diff --git a/keyserver/src/push/encrypted-notif-utils-api.js b/keyserver/src/push/encrypted-notif-utils-api.js
--- a/keyserver/src/push/encrypted-notif-utils-api.js
+++ b/keyserver/src/push/encrypted-notif-utils-api.js
@@ -52,7 +52,7 @@
   uploadLargeNotifPayload: blobServiceUpload,
   getNotifByteSize: (serializedPayload: string) =>
     Buffer.byteLength(serializedPayload),
-  getEncryptedNotifHash: (serializedNotification: string) =>
+  getEncryptedNotifHash: async (serializedNotification: string) =>
     getOlmUtility().sha256(serializedNotification),
 };
 
diff --git a/lib/push/crypto.js b/lib/push/crypto.js
--- a/lib/push/crypto.js
+++ b/lib/push/crypto.js
@@ -171,9 +171,10 @@
       dbPersistCondition,
     );
 
-    const encryptedPayloadHash = encryptedNotifUtilsAPI.getEncryptedNotifHash(
-      serializedPayload.body,
-    );
+    const encryptedPayloadHash =
+      await encryptedNotifUtilsAPI.getEncryptedNotifHash(
+        serializedPayload.body,
+      );
 
     return {
       notification: {
@@ -242,9 +243,10 @@
         unencryptedSerializedPayload,
       );
 
-    const encryptedPayloadHash = encryptedNotifUtilsAPI.getEncryptedNotifHash(
-      serializedPayload.body,
-    );
+    const encryptedPayloadHash =
+      await encryptedNotifUtilsAPI.getEncryptedNotifHash(
+        serializedPayload.body,
+      );
 
     return {
       notification: {
diff --git a/lib/types/notif-types.js b/lib/types/notif-types.js
--- a/lib/types/notif-types.js
+++ b/lib/types/notif-types.js
@@ -396,5 +396,5 @@
     | { +blobUploadError: string },
   >,
   +getNotifByteSize: (serializedNotification: string) => number,
-  +getEncryptedNotifHash: (serializedNotification: string) => string,
+  +getEncryptedNotifHash: (serializedNotification: string) => Promise<string>,
 };
diff --git a/native/push/encrypted-notif-utils-api.js b/native/push/encrypted-notif-utils-api.js
new file mode 100644
--- /dev/null
+++ b/native/push/encrypted-notif-utils-api.js
@@ -0,0 +1,42 @@
+// @flow
+
+import type { EncryptedNotifUtilsAPI } from 'lib/types/notif-types.js';
+
+import { commUtilsModule } from '../native-modules.js';
+
+const encryptedNotifUtilsAPI: EncryptedNotifUtilsAPI = {
+  encryptSerializedNotifPayload: async (
+    cryptoID: string,
+    unencryptedPayload: string,
+    encryptedPayloadSizeValidator?: (
+      encryptedPayload: string,
+      type: '1' | '0',
+    ) => boolean,
+  ) => {
+    // The "mock" implementation below will be replaced with proper
+    // implementation after olm notif sessions initialization is
+    // implemented. for now it is actually beneficial to return
+    // original string as encrypted string since it allows for
+    // better testing as we can verify which data are encrypted
+    // and which aren't.
+    return {
+      encryptedData: { body: unencryptedPayload, type: 1 },
+      sizeLimitViolated: encryptedPayloadSizeValidator
+        ? !encryptedPayloadSizeValidator(unencryptedPayload, '1')
+        : false,
+    };
+  },
+  uploadLargeNotifPayload: async () => ({ blobUploadError: 'not_implemented' }),
+  getNotifByteSize: (serializedNotification: string) => {
+    return commUtilsModule.encodeStringToUTF8ArrayBuffer(serializedNotification)
+      .byteLength;
+  },
+  getEncryptedNotifHash: async (serializedNotification: string) => {
+    const notifAsArrayBuffer = commUtilsModule.encodeStringToUTF8ArrayBuffer(
+      serializedNotification,
+    );
+    return commUtilsModule.sha256(notifAsArrayBuffer);
+  },
+};
+
+export default encryptedNotifUtilsAPI;
diff --git a/web/push-notif/encrypted-notif-utils-api.js b/web/push-notif/encrypted-notif-utils-api.js
new file mode 100644
--- /dev/null
+++ b/web/push-notif/encrypted-notif-utils-api.js
@@ -0,0 +1,38 @@
+// @flow
+
+import type { EncryptedNotifUtilsAPI } from 'lib/types/notif-types.js';
+
+const encryptedNotifUtilsAPI: EncryptedNotifUtilsAPI = {
+  encryptSerializedNotifPayload: async (
+    cryptoID: string,
+    unencryptedPayload: string,
+    encryptedPayloadSizeValidator?: (
+      encryptedPayload: string,
+      type: '1' | '0',
+    ) => boolean,
+  ) => {
+    // The "mock" implementation below will be replaced with proper
+    // implementation after olm notif sessions initialization is
+    // implemented. for now it is actually beneficial to return
+    // original string as encrypted string since it allows for
+    // better testing as we can verify which data are encrypted
+    // and which aren't.
+    return {
+      encryptedData: { body: unencryptedPayload, type: 1 },
+      sizeLimitViolated: encryptedPayloadSizeValidator
+        ? !encryptedPayloadSizeValidator(unencryptedPayload, '1')
+        : false,
+    };
+  },
+  uploadLargeNotifPayload: async () => ({ blobUploadError: 'not_implemented' }),
+  getNotifByteSize: (serializedNotification: string) => {
+    return new Blob([serializedNotification]).size;
+  },
+  getEncryptedNotifHash: async (serializedNotification: string) => {
+    const notificationBytes = new TextEncoder().encode(serializedNotification);
+    const hashBytes = await crypto.subtle.digest('SHA-256', notificationBytes);
+    return btoa(String.fromCharCode(...new Uint8Array(hashBytes)));
+  },
+};
+
+export default encryptedNotifUtilsAPI;