diff --git a/docs/GatewayConfiguration.md b/docs/GatewayConfiguration.md
index 9ed343b..e26406a 100644
--- a/docs/GatewayConfiguration.md
+++ b/docs/GatewayConfiguration.md
@@ -255,6 +255,88 @@ When LDAP is enabled, `Server`, `SearchBase`, `ServiceAccountDn`,
 must be in range. See `glauth.md` for the shared dev instance and the
 dev→production hardening posture.
 
+## Secrets Master Key
+
+`${secret:...}` tokens in configuration — currently just
+`MxGateway:Ldap:ServiceAccountPassword` (see above) — are resolved at startup
+from an encrypted local SQLite store, bound from the top-level `Secrets`
+section (a sibling of `MxGateway`, not nested under it):
+
+| Option | Default | Description |
+|--------|---------|-------------|
+| `Secrets:SqlitePath` | `mxgateway-secrets.db` | Path to the encrypted secrets store, resolved relative to the app content root when not rooted. |
+| `Secrets:MasterKey:Source` | `Environment` | Key-encryption-key (KEK) provider. `Environment` reads a base64-encoded 32-byte key from an env var; `Dpapi` uses a machine-bound key file instead (see below). |
+| `Secrets:MasterKey:EnvVarName` | `ZB_SECRETS_MASTER_KEY` | Env var name the `Environment` provider reads the KEK from. |
+
+The gateway always opens (and, if needed, creates and schema-migrates) this
+store at startup, even when zero `${secret:}` tokens are configured.
+
+### Delivering the master key on the NSSM box
+
+MxAccessGateway is a single Windows/NSSM box with no clustering, so the
+simplest posture applies: the `Environment` provider delivers
+`ZB_SECRETS_MASTER_KEY` via the NSSM service environment — the same mechanism
+already used for `MxGateway__Ldap__ServiceAccountPassword` overrides and the
+other `AppEnvironmentExtra` entries (see `wonder-app-vd03`/`windev` deploy
+notes). The key is **never committed** to `appsettings.json` or the repo.
+
+```text
+nssm get MxAccessGw AppEnvironmentExtra
+nssm set MxAccessGw AppEnvironmentExtra ZB_SECRETS_MASTER_KEY=<base64-32-bytes>
+```
+
+`AppEnvironmentExtra` is the same newline-separated `KEY=VALUE` block that
+already carries `ASPNETCORE_ENVIRONMENT`, `MxGateway__Worker__ExecutablePath`,
+and the API-key pepper (see the `wonder-app-vd03`/`windev` deploy notes) —
+`nssm set ... AppEnvironmentExtra` **replaces the entire block**, so always
+`get` first and re-include every existing entry alongside the new key, then
+restart the service for it to take effect.
+
+### Seeding a secret
+
+Once the master key is in place, seed (or update) a secret on the box with
+the `secret` CLI (`ZB.MOM.WW.Secrets.Cli`). The CLI is config-driven — it
+reads `Secrets:SqlitePath` and the master-key settings from its own
+`appsettings.json` plus environment variables, exactly like the gateway — so
+run it with the same `ZB_SECRETS_MASTER_KEY` set and, if it isn't colocated
+with a config that already points at the gateway's store, an explicit
+`Secrets__SqlitePath` override so both processes read/write the same file:
+
+```text
+set ZB_SECRETS_MASTER_KEY=<base64-32-bytes>
+set Secrets__SqlitePath=C:\ProgramData\MxGateway\mxgateway-secrets.db
+secret set ldap/mxgateway/bind <the real LDAP bind password>
+```
+
+A mismatched `Secrets:SqlitePath` between the CLI and the gateway silently
+seeds a different store, and the gateway then fails closed at startup as if
+the secret were never set.
+
+### Fail-closed
+
+If a referenced `${secret:...}` token cannot be resolved — the secret is
+missing from the store, or `ZB_SECRETS_MASTER_KEY` is absent or wrong — the
+gateway refuses to start (`SecretNotFoundException`, or a master-key error
+raised before that). There is no fallback to a blank or leaked credential.
+
+### Alternative: DPAPI
+
+`Secrets:MasterKey:Source = Dpapi` uses a machine-bound key file instead of an
+env var — stronger at-rest binding (the key never appears in the service
+environment or process listing) at the cost of tying the store to one
+machine, which complicates DR restore (the key file does not travel with a
+backup/restore of the `.db` file alone). Not used on the current NSSM
+deployments; documented here as the alternative posture if the env-var
+exposure becomes a concern.
+
+### Rotation / DR
+
+Losing the KEK makes the store permanently unrecoverable — there is no
+recovery path other than re-seeding every secret from scratch. Back the KEK
+up in the org secret vault, not just on the box. Full KEK rotation
+(re-wrapping every secret under a new key, `RewrapAll`) is a future item and
+is **not yet available**.
+
 ## Protocol Options
 
 | Option | Default | Description |