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Block Cipher Mode

The method that determines how a block cipher like AES is applied to data longer than a single block, such as CBC, CTR, or GCM. The mode chosen critically affects security, and a poor choice can break confidentiality even with a strong cipher.

A block cipher only encrypts one fixed-size block at a time, so a mode of operation defines how blocks are chained and combined to encrypt arbitrary data. Modes differ in whether they provide integrity, need a unique nonce, or leak patterns; authenticated modes like GCM provide both confidentiality and integrity and are preferred.

Introduced in: Cryptography Essentials

Examples

  • Using AES-GCM to get authenticated encryption with confidentiality and integrity.
  • Avoiding ECB mode because it leaks repeating patterns in data.
  • Ensuring a unique nonce per message when using CTR or GCM.

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