Signature support: decouple mupdf from the pkcs7 implementation

The mupdf build included an implimentation of the pkcs7 functions that
are needed for signing documents and verifying signatures, the
implementation being either an openssl-based one, or a stub that returned
errors. This commit removes the pkcs7 functions from the main mupdf
library.

For the sake of verification, there wasn't really a need for the pkcs7
functions to be part of mupdf. It was only the checking function that used
them. The checking function is now provided as a helper, outside of the
main build. The openssl-based pkcs7 functions area also supplied as a
helper. Users wishing to verify signatures can either use the checking
function directly, or use the source on which to base their own.

Document signing requires more integration between mupdf and pkcs7
because part of the process is performed at time of signing and part when
saving the document. Mupdf already had a pdf_pkcs7_signer object that
kept information between the two phases. That object has now been extended
to include the pkcs7 functions involved in signing, and the signing
function now requires such an object, rather than a file path to a
certificate. The openssl-based pkcs7 helper provides a function that, given
the path to a certificate, will return a pdf_pkcs7_signer object.

The intention is that different implementations can be produced for
different platforms, based on cryptographic routines built into the
operationg system. In each case, for the sake of document signing, the
routines would be wrapped up as a pdf_pkcs7_signer object.

7 files changed, 84 insertions, 65 deletions

diff --git a/include/mupdf/helpers/pkcs7-check.h b/include/mupdf/helpers/pkcs7-check.h
new file mode 100644
index 00000000..3ff529d7
--- /dev/null
+++ b/include/mupdf/helpers/pkcs7-check.h
@@ -0,0 +1,12 @@
+#ifndef MUPDF_PKCS7_CHECK_H
+#define MUPDF_PKCS7_CHECK_H
+
+/*
+	pdf_check_signature: check a signature's certificate chain and digest
+
+	This is a helper function defined to provide compatibility with older
+	versions of mupdf
+*/
+int pdf_check_signature(fz_context *ctx, pdf_document *doc, pdf_widget *widget, char *ebuf, int ebufsize);
+
+#endif
diff --git a/include/mupdf/helpers/pkcs7-openssl.h b/include/mupdf/helpers/pkcs7-openssl.h
new file mode 100644
index 00000000..a6268839
--- /dev/null
+++ b/include/mupdf/helpers/pkcs7-openssl.h
@@ -0,0 +1,24 @@
+#ifndef MUPDF_PKCS7_OPENSSL_H
+#define MUPDF_PKCS7_OPENSSL_H
+
+/* This an example pkcs7 implementation using openssl. These are the types of functions that you
+ * will likely need to sign documents and check signatures within documents. In particular, to
+ * sign a document, you need a function that derives a pdf_pkcs7_signer object from a certificate
+ * stored by the operating system or within a file. */
+
+/* Check a signature's digest against ranges of bytes drawn from a stream */
+SignatureError pkcs7_openssl_check_digest(fz_context *ctx, fz_stream *stm, char *sig, int sig_len);
+
+/* Check a singature's certificate is trusted */
+SignatureError pkcs7_openssl_check_certificate(char *sig, int sig_len);
+
+/* Obtain the designated name information from signature's certificate */
+pdf_pkcs7_designated_name *pkcs7_openssl_designated_name(fz_context *ctx, char *sig, int sig_len);
+
+/* Free the resources associated with designated name information */
+void pkcs7_opensll_drop_designated_name(fz_context *ctx, pdf_pkcs7_designated_name *dn);
+
+/* Read the certificate and private key from a pfx file, holding it as an opaque structure */
+pdf_pkcs7_signer *pkcs7_openssl_read_pfx(fz_context *ctx, const char *pfile, const char *pw);
+
+#endif
diff --git a/include/mupdf/pdf.h b/include/mupdf/pdf.h
index f593e680..eab70ee9 100644
--- a/include/mupdf/pdf.h
+++ b/include/mupdf/pdf.h
@@ -31,8 +31,6 @@ extern "C" {
 
 #include "mupdf/pdf/clean.h"
 
-#include "mupdf/pdf/pdf-pkcs7.h"
-
 #ifdef __cplusplus
 }
 #endif
diff --git a/include/mupdf/pdf/crypt.h b/include/mupdf/pdf/crypt.h
index 846ed5bb..c01d3978 100644
--- a/include/mupdf/pdf/crypt.h
+++ b/include/mupdf/pdf/crypt.h
@@ -39,14 +39,9 @@ fz_stream *pdf_signature_widget_hash_bytes(fz_context *ctx, pdf_document *doc, p
 int pdf_signature_widget_contents(fz_context *ctx, pdf_document *doc, pdf_widget *widget, char **contents);
 
 /*
-	pdf_check_signature: check a signature's certificate chain and digest
-*/
-int pdf_check_signature(fz_context *ctx, pdf_document *doc, pdf_widget *widget, char *ebuf, int ebufsize);
-
-/*
 	pdf_sign_signature: sign a signature form field
 */
-void pdf_sign_signature(fz_context *ctx, pdf_document *doc, pdf_widget *widget, const char *sigfile, const char *password);
+void pdf_sign_signature(fz_context *ctx, pdf_document *doc, pdf_widget *widget, pdf_pkcs7_signer *signer);
 
 void pdf_encrypt_data(fz_context *ctx, pdf_crypt *crypt, int num, int gen, void (*fmt_str_out)(fz_context *, void *, const unsigned char *, int), void *arg, const unsigned char *s, int n);
 
diff --git a/include/mupdf/pdf/document.h b/include/mupdf/pdf/document.h
index ef861145..124de254 100644
--- a/include/mupdf/pdf/document.h
+++ b/include/mupdf/pdf/document.h
@@ -535,8 +535,55 @@ void pdf_update_page(fz_context *ctx, pdf_page *page);
 */
 int pdf_has_unsaved_changes(fz_context *ctx, pdf_document *doc);
 
+typedef enum
+{
+	SignatureError_Okay,
+	SignatureError_NoSignatures,
+	SignatureError_NoCertificate,
+	SignatureError_DocumentChanged,
+	SignatureError_SelfSigned,
+	SignatureError_SelfSignedInChain,
+	SignatureError_NotTrusted,
+	SignatureError_Unknown
+} SignatureError;
+
+typedef struct pdf_pkcs7_designated_name_s
+{
+	char *cn;
+	char *o;
+	char *ou;
+	char *email;
+	char *c;
+}
+pdf_pkcs7_designated_name;
+
+/* Object that can perform the cryptographic operation necessary for document signing */
 typedef struct pdf_pkcs7_signer_s pdf_pkcs7_signer;
 
+/* Increment the reference count for a signer object */
+typedef pdf_pkcs7_signer *(pdf_pkcs7_keep_fn)(pdf_pkcs7_signer *signer);
+
+/* Drop a reference for a signer object */
+typedef void (pdf_pkcs7_drop_fn)(pdf_pkcs7_signer *signer);
+
+/* Obtain the designated name information from a signer object */
+typedef pdf_pkcs7_designated_name *(pdf_pkcs7_designated_name_fn)(pdf_pkcs7_signer *signer);
+
+/* Free the resources associated with previously obtained designated name information */
+typedef void (pdf_pkcs7_drop_designated_name_fn)(pdf_pkcs7_signer *signer, pdf_pkcs7_designated_name *name);
+
+/* Create a signature based on ranges of bytes drawn from a steam */
+typedef int (pdf_pkcs7_create_digest_fn)(pdf_pkcs7_signer *signer, fz_stream *in, unsigned char *digest, int *digest_len);
+
+struct pdf_pkcs7_signer_s
+{
+	pdf_pkcs7_keep_fn *keep;
+	pdf_pkcs7_drop_fn *drop;
+	pdf_pkcs7_designated_name_fn *designated_name;
+	pdf_pkcs7_drop_designated_name_fn *drop_designated_name;
+	pdf_pkcs7_create_digest_fn *create_digest;
+};
+
 /* Unsaved signature fields */
 typedef struct pdf_unsaved_sig_s pdf_unsaved_sig;
 
diff --git a/include/mupdf/pdf/field.h b/include/mupdf/pdf/field.h
index c3509938..5b25dc3f 100644
--- a/include/mupdf/pdf/field.h
+++ b/include/mupdf/pdf/field.h
@@ -51,6 +51,5 @@ char *pdf_field_name(fz_context *ctx, pdf_document *doc, pdf_obj *field);
 void pdf_field_set_display(fz_context *ctx, pdf_document *doc, pdf_obj *field, int d);
 pdf_obj *pdf_lookup_field(fz_context *ctx, pdf_obj *form, char *name);
 void pdf_field_reset(fz_context *ctx, pdf_document *doc, pdf_obj *field);
-int pdf_signatures_supported(fz_context *ctx);
 
 #endif
diff --git a/include/mupdf/pdf/pdf-pkcs7.h b/include/mupdf/pdf/pdf-pkcs7.h
deleted file mode 100644
index 237a036d..00000000
--- a/include/mupdf/pdf/pdf-pkcs7.h
+++ /dev/null
@@ -1,56 +0,0 @@
-#ifndef MUPDF_PDF_PKCS7_H
-#define MUPDF_PDF_PKCS7_H
-
-typedef enum
-{
-	SignatureError_Okay,
-	SignatureError_NoSignatures,
-	SignatureError_NoCertificate,
-	SignatureError_DocumentChanged,
-	SignatureError_SelfSigned,
-	SignatureError_SelfSignedInChain,
-	SignatureError_NotTrusted,
-	SignatureError_Unknown
-} SignatureError;
-
-typedef struct pdf_pkcs7_designated_name_s
-{
-	char *cn;
-	char *o;
-	char *ou;
-	char *email;
-	char *c;
-}
-pdf_pkcs7_designated_name;
-
-/* Check a signature's digest against ranges of bytes drawn from a stream */
-SignatureError pdf_pkcs7_check_digest(fz_context *ctx, fz_stream *stm, char *sig, int sig_len);
-
-/* Check a singature's certificate is trusted */
-SignatureError pdf_pkcs7_check_certificate(char *sig, int sig_len);
-
-/* Obtain the designated name information from signature's certificate */
-pdf_pkcs7_designated_name *pdf_cert_designated_name(fz_context *ctx, char *sig, int sig_len);
-
-/* Free the resources associated with designated name information */
-void pdf_pkcs7_drop_designated_name(fz_context *ctx, pdf_pkcs7_designated_name *dn);
-
-/* Read the certificate and private key from a pfx file, holding it as an opaque structure */
-pdf_pkcs7_signer *pdf_pkcs7_read_pfx(fz_context *ctx, const char *pfile, const char *pw);
-
-/* Increment the reference count for a signer object */
-pdf_pkcs7_signer *pdf_pkcs7_keep_signer(fz_context *ctx, pdf_pkcs7_signer *signer);
-
-/* Drop a reference for a signer object */
-void pdf_pkcs7_drop_signer(fz_context *ctx, pdf_pkcs7_signer *signer);
-
-/* Obtain the designated name information from a signer object */
-pdf_pkcs7_designated_name *pdf_pkcs7_signer_designated_name(fz_context *ctx, pdf_pkcs7_signer *signer);
-
-/* Create a signature based on ranges of bytes drawn from a steam */
-int pdf_pkcs7_create_digest(fz_context *ctx, fz_stream *in, pdf_pkcs7_signer *signer, unsigned char *digest, int *digest_len);
-
-/* Report whether pkcs7 is supported in the current build */
-int pdf_pkcs7_supported(fz_context *ctx);
-
-#endif