Hashing & Encryption : Session 1

The opening session introduced the two most foundational pillars of applied cryptography: hashing, which guarantees data integrity, and encryption, which guarantees confidentiality. The session moved from the properties of secure hash functions through the historical weaknesses of algorithms like MD5 and SHA-1, then into symmetric and asymmetric encryption, cipher modes of operation, and the concept of hybrid encryption used in almost every real-world secure system.

Part A — Hashing

The session opened by defining a cryptographic hash function as a deterministic algorithm that takes an input (or "message") of arbitrary length and returns a fixed-length output, called a hash value or digest. Participants worked through why this fixed-length property is useful  regardless of whether the input is a single character or a 10 GB file, the output is always the same size (e.g., 256 bits for SHA-256).

The discussion then moved into the three core security properties every cryptographic hash function must satisfy:

1. Pre-image resistance : Given a hash output h, it should be computationally infeasible to find any input m such that hash(m) = h.
2. 
   
3. Second pre-image resistance : Given an input m1, it should be infeasible to find a different input m2 such that hash(m1) = hash(m2).
4. 
   
5. Collision resistance :  It should be infeasible to find any two distinct inputs m1 and m2 such that hash(m1) = hash(m2).
6. 
   

The group examined the historical progression of hash algorithms:

MD5 (now broken due to practical collision attacks), SHA-1 (deprecated after the SHAttered collision attack demonstrated by Google in 2017), and the SHA-2 family (SHA-256, SHA-512), which remains widely used today.

SHA-3, based on the Keccak sponge construction, was introduced as a structurally different alternative offering resilience against future attacks that might target the Merkle-Damgård construction used by SHA-2.

A live demonstration showed how flipping a single bit in the input (the "avalanche effect") completely changes the output hash reinforcing that hash functions are designed to make outputs appear random and unrelated to small input changes.

Practical applications discussed included password storage (and why salting is required to defend against rainbow table attacks), file integrity verification via checksums, and hashing's role as a building block for Merkle trees and blockchains, previewed for later sessions.

Part B — Encryption

The session then transitioned into encryption, contrasting the two major categories:

• Symmetric encryption, where the same key is used to both encrypt and decrypt data. AES (Advanced Encryption Standard) was used as the primary example, with a walkthrough of its block cipher structure operating on 128-bit blocks with key sizes of 128, 192, or 256 bits.
• 
  
• Asymmetric (public-key) encryption, where a mathematically linked key pair  a public key and a private key  is used, allowing anyone to encrypt data with the public key while only the holder of the private key can decrypt it. RSA was used as the primary example.
• 
  

Participants discussed the performance trade-off between the two: symmetric encryption is extremely fast and suitable for bulk data, while asymmetric encryption is computationally expensive but solves the key distribution problem you don't need to have shared a secret key in advance.

The session covered modes of operation for block ciphers:

• ECB (Electronic Codebook) insecure for most use cases because identical plaintext blocks produce identical ciphertext blocks, leaking structural patterns.
• 
  
• CBC (Cipher Block Chaining) : Each block is XORed with the previous ciphertext block before encryption, removing the pattern-leakage problem but requiring careful handling of initialization vectors (IVs).
• 
  
• GCM (Galois/Counter Mode) : The modern preferred mode, providing both confidentiality and built-in authentication (AEAD), preventing undetected tampering with ciphertext.
• 
  

The concept of hybrid encryption was introduced as the practical solution used in almost every real-world secure system (including TLS): asymmetric encryption is used only to securely exchange a symmetric session key, and all bulk data is then encrypted using the fast symmetric cipher with that session key.

This directly previewed why Diffie-Hellman (the topic of Session 2) is so important it offers another way to establish that shared symmetric key.

Key management challenges were discussed as a closing point: how keys should be generated, stored, rotated, and revoked, and why poor key management not weak algorithms is one of the most common causes of real-world cryptographic failures.

This combined session laid the two most foundational pillars of applied cryptography integrity via hashing and confidentiality via encryption establishing vocabulary and primitives (hash functions, block ciphers, key pairs, modes of operation) referenced throughout the rest of the series.