EventId's in Nostr - from CGPT4

The mathematical operation used to derive the event.id in your getSignedEvent function is the SHA-256 hash function, applied to a string representation of the event's data.

async function getSignedEvent(event, privateKey) {

          var eventData = JSON.stringify([

          0,            // Reserved for future use

          event['pubkey'],  // The sender's public key

          event['created_at'],  // Unix timestamp

          event['kind'],    // Message "kind" or type

          event['tags'],    // Tags idnetify replies/recipients

          event['content']  // Your note contents

          ])

          event.id = sha256( eventData ).toString( 'hex' )

          event.sig = await schnorr.sign( event.id, privateKey )

          return event

       }        

Let's break down this process:

String Representation of Event Data:

First, a string eventData is created from the event object. This string is a JSON representation that includes several fields of the event object, such as the sender's public key (event['pubkey']), timestamp (event['created_at']), message type (event['kind']), tags (event['tags']), and content (event['content']).

These fields are combined into an array and then serialized into a JSON string.

Applying the SHA-256 Hash Function:

The SHA-256 hash function is then applied to this string. SHA-256 stands for Secure Hash Algorithm 256-bit and is a member of the SHA-2 cryptographic hash functions designed by the NSA.

Cryptographic hash functions like SHA-256 have several key properties:

• Deterministic: The same input will always produce the same output.
• Fixed Size: Regardless of the input size, the output (hash) is always of a fixed length. In the case of SHA-256, the output is 256 bits (32 bytes) long.
• Fast Computation: The hash value is quick to compute for any given input.
• Pre-image Resistance: Given a hash value, it's computationally infeasible to find the original input.
• Small Changes in Input Produce Significant Changes: Even a tiny change in the input will produce a significantly different output.
• Collision Resistance: It's extremely unlikely (though not impossible) that two different inputs will produce the same output hash.

Creating event.id:

The output of the SHA-256 hash function (applied to eventData) is then converted to a hexadecimal string. This hex string becomes the value of event.id.

This event.id serves as a unique identifier for the event, and due to the properties of the SHA-256 hash, it also ensures the integrity of the event's data. Any change in the event's data would result in a completely different event.id.

In summary, the event.id is derived through a process of serialization (converting the event data to a JSON string) and hashing (applying SHA-256 to this string). This process is fundamental in many cryptographic applications, including blockchain and digital signatures, as it ensures data integrity and uniqueness.