From 3cd9dac7e0976643c6e4b6537cf363b2b12d422f Mon Sep 17 00:00:00 2001
From: Werner Koch <wk@gnupg.org>
Date: Tue, 19 May 2020 14:28:40 +0200
Subject: [PATCH] common: New function to extract Q from an ECC key.

* common/sexputil.c (get_ecc_q_from_canon_sexp): New.

Signed-off-by: Werner Koch <wk@gnupg.org>
---
 common/sexputil.c | 90 ++++++++++++++++++++++++++++++++++++++++++++++-
 common/util.h     |  4 +++
 2 files changed, 93 insertions(+), 1 deletion(-)

diff --git a/common/sexputil.c b/common/sexputil.c
index 7a67f2b7c..b007b71f6 100644
--- a/common/sexputil.c
+++ b/common/sexputil.c
@@ -199,7 +199,7 @@ make_canon_sexp_pad (gcry_sexp_t sexp, int secure,
 }
 
 /* Return the so called "keygrip" which is the SHA-1 hash of the
-   public key parameters expressed in a way depended on the algorithm.
+   public key parameters expressed in a way dependend on the algorithm.
 
    KEY is expected to be an canonical encoded S-expression with a
    public or private key. KEYLEN is the length of that buffer.
@@ -512,6 +512,94 @@ get_rsa_pk_from_canon_sexp (const unsigned char *keydata, size_t keydatalen,
 }
 
 
+/* Return the public key parameter Q of a public RSA or ECC key
+ * expressed as an canonical encoded S-expression.  */
+gpg_error_t
+get_ecc_q_from_canon_sexp (const unsigned char *keydata, size_t keydatalen,
+                           unsigned char const **r_q, size_t *r_qlen)
+{
+  gpg_error_t err;
+  const unsigned char *buf, *tok;
+  size_t buflen, toklen;
+  int depth, last_depth1, last_depth2;
+  const unsigned char *ecc_q = NULL;
+  size_t ecc_q_len;
+
+  *r_q = NULL;
+  *r_qlen = 0;
+
+  buf = keydata;
+  buflen = keydatalen;
+  depth = 0;
+  if ((err = parse_sexp (&buf, &buflen, &depth, &tok, &toklen)))
+    return err;
+  if ((err = parse_sexp (&buf, &buflen, &depth, &tok, &toklen)))
+    return err;
+  if (!tok || toklen != 10 || memcmp ("public-key", tok, toklen))
+    return gpg_error (GPG_ERR_BAD_PUBKEY);
+  if ((err = parse_sexp (&buf, &buflen, &depth, &tok, &toklen)))
+    return err;
+  if ((err = parse_sexp (&buf, &buflen, &depth, &tok, &toklen)))
+    return err;
+  if (tok && toklen == 3 && !memcmp ("ecc", tok, toklen))
+    ;
+  else if (tok && toklen == 5 && (!memcmp ("ecdsa", tok, toklen)
+                                  || !memcmp ("eddsa", tok, toklen)))
+    ;
+  else
+    return gpg_error (GPG_ERR_WRONG_PUBKEY_ALGO);
+
+  last_depth1 = depth;
+  while (!(err = parse_sexp (&buf, &buflen, &depth, &tok, &toklen))
+         && depth && depth >= last_depth1)
+    {
+      if (tok)
+        return gpg_error (GPG_ERR_UNKNOWN_SEXP);
+      if ((err = parse_sexp (&buf, &buflen, &depth, &tok, &toklen)))
+        return err;
+      if (tok && toklen == 1)
+        {
+          const unsigned char **mpi;
+          size_t *mpi_len;
+
+          switch (*tok)
+            {
+            case 'q': mpi = &ecc_q; mpi_len = &ecc_q_len; break;
+            default:  mpi = NULL;   mpi_len = NULL; break;
+            }
+          if (mpi && *mpi)
+            return gpg_error (GPG_ERR_DUP_VALUE);
+
+          if ((err = parse_sexp (&buf, &buflen, &depth, &tok, &toklen)))
+            return err;
+          if (tok && mpi)
+            {
+              *mpi = tok;
+              *mpi_len = toklen;
+            }
+        }
+
+      /* Skip to the end of the list. */
+      last_depth2 = depth;
+      while (!(err = parse_sexp (&buf, &buflen, &depth, &tok, &toklen))
+             && depth && depth >= last_depth2)
+        ;
+      if (err)
+        return err;
+    }
+
+  if (err)
+    return err;
+
+  if (!ecc_q || !ecc_q_len)
+    return gpg_error (GPG_ERR_BAD_PUBKEY);
+
+  *r_q = ecc_q;
+  *r_qlen = ecc_q_len;
+  return 0;
+}
+
+
 /* Return the algo of a public KEY of SEXP. */
 int
 get_pk_algo_from_key (gcry_sexp_t key)
diff --git a/common/util.h b/common/util.h
index b5bdd0385..834438448 100644
--- a/common/util.h
+++ b/common/util.h
@@ -197,6 +197,10 @@ gpg_error_t get_rsa_pk_from_canon_sexp (const unsigned char *keydata,
                                         size_t *r_nlen,
                                         unsigned char const **r_e,
                                         size_t *r_elen);
+gpg_error_t get_ecc_q_from_canon_sexp (const unsigned char *keydata,
+                                       size_t keydatalen,
+                                       unsigned char const **r_q,
+                                       size_t *r_qlen);
 
 int get_pk_algo_from_key (gcry_sexp_t key);
 int get_pk_algo_from_canon_sexp (const unsigned char *keydata,