From fecb3ea832972630ebba2d4703be91a59e791fa6 Mon Sep 17 00:00:00 2001
From: Matt Zimmerman <mdz@alcor.net>
Date: Tue, 18 Aug 2026 12:26:35 -0700
Subject: [PATCH] fix: https://github.com/ledongthuc/pdf/issues/82

Implement step (g) of PDF 32000-1:2008 Algorithm 2: when R>=4 and
/EncryptMetadata is false, append 0xFFFFFFFF to the MD5 hash before
deriving the encryption key. Without this, AES-128 PDFs with cleartext
metadata (common from e-signature tools) fail with "invalid password"
even when the actual password is empty.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
---
 read.go                               |   5 +++++
 read_test.go                          |  25 +++++++++++++++++++++++++
 testdata/encrypted_metadata_false.pdf | Bin 0 -> 1100 bytes
 3 files changed, 30 insertions(+)
 create mode 100644 testdata/encrypted_metadata_false.pdf

diff --git a/read.go b/read.go
index 5cadd7c5..31148b3e 100644
--- a/read.go
+++ b/read.go
@@ -982,6 +982,11 @@ func (r *Reader) initEncrypt(password string) error {
 	h.Write([]byte(O))
 	h.Write([]byte{byte(P), byte(P >> 8), byte(P >> 16), byte(P >> 24)})
 	h.Write([]byte(ID))
+	if R >= 4 {
+		if encryptMetadata, ok := encrypt["EncryptMetadata"].(bool); ok && !encryptMetadata {
+			h.Write([]byte{0xff, 0xff, 0xff, 0xff})
+		}
+	}
 	key := h.Sum(nil)
 
 	if R >= 3 {
diff --git a/read_test.go b/read_test.go
index edea184e..32966764 100644
--- a/read_test.go
+++ b/read_test.go
@@ -2,6 +2,8 @@ package pdf
 
 import (
 	"bytes"
+	"context"
+	"io"
 	"strings"
 	"testing"
 )
@@ -151,3 +153,26 @@ func TestNewReaderMaliciousPDF(t *testing.T) {
 		t.Fatal("expected error from malicious PDF, got nil")
 	}
 }
+
+// Regression test for https://github.com/ledongthuc/pdf/issues/82:
+// AES-128 encrypted PDFs (V=4, R=4) with /EncryptMetadata false and an empty
+// user password must open successfully instead of returning ErrInvalidPassword.
+func TestOpenEncryptedMetadataFalse(t *testing.T) {
+	f, r, err := Open("testdata/encrypted_metadata_false.pdf")
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	defer f.Close()
+
+	b, err := r.GetPlainText(context.Background())
+	if err != nil {
+		t.Fatalf("GetPlainText: %v", err)
+	}
+	text, err := io.ReadAll(b)
+	if err != nil {
+		t.Fatalf("ReadAll: %v", err)
+	}
+	if !strings.Contains(string(text), "hello world") {
+		t.Fatalf("unexpected text: %q", text)
+	}
+}
diff --git a/testdata/encrypted_metadata_false.pdf b/testdata/encrypted_metadata_false.pdf
new file mode 100644
index 0000000000000000000000000000000000000000..1431736cd734ae6fdc05bb6b2a2ebce0d974e703
GIT binary patch
literal 1100
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zos|kH6g*JDT5K&NR{jQS3mXwEf-`$}xe%OXcV?fN_hX*-c_$ik^+`O%y@@Y>`Uf5c
z1i5_RRVv^&#JcW6V38*Hw;pY2@Mnb)oqQdtRZnNi9dz3xgW0^78NlG*Xsa%)04YsZ
zN5%txV^g<1HqDIAOc&@N+`Lw8F_Z6DFUURtLV)dSeKka35027m>TcdE5}T*XxSrdc
za)E)Bj;d9g>(K;`r0znE;@{NSy4ipTStsp|(S@Bm!st1j<VriEt|_$G^d6pE>d%~B
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z7Q>aql0B$Qd2GxrKjLM!UUJehYPg#2nT^`EW6__>lf^Ba*~-@E!Jn?RmILsIWMJ;N
zbM)1+q<FMt=Cu*ZDgaW^=~^|6m}(i3DN(4gWe|bC2o=I9!%~r0g^b8B!3v`|2$CSu
zoO31wO$lKnm59(tGQy*nXd<zaEMZ|RZRo`sRP5A%NeX_`%END&&AR)#+t{fX|13cz
zj6%W~O@mky6mq3y$aN?+5{h!oBtikEl*ciWv8EVP6k<VDj1i3^nqV7hX%KPQ#v1dv
zQ|Qz~5PIlv0B4LaNZ}B}wsvknHpaMB*)a?UR{6)UfZ5JFhEen%EIfjlLbN+ps!s9E
zJa?^E_G+vBSVIW@wK-U+1Sp`f;zUIWjiOY9n1_ra62wGmlHzJb@{kI`<3L!>9c@T5
kg@jZJ;V7l5YP)Wz0C2i$4%$H!req~Tp*Jy6TdaHk0Ee?PXaE2J

literal 0
HcmV?d00001