OFB (Output FeedBack) as a Stream Cipher

Counter Mode – The Counter Mode or CTR is a simple counter based block cipher implementation. Every time a counter initiated value is encrypted and given as input to XOR with plaintext which results in ciphertext block. The CTR mode is independent of feedback use and thus can be implemented in parallel. Its simple implementation is shown below: How to choose an AES encryption mode (CBC ECB CTR OCB CFB OFB: An IV-based encryption scheme, the mode is secure as a probabilistic encryption scheme, achieving indistinguishability from random bits, assuming a random IV. Confidentiality is not achieved if the IV is a nonce, although a fixed sequence of IVs (eg, a counter) does work fine. Ciphertexts are highly malleable. PHP: Predefined Constants - Manual

## OFB mode is which is output feedback mode, each block ciphertext is generated by this way: use the IV encrypt with the key, then xor the output with the plaintext, in the next step, the value to encrypt with the key is the output of previous step before xor with plain text (which is feedback mechanism)

OFB (Output FeedBack) as a Stream Cipher Note that there are some special cases of OFB variations: The 64-bit OFB variation is the original OFB operation mode. The 8-bit OFB variation is a stream cipher that takes 1 byte (8 bits) from the plaintext message at a time. The 1-bit OFB variation is a stream cipher that takes 1 bit from the plaintext message at a time. Lab 4.docx - LAB 4 REPORT Course code CRY302 Student name OFB mode is which is output feedback mode, each block ciphertext is generated by this way: use the IV encrypt with the key, then xor the output with the plaintext, in the next step, the value to encrypt with the key is the output of previous step before xor with plain text (which is feedback mechanism)

### Lab 4.docx - LAB 4 REPORT Course code CRY302 Student name

Counter Mode – The Counter Mode or CTR is a simple counter based block cipher implementation. Every time a counter initiated value is encrypted and given as input to XOR with plaintext which results in ciphertext block. The CTR mode is independent of feedback use and thus can be implemented in parallel. Its simple implementation is shown below: OFB: An IV-based encryption scheme, the mode is secure as a probabilistic encryption scheme, achieving indistinguishability from random bits, assuming a random IV. Confidentiality is not achieved if the IV is a nonce, although a fixed sequence of IVs (eg, a counter) does work fine. The OFB mode requires that the IV is a nonce, i.e., the IV 33 * must be unique for each execution of the mode under the given key. 34 * Refer to SP 800-38A for more details