Kripto 14: Kriptografi Modern (Bagian 2: Stream Cipher)

Summary

This video covers the second part of modern cryptography, focusing on stream ciphers. Stream ciphers encrypt plaintext bit-by-bit, using a key stream generated by systems like Vernam cipher and Linear Feedback Shift Registers (LFSR). The video explains encryption and decryption processes, highlighting key issues like key generation, bit-wise operations, and potential vulnerabilities. The importance of truly random key streams for security is discussed, along with common attacks such as non-PTT, ciphertext-only, and bit-flip attacks. The section concludes with a preview of block ciphers in the next video.

Takeaways

• 😀 Stream cipher operates on individual bits of data for encryption and decryption.
• 😀 Block cipher operates on blocks of data, with encryption and decryption done on chunks like 64-bit blocks.
• 😀 Stream cipher encryption involves XORing bits of plaintext with corresponding bits from a key stream.
• 😀 Vernam cipher, introduced by military expert Vernam, is the foundation of stream ciphers, utilizing the concept of one-time pad encryption.
• 😀 Stream cipher's security level depends on the randomness and non-repeating nature of the key stream.
• 😀 A stream cipher's encryption and decryption rely on the same key stream, generated by a key stream generator.
• 😀 The key stream generator can be a hardware system such as Linear Feedback Shift Register (LFSR), which generates a sequence of bits.
• 😀 A LFSR's feedback function XORs bits from the shift register to produce new bits, forming the key stream.
• 😀 Security weaknesses in stream ciphers can arise if the key stream repeats or is predictable, making the cipher vulnerable to attacks.
• 😀 Attacks on stream ciphers include known plaintext attacks, where the attacker can XOR ciphertext with known plaintext to reveal the key.
• 😀 Periodic key stream repetition can lead to vulnerabilities, especially when the same key stream is used for multiple plaintexts.
• 😀 A robust key stream generator ensures that the key stream is truly random and does not repeat, providing secure encryption similar to a one-time pad.

Q & A

What is the primary difference between stream ciphers and block ciphers?

Stream ciphers encrypt data bit by bit, while block ciphers operate on fixed-size blocks of bits, such as 64-bit blocks. This difference affects their encryption and decryption processes.

What is the key stream in a stream cipher?

The key stream in a stream cipher is a sequence of bits used to encrypt the plaintext. It is combined with the plaintext using the XOR operation to produce the ciphertext.

How does the Vernam Cipher work, and why is it considered a form of perfect security?

The Vernam Cipher uses a key that is as long as the plaintext and is used only once. It is considered a form of perfect security because the key is completely random and never reused, making the ciphertext indistinguishable from random data.

What is the role of a key stream generator in stream ciphers?

A key stream generator produces a sequence of bits that are combined with the plaintext to encrypt it. It must generate a key stream that is unpredictable and non-repeating for strong security.

What are the three cases for key stream generation described in the transcript?

The three cases are: 1) The key stream is entirely zero, which results in the ciphertext being identical to the plaintext; 2) The key stream repeats periodically, which reduces security; 3) The key stream is truly random, providing perfect security similar to the One-Time Pad.

What is a Linear Feedback Shift Register (LFSR), and how is it used in stream ciphers?

An LFSR is a type of key stream generator that shifts and modifies bits within a register to produce a sequence of output bits. It is used in stream ciphers to generate the key stream that is combined with the plaintext during encryption.

How does an LFSR work with a 4-bit register, and what is the process of key stream generation?

In an LFSR with a 4-bit register, the register is initialized with a set of bits. The feedback function XORs selected bits and shifts the register to generate new bits, which are then used as the key stream. The process repeats until the sequence is complete.

What are the potential security weaknesses in stream ciphers as described in the transcript?

Stream ciphers are vulnerable to weak key stream generation, such as when the key stream is predictable, periodic, or entirely zero. This can make the encryption insecure and susceptible to attacks like known-plaintext or ciphertext-only attacks.

What is a known-plaintext attack in the context of stream ciphers?

A known-plaintext attack occurs when an attacker knows both a portion of the plaintext and its corresponding ciphertext. Using XOR operations, the attacker can deduce the key stream, compromising the encryption.

What is a ciphertext-only attack, and how does it exploit the periodicity of key streams?

A ciphertext-only attack involves the attacker having access only to ciphertexts. If the same key stream is used repeatedly, the attacker can XOR different ciphertexts to expose patterns in the key stream, eventually leading to the decryption of the messages.