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

@@ -5,6 +5,7 @@ import (
 	"context"
 	"crypto/sha256"
 	"encoding/hex"
+	"io"
 	"strings"
 	"testing"
 
@@ -58,12 +59,19 @@ func TestFetchAndDecryptBlobVerifiesHash(t *testing.T) {
 	ctx := context.Background()
 
 	t.Run("correct hash succeeds", func(t *testing.T) {
-		result, err := tv.FetchAndDecryptBlob(ctx, correctHash, int64(len(encryptedData)), identity)
+		rc, err := tv.FetchAndDecryptBlob(ctx, correctHash, int64(len(encryptedData)), identity)
 		if err != nil {
 			t.Fatalf("expected success, got error: %v", err)
 		}
-		if !bytes.Equal(result.Data, plaintext) {
-			t.Fatalf("decrypted data mismatch: got %q, want %q", result.Data, plaintext)
+		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)
 		}
 	})
 
@@ -75,7 +83,13 @@ func TestFetchAndDecryptBlobVerifiesHash(t *testing.T) {
 		mockStorage.data[fakePath] = encryptedData
 		mockStorage.mu.Unlock()
 
-		_, err := tv.FetchAndDecryptBlob(ctx, fakeHash, int64(len(encryptedData)), identity)
+		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")
 		}

internal/vaultik/blob_fetch_stub.go

@@ -5,49 +5,79 @@ import (
 	"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.
-type FetchAndDecryptBlobResult struct {
-	Data []byte
+// hashVerifyReader wraps a reader and computes a double-SHA-256 hash of all
+// data read through it. The hash is verified against the expected blob hash
+// 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 and decompresses it, then
-// verifies that the double-SHA-256 hash of the plaintext matches blobHash.
-// This ensures blob integrity end-to-end during restore operations.
-func (v *Vaultik) FetchAndDecryptBlob(ctx context.Context, blobHash string, expectedSize int64, identity age.Identity) (*FetchAndDecryptBlobResult, error) {
+func (h *hashVerifyReader) Read(p []byte) (int, 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)
-	if err != nil {
-		return nil, fmt.Errorf("reading blob data: %w", err)
-	}
-
-	// Verify blob integrity: compute double-SHA-256 of the decrypted plaintext
-	// and compare to the expected blob hash. The blob hash is SHA256(SHA256(plaintext))
-	// as produced by blobgen.Writer.Sum256().
-	firstHash := sha256.Sum256(data)
-	secondHash := sha256.Sum256(firstHash[:])
-	actualHashHex := hex.EncodeToString(secondHash[:])
-	if actualHashHex != blobHash {
-		return nil, fmt.Errorf("blob hash mismatch: expected %s, got %s", blobHash[:16], actualHashHex[:16])
-	}
-
-	return &FetchAndDecryptBlobResult{Data: data}, nil
+	return &hashVerifyReader{
+		reader:   reader,
+		fetcher:  rc,
+		hasher:   sha256.New(),
+		blobHash: blobHash,
+	}, 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