2 Symmetric Encryption and Authenticated Encryption Free Online FlashCards

What key arrangement does symmetric encryption use?

Symmetric encryption uses the same secret key to encrypt and decrypt data; both parties must protect that key.

What do AES and a mode of operation each do?

AES transforms fixed-size blocks under a key. A mode of operation specifies how to apply the cipher to messages of varying length.

Why can ECB reveal patterns in plaintext?

ECB encrypts each block independently, so identical plaintext blocks produce identical ciphertext blocks under the same key, revealing patterns.

How does CBC process blocks, and what does it require?

CBC combines each plaintext block with the preceding ciphertext block before encryption. It requires an initialization vector and typically padding.

How does CTR produce ciphertext?

CTR encrypts successive counter values to produce a keystream, then combines that stream with the plaintext. It needs no padding and supports parallel processing.

What do CFB and OFB have in common?

CFB and OFB turn a block cipher into stream-like modes, but neither provides authentication by itself.

What protections does AEAD combine?

AEAD combines encryption with an authentication check, protecting plaintext confidentiality and detecting attempts to alter ciphertext.

How does AEAD treat associated data (AAD)?

Associated data (AAD) is authenticated but not encrypted. It can protect visible information, such as a protocol version or message header, from alteration.

What authentication-related value accompanies AEAD ciphertext?

An AEAD operation outputs ciphertext and an authentication tag; the tag supports checking whether the protected data was altered.

What can AEAD authentication not establish about the creator?

AEAD shows that data was produced by someone with the shared key, but it cannot identify which key holder created it or provide public-key nonrepudiation.

What is a nonce, and why must it not be reused with a GCM key?

A nonce is used once within the relevant key context and usually need not be secret. Reusing a nonce with the same key in common AEAD constructions such as GCM can severely undermine confidentiality and authentication.

What protection does CBC fail to provide?

CBC provides confidentiality only; it does not authenticate ciphertext. A legacy CBC system needs a separate, correctly designed authentication mechanism.