually 56 bits long, with an extra 8 bits of redundant padding, that can be computed provided you know the original 56 bits.This is far too short to be secure today.
