[PATCH v2 05/18] pgp: factor out rsa_pad

[Daniel Axtens]

rsa_pad does the PKCS#1 v1.5 padding for the RSA signature scheme.
We want to use it in other RSA signature verification applications.

I considered and rejected putting it in lib/crypto.c. That file doesn't
currently require any MPI functions, but rsa_pad does. That's not so
much of a problem for the grub kernel and modules, but crypto.c also
gets built into all the grub utilities. So - despite the utils not
using any asymmetric ciphers -  we would need to built the entire MPI
infrastructure in to them.

A better and simpler solution is just to spin rsa_pad out into its own
PKCS#1 v1.5 module.

Signed-off-by: Daniel Axtens <dja@axtens.net>
---
 grub-core/Makefile.core.def |  8 +++++
 grub-core/commands/pgp.c    | 28 ++----------------
 grub-core/lib/pkcs1_v15.c   | 59 +++++++++++++++++++++++++++++++++++++
 include/grub/pkcs1_v15.h    | 27 +++++++++++++++++
 4 files changed, 96 insertions(+), 26 deletions(-)
 create mode 100644 grub-core/lib/pkcs1_v15.c
 create mode 100644 include/grub/pkcs1_v15.h

diff --git a/grub-core/Makefile.core.def b/grub-core/Makefile.core.def
index b5f47fc41b54..ad52e54098d6 100644
--- a/grub-core/Makefile.core.def
+++ b/grub-core/Makefile.core.def
@@ -2476,6 +2476,14 @@ module = {
   cppflags = '$(CPPFLAGS_GCRY)';
 };
 
+module = {
+  name = pkcs1_v15;
+  common = lib/pkcs1_v15.c;
+
+  cflags = '$(CFLAGS_GCRY) -Wno-redundant-decls -Wno-sign-compare';
+  cppflags = '$(CPPFLAGS_GCRY)';
+};
+
 module = {
   name = all_video;
   common = lib/fake_module.c;
diff --git a/grub-core/commands/pgp.c b/grub-core/commands/pgp.c
index bbf6871fe71f..2032afa4ec69 100644
--- a/grub-core/commands/pgp.c
+++ b/grub-core/commands/pgp.c
@@ -24,6 +24,7 @@
 #include <grub/file.h>
 #include <grub/command.h>
 #include <grub/crypto.h>
+#include <grub/pkcs1_v15.h>
 #include <grub/i18n.h>
 #include <grub/gcrypt/gcrypt.h>
 #include <grub/pubkey.h>
@@ -411,32 +412,7 @@ static int
 rsa_pad (gcry_mpi_t *hmpi, grub_uint8_t *hval,
         const gcry_md_spec_t *hash, struct grub_public_subkey *sk)
 {
-  grub_size_t tlen, emlen, fflen;
-  grub_uint8_t *em, *emptr;
-  unsigned nbits = gcry_mpi_get_nbits (sk->mpis[0]);
-  int ret;
-  tlen = hash->mdlen + hash->asnlen;
-  emlen = (nbits + 7) / 8;
-  if (emlen < tlen + 11)
-    return 1;
-
-  em = grub_malloc (emlen);
-  if (!em)
-    return 1;
-
-  em[0] = 0x00;
-  em[1] = 0x01;
-  fflen = emlen - tlen - 3;
-  for (emptr = em + 2; emptr < em + 2 + fflen; emptr++)
-    *emptr = 0xff;
-  *emptr++ = 0x00;
-  grub_memcpy (emptr, hash->asnoid, hash->asnlen);
-  emptr += hash->asnlen;
-  grub_memcpy (emptr, hval, hash->mdlen);
-
-  ret = gcry_mpi_scan (hmpi, GCRYMPI_FMT_USG, em, emlen, 0);
-  grub_free (em);
-  return ret;
+  return grub_crypto_rsa_pad(hmpi, hval, hash, sk->mpis[0]);
 }
 
 struct grub_pubkey_context
diff --git a/grub-core/lib/pkcs1_v15.c b/grub-core/lib/pkcs1_v15.c
new file mode 100644
index 000000000000..dbacd563d014
--- /dev/null
+++ b/grub-core/lib/pkcs1_v15.c
@@ -0,0 +1,59 @@
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2013  Free Software Foundation, Inc.
+ *
+ *  GRUB is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  GRUB is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <grub/dl.h>
+#include <grub/gcrypt/gcrypt.h>
+
+GRUB_MOD_LICENSE ("GPLv3+");
+
+/*
+ * Given a hash value 'hval', of hash specification 'hash', perform
+ * the EMSA-PKCS1-v1_5 padding suitable for a key with modulus 'mod'
+ * (see RFC 8017 s 9.2) and place the result in 'hmpi'.
+ */
+gcry_err_code_t
+grub_crypto_rsa_pad (gcry_mpi_t * hmpi, grub_uint8_t * hval,
+                    const gcry_md_spec_t * hash, gcry_mpi_t mod)
+{
+  grub_size_t tlen, emlen, fflen;
+  grub_uint8_t *em, *emptr;
+  unsigned nbits = gcry_mpi_get_nbits (mod);
+  int ret;
+  tlen = hash->mdlen + hash->asnlen;
+  emlen = (nbits + 7) / 8;
+  if (emlen < tlen + 11)
+    return GPG_ERR_TOO_SHORT;
+
+  em = grub_malloc (emlen);
+  if (!em)
+    return 1;
+
+  em[0] = 0x00;
+  em[1] = 0x01;
+  fflen = emlen - tlen - 3;
+  for (emptr = em + 2; emptr < em + 2 + fflen; emptr++)
+    *emptr = 0xff;
+  *emptr++ = 0x00;
+  grub_memcpy (emptr, hash->asnoid, hash->asnlen);
+  emptr += hash->asnlen;
+  grub_memcpy (emptr, hval, hash->mdlen);
+
+  ret = gcry_mpi_scan (hmpi, GCRYMPI_FMT_USG, em, emlen, 0);
+  grub_free (em);
+  return ret;
+}
diff --git a/include/grub/pkcs1_v15.h b/include/grub/pkcs1_v15.h
new file mode 100644
index 000000000000..5c338c84a158
--- /dev/null
+++ b/include/grub/pkcs1_v15.h
@@ -0,0 +1,27 @@
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2013  Free Software Foundation, Inc.
+ *
+ *  GRUB is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  GRUB is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+ * Given a hash value 'hval', of hash specification 'hash', perform
+ * the EMSA-PKCS1-v1_5 padding suitable for a key with modulus 'mod'
+ * (See RFC 8017 s 9.2)
+ */
+gcry_err_code_t
+grub_crypto_rsa_pad (gcry_mpi_t * hmpi, grub_uint8_t * hval,
+                    const gcry_md_spec_t * hash, gcry_mpi_t mod);
+
-- 
2.25.1