TLS und Kryptographie

TLS - Transport Layer Security

• HTTPS utilizes TLS for secure internet browsing
• TLS employs a handshake protocol, utilizing asymmetric encryption and digital signatures to negotiate a shared secret key for secure communication.
• The handshake protocol facilitates the exchange of certificates and keys to establish a secure channel.
• The record protocol then employs symmetric encryption and authentication to protect application data.

Goals of Cryptography

• Privacy: Data confidentiality, only intended recipients can read the data.
• Authenticity: Verifying the source of the data, ensuring the data has not been tampered with.
• Integrity: Guaranteeing the data hasn't been altered or corrupted.
• Non-Repudiation: Preventing the sender from denying that they sent the data or that it was tampered with.

Security by Obscurity

• A security system should be secure even with complete knowledge of the system except for the secret key.

Shannon Model, Four Main Attack Scenarios

• KCA: Known Ciphertext Attack
• CPA: Chosen Plaintext Attack
• CCA: Chosen Ciphertext Attack

V1 - Ciphers

• A cipher is defined by three sets:
  • P (Plaintext)
  • C (Ciphertext)
  • K (Keys)
• And three efficient algorithms for encryption(E), decryption (D) and key generation (G).
• Julius Caesar cipher as an example of a substitution cipher with 25 combinations.
• A substitution cipher uses permutations of the alphabet to encrypt characters. Key generation randomly chooses a permutation from S₂₆.

Attacking the Substitution Cipher

• English and German alphabets show patterns and frequencies of letters that attackers can explore to deduce more information on the encrypted text .

Perfect Cryptosystems

• A cipher is perfect if the probability of the plaintext does not depend on the ciphertext

OTP or "Vernam-Cipher"

• The one-time pad is a type of encryption where every message is encrypted using a unique key.
• OTP use XOR operations between the message and the unique key
• The key must be as long as the message
• Reusing keys significantly compromises security.

Stream Ciphers

• Generates a long pseudorandom key stream from a short cipher key
• Stream ciphers are susceptible to the reuse of the same key or initial state.
• Stream ciphers lack authenticity, making them vulnerable to attacks where an adversary might be able to tamper with the message without being detected.

Stream Ciphers - Weaknesses

• Pseudorandom key streams that are not truly random.
• Lack of authenticity, meaning an adversary can modify the data without being detected.
• Reusing the same cipher key or states.

Linear Feedback Shift Registers (LFSR)

• Defined by a linear equation
• Efficient in hardware
• The cycle length for nonzero states is maximal(2^n - 1) when all bits in LFSR are not zero.
• Terribly insecure due to linearity.

What is secure?

• A cryptosystem is secure against a particular class of attacks, if there are no efficient algorithms that successfully attack it with a significant probability.

V2 - Passwords and Entrophy

• Passwords should be long, complex, and memorable
• Offline and online adversary concepts and measures to protect against them

NIST-Richtlinien zu Passwortsicherheit

• Passwörter mit mindestens 8 Zeichen lang
• Mindestens 64 Zeichen
• Alle Unicode Zeichen erlaubt
• Keine periodischen Passwortänderungen vorschreiben
• Passwortänderungen nach Server Kompromittierung
• Keine Hinweise zu Passwörtern
• Keine Fragen wie: „Was ist der Name von...?"

Entropy

• Represents the amount of information provided by a source
• A source providing the same data consistently has zero entropy.
• Random bit sequences have high entropy

V3 - Block Ciphers

• Block ciphers encrypt data in fixed-size blocks
• CPA on PRFGs involves differentiating between a pseudo-random function family and a random function.
• CPA on PRPGs involves differentiating between a pseudo-random permutation family and a random permutation.
• Security is achieved when no efficient adversary can differentiate the system from a random function/permutation.
• Luby-Rackoff/Feistel-Ciphers are block ciphers structures, that are not CP secure

P1 and P2

• P1 and P2 are vulnerable to CPA due to using invertible round functions.

P3 and P4

• P3 and P4 are CPA-secure due to using non-invertible round functions

V4 - Attacks

• Differential Cryptanalysis
• Exploiting the small key size in DES
• Brute-force attacks are viable due to the 56 bit key size

Side-Channel Attacks

• Attacks exploiting the physical implementation of a cryptosystem, such as timing, power consumption, and electromagnetic radiation.
• Insider Attacks
• Attacks by authorized individuals utilizing their access to exploit weaknesses.
• Outsider Attacks
• Attacks from unauthorized individuals attempting to breach the system.

V5 - Foundations of P. K. Cryptographie

• Public-key cryptography uses pairs of keys – a public key for encryption and a private key for decryption.
• No secure channel is required for key exchange, in contrast to private-key cryptography.
• Shamir's No-Key Protocol
• A method of exchanging keys without prior communication.
• Participants create and swap padlocks twice.

V6 - Insecure Systems from Secure Building Blocks

• Naive use of stream ciphers with reused keys, leading to security risks.
• Naive use of block ciphers, where blocks are too short for various uses.
• ECB (Electronic CodeBook) mode is not secure and can be exploited.
• CBC (Cipher Block Chaining) mode is susceptible to manipulation when the initial vector (IV) is predictable.

V7 - Secure PK Crypto

• Secure public-key encryption and digital signatures.

V8 - Authentication & Authenticated Encryption

• Authentication provides assurance about the source and integrity of a message, whereas encryption protects the confidentiality of a message.
• Message Authentication Code (MAC) is an algorithm that generates a tag from a message to authenticate a message, using a key.
• SUF-CMA (Strongly Unforgeable under Chosen-Message Attacks) is a way to secure message authentication codes.

V9 - Secure PK Crypto - Further Aspects

• RSA is a widely used public-key cryptosystem
• It provides encryption and digital signatures
• The RSA function has desirable properties as a trapdoor one-way function
• Low-entropy plaintexts or messages can be problematic with public-key cryptography
• Secure message encryption and digital signatures can rely on the Random Oracle Model (ROM).

Dieses Quiz behandelt die Grundlagen von TLS (Transport Layer Security) und die Ziele der Kryptographie. Sie erfahren, wie der Handshake-Protokollprozess sichergestellt wird, um eine sichere Kommunikation zu ermöglichen. Zusätzlich werden die Konzepte von Datenschutz, Authentizität und Integrität thematisiert.