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+# `ICRC-154`: Privileged Pause, Unpause & Deactivate API
+
+| Status |
+|:------:|
+| Draft  |
+
+## Introduction & Motivation
+
+For operational safety, incident response, and regulatory compliance, ledgers often need a way to **pause** (temporarily disable critical state transitions) or **deactivate** (enter a long-lived/terminal safe state) the system. Without a standard interface and canonical on-chain recording, integrators cannot reliably determine the current operational state or attribute transitions to authorized actors.
+
+**ICRC-154** standardizes three privileged methods—`pause`, `unpause`, and `deactivate`—and defines the canonical mapping from method inputs to the `tx` field of the corresponding typed blocks (as defined in **ICRC-124**). It also includes lightweight query methods for current status.
+
+- Privileged methods (authorized principals only):
+  - `icrc154_pause`, `icrc154_unpause`, `icrc154_deactivate`
+- Canonical `tx` mapping with namespaced `op` values (`"154..."`), caller identity, and optional human-readable `reason`.
+- Recording uses **ICRC-124** block kinds (this standard **does not** add new block types).
+- Read-only queries: `icrc154_is_paused`, `icrc154_is_deactivated`.
+
+## Overview
+
+ICRC-154 standardizes privileged **pause/unpause/deactivate** controls for ICRC ledgers.
+
+Specifically, it defines:
+
+- **APIs** to pause, unpause, and deactivate the ledger (privileged only).
+- **Canonical `tx` mapping** rules ensuring deterministic, auditable block content.
+- **Use of ICRC-124 block kinds** to record these actions:
+  - `btype = "124pause"`, `btype = "124unpause"`, `btype = "124deactivate"`
+- **Status queries** for quick integration.
+
+This enables wallets, explorers, and auditors to:
+
+- Determine, on-chain, whether the ledger is currently **paused** or **deactivated**.
+- Attribute actions to a specific authorized caller with an optional `reason`.
+- Interoperate across ledgers that implement the same API and block semantics.
+
+## Dependencies
+
+This standard does not introduce new block kinds.
+
+- **ICRC-3** — Block log format, hashing, certification, and canonical `tx` mapping rules.
+- **ICRC-124** — Defines the typed block kinds that ICRC-154 uses:
+  - `btype = "124pause"`
+  - `btype = "124unpause"`
+  - `btype = "124deactivate"`
+
+A ledger implementing ICRC-154 MUST:
+- Emit the appropriate **ICRC-124** block on each successful call.
+- Populate `tx.op` with namespaced values **introduced by this standard**:
+  `"154pause"`, `"154unpause"`, `"154deactivate"`.
+
+
+## Common Elements
+
+This standard follows the conventions defined in ICRC-3 and used across
+ICRC-122, 123, and 124.
+
+- **Principals**  
+  Represented as `variant { Blob = <principal_bytes> }`.
+
+- **Timestamps**  
+  The `ts` value in the transaction (`tx`) corresponds to the
+  caller-supplied `created_at_time` argument.  
+  Timestamps are measured in **nanoseconds since the Unix epoch** and
+  encoded as `Nat` (MUST fit into `nat64`).
+
+- **Parent Hash**  
+  Each block includes `phash : Blob`, the hash of its parent block, unless it
+  is the genesis block (where `phash` is omitted).
+
+- **Caller Tracking**  
+  Each block includes `tx.caller`, encoded as
+  `variant { Blob = <caller_principal_bytes> }`, to identify the authorized
+  entity performing the privileged operation.
+
+
+
+### `icrc154_pause`
+
+Temporarily move the ledger into a **paused** state (semantics per ICRC-124).
+
+#### Arguments
+
+```
+type PauseArgs = record {
+  reason          : opt text;
+  created_at_time : nat64;
+};
+
+type PauseError = variant {
+  Unauthorized : text;                // caller not permitted
+  AlreadyPaused : text;               // ledger is already paused
+  AlreadyDeactivated : text;          // cannot pause when deactivated
+  Duplicate : record { duplicate_of : nat };
+  GenericError : record { error_code : nat; message : text };
+};
+
+icrc154_pause : (PauseArgs) -> (variant { Ok : nat; Err : PauseError });
+```
+
+#### Semantics
+
+**Authorization**  
+- The method **MUST** be callable only by a **controller** of the ledger or
+  other explicitly authorized principals.  
+- Unauthorized calls **MUST** fail with `Unauthorized`.
+
+**Effect (on success, non-retroactive)**  
+- Transition the ledger into the **paused** state, following the semantics of
+  ICRC-124.  
+- Append a new block with `btype = "124pause"`.  
+- The block’s `tx` field **MUST** be constructed **exactly** as defined in
+  **Canonical `tx` Mapping** (same keys, types, and encodings), with `ts`
+  derived from the `created_at_time` argument.  
+- On success, return the index of the newly appended block.
+
+**Return value**  
+- On success: `variant { Ok : nat }`, where `nat` is the block index.  
+- On failure: `variant { Err : PauseError }`.
+
+**Deduplication & idempotency**  
+- The ledger **MUST** perform deduplication using `created_at_time`.  
+- If a duplicate is detected, the ledger **MUST NOT** append a new block and
+  **MUST** return  
+  `Err(Duplicate { duplicate_of = <index> })`.
+
+**Error cases (normative)**  
+- `Unauthorized` — caller not permitted.  
+- `AlreadyPaused` — ledger already paused.  
+- `AlreadyDeactivated` — cannot pause when deactivated.  
+- `Duplicate { duplicate_of }` — duplicate transaction detected.  
+- `GenericError { error_code, message }` — any other failure.
+
+**Clarifications**  
+- The `tx` field uses **`ts`** for the caller-supplied timestamp
+  (`created_at_time`).  
+- Optional fields **MUST** be omitted from `tx` if not supplied.  
+- Representation-independent hashing (ICRC-3) applies; field presence and value
+  determine the hash, not field order.
+
+#### Canonical `tx` Mapping (normative)
+
+| Field   | Type (ICRC-3 `Value`) | Source / Encoding Rule |
+|----------|-----------------------|-------------------------|
+| `op`     | `Text`                | Constant `"154pause"`. |
+| `ts`     | `Nat`                 | From `created_at_time` (ns since Unix epoch; MUST fit in `nat64`). |
+| `caller` | `Blob`                | Principal of the caller. |
+| `reason` | `Text` (optional)     | From `reason` argument; omit if absent. |
+
+
+
+### `icrc154_unpause`
+
+Return the ledger to **unpaused** operation (semantics per ICRC-124).
+
+#### Arguments
+```
+type UnpauseArgs = record {
+  reason          : opt text;
+  created_at_time : nat64;
+};
+
+type UnpauseError = variant {
+  Unauthorized : text;
+  NotPaused : text;                   // ledger is not currently paused
+  AlreadyDeactivated : text;          // cannot unpause when deactivated
+  Duplicate : record { duplicate_of : nat };
+  GenericError : record { error_code : nat; message : text };
+};
+
+icrc154_unpause : (UnpauseArgs) -> (variant { Ok : nat; Err : UnpauseError });
+```
+
+#### Semantics
+
+**Authorization**  
+- The method **MUST** be callable only by a **controller** of the ledger or
+  other explicitly authorized principals.  
+- Unauthorized calls **MUST** fail with `Unauthorized`.
+
+**Effect (on success, non-retroactive)**  
+- Transition the ledger from **paused** to **unpaused** (per ICRC-124).  
+- Append a new block with `btype = "124unpause"`.  
+- The block’s `tx` field **MUST** be constructed **exactly** as defined in
+  **Canonical `tx` Mapping** (same keys, types, and encodings), with `ts`
+  derived from the `created_at_time` argument.  
+- On success, return the index of the newly appended block.
+
+**Return value**  
+- On success: `variant { Ok : nat }` where `nat` is the block index.  
+- On failure: `variant { Err : UnpauseError }`.
+
+**Deduplication & idempotency**  
+- The ledger **MUST** perform deduplication (e.g., using `created_at_time`).  
+- If a duplicate is detected, the ledger **MUST NOT** append a new block and
+  **MUST** return `Err(Duplicate { duplicate_of = <index> })`.
+
+**Error cases (normative)**  
+- `Unauthorized` — caller not permitted.  
+- `NotPaused` — ledger is not currently paused.  
+- `AlreadyDeactivated` — cannot unpause when deactivated.  
+- `Duplicate { duplicate_of }` — semantically identical transaction already accepted.  
+- `GenericError { error_code, message }` — any other failure preventing a valid block.
+
+**Clarifications**  
+- The `tx` field uses **`ts`** for the caller-supplied timestamp (`created_at_time`).  
+- Optional fields **MUST** be omitted from `tx` if not supplied.  
+- Representation-independent hashing (ICRC-3)
+
+#### Canonical `tx` Mapping (normative)
+
+| Field   | Type (ICRC-3 `Value`) | Source / Encoding Rule |
+|---------|------------------------|-------------------------|
+| `op`    | `Text`                 | **Constant** `"154unpause"`. |
+| `ts`    | `Nat`                  | From `created_at_time`. |
+| `caller`| `Blob`                 | Principal of the caller. |
+| `reason`| `Text` *(optional)*    | From `reason` if provided; **omit** if absent. |
+
+
+### `icrc154_deactivate`
+
+Move the ledger into a **deactivated** state (long-lived/terminal safe state as per ICRC-124).
+
+#### Arguments
+```
+type DeactivateArgs = record {
+  reason          : opt text;
+  created_at_time : nat64;
+};
+
+type DeactivateError = variant {
+  Unauthorized : text;
+  AlreadyDeactivated : text;          // ledger already deactivated
+  Duplicate : record { duplicate_of : nat };
+  GenericError : record { error_code : nat; message : text };
+};
+
+icrc154_deactivate : (DeactivateArgs) -> (variant { Ok : nat; Err : DeactivateError });
+```
+
+#### Semantics
+
+**Authorization**  
+- The method **MUST** be callable only by a **controller** of the ledger or
+  other explicitly authorized principals.  
+- Unauthorized calls **MUST** fail with `Unauthorized`.
+
+**Effect (on success, non-retroactive)**  
+- Transition the ledger into the **deactivated** state (per ICRC-124).  
+- Append a new block with `btype = "124deactivate"`.  
+- The block’s `tx` field **MUST** be constructed **exactly** as defined in
+  **Canonical `tx` Mapping** (same keys, types, and encodings), with `ts`
+  derived from the `created_at_time` argument.  
+- On success, return the index of the newly appended block.
+
+**Return value**  
+- On success: `variant { Ok : nat }` where `nat` is the block index.  
+- On failure: `variant { Err : DeactivateError }`.
+
+**Deduplication & idempotency**  
+- The ledger **MUST** perform deduplication (e.g., using `created_at_time`).  
+- If a duplicate is detected, the ledger **MUST NOT** append a new block and
+  **MUST** return `Err(Duplicate { duplicate_of = <index> })`.
+
+**Error cases (normative)**  
+- `Unauthorized` — caller not permitted.  
+- `AlreadyDeactivated` — ledger is already deactivated.  
+- `Duplicate { duplicate_of }` — semantically identical transaction already accepted.  
+- `GenericError { error_code, message }` — any other failure preventing a valid block.
+
+**Clarifications**  
+- The `tx` field uses **`ts`** for the caller-supplied timestamp (`created_at_time`).  
+- Optional fields **MUST** be omitted from `tx` if not supplied.  
+- Representation-independent hashing (ICRC-3) applies; field presence and value determine the hash, not field order.
+
+#### Canonical `tx` Mapping (normative)
+
+| Field   | Type (ICRC-3 `Value`) | Source / Encoding Rule |
+|---------|------------------------|-------------------------|
+| `op`    | `Text`                 | **Constant** `"154deactivate"`. |
+| `ts`    | `Nat`                  | From `created_at_time`. |
+| `caller`| `Blob`                 | Principal of the caller. |
+| `reason`| `Text` *(optional)*    | From `reason` if provided; **omit** if absent. |
+
+
+
+## Query & Introspection Methods
+
+These read-only methods expose the current operational state. They **do not** produce blocks.
+
+### `icrc154_is_paused`
+```
+icrc154_is_paused : () -> (bool) query;
+```
+- Returns `true` iff the ledger is currently **paused** (per ICRC-124).
+- Returns `false` otherwise.
+
+### `icrc154_is_deactivated`
+```
+icrc154_is_deactivated : () -> (bool) query;
+```
+
+- Returns `true` iff the ledger is currently **deactivated** (per ICRC-124).
+- Returns `false` otherwise.
+
+**Notes:**
+- If the ledger is **deactivated**, it is implementation-defined in ICRC-124 whether certain operations are permanently disabled; `icrc154_unpause` MUST return `AlreadyDeactivated` in that state.
+- Answers reflect the ledger state **at query time**.
+
+## Notes on Semantics & Scope
+
+- **Authorizations**: Methods are **privileged**; ledgers MUST restrict calls to authorized principals (policy/RBAC is ledger-defined or standardized separately).
+- **Deduplication**: `created_at_time` is used for replay protection/deduplication (as in ICRC-1). Duplicate calls MUST return `Duplicate { duplicate_of = <block_index> }` and MUST NOT produce a new block.
+- **Operational semantics**: The precise operational effects of pause/unpause/deactivate (which operations are blocked, permanence of deactivation, etc.) are **entirely defined by ICRC-124**. ICRC-154 only defines the interface and canonical `tx` mapping.
+- **No fees**: ICRC-154 calls do not involve fees; ledgers MUST NOT include a top-level `fee` field for these blocks.
+
+## Reporting Compliance
+
+### Supported Standards
+
+Ledgers implementing ICRC-154 MUST indicate compliance through the
+`icrc1_supported_standards` and `icrc10_supported_standards` methods by including:
+```
+variant { Vec = vec {
+  record {
+    "name"; variant { Text = "ICRC-154" };
+    "url";  variant { Text = "https://github.com/dfinity/ICRC/blob/main/ICRCs/ICRC-154.md" };
+  }
+}};
+```
+### Supported Block Types
+
+ICRC-154 extends **ICRC-124** and does not introduce new block kinds.
+Accordingly, ledgers implementing ICRC-154 MUST already advertise support for the
+relevant ICRC-124 block kinds (`124pause`, `124unpause`, `124deactivate`) as required by ICRC-124.
+No additional block types need to be reported.
+
+
+
+### Example calls and resulting blocks
+
+#### Call
+```
+icrc154_pause({
+  created_at_time = 1_753_700_000_000_000_000 : nat64;
+  reason          = ?"System maintenance window";
+})
+```
+
+
+#### Resulting Block```variant {
+  Map = vec {
+    record { "btype"; variant { Text = "124pause" } };
+    record { "phash"; variant { Blob = blob "\aa\bb\cc\dd\ee\ff\00\11\22\33\44\55\66\77\88\99\01\23\45\67\89\ab\cd\ef\10\32\54\76\98\ba\dc\fe" } }; // illustrative
+    record { "ts";    variant { Nat = 1_753_700_001_000_000_000 : Nat } };
+    record {
+      "tx";
+      variant {
+        Map = vec {
+          record { "op";     variant { Text = "154pause" } };
+          record { "ts";     variant { Nat  = 1_753_700_000_000_000_000 : Nat } };
+          record { "caller"; variant { Blob = blob "\00\00\00\00\02\30\02\17\01\01" } };
+          record { "reason"; variant { Text = "System maintenance window" } };
+        }
+      };
+    };
+  }
+};
+```
+
+
+
+### Example: `icrc154_unpause`
+
+**Call with parameters**
+```motoko
+icrc154_unpause({
+  created_at_time = 1_753_700_500_000_000_000 : nat64;
+  reason          = ?"Maintenance completed";
+})
+
+```
+
+#### Resulting Block
+```
+variant {
+  Map = vec {
+    record { "btype"; variant { Text = "124pause" } };
+    record { "phash"; variant { Blob = blob "\aa\bb\cc\dd\ee\ff\00\11\22\33\44\55\66\77\88\99\01\23\45\67\89\ab\cd\ef\10\32\54\76\98\ba\dc\fe" } }; // illustrative
+    record { "ts";    variant { Nat = 1_753_700_001_000_000_000 : Nat } };
+    record {
+      "tx";
+      variant {
+        Map = vec {
+          record { "op";     variant { Text = "154pause" } };
+          record { "ts";     variant { Nat  = 1_753_700_000_000_000_000 : Nat } };
+          record { "caller"; variant { Blob = blob "\00\00\00\00\02\30\02\17\01\01" } };
+          record { "reason"; variant { Text = "System maintenance window" } };
+        }
+      };
+    };
+  }
+};
+```
+
+
+### Example: `icrc154_deactivate`
+
+
+```
+icrc154_deactivate({
+  created_at_time = 1_753_800_000_000_000_000 : nat64;
+  reason          = ?"Regulatory directive";
+})
+```
+
+#### Resulting Block
+```
+variant {
+  Map = vec {
+    record { "btype"; variant { Text = "124deactivate" } };
+    record { "phash"; variant { Blob = blob "\2d\86\7f\34\c7\2d\1e\2d\00\84\10\a4\00\b0\b6\4c\3e\02\96\c9\e8\55\6f\dd\72\68\e8\df\8d\8e\8a\ee" } }; // illustrative
+    record { "ts";    variant { Nat = 1_753_800_001_000_000_000 : Nat } };
+    record {
+      "tx";
+      variant {
+        Map = vec {
+          record { "op";     variant { Text = "154deactivate" } };
+          record { "ts";     variant { Nat  = 1_753_800_000_000_000_000 : Nat } };
+          record { "caller"; variant { Blob = blob "\00\00\00\00\02\30\02\17\01\01" } };
+          record { "reason"; variant { Text = "Regulatory directive" } };
+        }
+      };
+    };
+  }
+};
+```
+