From b325887fad0568b5b5b9c4f43e5a5c0fcaae32c2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Manuel=20P=C3=A9gouri=C3=A9-Gonnard?= <mpg@elzevir.fr>
Date: Sun, 10 Feb 2013 12:06:19 +0100
Subject: [PATCH] Add ecp_tls_write_group()

---
 include/polarssl/ecp.h | 22 +++++++++++++++++++++-
 library/ecp.c          | 41 +++++++++++++++++++++++++++++++++++------
 2 files changed, 56 insertions(+), 7 deletions(-)

diff --git a/include/polarssl/ecp.h b/include/polarssl/ecp.h
index a810a62dc..0919cbe1c 100644
--- a/include/polarssl/ecp.h
+++ b/include/polarssl/ecp.h
@@ -53,6 +53,11 @@ typedef struct
 }
 ecp_point;
 
+/*
+ * RFC 4492 defines an enum NamedCurve with two-bytes values
+ */
+typedef uint16_t ecp_group_id;
+
 /**
  * \brief           ECP group structure
  *
@@ -70,6 +75,7 @@ ecp_point;
  */
 typedef struct
 {
+    ecp_group_id id;    /*!<  RFC 4492 group ID                 */
     mpi P;              /*!<  prime modulus of the base field   */
     mpi B;              /*!<  constant term in the equation     */
     ecp_point G;        /*!<  generator of the subgroup used    */
@@ -284,7 +290,7 @@ int ecp_point_read_binary( const ecp_group *grp, ecp_point *P,
  * \note            Index should be a value of RFC 4492's enum NamdeCurve,
  *                  possibly in the form of a POLARSSL_ECP_DP_XXX macro.
  */
-int ecp_use_known_dp( ecp_group *grp, uint16_t index );
+int ecp_use_known_dp( ecp_group *grp, ecp_group_id id );
 
 /**
  * \brief           Set a group from a TLS ECParameters record
@@ -299,6 +305,20 @@ int ecp_use_known_dp( ecp_group *grp, uint16_t index );
  */
 int ecp_tls_read_group( ecp_group *grp, const unsigned char *buf, size_t len );
 
+/**
+ * \brief           Write the TLS ECParameters record for a group
+ *
+ * \param grp       ECP group used
+ * \param olen      Number of bytes actually written
+ * \param buf       Buffer to write to
+ * \param blen      Buffer length
+ *
+ * \return          0 if successful,
+ *                  or POLARSSL_ERR_ECP_BUFFER_TOO_SMALL
+ */
+int ecp_tls_write_group( const ecp_group *grp, size_t *olen,
+                         unsigned char *buf, size_t blen );
+
 /**
  * \brief           Import a point from a TLS ECPoint record
  *
diff --git a/library/ecp.c b/library/ecp.c
index 5520d7393..41fc9dbac 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -534,9 +534,11 @@ cleanup:
 /*
  * Set a group using well-known domain parameters
  */
-int ecp_use_known_dp( ecp_group *grp, uint16_t index )
+int ecp_use_known_dp( ecp_group *grp, ecp_group_id id )
 {
-    switch( index )
+    grp->id = id;
+
+    switch( id )
     {
         case POLARSSL_ECP_DP_SECP192R1:
             grp->modp = ecp_mod_p192;
@@ -574,7 +576,7 @@ int ecp_use_known_dp( ecp_group *grp, uint16_t index )
  */
 int ecp_tls_read_group( ecp_group *grp, const unsigned char *buf, size_t len )
 {
-    uint16_t namedcurve;
+    ecp_group_id id;
 
     /*
      * We expect at least three bytes (see below)
@@ -589,10 +591,37 @@ int ecp_tls_read_group( ecp_group *grp, const unsigned char *buf, size_t len )
         return( POLARSSL_ERR_ECP_BAD_INPUT_DATA );
 
     /*
-     * Next two bytes are the namedcurve
+     * Next two bytes are the namedcurve value
      */
-    namedcurve = 256 * buf[0] + buf[1];
-    return ecp_use_known_dp( grp, namedcurve );
+    id = 256 * buf[0] + buf[1];
+    return ecp_use_known_dp( grp, id );
+}
+
+/*
+ * Write the ECParameters record corresponding to a group (RFC 4492)
+ */
+int ecp_tls_write_group( const ecp_group *grp, size_t *olen,
+                         unsigned char *buf, size_t blen )
+{
+    /*
+     * We are going to write 3 bytes (see below)
+     */
+    *olen = 3;
+    if( blen < *olen )
+        return( POLARSSL_ERR_ECP_BUFFER_TOO_SMALL );
+
+    /*
+     * First byte is curve_type, always named_curve
+     */
+    *buf++ = POLARSSL_ECP_TLS_NAMED_CURVE;
+
+    /*
+     * Next two bytes are the namedcurve value
+     */
+    buf[0] = grp->id >> 8;
+    buf[1] = grp->id && 0xFF;
+
+    return 0;
 }
 
 /*