Kryptographie #5 - Das One-Time-Pad

Summary

In this video, the concept of the One-Time Pad (OTP) encryption method is explored, focusing on its theoretical perfection in security. The OTP uses a random, one-time key that is as long as the message to encrypt and decrypt it using XOR operations. While the OTP is unbreakable with a perfectly random key, it has practical limitations, such as the need for a secure key distribution system and the challenge of generating truly random keys. The video explains the mechanics of the OTP and its flaws, highlighting both its strength and its practical drawbacks in cryptography.

Takeaways

• 😀 The One-Time Pad is the only theoretically perfect secure cryptographic system that cannot be broken.
• 😀 A One-Time Pad can only be used once. Reusing it compromises its security.
• 😀 The One-Time Pad operates on binary data, where letters are converted into numbers using ASCII.
• 😀 ASCII assigns a unique number to each letter, which can be converted to binary for encryption purposes.
• 😀 Logical operators like XOR (exclusive OR) are used to encrypt messages with the One-Time Pad.
• 😀 The key for the One-Time Pad must be at least as long as the message being encrypted, ideally longer.
• 😀 A key must be perfectly random for the encryption to be secure, and any predictability or pattern makes it vulnerable.
• 😀 The biggest flaw of the One-Time Pad is the need for a new random key for every message, making key distribution difficult.
• 😀 XOR works by comparing corresponding bits of the message and the key, generating a secure ciphertext.
• 😀 Decoding a One-Time Pad involves applying XOR again between the ciphertext and the key to retrieve the original message.
• 😀 The security of the One-Time Pad is only guaranteed if the key is truly random, which is essential for preventing frequency analysis and ensuring encryption strength.

Q & A

What is the One-Time Pad in cryptography?

The One-Time Pad (OTP) is a cryptographic system that is considered perfectly secure when used properly. Its security is based on the key being used only once, ensuring that the encryption cannot be broken if the system is followed correctly.

Why can the key in a One-Time Pad only be used once?

The key in a One-Time Pad can only be used once because reusing the key would allow for patterns to emerge, which could compromise the security of the encryption, making it vulnerable to attacks.

How is the message in a One-Time Pad encrypted?

The message is first converted into binary, and then an XOR (exclusive OR) operation is applied between the binary message and a randomly generated binary key of the same length. The result of the XOR operation is the encrypted message.

What is ASCII, and why is it important in the One-Time Pad process?

ASCII (American Standard Code for Information Interchange) is a character encoding standard that assigns a unique number to each letter, digit, or symbol. In the One-Time Pad process, ASCII is used to convert letters and symbols into binary strings, which are then used for encryption and decryption.

How does the XOR operation work in the One-Time Pad?

The XOR operation compares corresponding bits from two binary strings, and returns 1 if the bits are different and 0 if they are the same. For example, 1 XOR 1 = 0, 1 XOR 0 = 1, and 0 XOR 0 = 0.

What is the major flaw of the One-Time Pad?

The major flaw of the One-Time Pad is the requirement for a key that is at least as long as the message being sent. Additionally, the key must be truly random, never reused, and securely distributed to both the sender and the recipient.

Why does the One-Time Pad require a perfectly random key?

The key must be perfectly random because any predictability or pattern in the key could make the encryption vulnerable. A truly random key ensures that the encryption is unbreakable and that the ciphertext appears completely random.

How is the One-Time Pad decoded?

To decode the message, the recipient uses the same key that was used for encryption and performs the XOR operation again between the ciphertext and the key. This process reverses the encryption, returning the original message.

What happens if the key in a One-Time Pad is shorter than the message?

If the key is shorter than the message, it must be repeated or truncated, which leads to vulnerabilities similar to those in the Vigenère cipher. In this case, patterns in the key can be exploited to break the encryption.

Why is the One-Time Pad not commonly used despite being perfectly secure?

The One-Time Pad is not commonly used because it requires the key to be as long as the message, perfectly random, and securely shared between the sender and recipient. These requirements make it impractical for regular use in most scenarios, especially when it comes to large-scale communication.