Cryptography: Advanced Encryption Standard - Facts

1. What was the original set of criteria used by NIST to evaluate candidate AES ciphers?

• Security: Actual security; randomness; soundness, other security factors.
• Cost: Licensing requirements; computational efficiency; memory requirements.
• Algorithm and Implementation Characteristics: Flexibility; hardware and software suitability; simplicity.

1. What was the final set of criteria used by NIST to evaluate candidate AES ciphers?

General security; software implementations; restricted-space environments; hardware implementations; attacks on implementations; encryption vs. decryption; key agility; other versatility and flexibility; potential for instruction-level parallelism. 

1. What is the difference between Rijndael and AES 

The basic idea behind power analysis is the observation that the power consumed by a smart card at any particular time during the cryptographic operation is related to the instruction being executed and to the data being processed.

Cryptography: Cryptanalytic Attacks Examples

 

Known Ciphertext Attack

Example:

• Eve steals a bunch of ciphertexts from Alice's handbag, but has no idea what they mean.

Known Plaintext Attack

Example:

• Eve overhears an encrypted communication from Bob to Alice, and later observes them meeting at Baker Street - Eve can now guess that the communication contained the word "baker street" somewhere, a form of known plaintext attack.

Chosen Plaintext Attack

Example:

• A poorly designed file storage system uses the same key to encrypt everyone's files, and lets anyone see anyone's files (in encrypted form). Eve knows Bob uses the service, she registers as well and starts encrypting arbitrary files (which she can choose) and looks at the resulting ciphertext. From this she obtains the service's encryption key, and decrypts Bob's stuff.

 
 
Chosen Ciphertext Attack

Example:

• Eve breaks into Bob's house while he is sleeping, and replaces the ciphertext he was going to send to Alice tomorrow with a new one of her choosing. She then eavesdrops on their communications (encrypted or not) the next day to try and work out what Alice read when she decrypted the fake ciphertext (variants of this involve Eve not just creating a new ciphertext, but modifying the existing one slightly).

Three Dimensions of Cryptographic Systems

•The type of operations used for transforming plaintext to ciphertext.

–All encryption algorithms are based on two general principles:

•Substitution: each element in the plaintext is mapped into another element, and

•Transposition: elements in the plaintext are rearranged.

–The fundamental requirement is that no information be lost (that is, that all operations are reversible).

–Most systems involve multiple stages of substitutions and transpositions.

•The number of keys used.

–If both sender and receiver use the same key, the system is referred to as symmetric, single-key, secret-key, or conventional encryption.

–If the sender and receiver use different keys, the system is referred to as asymmetric, two-key, or public-key encryption.

•The way in which the plaintext is processed.

–A block cipher processes the input one block of elements at a time, producing an output block for each input block.

–A stream cipher processes the input elements continuously, producing output one element at a time, as it goes along.