diff --git a/src/model/decrypt.ts b/src/model/decrypt.ts
new file mode 100644
index 0000000..905596c
--- /dev/null
+++ b/src/model/decrypt.ts
@@ -0,0 +1,23 @@
+// Stub: see the README "Development workflow" section for TDD policy.
+
+import type {
+    Collection,
+    EnteFile,
+    RawCollection,
+    RawEnteFile,
+} from "./types.js";
+
+export const decryptCollection = (
+    _raw: RawCollection,
+    _masterKey: Uint8Array,
+    _currentUserID?: number,
+): Collection => {
+    throw new Error("model.decryptCollection not implemented");
+};
+
+export const decryptFile = (
+    _raw: RawEnteFile,
+    _collectionKey: Uint8Array,
+): EnteFile => {
+    throw new Error("model.decryptFile not implemented");
+};
diff --git a/src/model/index.ts b/src/model/index.ts
new file mode 100644
index 0000000..7d337ed
--- /dev/null
+++ b/src/model/index.ts
@@ -0,0 +1,12 @@
+export { decryptCollection, decryptFile } from "./decrypt.js";
+export type {
+    Collection,
+    CollectionType,
+    EnteFile,
+    FileBlob,
+    FileMetadata,
+    FileType,
+    Microseconds,
+    RawCollection,
+    RawEnteFile,
+} from "./types.js";
diff --git a/src/model/types.ts b/src/model/types.ts
new file mode 100644
index 0000000..79551a0
--- /dev/null
+++ b/src/model/types.ts
@@ -0,0 +1,74 @@
+export type Microseconds = number;
+
+export type CollectionType =
+    | "album"
+    | "folder"
+    | "favorites"
+    | "uncategorized"
+    | "unknown";
+
+export interface Collection {
+    id: number;
+    ownerID: number;
+    key: Uint8Array;
+    name: string;
+    type: CollectionType;
+    updationTime: Microseconds;
+    isShared: boolean;
+}
+
+export type FileType = "image" | "video" | "livePhoto" | "unknown";
+
+export interface FileMetadata {
+    title: string;
+    fileType: FileType;
+    creationTime: Microseconds;
+    modificationTime: Microseconds;
+    latitude?: number;
+    longitude?: number;
+    hash?: string;
+}
+
+export interface FileBlob {
+    decryptionHeader: string;
+    size?: number;
+}
+
+export interface EnteFile {
+    id: number;
+    collectionID: number;
+    ownerID: number;
+    key: Uint8Array;
+    metadata: FileMetadata;
+    file: FileBlob;
+    thumbnail: FileBlob;
+    updationTime: Microseconds;
+}
+
+// Raw shapes as they arrive from the Ente API, before decryption.
+
+export interface RawCollection {
+    id: number;
+    owner: { id: number };
+    encryptedKey: string;
+    keyDecryptionNonce: string;
+    encryptedName?: string;
+    nameDecryptionNonce?: string;
+    type: string;
+    updationTime: number;
+    isDeleted?: boolean;
+}
+
+export interface RawEnteFile {
+    id: number;
+    collectionID: number;
+    ownerID: number;
+    encryptedKey: string;
+    keyDecryptionNonce: string;
+    metadata: { encryptedData: string; decryptionHeader: string };
+    info?: { fileSize?: number; thumbSize?: number };
+    file: { decryptionHeader: string };
+    thumbnail: { decryptionHeader: string };
+    updationTime: number;
+    isDeleted?: boolean;
+}
diff --git a/test/model/decrypt.test.ts b/test/model/decrypt.test.ts
new file mode 100644
index 0000000..2e34f1e
--- /dev/null
+++ b/test/model/decrypt.test.ts
@@ -0,0 +1,267 @@
+/**
+ * Tests for `model.decryptCollection` and `model.decryptFile`.
+ *
+ * These two functions turn the raw encrypted JSON blobs the Ente server
+ * returns into the decrypted Collection and EnteFile objects that the
+ * rest of quack works with.
+ *
+ * ## Collection decryption
+ *
+ *   The server stores each collection's encryption key sealed under the
+ *   owner's master key (secretbox). The collection's name is then sealed
+ *   under that collection key (also secretbox). `decryptCollection`:
+ *
+ *     1. decryptBox(encryptedKey, keyDecryptionNonce, masterKey) -> collectionKey
+ *     2. decryptBox(encryptedName, nameDecryptionNonce, collectionKey) -> name (UTF-8)
+ *     3. Maps the string `type` field to a CollectionType union member
+ *     4. Returns a Collection with decrypted key, name, and type
+ *
+ *   If the collection is shared (owner != current user), `isShared` is true.
+ *
+ * ## File decryption
+ *
+ *   Each file has its own encryption key, sealed under the collection key
+ *   (secretbox). File metadata (title, creation time, geolocation, etc.)
+ *   is a JSON blob sealed under the file key (also secretbox, where the
+ *   field called `decryptionHeader` is the nonce). `decryptFile`:
+ *
+ *     1. decryptBox(encryptedKey, keyDecryptionNonce, collectionKey) -> fileKey
+ *     2. decryptBox(metadata.encryptedData, metadata.decryptionHeader, fileKey) -> JSON
+ *     3. Parses the JSON into a typed FileMetadata
+ *     4. Returns an EnteFile with decrypted key, metadata, and the raw
+ *        file/thumbnail decryption headers (used later for download)
+ *
+ * The tests build synthetic encrypted payloads using libsodium directly.
+ */
+
+import sodium from "libsodium-wrappers-sumo";
+import { beforeAll, describe, expect, it } from "vitest";
+import { init, toBase64 } from "../../src/crypto/index.js";
+import { decryptCollection, decryptFile } from "../../src/model/index.js";
+import type { RawCollection, RawEnteFile } from "../../src/model/index.js";
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+const secretboxEncrypt = (
+    plaintext: Uint8Array,
+    key: Uint8Array,
+): { ciphertext: Uint8Array; nonce: Uint8Array } => {
+    const nonce = sodium.randombytes_buf(sodium.crypto_secretbox_NONCEBYTES);
+    const ciphertext = sodium.crypto_secretbox_easy(plaintext, nonce, key);
+    return { ciphertext, nonce };
+};
+
+const buildRawCollection = (
+    masterKey: Uint8Array,
+    opts?: { name?: string; type?: string; ownerID?: number },
+): { raw: RawCollection; collectionKey: Uint8Array } => {
+    const collectionKey = sodium.crypto_secretbox_keygen();
+    const { ciphertext: encKey, nonce: keyNonce } = secretboxEncrypt(
+        collectionKey,
+        masterKey,
+    );
+    const name = opts?.name ?? "Vacation 2025";
+    const { ciphertext: encName, nonce: nameNonce } = secretboxEncrypt(
+        new TextEncoder().encode(name),
+        collectionKey,
+    );
+    const raw: RawCollection = {
+        id: 100,
+        owner: { id: opts?.ownerID ?? 1 },
+        encryptedKey: toBase64(encKey),
+        keyDecryptionNonce: toBase64(keyNonce),
+        encryptedName: toBase64(encName),
+        nameDecryptionNonce: toBase64(nameNonce),
+        type: opts?.type ?? "album",
+        updationTime: 1700000000000000,
+    };
+    return { raw, collectionKey };
+};
+
+const buildRawFile = (
+    collectionKey: Uint8Array,
+    opts?: { title?: string; fileType?: number; creationTime?: number },
+): RawEnteFile => {
+    const fileKey = sodium.crypto_secretbox_keygen();
+    const { ciphertext: encFileKey, nonce: fileKeyNonce } = secretboxEncrypt(
+        fileKey,
+        collectionKey,
+    );
+
+    const metadata = {
+        title: opts?.title ?? "IMG_0001.jpg",
+        fileType: opts?.fileType ?? 0,
+        creationTime: opts?.creationTime ?? 1700000000000000,
+        modificationTime: 1700000000000000,
+        latitude: 48.8566,
+        longitude: 2.3522,
+        hash: "abcdef1234567890",
+    };
+    const metadataBytes = new TextEncoder().encode(JSON.stringify(metadata));
+    const { ciphertext: encMeta, nonce: metaNonce } = secretboxEncrypt(
+        metadataBytes,
+        fileKey,
+    );
+
+    return {
+        id: 200,
+        collectionID: 100,
+        ownerID: 1,
+        encryptedKey: toBase64(encFileKey),
+        keyDecryptionNonce: toBase64(fileKeyNonce),
+        metadata: {
+            encryptedData: toBase64(encMeta),
+            decryptionHeader: toBase64(metaNonce),
+        },
+        file: { decryptionHeader: toBase64(sodium.randombytes_buf(24)) },
+        thumbnail: { decryptionHeader: toBase64(sodium.randombytes_buf(24)) },
+        updationTime: 1700000000000000,
+    };
+};
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+describe("model.decryptCollection", () => {
+    beforeAll(async () => {
+        await init();
+        await sodium.ready;
+    });
+
+    it("decrypts the collection key and name from a raw server response", () => {
+        const masterKey = sodium.crypto_secretbox_keygen();
+        const { raw, collectionKey } = buildRawCollection(masterKey, {
+            name: "Summer Photos",
+        });
+
+        const col = decryptCollection(raw, masterKey, 1);
+
+        expect(col.id).toBe(100);
+        expect(col.key).toEqual(collectionKey);
+        expect(col.name).toBe("Summer Photos");
+        expect(col.type).toBe("album");
+        expect(col.ownerID).toBe(1);
+        expect(col.updationTime).toBe(1700000000000000);
+        expect(col.isShared).toBe(false);
+    });
+
+    it("sets isShared when owner != current user", () => {
+        const masterKey = sodium.crypto_secretbox_keygen();
+        const { raw } = buildRawCollection(masterKey, { ownerID: 99 });
+
+        const col = decryptCollection(raw, masterKey, 1);
+        expect(col.isShared).toBe(true);
+    });
+
+    it("maps known type strings to CollectionType", () => {
+        const masterKey = sodium.crypto_secretbox_keygen();
+        for (const type of ["album", "folder", "favorites", "uncategorized"]) {
+            const { raw } = buildRawCollection(masterKey, { type });
+            const col = decryptCollection(raw, masterKey, 1);
+            expect(col.type).toBe(type);
+        }
+    });
+
+    it("maps unrecognised type strings to 'unknown'", () => {
+        const masterKey = sodium.crypto_secretbox_keygen();
+        const { raw } = buildRawCollection(masterKey, {
+            type: "someFutureType",
+        });
+        const col = decryptCollection(raw, masterKey, 1);
+        expect(col.type).toBe("unknown");
+    });
+
+    it("handles a collection with no encrypted name gracefully", () => {
+        // Some special collections (e.g. uncategorized) may have no name.
+        const masterKey = sodium.crypto_secretbox_keygen();
+        const { raw } = buildRawCollection(masterKey);
+        delete raw.encryptedName;
+        delete raw.nameDecryptionNonce;
+
+        const col = decryptCollection(raw, masterKey, 1);
+        expect(col.name).toBe("");
+    });
+
+    it("throws when the master key is wrong", () => {
+        const masterKey = sodium.crypto_secretbox_keygen();
+        const wrongKey = sodium.crypto_secretbox_keygen();
+        const { raw } = buildRawCollection(masterKey);
+
+        expect(() => decryptCollection(raw, wrongKey, 1)).toThrow();
+    });
+});
+
+describe("model.decryptFile", () => {
+    beforeAll(async () => {
+        await init();
+        await sodium.ready;
+    });
+
+    it("decrypts the file key and metadata from a raw server response", () => {
+        const masterKey = sodium.crypto_secretbox_keygen();
+        const { collectionKey } = buildRawCollection(masterKey);
+        const raw = buildRawFile(collectionKey, {
+            title: "sunset.heic",
+            fileType: 0,
+            creationTime: 1710000000000000,
+        });
+
+        const file = decryptFile(raw, collectionKey);
+
+        expect(file.id).toBe(200);
+        expect(file.collectionID).toBe(100);
+        expect(file.ownerID).toBe(1);
+        expect(file.key.length).toBe(32);
+        expect(file.metadata.title).toBe("sunset.heic");
+        expect(file.metadata.fileType).toBe("image");
+        expect(file.metadata.creationTime).toBe(1710000000000000);
+        expect(file.metadata.latitude).toBeCloseTo(48.8566);
+        expect(file.metadata.longitude).toBeCloseTo(2.3522);
+    });
+
+    it("maps fileType numbers to FileType strings", () => {
+        // Ente uses: 0=image, 1=video, 2=livePhoto
+        const masterKey = sodium.crypto_secretbox_keygen();
+        const { collectionKey } = buildRawCollection(masterKey);
+
+        const mapping: [number, string][] = [
+            [0, "image"],
+            [1, "video"],
+            [2, "livePhoto"],
+            [99, "unknown"],
+        ];
+        for (const [num, expected] of mapping) {
+            const raw = buildRawFile(collectionKey, { fileType: num });
+            const file = decryptFile(raw, collectionKey);
+            expect(file.metadata.fileType).toBe(expected);
+        }
+    });
+
+    it("preserves the file and thumbnail decryption headers", () => {
+        // These headers are passed to initStreamPull later when
+        // downloading and decrypting the actual file content. The
+        // decryptFile function must pass them through unchanged.
+        const masterKey = sodium.crypto_secretbox_keygen();
+        const { collectionKey } = buildRawCollection(masterKey);
+        const raw = buildRawFile(collectionKey);
+
+        const file = decryptFile(raw, collectionKey);
+
+        expect(file.file.decryptionHeader).toBe(raw.file.decryptionHeader);
+        expect(file.thumbnail.decryptionHeader).toBe(
+            raw.thumbnail.decryptionHeader,
+        );
+    });
+
+    it("throws when the collection key is wrong", () => {
+        const masterKey = sodium.crypto_secretbox_keygen();
+        const { collectionKey } = buildRawCollection(masterKey);
+        const wrongKey = sodium.crypto_secretbox_keygen();
+        const raw = buildRawFile(collectionKey);
+
+        expect(() => decryptFile(raw, wrongKey)).toThrow();
+    });
+});