PK: add nice interface functions

Also fix a const-corectness issue.
diff --git a/library/error.c b/library/error.c
index 23f4a85..333a8f8 100644
--- a/library/error.c
+++ b/library/error.c
@@ -373,6 +373,8 @@
             snprintf( buf, buflen, "SSL - Processing of the NewSessionTicket handshake message failed" );
         if( use_ret == -(POLARSSL_ERR_SSL_SESSION_TICKET_EXPIRED) )
             snprintf( buf, buflen, "SSL - Session ticket has expired" );
+        if( use_ret == -(POLARSSL_ERR_SSL_PK_TYPE_MISMATCH) )
+            snprintf( buf, buflen, "SSL - Public key type mismatch (eg, asked for RSA key exchange and presented EC key)" );
 #endif /* POLARSSL_SSL_TLS_C */
 
 #if defined(POLARSSL_X509_PARSE_C)
diff --git a/library/pk.c b/library/pk.c
index 9f33641..ce3b88a 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -124,3 +124,39 @@
 
     return( 0 );
 }
+
+/*
+ * Tell if a PK can do the operations of the given type
+ */
+int pk_can_do( pk_context *ctx, pk_type_t type )
+{
+    /* null of NONE context can't do anything */
+    if( ctx == NULL || ctx->info == NULL )
+        return( 0 );
+
+    return( ctx->info->can_do( type ) );
+}
+
+/*
+ * Verify a signature
+ */
+int pk_verify( pk_context *ctx,
+               const unsigned char *hash, const md_info_t *md_info,
+               const unsigned char *sig, size_t sig_len )
+{
+    if( ctx == NULL || ctx->info == NULL )
+        return( POLARSSL_ERR_PK_TYPE_MISMATCH ); // TODO
+
+    return( ctx->info->verify_func( ctx->data, hash, md_info, sig, sig_len ) );
+}
+
+/*
+ * Get key size in bits
+ */
+size_t pk_get_size( const pk_context *ctx )
+{
+    if( ctx == NULL || ctx->info == NULL )
+        return( 0 );
+
+    return( ctx->info->get_size( ctx->data ) );
+}
diff --git a/library/pk_wrap.c b/library/pk_wrap.c
index 50e8db5..239ff78 100644
--- a/library/pk_wrap.c
+++ b/library/pk_wrap.c
@@ -53,9 +53,9 @@
     return( type == POLARSSL_PK_RSA );
 }
 
-static size_t rsa_get_size( void * ctx )
+static size_t rsa_get_size( const void * ctx )
 {
-    return( mpi_size( &((rsa_context *) ctx)->N ) * 8 );
+    return( 8 * ((rsa_context *) ctx)->len );
 }
 
 static int rsa_verify_wrap( void *ctx,
@@ -101,7 +101,7 @@
     return( type == POLARSSL_PK_ECDSA );
 }
 
-static size_t ecdsa_get_size( void *ctx )
+static size_t ecdsa_get_size( const void *ctx )
 {
     return( ((ecdsa_context *) ctx)->grp.pbits );
 }
@@ -152,7 +152,7 @@
             type == POLARSSL_PK_ECDSA );
 }
 
-static size_t eckey_get_size( void *ctx )
+static size_t eckey_get_size( const void *ctx )
 {
     return( ((ecp_keypair *) ctx)->grp.pbits );
 }
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 6674348..1c2c395 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -1346,12 +1346,15 @@
             return( POLARSSL_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE );
         }
 
-        /* EC NOT IMPLEMENTED YET */
-        if( ssl->session_negotiate->peer_cert->pk.type != POLARSSL_PK_RSA )
-            return( POLARSSL_ERR_SSL_FEATURE_UNAVAILABLE );
+        if( ! pk_can_do( &ssl->session_negotiate->peer_cert->pk,
+                         POLARSSL_PK_RSA ) )
+        {
+            SSL_DEBUG_MSG( 1, ( "bad server key exchange message" ) );
+            return( POLARSSL_ERR_SSL_PK_TYPE_MISMATCH );
+        }
 
-        if( (unsigned int)( end - p ) !=
-            pk_rsa( ssl->session_negotiate->peer_cert->pk )->len )
+        if( 8 * (unsigned int)( end - p ) !=
+            pk_get_size( &ssl->session_negotiate->peer_cert->pk ) )
         {
             SSL_DEBUG_MSG( 1, ( "bad server key exchange message" ) );
             return( POLARSSL_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE );
@@ -1795,12 +1798,15 @@
         if( ret != 0 )
             return( ret );
 
-        /* EC NOT IMPLEMENTED YET */
-        if( ssl->session_negotiate->peer_cert->pk.type != POLARSSL_PK_RSA )
-            return( POLARSSL_ERR_SSL_FEATURE_UNAVAILABLE );
+        if( ! pk_can_do( &ssl->session_negotiate->peer_cert->pk,
+                         POLARSSL_PK_RSA ) )
+        {
+            SSL_DEBUG_MSG( 1, ( "certificate key type mismatch" ) );
+            return( POLARSSL_ERR_SSL_PK_TYPE_MISMATCH );
+        }
 
         i = 4;
-        n = pk_rsa( ssl->session_negotiate->peer_cert->pk )->len;
+        n = pk_get_size( &ssl->session_negotiate->peer_cert->pk ) / 8;
 
         if( ssl->minor_ver != SSL_MINOR_VERSION_0 )
         {
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 36c4f2f..0780da5 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -2517,10 +2517,13 @@
     }
 
     /* EC NOT IMPLEMENTED YET */
-    if( ssl->session_negotiate->peer_cert->pk.type != POLARSSL_PK_RSA )
+    if( ! pk_can_do( &ssl->session_negotiate->peer_cert->pk,
+                     POLARSSL_PK_RSA ) )
+    {
         return( POLARSSL_ERR_SSL_FEATURE_UNAVAILABLE );
+    }
 
-    n1 = pk_rsa( ssl->session_negotiate->peer_cert->pk )->len;
+    n1 = pk_get_size( &ssl->session_negotiate->peer_cert->pk ) / 8;
     n2 = ( ssl->in_msg[4 + n] << 8 ) | ssl->in_msg[5 + n];
 
     if( n + n1 + 6 != ssl->in_hslen || n1 != n2 )
diff --git a/library/x509parse.c b/library/x509parse.c
index a8fcc0b..225f45d 100644
--- a/library/x509parse.c
+++ b/library/x509parse.c
@@ -3147,7 +3147,7 @@
     }
 
     ret = snprintf( p, n, "\n%s%-" BC "s: %d bits\n", prefix, key_size_str,
-                          (int) crt->pk.info->get_size( crt->pk.data ) );
+                          (int) pk_get_size( &crt->pk ) );
     SAFE_SNPRINTF();
 
     return( (int) ( size - n ) );
@@ -3399,9 +3399,9 @@
 
         md( md_info, crl_list->tbs.p, crl_list->tbs.len, hash );
 
-        if( ca->pk.info->can_do( crl_list->sig_pk ) == 0 ||
-                ca->pk.info->verify_func( ca->pk.data, hash, md_info,
-                                    crl_list->sig.p, crl_list->sig.len ) != 0 )
+        if( pk_can_do( &ca->pk, crl_list->sig_pk ) == 0 ||
+            pk_verify( &ca->pk, hash, md_info,
+                       crl_list->sig.p, crl_list->sig.len ) != 0 )
         {
             flags |= BADCRL_NOT_TRUSTED;
             break;
@@ -3516,9 +3516,9 @@
 
         md( md_info, child->tbs.p, child->tbs.len, hash );
 
-        if( trust_ca->pk.info->can_do( child->sig_pk ) == 0 ||
-            trust_ca->pk.info->verify_func( trust_ca->pk.data, hash, md_info,
-                                        child->sig.p, child->sig.len ) != 0 )
+        if( pk_can_do( &trust_ca->pk, child->sig_pk ) == 0 ||
+            pk_verify( &trust_ca->pk, hash, md_info,
+                       child->sig.p, child->sig.len ) != 0 )
         {
             trust_ca = trust_ca->next;
             continue;
@@ -3593,9 +3593,9 @@
     {
         md( md_info, child->tbs.p, child->tbs.len, hash );
 
-        if( parent->pk.info->can_do( child->sig_pk ) == 0 ||
-                parent->pk.info->verify_func( parent->pk.data, hash, md_info,
-                                        child->sig.p, child->sig.len ) != 0 )
+        if( pk_can_do( &parent->pk, child->sig_pk ) == 0 ||
+            pk_verify( &parent->pk, hash, md_info,
+                       child->sig.p, child->sig.len ) != 0 )
         {
             *flags |= BADCERT_NOT_TRUSTED;
         }