Add adapted appendedzip implementation

This is from [Spyre](https://github.com/spyre-project/spyre), with
crypto zip support removed and a function renamed for consistency

appendedzip/appendedzip.go

@@ -0,0 +1,55 @@
+package appendedzip
+
+import (
+	"archive/zip"
+	"bytes"
+	"errors"
+	"io"
+	"os"
+)
+
+func OpenFile(file string) (*zip.Reader, error) {
+	f, err := os.Open(file)
+	if err != nil {
+		return nil, err
+	}
+	fi, err := f.Stat()
+	if err != nil {
+		return nil, err
+	}
+	return NewReader(f, fi.Size())
+}
+
+// OpenReader searches for ZIP beginning-of-file signatures ('P' 'K'
+// 03 04) in r and tries to read the file starting at that offset
+// using an encryption-enabled archive/zip, returning a *zip.Reader
+// for the first valid entry, or an error.
+func NewReader(r io.ReaderAt, size int64) (*zip.Reader, error) {
+	const BUFSIZE = 4096
+	var buf [BUFSIZE + 4]byte
+	for i := int64(0); (i-1)*BUFSIZE < size; i++ {
+		len, err := r.ReadAt(buf[:], i*BUFSIZE)
+		if err != nil && err != io.EOF {
+			break
+		}
+
+		n := 0
+		for {
+			m := bytes.Index(buf[n:len], []byte("PK\x03\x04"))
+			if m == -1 {
+				break
+			}
+			off := i*BUFSIZE + int64(n+m)
+			ssize := size - int64(off)
+			sr := io.NewSectionReader(r, int64(off), ssize)
+			if zr, ze := zip.NewReader(sr, ssize+1); ze == nil {
+				return zr, nil
+			}
+			n += m + 1
+		}
+		if err == io.EOF {
+			break
+		}
+	}
+	return nil, errors.New("No zip file found")
+}

appendedzip/appendedzip_test.go

@@ -0,0 +1,39 @@
+package appendedzip
+
+import (
+	"archive/zip"
+	"os"
+	"testing"
+)
+
+func TestAppendedZip(t *testing.T) {
+	for _, sample := range []string{
+		"data512",
+		"data8192",
+		"data8190",
+		"doublezip",
+		"fakeheader8192",
+		"fakeheader512",
+		"regular",
+	} {
+		t.Log("Reading " + sample + " ...")
+		f, err := os.Open("testdata/" + sample)
+		if _, err := zip.OpenReader("testdata/" + sample); err == nil {
+			t.Logf("Note: %s can be opened by archive/zip", sample)
+		}
+		if err != nil {
+			t.Fatal(err)
+		}
+		fi, err := f.Stat()
+		if err != nil {
+			t.Fatal(err)
+		}
+		zr, err := NewReader(f, fi.Size())
+		if err != nil {
+			t.Error(err)
+		} else {
+			t.Log("found zip in " + sample)
+			t.Logf("%+v", zr)
+		}
+	}
+}

scanner/main.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"archive/zip"
 	"bytes"
 	"flag"
 	"fmt"
@@ -11,6 +10,7 @@ import (
 	"path/filepath"
 	"strings"
 
+	zip "github.com/hillu/local-log4j-vuln-scanner/appendedzip"
 	"github.com/hillu/local-log4j-vuln-scanner/filter"
 )