Block Cipher Modes

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Electronic Code Book (ECB)

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Most basic, weak, and unsecure mode. Each block is processed separately. No Salt or IV is used and the same key will be used to encrypt each block. This means if a given plaintext is encrypted in ECB and results in a given ciphertext, that same ciphertext will be output EVERY TIME the same plaintext is encrypted.

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Cipher Block Chaining (CBC)

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Minor step up from ECB with the incorporation of an initialization vector for the first block. Results of encryption from previous block is XOR’d with plaintext of the current block. That result is input into to encryption process of the current block.

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Cipher Feedback (CFB)

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Converts the block cipher into a self-synchronizing stream cipher. Current block takes output of the XOR ⊕ process vs from the cipher stage of the previous block (difference between CFB and OFB).

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Output Feedback (OFB)

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Converts the block cipher to a synchronous stream output. Current block takes output from the cipher stage vs from the output of the XOR process of the previous block (difference between CFB and OFB). The first stage takes the data blocks and X-ORs it with the encrypted version of the IV value. The output of the first stage encryption is then feed into the next stage, and encrypted, with the output being X-OR’ed with the second block.

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Counter Mode (CTR)

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Converts the block cipher into a stream cipher. Generates a counter value and a nonce, and encrypts this, in order to EX-OR with the plain text block. The advantage of CTR is that each block is processed independent of the others, facilitating ability to conduct parallel processing of blocks. i.e., feedback from other stages to feed into the current one is not required.