Hashing

Overview

Cryptographic hash functions transform data of any size into a fixed-length message digest or checksum, with SHA256 producing a 256-bit (64 hexadecimal digit) output regardless of input size. These functions are deterministic and demonstrate the avalanche effect—even a single-bit change in input produces a completely different hash value. Hash functions serve critical security purposes including secure password storage and detecting file modifications or tampering. Older algorithms like MD5 and SHA-1 are now cryptographically broken and vulnerable to collision attacks.

Key Information

• Fixed Output Size: Hash functions always produce the same length output regardless of input file size (e.g., SHA256 always produces 256 bits or 64 hex digits)
• Avalanche Effect: A single-bit change in input data produces a drastically different hash value, enabling reliable tamper detection
• Primary Applications: Password storage (with salting), file integrity verification, and detecting both intentional tampering and transfer errors
• Secure vs. Broken Algorithms: SHA224, SHA256, SHA384, SHA512, and RIPEMD160 are currently secure; MD5 and SHA-1 are cryptographically broken and susceptible to hash collisions
• HMAC Authentication: HMAC combines a hash function with a secret key using inner/outer padding (ipad/opad) to provide message authentication.

Notes

sha256sum file

hmac256 key file

Task

1. What is the SHA256 checksum of the file order.json?
   1. sha256sum order.json
   2. 2c34b68669427d15f76a1c06ab941e3e6038dacdfb9209455c87519a3ef2c660
2. Open the file order.json and change the amount from 1000 to 9000. What is the new SHA256 checksum?
   1. sha256sum order.json
   2. 11faeec5edc2a2bad82ab116bbe4df0f4bc6edd96adac7150bb4e6364a238466
3. Using SHA256 and the key 3RfDFz82, what is the HMAC of order.txt?
   1. hmac256 3RfDFz82 order.json
   2. c7e4de386a09ef970300243a70a444ee2a4ca62413aeaeb7097d43d2c5fac89f

Conclusion

Cryptographic hash functions are fundamental security tools that provide both data integrity verification and secure password storage mechanisms. Understanding the difference between secure algorithms (SHA-256 family) and broken ones (MD5, SHA-1) is essential for implementing modern cybersecurity solutions. HMAC extends basic hashing by incorporating secret keys, making it suitable for message authentication in scenarios requiring both integrity and authenticity verification.

Resources

• TryHackMe: Intro to Cryptography