[7.15] In what mode is the AES256 algorithm used for encrypted backups?

Question

BlackTom AI · Accepted Answer

Examining the question, we need to determine which AES-256 mode is used for encrypted backups.
Option 1: OFB (Output Feedback) – This mode turns a block cipher into a stream cipher by feeding the output back as input. While OFB has certain properties like error propagation being limited to a single block, it is not the standard mode used for typical full-disk or backup encryption routines, which favor block chaining to provide ciphertext feedback and integrity considerations.
Option 2: CBC (Cipher Block Chaining) – This mode operates by XORing each plaintext block with the previous ciphertext block before encryption, which creates a dependency chain that helps obscure patterns across blocks. In many encrypted backup implementations, CBC is chosen because it provides strong diffusion across blocks and is well-supported by AES-256 in common cryptographic libraries used for backups. This makes it a common default for encrypted backups.
Option 3: ECB (Enhanced Cipher Blocking) – This option misnames a real mode; the standard is ECB (Electronic Codebook). ECB encrypts each block independently, which can reveal patterns in the plaintext if the same block occurs more than once. This lack of diffusion makes ECB unsuitable for secure backups, as identical plaintext blocks yield identical ciphertext blocks, leaking structural information.
Option 4: CFB (Cipher Forensic Block) – This appears to be a mischaracterization of CFB (Cipher Feedback) mode. CFB turns a block cipher into a self-synchronizing stream cipher, which has different fault-tolerance and error propagation characteristics. While CFB is a legitimate mode, it is not the typical default for backup encryption and is less commonly used as the standard mode for AES-256 backups compared to CBC in many common implementations.
In summary, the reasoning favors CBC because it provides a balanced, well-supported approach for encrypting blocks with AES-256 in backup scenarios, delivering good diffusion and compatibility with standard cryptographic toolchains. The other modes described either do not reflect standard naming or carry properties that make them less suitable as a general default for encrypted backups, such as lack of diffusion in ECB, or specialized streaming properties in OFB and CFB that are not as commonly chosen for bulk backup encryption.

[7.15] In what mode is the AES256 algorithm used for encrypted backups? 单项选择题

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