sm: Support encryption using ECDH keys.

* sm/decrypt.c (hash_ecc_cms_shared_info): Make global.
* sm/encrypt.c (ecdh_encrypt): New.
(encrypt_dek): Add arg PK_ALGO and support ECDH.
(gpgsm_encrypt): Pass PK_ALGO.
--

Note: This has only been tested with a messages created and decrypted
by GnuPG.

GnuPG-bug-id: 4098
Signed-off-by: Werner Koch <wk@gnupg.org>
Backported-from-master: d5051e31a8
GnuPG-bug-id: 6253

sm/decrypt.c

@@ -1,7 +1,7 @@
 /* decrypt.c - Decrypt a message
  * Copyright (C) 2001, 2003, 2010 Free Software Foundation, Inc.
  * Copyright (C) 2001-2019 Werner Koch
- * Copyright (C) 2015-2021 g10 Code GmbH
+ * Copyright (C) 2015-2020 g10 Code GmbH
  *
  * This file is part of GnuPG.
  *
@@ -88,7 +88,7 @@ string_from_gcry_buffer (gcry_buffer_t *buffer)
  *      entityUInfo [0] EXPLICIT OCTET STRING OPTIONAL,
  *      suppPubInfo [2] EXPLICIT OCTET STRING  }
  * as described in RFC-5753, 7.2.  */
-static gpg_error_t
+gpg_error_t
 hash_ecc_cms_shared_info (gcry_md_hd_t hash_hd, const char *wrap_algo_str,
                           unsigned int keylen,
                           const void *ukm, unsigned int ukmlen)

sm/encrypt.c

@@ -1,6 +1,8 @@
 /* encrypt.c - Encrypt a message
  * Copyright (C) 2001, 2003, 2004, 2007, 2008,
  *               2010 Free Software Foundation, Inc.
+ * Copyright (C) 2001-2019 Werner Koch
+ * Copyright (C) 2015-2020 g10 Code GmbH
  *
  * This file is part of GnuPG.
  *
@@ -16,6 +18,7 @@
  *
  * You should have received a copy of the GNU General Public License
  * along with this program; if not, see <https://www.gnu.org/licenses/>.
+ * SPDX-License-Identifier: GPL-3.0-or-later
  */
 
 #include <config.h>
@@ -144,7 +147,7 @@ init_dek (DEK dek)
   return 0;
 }
 
-
+/* Encrypt an RSA session key.  */
 static int
 encode_session_key (DEK dek, gcry_sexp_t * r_data)
 {
@@ -165,10 +168,282 @@ encode_session_key (DEK dek, gcry_sexp_t * r_data)
 }
 
 
+/* Encrypt DEK using ECDH.  S_PKEY is the public key.  On success the
+ * result is stored at R_ENCVAL.  Example of a public key:
+ *
+ *   (public-key (ecc (curve "1.3.132.0.34") (q #04B0[...]B8#)))
+ *
+ */
+static gpg_error_t
+ecdh_encrypt (DEK dek, gcry_sexp_t s_pkey, gcry_sexp_t *r_encval)
+{
+  gpg_error_t err;
+  gcry_sexp_t l1;
+  char *curvebuf = NULL;
+  const char *curve;
+  unsigned int curvebits;
+  const char *encr_algo_str;
+  const char *wrap_algo_str;
+  int hash_algo, cipher_algo;
+  unsigned int keylen, hashlen;
+  unsigned char key[32];
+  gcry_sexp_t s_data = NULL;
+  gcry_sexp_t s_encr = NULL;
+  gcry_buffer_t ioarray[2] = { {0}, {0} };
+  unsigned char *secret;  /* Alias for ioarray[0].  */
+  unsigned int secretlen;
+  unsigned char *pubkey;  /* Alias for ioarray[1].  */
+  unsigned int pubkeylen;
+  gcry_cipher_hd_t cipher_hd = NULL;
+  unsigned char *result = NULL;
+  unsigned int resultlen;
+
+  *r_encval = NULL;
+
+  /* Figure out the encryption and wrap algo OIDs.  */
+  /* Get the curve name if any,  */
+  l1 = gcry_sexp_find_token (s_pkey, "curve", 0);
+  if (l1)
+    {
+      curvebuf = gcry_sexp_nth_string (l1, 1);
+      gcry_sexp_release (l1);
+    }
+  if (!curvebuf)
+    {
+      err = gpg_error (GPG_ERR_INV_CURVE);
+      log_error ("%s: invalid public key: no curve\n", __func__);
+      goto leave;
+    }
+
+  /* We need to use our OpenPGP mapping to turn a curve name into its
+   * canonical numerical OID.  We also use this to get the size of the
+   * curve which we need to figure out a suitable hash algo.  We
+   * should have a Libgcrypt function to do this; see bug report #4926.  */
+  curve = openpgp_curve_to_oid (curvebuf, &curvebits, NULL);
+  if (!curve)
+    {
+      err = gpg_error (GPG_ERR_UNKNOWN_CURVE);
+      log_error ("%s: invalid public key: %s\n", __func__, gpg_strerror (err));
+      goto leave;
+    }
+  xfree (curvebuf);
+  curvebuf = NULL;
+
+  /* Our mapping matches the recommended algorithms from RFC-5753 but
+   * not supporing the short curves which would require 3DES.  */
+  if (curvebits < 255)
+    {
+      err = gpg_error (GPG_ERR_UNKNOWN_CURVE);
+      log_error ("%s: curve '%s' is not supported\n", __func__, curve);
+      goto leave;
+    }
+  else if (curvebits <= 256)
+    {
+      /* dhSinglePass-stdDH-sha256kdf-scheme */
+      encr_algo_str = "1.3.132.1.11.1";
+      wrap_algo_str = "2.16.840.1.101.3.4.1.5";
+      hash_algo     = GCRY_MD_SHA256;
+      hashlen       = 32;
+      cipher_algo   = GCRY_CIPHER_AES128;
+      keylen        = 16;
+    }
+  else if (curvebits <= 384)
+    {
+      /* dhSinglePass-stdDH-sha384kdf-scheme */
+      encr_algo_str = "1.3.132.1.11.2";
+      wrap_algo_str = "2.16.840.1.101.3.4.1.25";
+      hash_algo     = GCRY_MD_SHA384;
+      hashlen       = 48;
+      cipher_algo   = GCRY_CIPHER_AES256;
+      keylen        = 24;
+    }
+  else
+    {
+      /* dhSinglePass-stdDH-sha512kdf-scheme*/
+      encr_algo_str = "1.3.132.1.11.3";
+      wrap_algo_str = "2.16.840.1.101.3.4.1.45";
+      hash_algo     = GCRY_MD_SHA512;
+      hashlen       = 64;
+      cipher_algo   = GCRY_CIPHER_AES256;
+      keylen        = 32;
+    }
+
+
+  /* Create a secret and an ephemeral key.  */
+  {
+    char *k;
+    k = gcry_random_bytes_secure ((curvebits+7)/8, GCRY_STRONG_RANDOM);
+    if (DBG_CRYPTO)
+      log_printhex (k, (curvebits+7)/8, "ephm. k .:");
+    err = gcry_sexp_build (&s_data, NULL, "%b", (int)(curvebits+7)/8, k);
+    xfree (k);
+  }
+  if (err)
+    {
+      log_error ("%s: error building ephemeral secret: %s\n",
+                 __func__, gpg_strerror (err));
+      goto leave;
+    }
+
+  err = gcry_pk_encrypt (&s_encr, s_data, s_pkey);
+  if (err)
+    {
+      log_error ("%s: error encrypting ephemeral secret: %s\n",
+                 __func__, gpg_strerror (err));
+      goto leave;
+    }
+  err = gcry_sexp_extract_param (s_encr, NULL, "&se",
+                                 &ioarray+0, ioarray+1, NULL);
+  if (err)
+    {
+      log_error ("%s: error extracting ephemeral key and secret: %s\n",
+                 __func__, gpg_strerror (err));
+      goto leave;
+    }
+  secret    = ioarray[0].data;
+  secretlen = ioarray[0].len;
+  pubkey    = ioarray[1].data;
+  pubkeylen = ioarray[1].len;
+
+  if (DBG_CRYPTO)
+    {
+      log_printhex (pubkey, pubkeylen, "pubkey ..:");
+      log_printhex (secret, secretlen, "secret ..:");
+    }
+
+  /* Extract X coordinate from SECRET.  */
+  if (secretlen < 5)  /* 5 because N could be reduced to (n-1)/2.  */
+    err = gpg_error (GPG_ERR_BAD_DATA);
+  else if (*secret == 0x04)
+    {
+      secretlen--;
+      memmove (secret, secret+1, secretlen);
+      if ((secretlen & 1))
+        {
+          err = gpg_error (GPG_ERR_BAD_DATA);
+          goto leave;
+        }
+      secretlen /= 2;
+    }
+  else if (*secret == 0x40 || *secret == 0x41)
+    {
+      secretlen--;
+      memmove (secret, secret+1, secretlen);
+    }
+  else
+    err = gpg_error (GPG_ERR_BAD_DATA);
+  if (err)
+    goto leave;
+
+  if (DBG_CRYPTO)
+    log_printhex (secret, secretlen, "ECDH X ..:");
+
+  /* Derive a KEK (key wrapping key) using MESSAGE and SECRET_X.
+   * According to SEC1 3.6.1 we should check that
+   *   SECRETLEN + UKMLEN + 4 < maxhashlen
+   * However, we have no practical limit on the hash length and thus
+   * there is no point in checking this.  The second check that
+   *   KEYLEN < hashlen*(2^32-1)
+   * is obviously also not needed.  Because with our allowed
+   * parameters KEYLEN is always less or equal to HASHLEN so that we
+   * do not need to iterate at all.
+   */
+  log_assert (gcry_md_get_algo_dlen (hash_algo) == hashlen);
+  {
+    gcry_md_hd_t hash_hd;
+    err = gcry_md_open (&hash_hd, hash_algo, 0);
+    if (err)
+      goto leave;
+    gcry_md_write(hash_hd, secret, secretlen);
+    gcry_md_write(hash_hd, "\x00\x00\x00\x01", 4);  /* counter */
+    err = hash_ecc_cms_shared_info (hash_hd, wrap_algo_str, keylen, NULL, 0);
+    gcry_md_final (hash_hd);
+    log_assert (keylen <= sizeof key && keylen <= hashlen);
+    memcpy (key, gcry_md_read (hash_hd, 0), keylen);
+    gcry_md_close (hash_hd);
+    if (err)
+      goto leave;
+  }
+
+  if (DBG_CRYPTO)
+    log_printhex (key, keylen, "KEK .....:");
+
+  /* Wrap the key.  */
+  if ((dek->keylen % 8) || dek->keylen < 16)
+    {
+      log_error ("%s: can't use a session key of %u bytes\n",
+                 __func__, dek->keylen);
+      err = gpg_error (GPG_ERR_BAD_DATA);
+      goto leave;
+    }
+
+  resultlen = dek->keylen + 8;
+  result = xtrymalloc_secure (resultlen);
+  if (!result)
+    {
+      err = gpg_error_from_syserror ();
+      goto leave;
+    }
+
+  err = gcry_cipher_open (&cipher_hd, cipher_algo, GCRY_CIPHER_MODE_AESWRAP, 0);
+  if (err)
+    {
+      log_error ("%s: failed to initialize AESWRAP: %s\n",
+                 __func__, gpg_strerror (err));
+      goto leave;
+    }
+
+  err = gcry_cipher_setkey (cipher_hd, key, keylen);
+  wipememory (key, sizeof key);
+  if (err)
+    {
+      log_error ("%s: failed in gcry_cipher_setkey: %s\n",
+                 __func__, gpg_strerror (err));
+      goto leave;
+    }
+
+  err = gcry_cipher_encrypt (cipher_hd, result, resultlen,
+                             dek->key, dek->keylen);
+  if (err)
+    {
+      log_error ("%s: failed in gcry_cipher_encrypt: %s\n",
+                 __func__, gpg_strerror (err));
+      goto leave;
+    }
+
+  if (DBG_CRYPTO)
+    log_printhex (result, resultlen, "w(CEK) ..:");
+
+  err = gcry_sexp_build (r_encval, NULL,
+                         "(enc-val(ecdh(e%b)(s%b)(encr-algo%s)(wrap-algo%s)))",
+                         (int)pubkeylen, pubkey,
+                         (int)resultlen, result,
+                         encr_algo_str,
+                         wrap_algo_str,
+                         NULL);
+  if (err)
+    log_error ("%s: failed building final S-exp: %s\n",
+               __func__, gpg_strerror (err));
+
+ leave:
+  gcry_cipher_close (cipher_hd);
+  wipememory (key, sizeof key);
+  xfree (result);
+  xfree (ioarray[0].data);
+  xfree (ioarray[1].data);
+  gcry_sexp_release (s_data);
+  gcry_sexp_release (s_encr);
+  xfree (curvebuf);
+  return err;
+}
+
+
 /* Encrypt the DEK under the key contained in CERT and return it as a
-   canonical S-Exp in encval. */
+ * canonical S-expressions at ENCVAL. PK_ALGO is the public key
+ * algorithm which the caller has already retrieved from CERT.  */
 static int
-encrypt_dek (const DEK dek, ksba_cert_t cert, unsigned char **encval)
+encrypt_dek (const DEK dek, ksba_cert_t cert, int pk_algo,
+             unsigned char **encval)
 {
   gcry_sexp_t s_ciph, s_data, s_pkey;
   int rc;
@@ -198,21 +473,38 @@ encrypt_dek (const DEK dek, ksba_cert_t cert, unsigned char **encval)
       return rc;
     }
 
+  if (DBG_CRYPTO)
+    {
+      log_printsexp (" pubkey:", s_pkey);
+      log_printhex (dek->key, dek->keylen, "CEK .....:");
+    }
+
   /* Put the encoded cleartext into a simple list. */
   s_data = NULL; /* (avoid compiler warning) */
+  if (pk_algo == GCRY_PK_ECC)
+    {
+      rc = ecdh_encrypt (dek, s_pkey, &s_ciph);
+    }
+  else
+    {
       rc = encode_session_key (dek, &s_data);
       if (rc)
         {
-      gcry_sexp_release (s_pkey);
           log_error ("encode_session_key failed: %s\n", gpg_strerror (rc));
           return rc;
         }
+      if (DBG_CRYPTO)
+        log_printsexp ("   data:", s_data);
 
       /* pass it to libgcrypt */
       rc = gcry_pk_encrypt (&s_ciph, s_data, s_pkey);
+    }
   gcry_sexp_release (s_data);
   gcry_sexp_release (s_pkey);
 
+  if (DBG_CRYPTO)
+    log_printsexp ("enc-val:", s_ciph);
+
   /* Reformat it. */
   if (!rc)
     {
@@ -387,7 +679,7 @@ gpgsm_encrypt (ctrl_t ctrl, certlist_t recplist, int data_fd, estream_t out_fp)
   err = ksba_cms_set_reader_writer (cms, reader, writer);
   if (err)
     {
-      log_debug ("ksba_cms_set_reader_writer failed: %s\n",
+      log_error ("ksba_cms_set_reader_writer failed: %s\n",
                  gpg_strerror (err));
       rc = err;
       goto leave;
@@ -402,7 +694,7 @@ gpgsm_encrypt (ctrl_t ctrl, certlist_t recplist, int data_fd, estream_t out_fp)
     err = ksba_cms_set_content_type (cms, 1, KSBA_CT_DATA);
   if (err)
     {
-      log_debug ("ksba_cms_set_content_type failed: %s\n",
+      log_error ("ksba_cms_set_content_type failed: %s\n",
                  gpg_strerror (err));
       rc = err;
       goto leave;
@@ -500,7 +792,7 @@ gpgsm_encrypt (ctrl_t ctrl, certlist_t recplist, int data_fd, estream_t out_fp)
           && !gnupg_pk_is_compliant (CO_DE_VS, pk_algo, 0, NULL, nbits, NULL))
         compliant = 0;
 
-      rc = encrypt_dek (dek, cl->cert, &encval);
+      rc = encrypt_dek (dek, cl->cert, pk_algo, &encval);
       if (rc)
         {
           audit_log_cert (ctrl->audit, AUDIT_ENCRYPTED_TO, cl->cert, rc);

sm/gpgsm.h

@@ -390,6 +390,10 @@ int gpgsm_encrypt (ctrl_t ctrl, certlist_t recplist,
                    int in_fd, estream_t out_fp);
 
 /*-- decrypt.c --*/
+gpg_error_t hash_ecc_cms_shared_info (gcry_md_hd_t hash_hd,
+                                      const char *wrap_algo_str,
+                                      unsigned int keylen,
+                                      const void *ukm, unsigned int ukmlen);
 int gpgsm_decrypt (ctrl_t ctrl, int in_fd, estream_t out_fp);
 
 /*-- certreqgen.c --*/