Add NIP-XX: Time Capsules specification
This NIP defines time-locked capsules: encrypted Nostr events that become readable only at/after a target timestamp or when a threshold of designated witnesses publish unlock shares. It includes specifications for event kinds, unlock modes, protocol flow, client behavior, security considerations, and examples.
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# NIP-XX: Time Capsules
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`draft` `optional`
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This NIP defines time-locked capsules: encrypted Nostr events that become readable only at/after a target timestamp or when a threshold of designated witnesses publish unlock shares. This enables delayed revelation, threshold cryptography, digital inheritance, and whistleblowing protection.
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Time-locked capsules allow content to be:
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- Released automatically after a specific timestamp
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- Unlocked when multiple witnesses collaborate
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- Made accessible after long periods for digital inheritance
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- Protected with built-in delays for sensitive material
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## Event Kinds
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Permalink: Event Kinds
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- `1990`: Time Capsule (regular)
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- `30095`: Time Capsule (parameterized replaceable; keyed by `d` tag)
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- `1991`: Time Capsule Unlock Share
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- `1992`: Time Capsule Share Distribution
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## Specification
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Permalink: Specification
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### Time Capsule Events (kinds `1990` and `30095`)
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Permalink: Time Capsule Events
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A time capsule event contains encrypted content and unlock conditions.
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#### Required tags
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- `u`: Unlock configuration in format `["u","<mode>","<param1>","<value1>",...]`
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- `p`: Witness pubkeys (one or more) - `["p","<witness_pubkey_hex>"]`
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- `w-commit`: Merkle root commitment - `["w-commit","<hex_merkle_root>"]`
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- `enc`: Encryption method - `["enc","nip44:v2"]`
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- `loc`: Storage location - `["loc","inline"|"https"|"blossom"|"ipfs"]`
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#### Optional tags
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- `d`: Identifier (required for kind `30095`) - `["d","<capsule-id>"]`
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- `uri`: External content URI (required when `loc != "inline"`) - `["uri","<url>"]`
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- `sha256`: Content integrity hash - `["sha256","<hex_hash>"]`
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- `expiration`: Expiration timestamp per NIP-40 - `["expiration","<unix>"]`
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- `alt`: Human-readable description - `["alt","<description>"]`
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#### Content
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The `content` field MUST contain a base64-encoded NIP-44 v2 encrypted payload. When `loc` is `"inline"`, the entire encrypted content is in this field. When `loc` is external, this field MAY be empty and the `uri` tag points to the encrypted content.
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### Unlock Modes
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Permalink: Unlock Modes
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#### Threshold Mode
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```plaintext
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["u","threshold","t","<t>","n","<n>","T","<unix_unlock_time>"]
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```
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- **t**-of-**n** witnesses must provide shares at/after timestamp `T`
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- Prevents unilateral early disclosure but not collusion of any `t` witnesses
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#### Scheduled Mode
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```plaintext
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["u","scheduled","T","<unix_unlock_time>"]
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```
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- Indicates time-based operational release where witnesses or services intend to post shares after `T`
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- This mode is not a cryptographic timelock; a future revision may define a VDF-based trustless mode
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Implementations MUST parse unknown `u` modes conservatively and treat them as unsupported.
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### Unlock Share Events (kind `1991`)
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Permalink: Unlock Share Events
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A witness posts one share after the unlock timestamp (with optional skew tolerance).
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#### Required tags
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- `e`: Capsule event reference - `["e","<capsule_event_id>"]`
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- `a`: Addressable reference (if capsule is parameterized replaceable) - `["a","30095:<pubkey_hex>:<d>"]`
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- `p`: Witness pubkey - `["p","<witness_pubkey_hex>"]`
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- `T`: Unlock time from capsule - `["T","<unix_timestamp>"]`
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#### Content
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- Base64 Shamir share for threshold mode
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- MAY be gift-wrapped (per NIP-59) to reduce metadata leakage
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- Clients MUST access the plaintext share after timestamp `T`
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### Share Distribution Events (kind `1992`)
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Permalink: Share Distribution Events
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Automates delivery of per-witness shares immediately after capsule creation.
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#### Required tags
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- `e`: Capsule event reference - `["e","<capsule_event_id>"]`
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- `a`: Addressable reference (if capsule is parameterized replaceable) - `["a","30095:<pubkey_hex>:<d>"]`
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- `p`: Recipient witness - `["p","<witness_pubkey_hex>"]`
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- `share-idx`: Share index - `["share-idx","<0..n-1>"]`
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- `enc`: Encryption method - `["enc","nip44:v2"]`
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#### Content
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NIP-44 v2 ciphertext containing the Shamir share destined for the witness. Only the intended witness can decrypt.
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#### Validation Rules
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- Event MUST be authored by the same pubkey as the capsule
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- The target `p` MUST appear in the capsule's witness list
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- `share-idx` MUST be within `[0, n-1]`
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## Protocol Flow
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Permalink: Protocol Flow
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1. **Create Capsule** (kind `1990` or `30095`)
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- Author generates random key `K` and encrypts payload with NIP-44 v2 → `C`
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- Selects witnesses (p tags), sets threshold `t`, witness count `n`, unlock time `T`
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- Computes `w-commit` over ordered witnesses
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- Publishes capsule with `content=C`, unlock config, witness list, commitment, storage location
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2. **Distribute Shares** (kind `1992`) *(recommended)*
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- Split `K` using Shamir's Secret Sharing (t, n)
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- For each witness, publish `1992` with NIP-44 encrypted share for that witness
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- Include `share-idx` to maintain ordering
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3. **Unlock** (kind `1991`)
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- At/after timestamp `T` (± skew tolerance), witnesses publish `1991` with plaintext shares
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- Clients collect any `t` valid shares, reconstruct `K`, and decrypt `C`
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## Relay Behavior
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Permalink: Relay Behavior
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### Validation
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Relays MUST:
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- Ensure required tags exist and are well-formed
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- For `1991`, reject shares where `now < T - skew` (recommended skew = 300 seconds)
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- For `1992`, validate author matches capsule author and recipient witness is in capsule's witness list
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### Indexing
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Relays SHOULD:
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- Index `p` tags (witnesses) and `e` tags (capsule references) for discovery
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- Not rely on custom tag filters beyond NIP-01
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### NIP-11 Capability Advertisement
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Permalink: NIP-11 Capability Advertisement
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Relays implementing this NIP SHOULD advertise their support in their NIP-11 document:
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```json
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{
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"supported_nips": [1, 11, ...],
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"software": "...",
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"version": "...",
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"capsules": {
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"v": "1",
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"modes": ["threshold","scheduled"],
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"max_inline_bytes": 131072
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}
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}
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```
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### Error Handling
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Early share rejection SHOULD use clear error messages per NIP-01 (e.g., `["OK", <event_id>, false, "invalid: too early"]`).
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## Client Behavior
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Permalink: Client Behavior
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- **Creation**: Generate `K`, encrypt payload with NIP-44 v2, produce capsule event, compute `w-commit`, publish
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- **Distribution**: Publish `1992` per witness with NIP-44 encrypted share; store local copy
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- **Monitoring**: Track timestamp `T`, watch for `1991` from witnesses; tolerate skew ±300s
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- **Reconstruction**: Verify witness membership via `w-commit`, collect any `t` valid shares, reconstruct `K`, decrypt content
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- **Integrity**: When `loc != inline`, fetch `uri`, verify `sha256` hash before decryption
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- **Discovery**: Use standard filters, e.g., witnesses look up:
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```json
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{ "kinds": [1992], "#p": ["<witness_pubkey_hex>"] }
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```
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## Security Considerations
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Permalink: Security Considerations
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- **Witness Collusion**: Threshold prevents unilateral early disclosure but not collusion of any `t` witnesses. Choose diverse witnesses and set `t` accordingly.
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- **Early Disclosure**: Enforce timestamp `T` at relays (reject pre-`T - skew`) and at clients (ignore early shares).
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- **Time Manipulation**: Use trusted time sources where possible; keep small skew windows.
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- **External Storage Integrity**: Include `sha256` for any `uri` content.
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- **Spam/DoS**: Rate-limit `1991/1992` per capsule and per witness.
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## Examples
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Permalink: Examples
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### Time Capsule (kind 1990, threshold 2/3)
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```json
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{
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"kind": 1990,
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"pubkey": "a2b3c4d5...",
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"created_at": 1735689600,
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"content": "base64_encoded_nip44v2_ciphertext",
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"tags": [
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["u","threshold","t","2","n","3","T","1735776000"],
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["p","f7234bd4..."],
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["p","a1a2a3a4..."],
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["p","b1b2b3b4..."],
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["w-commit","3a5f...c9"],
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["enc","nip44:v2"],
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["loc","inline"],
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["alt","Secret message requiring 2 of 3 witnesses"]
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]
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}
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```
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### Time Capsule (kind 30095, external storage)
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```json
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{
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"kind": 30095,
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"pubkey": "a2b3c4d5...",
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"created_at": 1735689600,
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"content": "",
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"tags": [
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["d","capsule-2025-07"],
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["u","threshold","t","3","n","5","T","1736000000"],
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["p","w1..."],
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["p","w2..."],
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["p","w3..."],
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["p","w4..."],
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["p","w5..."],
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["w-commit","9c01...ab"],
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["enc","nip44:v2"],
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["loc","https"],
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["uri","https://media.example/caps/abc"],
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["sha256","c0ffee..."],
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["alt","External ciphertext with integrity hash"]
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]
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}
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```
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### Unlock Share (kind 1991)
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```json
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{
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"kind": 1991,
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"pubkey": "a1a2a3a4...",
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"created_at": 1735776100,
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"content": "base64_shamir_share",
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"tags": [
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["e","...capsule_event_id..."],
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["a","30095:a2b3c4d5...:capsule-2025-07"],
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["p","a1a2a3a4..."],
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["T","1735776000"]
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]
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}
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```
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### Share Distribution (kind 1992)
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```json
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{
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"kind": 1992,
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"pubkey": "a2b3c4d5...",
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"created_at": 1735689700,
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"content": "base64_nip44v2_encrypted_share_for_witness",
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"tags": [
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["e","...capsule_event_id..."],
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["a","30095:a2b3c4d5...:capsule-2025-07"],
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["p","a1a2a3a4..."],
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["share-idx","1"],
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["enc","nip44:v2"]
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]
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}
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```
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## Test Vectors
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Permalink: Test Vectors
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### Test Vector A: Threshold 2-of-3
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- Witnesses (ordered pubkeys): `hex_pubkey_A`, `hex_pubkey_B`, `hex_pubkey_C`
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- `w-commit` = MerkleRoot([(0, `hex_pubkey_A`), (1, `hex_pubkey_B`), (2, `hex_pubkey_C`)])
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- `T` = `1735776000`
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- Shares: `S0,S1,S2`; any two reconstruct `K`
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- Ciphertext: `C = NIP44v2_Encrypt(K, "hello world")` → `content = base64(C)`
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Expected flow:
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- `1990` event as shown above
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- `1992` to `hex_pubkey_B` with `share-idx=1` (content = NIP-44 encrypted `S1` to `hex_pubkey_B`)
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- `1991` from `hex_pubkey_B` and `hex_pubkey_C` after `T` (plaintext shares)
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- Client reconstructs `K` and decrypts `C` → `"hello world"`
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## Rationale
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Permalink: Rationale
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- Uses new kinds to avoid overloading existing semantics; unaware nodes ignore unknown kinds
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- Leverages standard `p`/`e` tags for discovery; avoids non-standard tag filtering
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- `w-commit` binds the witness set to prevent tampering
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- Parameterized replaceable variant (`30095`) supports pre-`T` fixes via the `d` tag and `a` addressing
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## Backwards Compatibility
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Permalink: Backwards Compatibility
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New kinds are ignored by unaware relays/clients. The `alt` tag provides a human-readable hint for unknown kinds. Use of standard `p` and `e` tags preserves discoverability via existing filters.
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## Reference Implementation
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Permalink: Reference Implementation
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A reference implementation is provided in [Shugur Relay](https://github.com/Shugur-Network/relay) project:
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- Relay validation: `internal/relay/nips/nip_time_capsules.go`
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- Test suite: `tests/nips/test_time_capsules_comprehensive.sh`
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