app.go

@@ -167,7 +167,7 @@ func (a *App) Delete(connId, path string) error {
 	return exp.Delete(path)
 }
 
-func (a *App) QueueTransfer(id string, srcConnId, dstConnId, srcPath, dstPath, filename string, size int64) error {
+func (a *App) QueueTransfer(id string, srcConnId, dstConnId, srcPath, dstPath, filename string, size int64, verify bool) error {
 	srcExp, err := a.getExplorerForConnection(srcConnId)
 	if err != nil {
 		return err
@@ -185,7 +185,7 @@ func (a *App) QueueTransfer(id string, srcConnId, dstConnId, srcPath, dstPath, f
 		return err
 	}
 
-	return a.transferManager.QueueTransfer(id, srcPath, dstPath, filename, size, srcExp, dstExp)
+	return a.transferManager.QueueTransfer(id, srcPath, dstPath, filename, size, srcExp, dstExp, verify)
 }
 
 func (a *App) GetTransfers() []*transfer.Transfer {
@@ -232,7 +232,7 @@ func (a *App) PromptUploadFiles(connId, destPath string) error {
 			remotePath = destPath + fileName
 		}
 
-		a.QueueTransfer(id, "local", connId, localPath, remotePath, fileName, stat.Size())
+		a.QueueTransfer(id, "local", connId, localPath, remotePath, fileName, stat.Size(), false)
 	}
 	return nil
 }
@@ -272,7 +272,7 @@ func (a *App) PromptUploadDirectory(connId, destPath string) error {
 		}
 
 		id := uuid.New().String()
-		a.QueueTransfer(id, "local", connId, path, remotePath, info.Name(), info.Size())
+		a.QueueTransfer(id, "local", connId, path, remotePath, info.Name(), info.Size(), false)
 		return nil
 	})
 }
@@ -304,7 +304,7 @@ func (a *App) PromptDownload(connId, remotePath string) error {
 		size = stat.Size
 	}
 
-	return a.QueueTransfer(id, connId, "local", remotePath, localPath, fileName, size)
+	return a.QueueTransfer(id, connId, "local", remotePath, localPath, fileName, size, false)
 }
 
 type TransferItem struct {
@@ -314,7 +314,7 @@ type TransferItem struct {
 	Size  int64  `json:"size"`
 }
 
-func (a *App) TransferItems(srcConnId, dstConnId, dstPath string, items []TransferItem) error {
+func (a *App) TransferItems(srcConnId, dstConnId, dstPath string, items []TransferItem, verify bool) error {
 	for _, item := range items {
 		if !item.IsDir {
 			id := uuid.New().String()
@@ -328,18 +328,18 @@ func (a *App) TransferItems(srcConnId, dstConnId, dstPath string, items []Transf
 				remotePath = dstPath + item.Name
 			}
 
-			if err := a.QueueTransfer(id, srcConnId, dstConnId, item.Path, remotePath, item.Name, item.Size); err != nil {
+			if err := a.QueueTransfer(id, srcConnId, dstConnId, item.Path, remotePath, item.Name, item.Size, verify); err != nil {
 				return err
 			}
 		} else {
 			// Run remote walk in goroutine to not block UI
-			go a.transferRemoteDirectory(srcConnId, dstConnId, item.Path, dstPath)
+			go a.transferRemoteDirectory(srcConnId, dstConnId, item.Path, dstPath, verify)
 		}
 	}
 	return nil
 }
 
-func (a *App) transferRemoteDirectory(srcConnId, dstConnId, srcDirPath, dstBasePath string) {
+func (a *App) transferRemoteDirectory(srcConnId, dstConnId, srcDirPath, dstBasePath string, verify bool) {
 	baseName := filepath.Base(srcDirPath)
 	
 	newDstPath := dstBasePath
@@ -360,11 +360,11 @@ func (a *App) transferRemoteDirectory(srcConnId, dstConnId, srcDirPath, dstBaseP
 
 	for _, entry := range entries {
 		if entry.IsDir {
-			a.transferRemoteDirectory(srcConnId, dstConnId, entry.Path, newDstPath)
+			a.transferRemoteDirectory(srcConnId, dstConnId, entry.Path, newDstPath, verify)
 		} else {
 			id := uuid.New().String()
 			itemDstPath := newDstPath + "/" + entry.Name
-			a.QueueTransfer(id, srcConnId, dstConnId, entry.Path, itemDstPath, entry.Name, entry.Size)
+			a.QueueTransfer(id, srcConnId, dstConnId, entry.Path, itemDstPath, entry.Name, entry.Size, verify)
 		}
 	}
 }

frontend/index.html

@@ -176,6 +176,15 @@
                     <tbody id="transfer-dest-list"></tbody>
                 </table>
             </div>
+            
+            <div class="form-group" style="display: flex; align-items: center; gap: 0.5rem; flex-direction: row; margin-bottom: 1rem;">
+                <input type="checkbox" id="transfer-verify" style="width: auto;">
+                <label style="margin: 0; display: flex; flex-direction: column;">
+                    Verify Checksum (MD5)
+                    <span style="font-size: 0.75rem; color: var(--text-secondary); font-weight: normal;">Slower for non-S3 destinations as it requires re-reading the file over the network.</span>
+                </label>
+            </div>
+
             <div style="display: flex; justify-content: flex-end; gap: 1rem;">
                 <button class="btn" style="background: transparent; color: var(--text-primary); border-color: var(--border);" onclick="closeTransferModal()">Cancel</button>
                 <button class="btn btn-primary" onclick="executeTransfer()">Transfer Here</button>

frontend/src/main.js

@@ -528,6 +528,7 @@ window.openTransferModal = () => {
 
 window.closeTransferModal = () => {
     document.getElementById('transfer-modal').style.display = 'none';
+    document.getElementById('transfer-verify').checked = false;
 };
 
 window.loadTransferDestRoot = () => {

internal/explorer/explorer.go

@@ -21,4 +21,5 @@ type Explorer interface {
 	Rename(src, dst string) error
 	ReadFile(path string) (io.ReadCloser, error)
 	WriteFile(path string, r io.Reader, size int64) error
+	Checksum(path string) (string, error)
 }

internal/protocols/ftp/ftp.go

@@ -1,7 +1,9 @@
 package ftp
 
 import (
+	"crypto/md5"
 	"crypto/tls"
+	"encoding/hex"
 	"fmt"
 	"io"
 	"path/filepath"
@@ -177,3 +179,16 @@ func (e *FTPExplorer) ReadFile(path string) (io.ReadCloser, error) {
 func (e *FTPExplorer) WriteFile(path string, r io.Reader, size int64) error {
 	return e.client.Stor(path, r)
 }
+
+func (e *FTPExplorer) Checksum(path string) (string, error) {
+	r, err := e.ReadFile(path)
+	if err != nil {
+		return "", err
+	}
+	defer r.Close()
+	h := md5.New()
+	if _, err := io.Copy(h, r); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(h.Sum(nil)), nil
+}

internal/protocols/local/local.go

@@ -1,6 +1,8 @@
 package local
 
 import (
+	"crypto/md5"
+	"encoding/hex"
 	"io"
 	"os"
 	"path/filepath"
@@ -82,6 +84,19 @@ func (e *LocalExplorer) Rename(src, dst string) error {
 	return os.Rename(src, dst)
 }
 
+func (e *LocalExplorer) Checksum(path string) (string, error) {
+	r, err := e.ReadFile(path)
+	if err != nil {
+		return "", err
+	}
+	defer r.Close()
+	h := md5.New()
+	if _, err := io.Copy(h, r); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(h.Sum(nil)), nil
+}
+
 func (e *LocalExplorer) ReadFile(path string) (io.ReadCloser, error) {
 	return os.Open(path)
 }

internal/protocols/nfs/nfs_unix.go

@@ -3,6 +3,8 @@
 package nfs
 
 import (
+	"crypto/md5"
+	"encoding/hex"
 	"fmt"
 	"io"
 	"path/filepath"
@@ -125,3 +127,16 @@ func (e *NFSExplorer) WriteFile(path string, r io.Reader, size int64) error {
 	_, err = io.Copy(f, r)
 	return err
 }
+
+func (e *NFSExplorer) Checksum(path string) (string, error) {
+	r, err := e.ReadFile(path)
+	if err != nil {
+		return "", err
+	}
+	defer r.Close()
+	h := md5.New()
+	if _, err := io.Copy(h, r); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(h.Sum(nil)), nil
+}

internal/protocols/nfs/nfs_windows.go

@@ -25,3 +25,4 @@ func (e *NFSExplorer) Delete(path string) error { return nil }
 func (e *NFSExplorer) Rename(src, dst string) error { return nil }
 func (e *NFSExplorer) ReadFile(path string) (io.ReadCloser, error) { return nil, nil }
 func (e *NFSExplorer) WriteFile(path string, r io.Reader, size int64) error { return nil }
+func (e *NFSExplorer) Checksum(path string) (string, error) { return "", nil }

internal/protocols/s3/s3.go

@@ -307,3 +307,28 @@ func (e *S3Explorer) WriteFile(path string, r io.Reader, size int64) error {
 	})
 	return err
 }
+
+func (e *S3Explorer) Checksum(path string) (string, error) {
+	bucket, subpath, _, err := e.getBucketAndPath(path)
+	if err != nil {
+		return "", err
+	}
+	
+	if subpath == "" {
+		return "", fmt.Errorf("cannot calculate checksum of bucket")
+	}
+
+	input := &s3.HeadObjectInput{
+		Bucket: aws.String(bucket),
+		Key:    aws.String(subpath),
+	}
+	head, err := e.client.HeadObject(context.TODO(), input)
+	if err != nil {
+		return "", err
+	}
+
+	if head.ETag != nil {
+		return strings.Trim(*head.ETag, "\""), nil
+	}
+	return "", fmt.Errorf("no etag found")
+}

internal/protocols/sftp/sftp.go

@@ -1,6 +1,8 @@
 package sftp
 
 import (
+	"crypto/md5"
+	"encoding/hex"
 	"fmt"
 	"io"
 	"path/filepath"
@@ -162,3 +164,16 @@ func (e *SFTPExplorer) WriteFile(path string, r io.Reader, size int64) error {
 	_, err = io.Copy(f, r)
 	return err
 }
+
+func (e *SFTPExplorer) Checksum(path string) (string, error) {
+	r, err := e.ReadFile(path)
+	if err != nil {
+		return "", err
+	}
+	defer r.Close()
+	h := md5.New()
+	if _, err := io.Copy(h, r); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(h.Sum(nil)), nil
+}

internal/protocols/smb/smb.go

@@ -1,6 +1,8 @@
 package smb
 
 import (
+	"crypto/md5"
+	"encoding/hex"
 	"fmt"
 	"io"
 	"net"
@@ -243,3 +245,16 @@ func (e *SMBExplorer) WriteFile(path string, r io.Reader, size int64) error {
 	_, err = io.Copy(f, r)
 	return err
 }
+
+func (e *SMBExplorer) Checksum(path string) (string, error) {
+	r, err := e.ReadFile(path)
+	if err != nil {
+		return "", err
+	}
+	defer r.Close()
+	h := md5.New()
+	if _, err := io.Copy(h, r); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(h.Sum(nil)), nil
+}

internal/protocols/webdav/webdav.go

@@ -1,6 +1,8 @@
 package webdav
 
 import (
+	"crypto/md5"
+	"encoding/hex"
 	"fmt"
 	"io"
 	"strings"
@@ -102,3 +104,16 @@ func (e *WebDAVExplorer) ReadFile(path string) (io.ReadCloser, error) {
 func (e *WebDAVExplorer) WriteFile(path string, r io.Reader, size int64) error {
 	return e.client.WriteStream(path, r, 0644)
 }
+
+func (e *WebDAVExplorer) Checksum(path string) (string, error) {
+	r, err := e.ReadFile(path)
+	if err != nil {
+		return "", err
+	}
+	defer r.Close()
+	h := md5.New()
+	if _, err := io.Copy(h, r); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(h.Sum(nil)), nil
+}

internal/transfer/transfer.go

@@ -1,8 +1,12 @@
 package transfer
 
 import (
+	"crypto/md5"
+	"encoding/hex"
 	"errors"
+	"fmt"
 	"io"
+	"strings"
 	"sync"
 	"time"
 
@@ -29,6 +33,7 @@ type Transfer struct {
 	ETA         int     `json:"eta_seconds"`
 	Status      Status  `json:"status"`
 	Error       string  `json:"error,omitempty"`
+	Verify      bool    `json:"verify"`
 
 	srcExp explorer.Explorer
 	dstExp explorer.Explorer
@@ -81,12 +86,36 @@ func (m *Manager) doTransfer(t *Transfer) error {
 	}
 	defer r.Close()
 
+	hash := md5.New()
+	tr := io.TeeReader(r, hash)
+
 	pr := &progressReader{
-		r: r,
+		r: tr,
 		t: t,
 	}
 
-	return t.dstExp.WriteFile(t.Destination, pr, t.BytesTotal)
+	if err := t.dstExp.WriteFile(t.Destination, pr, t.BytesTotal); err != nil {
+		return err
+	}
+
+	if t.Verify {
+		expectedHash := hex.EncodeToString(hash.Sum(nil))
+		t.Status = StatusActive // Still active during verification
+
+		actualHash, err := t.dstExp.Checksum(t.Destination)
+		if err != nil {
+			return fmt.Errorf("verification failed: could not calculate destination checksum: %w", err)
+		}
+
+		if expectedHash != actualHash {
+			// S3 Multi-part ETags end with -N, we can't easily verify them with local md5
+			if !strings.Contains(actualHash, "-") {
+				return fmt.Errorf("verification failed: checksum mismatch (expected %s, got %s)", expectedHash, actualHash)
+			}
+		}
+	}
+
+	return nil
 }
 
 type progressReader struct {
@@ -109,7 +138,7 @@ func (pr *progressReader) Read(p []byte) (int, error) {
 	return n, err
 }
 
-func (m *Manager) QueueTransfer(id, srcPath, dstPath, filename string, size int64, srcExp, dstExp explorer.Explorer) error {
+func (m *Manager) QueueTransfer(id, srcPath, dstPath, filename string, size int64, srcExp, dstExp explorer.Explorer, verify bool) error {
 	if srcExp == nil || dstExp == nil {
 		return errors.New("invalid explorers")
 	}
@@ -121,6 +150,7 @@ func (m *Manager) QueueTransfer(id, srcPath, dstPath, filename string, size int6
 		Filename:    filename,
 		BytesTotal:  size,
 		Status:      StatusQueued,
+		Verify:      verify,
 		srcExp:      srcExp,
 		dstExp:      dstExp,
 	}