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XID Fundamentals

Expected Learning Outcomes

By the end of this document, you will:

  • Understand what XIDs are and why they're valuable
  • Know how XIDs are derived from cryptographic keys
  • Be familiar with basic XID document structure
  • Know when to use XIDs for pseudonymous identity

What Are XIDs?

An XID (eXtensible IDentifier, pronounced "zid") is a unique 32-byte identifier derived from a cryptographic key. It provides a stable digital identity that remains consistent even as the keys associated with it evolve over time.

For example, an XID might look like this:

7e1e25d7c4b9e4c92753f4476158e972be2fbbd9dffdd13b0561b5f1177826d3

For convenience, XIDs are often shown in shortened form using just the first few bytes:

🅧 7e1e25d7

Stability Through Change

One of the most powerful aspects of XIDs is that they maintain a stable identifier even as associated keys change:

  1. The XID is derived from the initial "inception" key
  2. Additional keys can be added or removed without affecting the XID
  3. The original key can eventually be rotated out entirely
  4. The identifier remains consistent throughout these changes

This stability allows for:

  • Secure key rotation without disrupting existing relationships
  • Addition of device-specific keys while maintaining the same identity
  • Recovery from key loss without losing your established identity
  • Progressive trust building over time with a consistent identifier

How XIDs Are Created

An XID is created as follows:

  1. Generate a cryptographic key pair (public and private keys)
  2. Take the SHA-256 hash of the public key material
  3. This hash becomes the stable identifier (the XID)

In the envelope-cli tool, the process looks like this:

PRIVATE_KEYS=$(envelope generate prvkeys)
PUBLIC_KEYS=$(envelope generate pubkeys "$PRIVATE_KEYS")
XID_DOC=$(envelope xid new --nickname "MyIdentifier" "$PUBLIC_KEYS")
XID=$(envelope xid id "$XID_DOC")

Note that $XID_DOC will contain the full document, while $XID will only contain the XID, each in ur:xid form.

XID Document Structure

An XID alone is just an identifier. The real power comes from the XID document, a Gordian Envelope containing structured data about the XID:


XID(7e1e25d7) [
   "name": "MyIdentifier"
   "publicKeys": ur:crypto-pubkeys/hdcxlkadjngh...
   "domain": "Software Development"
   "key": [
      ur:crypto-pubkeys/hdcxaeluhhfy...
      "Secondary Device Key"
      "sign"
   ]
]

The XID document contains:

  • The name of the identity
  • Public key material
  • Additional assertions about the identity
  • Additional keys with specific permissions
  • Other relevant information for verification

When to Use XIDs for Pseudonymous Identity

XIDs are important for situations where pseudonymity is required. This includes the following use cases:

  1. Privacy is required: You need to participate without revealing your real identity.
  2. Identity persistence matters: You need a stable identifier even as your keys or devices change.
  3. Trust must be verifiable: Others need to verify that your contributions come from the same identity.
  4. Cryptographic verification is important: You need to prove control without revealing identity.

In addition, XIDs can support progressive trust:

  1. Progressive disclosure is needed: You want to reveal information gradually as trust develops.

Opens source software development offers another convincing use case, as developers might wish to maintain their privacy, but users will need to trust the continuity of the software design. The tutorials explore this use case through the story of Amira.

Relationship to Other Concepts

XIDs work together with:

Check Your Understanding

  1. How is an XID derived from a cryptographic key?
  2. What is the difference between a XID and a XID document?
  3. What types of information can be included in a XID document?
  4. When would you want to use a pseudonymous XID rather than your real identity?

Next Steps

After understanding XID fundamentals, you can: