refactor: stream blob hash verification instead of buffering in memory

FetchAndDecryptBlob now returns io.ReadCloser with a hashVerifyReader
that computes the double-SHA-256 on-the-fly during reads. Hash is
verified on Close() after the stream is fully consumed. This avoids
loading entire blobs into memory, which could exceed available RAM.

Addresses review feedback on PR #39.

internal/vaultik/blob_fetch_hash_test.go

@@ -0,0 +1,100 @@
+package vaultik_test
+
+import (
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"io"
+	"strings"
+	"testing"
+
+	"filippo.io/age"
+	"git.eeqj.de/sneak/vaultik/internal/blobgen"
+	"git.eeqj.de/sneak/vaultik/internal/vaultik"
+)
+
+// TestFetchAndDecryptBlobVerifiesHash verifies that FetchAndDecryptBlob checks
+// the double-SHA-256 hash of the decrypted plaintext against the expected blob hash.
+func TestFetchAndDecryptBlobVerifiesHash(t *testing.T) {
+	identity, err := age.GenerateX25519Identity()
+	if err != nil {
+		t.Fatalf("generating identity: %v", err)
+	}
+
+	// Create test data and encrypt it using blobgen.Writer
+	plaintext := []byte("hello world test data for blob hash verification")
+	var encBuf bytes.Buffer
+	writer, err := blobgen.NewWriter(&encBuf, 1, []string{identity.Recipient().String()})
+	if err != nil {
+		t.Fatalf("creating blobgen writer: %v", err)
+	}
+	if _, err := writer.Write(plaintext); err != nil {
+		t.Fatalf("writing plaintext: %v", err)
+	}
+	if err := writer.Close(); err != nil {
+		t.Fatalf("closing writer: %v", err)
+	}
+	encryptedData := encBuf.Bytes()
+
+	// Compute correct double-SHA-256 hash of the plaintext (matches blobgen.Writer.Sum256)
+	firstHash := sha256.Sum256(plaintext)
+	secondHash := sha256.Sum256(firstHash[:])
+	correctHash := hex.EncodeToString(secondHash[:])
+
+	// Verify our hash matches what blobgen.Writer produces
+	writerHash := hex.EncodeToString(writer.Sum256())
+	if correctHash != writerHash {
+		t.Fatalf("hash computation mismatch: manual=%s, writer=%s", correctHash, writerHash)
+	}
+
+	// Set up mock storage with the blob at the correct path
+	mockStorage := NewMockStorer()
+	blobPath := "blobs/" + correctHash[:2] + "/" + correctHash[2:4] + "/" + correctHash
+	mockStorage.mu.Lock()
+	mockStorage.data[blobPath] = encryptedData
+	mockStorage.mu.Unlock()
+
+	tv := vaultik.NewForTesting(mockStorage)
+	ctx := context.Background()
+
+	t.Run("correct hash succeeds", func(t *testing.T) {
+		rc, err := tv.FetchAndDecryptBlob(ctx, correctHash, int64(len(encryptedData)), identity)
+		if err != nil {
+			t.Fatalf("expected success, got error: %v", err)
+		}
+		data, err := io.ReadAll(rc)
+		if err != nil {
+			t.Fatalf("reading stream: %v", err)
+		}
+		if err := rc.Close(); err != nil {
+			t.Fatalf("close (hash verification) failed: %v", err)
+		}
+		if !bytes.Equal(data, plaintext) {
+			t.Fatalf("decrypted data mismatch: got %q, want %q", data, plaintext)
+		}
+	})
+
+	t.Run("wrong hash fails", func(t *testing.T) {
+		// Use a fake hash that doesn't match the actual plaintext
+		fakeHash := strings.Repeat("ab", 32) // 64 hex chars
+		fakePath := "blobs/" + fakeHash[:2] + "/" + fakeHash[2:4] + "/" + fakeHash
+		mockStorage.mu.Lock()
+		mockStorage.data[fakePath] = encryptedData
+		mockStorage.mu.Unlock()
+
+		rc, err := tv.FetchAndDecryptBlob(ctx, fakeHash, int64(len(encryptedData)), identity)
+		if err != nil {
+			t.Fatalf("unexpected error opening stream: %v", err)
+		}
+		// Read all data — hash is verified on Close
+		_, _ = io.ReadAll(rc)
+		err = rc.Close()
+		if err == nil {
+			t.Fatal("expected error for mismatched hash, got nil")
+		}
+		if !strings.Contains(err.Error(), "hash mismatch") {
+			t.Fatalf("expected hash mismatch error, got: %v", err)
+		}
+	})
+}

internal/vaultik/blob_fetch_stub.go

@@ -2,38 +2,82 @@ package vaultik
 
 import (
 	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"fmt"
+	"hash"
 	"io"
 
 	"filippo.io/age"
 	"git.eeqj.de/sneak/vaultik/internal/blobgen"
 )
 
-// FetchAndDecryptBlobResult holds the result of fetching and decrypting a blob.
+// hashVerifyReader wraps a reader and computes a double-SHA-256 hash of all
-type FetchAndDecryptBlobResult struct {
+// data read through it. The hash is verified against the expected blob hash
-	Data []byte
+// when Close is called. This allows streaming blob verification without
+// buffering the entire blob in memory.
+type hashVerifyReader struct {
+	reader   io.ReadCloser // underlying decrypted blob reader
+	fetcher  io.ReadCloser // raw fetched stream (closed on Close)
+	hasher   hash.Hash     // running SHA-256 of plaintext
+	blobHash string        // expected double-SHA-256 hex
+	done     bool          // EOF reached
 }
 
-// FetchAndDecryptBlob downloads a blob, decrypts it, and returns the plaintext data.
+func (h *hashVerifyReader) Read(p []byte) (int, error) {
-func (v *Vaultik) FetchAndDecryptBlob(ctx context.Context, blobHash string, expectedSize int64, identity age.Identity) (*FetchAndDecryptBlobResult, error) {
+	n, err := h.reader.Read(p)
+	if n > 0 {
+		h.hasher.Write(p[:n])
+	}
+	if err == io.EOF {
+		h.done = true
+	}
+	return n, err
+}
+
+// Close verifies the hash (if the stream was fully read) and closes underlying readers.
+func (h *hashVerifyReader) Close() error {
+	readerErr := h.reader.Close()
+	fetcherErr := h.fetcher.Close()
+
+	if h.done {
+		firstHash := h.hasher.Sum(nil)
+		secondHasher := sha256.New()
+		secondHasher.Write(firstHash)
+		actualHashHex := hex.EncodeToString(secondHasher.Sum(nil))
+		if actualHashHex != h.blobHash {
+			return fmt.Errorf("blob hash mismatch: expected %s, got %s", h.blobHash[:16], actualHashHex[:16])
+		}
+	}
+
+	if readerErr != nil {
+		return readerErr
+	}
+	return fetcherErr
+}
+
+// FetchAndDecryptBlob downloads a blob, decrypts and decompresses it, and
+// returns a streaming reader that computes the double-SHA-256 hash on the fly.
+// The hash is verified when the returned reader is closed (after fully reading).
+// This avoids buffering the entire blob in memory.
+func (v *Vaultik) FetchAndDecryptBlob(ctx context.Context, blobHash string, expectedSize int64, identity age.Identity) (io.ReadCloser, error) {
 	rc, _, err := v.FetchBlob(ctx, blobHash, expectedSize)
 	if err != nil {
 		return nil, err
 	}
-	defer func() { _ = rc.Close() }()
 
 	reader, err := blobgen.NewReader(rc, identity)
 	if err != nil {
+		_ = rc.Close()
 		return nil, fmt.Errorf("creating blob reader: %w", err)
 	}
-	defer func() { _ = reader.Close() }()
 
-	data, err := io.ReadAll(reader)
+	return &hashVerifyReader{
-	if err != nil {
+		reader:   reader,
-		return nil, fmt.Errorf("reading blob data: %w", err)
+		fetcher:  rc,
-	}
+		hasher:   sha256.New(),
-
+		blobHash: blobHash,
-	return &FetchAndDecryptBlobResult{Data: data}, nil
+	}, nil
 }
 
 // FetchBlob downloads a blob and returns a reader for the encrypted data.

internal/vaultik/restore.go

@@ -494,11 +494,23 @@ func (v *Vaultik) restoreRegularFile(
 
 // downloadBlob downloads and decrypts a blob
 func (v *Vaultik) downloadBlob(ctx context.Context, blobHash string, expectedSize int64, identity age.Identity) ([]byte, error) {
-	result, err := v.FetchAndDecryptBlob(ctx, blobHash, expectedSize, identity)
+	rc, err := v.FetchAndDecryptBlob(ctx, blobHash, expectedSize, identity)
 	if err != nil {
 		return nil, err
 	}
-	return result.Data, nil
+
+	data, err := io.ReadAll(rc)
+	if err != nil {
+		_ = rc.Close()
+		return nil, fmt.Errorf("reading blob data: %w", err)
+	}
+
+	// Close triggers hash verification
+	if err := rc.Close(); err != nil {
+		return nil, err
+	}
+
+	return data, nil
 }
 
 // verifyRestoredFiles verifies that all restored files match their expected chunk hashes