Create Gaussian noise and ACDP composition rollout doc (#1185)

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docs/operations/enabling-gaussian-noise-and-acdp-pbm.md

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+# Enabling Gaussian Noise and ACDP Composition in PBM
+
+This guide describes how to enable Gaussian Differential Privacy(DP) noise for
+Multi-Party Computation(MPC) and direct measurements, and Almost Concentrated
+Differential Privacy(ACDP) composition mechanism in Privacy Budget Manager(PBM)
+to improve privacy accounting. Gaussian noise and ACDP composition can support
+more than 2x queries with the same privacy budget compared to Laplace noise and
+advanced composition in some parameter settings. It also enables the Measurement
+Consumer to change their per-query epsilon, delta values arbitrarily, which
+improves flexibility of privacy budgeting.
+
+## Steps
+
+### Prerequisites
+
+Before enabling Gaussian noise in the Kingdom, it is important to ensure that
+all Events Data Providers(EDPs) who implement a PBM are capable of accounting
+for Gaussian noise. They should also have implemented ACDP, but if they have
+not, they should be comfortable with Gaussian noise allowing fewer queries than
+Laplace noise.
+
+All EDPs with PBM using ACDP composition should support adding continuous
+Gaussian noise to their direct measurements. The Gaussian noiser implementation
+is provided in
+[`GaussianNoiser.kt`](../../src/main/kotlin/org/wfanet/measurement/eventdataprovider/noiser/GaussianNoiser.kt)
+
+### Configure MPC protocols to use discrete Gaussian noise in Kingdom
+
+1.  Change `noise_mechanism` to `DISCRETE_GAUSSIAN` in the Kingdom's
+    `Llv2ProtocolConfigConfig` for LLV2 based MPC reach and frequency
+    measurement. In the `dev` configuration, this is read from the
+    `llv2_protocol_config_config.textproto` file in the `certs-and-configs`
+    Kubernetes secret.
+2.  Change `noise_mechanism` to `DISCRETE_GAUSSIAN` in the Kingdom's
+    `RoLlv2ProtocolConfigConfig` for LLV2 based MPC reach only measurement. In
+    the `dev` configuration, this is read from the
+    `ro_llv2_protocol_config_config.textproto` file in the `certs-and-configs`
+    Kubernetes secret.
+
+### Ensure continuous Gaussian noise for direct protocol in Kingdom
+
+For direct measurements, ensure `CONTINUOUS_GAUSSIAN` is enabled for the Direct
+protocol in Kingdom. By default, both `CONTINUOUS_GAUSSIAN` and
+`CONTINUOUS_LAPLACE` are enabled. The prerequisite is EDPs reading possible
+noise mechanisms from ProtocolConfig.
+
+### EDPs with ACDP PBM
+
+1.  When a Kingdom is configured to use discrete Gaussian noise for MPC
+    requisitions, EDPs with PBM should update their requisition fulfillment
+    pipelines to use ACDP composition.
+
+    Note: If you have a testing environment with EDP simulators, you can update
+    those simulators to use ACDP by setting the value of the
+    `--composition-mechanism` option to `ACDP`.
+
+2.  Changing to ACDP composition also requires EDP to use continuous Gaussian
+    noise for direct measurements.
+
+3.  Note that in the PBM ACDP reference implementation, ACDP composition will
+    write privacy charges to a new table(PrivacyBucketAcdpCharges) which is
+    effectively resetting the ledger.
+
+## Verification
+
+When PBM is using ACDP composition, and the `noise_mechanism` passed in
+`ProtocolConfig.Protocol.LiquidLegionsV2` in MPC requisition is not
+`DISCRETE_GAUSSIAN`, the requisition will be refused. The following code snippet
+can be used to verify the `noise_mechanism`:
+
+```kotlin
+val llv2Protocol: ProtocolConfig.Protocol =
+        requireNotNull(requisition.protocolConfig.protocolsList.find { protocol -> protocol.hasLiquidLegionsV2() })
+val liquidLegionsV2: ProtocolConfig.LiquidLegionsV2 = llv2Protocol.liquidLegionsV2
+val llv2NoiseMechanism = liquidLegionsV2.noiseMechanism
+```
+
+`llv2NoiseMechanism` should be `DISCRETE_GAUSSIAN`.
+
+The noise and composition mechanisms changes can also be validated with logs in
+EDP simulator through integration tests:
+
+```shell
+INFO:  chargeLiquidLegionsV2PrivacyBudget with ACDP composition mechanism for requisition with DISCRETE_GAUSSIAN noise mechanism...
+```
+
+```shell
+INFO:  chargeDirectPrivacyBudget with ACDP composition mechanism...
+```
+
+The `ProtocolConfig` can also be verified in `MeasurementDetails` in the
+`Measurements` table in Kingdom.
+
+## Emergency Rollback
+
+The Gaussian noise and PBM ACDP composition features are designed with backward
+compatibility. If there’s a need to roll back the changes, follow Steps to
+revert back the configs.
+
+Note that in the PBM ACDP reference implementation, ACDP composition will write
+privacy charges to a different table(PrivacyBucketAcdpCharges) from the current
+PrivacyBucketCharges table. Rolling back on the EDP side has the effect of
+dropping all the ACDP privacy charges.