Use in-place decryption in pk_parse_pkcs8_encrypted_der

The stack buffer used to hold the decrypted key in pk_parse_pkcs8_encrypted_der
was statically sized to 2048 bytes, which is not enough for DER encoded 4096bit
RSA keys.

This commit resolves the problem by performing the key-decryption in-place,
circumventing the introduction of another stack or heap copy of the key.

There are two situations where pk_parse_pkcs8_encrypted_der is invoked:
1. When processing a PEM-encoded encrypted key in pk_parse_key.
   This does not need adaption since the PEM context used to hold the decoded
   key is already constructed and owned by pk_parse_key.
2. When processing a DER-encoded encrypted key in pk_parse_key.
   In this case, pk_parse_key calls pk_parse_pkcs8_encrypted_der with
   the buffer provided by the user, which is declared const. The commit
   therefore adds a small code paths making a copy of the keybuffer before
   calling pk_parse_pkcs8_encrypted_der.

library/pkparse.c

@@ -930,12 +930,12 @@ static int pk_parse_key_pkcs8_unencrypted_der(
 #if defined(POLARSSL_PKCS12_C) || defined(POLARSSL_PKCS5_C)
 static int pk_parse_key_pkcs8_encrypted_der(
                                     pk_context *pk,
-                                    const unsigned char *key, size_t keylen,
+                                    unsigned char *key, size_t keylen,
                                     const unsigned char *pwd, size_t pwdlen )
 {
     int ret, decrypted = 0;
     size_t len;
-    unsigned char buf[2048];
+    unsigned char *buf;
     unsigned char *p, *end;
     asn1_buf pbe_alg_oid, pbe_params;
 #if defined(POLARSSL_PKCS12_C)
@@ -943,8 +943,6 @@ static int pk_parse_key_pkcs8_encrypted_der(
     md_type_t md_alg;
 #endif
 
-    memset( buf, 0, sizeof( buf ) );
-
     p = (unsigned char *) key;
     end = p + keylen;
 
@@ -979,8 +977,7 @@ static int pk_parse_key_pkcs8_encrypted_der(
     if( ( ret = asn1_get_tag( &p, end, &len, ASN1_OCTET_STRING ) ) != 0 )
         return( POLARSSL_ERR_PK_KEY_INVALID_FORMAT + ret );
 
-    if( len > sizeof( buf ) )
+    buf = p;
-        return( POLARSSL_ERR_PK_BAD_INPUT_DATA );
 
     /*
      * Decrypt EncryptedData with appropriate PDE
@@ -1073,7 +1070,7 @@ int pk_parse_key( pk_context *pk,
         if( ( pk_info = pk_info_from_type( POLARSSL_PK_RSA ) ) == NULL )
             return( POLARSSL_ERR_PK_UNKNOWN_PK_ALG );
 
-        if( ( ret = pk_init_ctx( pk, pk_info                    ) ) != 0 ||
+        if( ( ret = pk_init_ctx( pk, pk_info ) ) != 0 ||
             ( ret = pk_parse_key_pkcs1_der( pk_rsa( *pk ),
                                             pem.buf, pem.buflen ) ) != 0 )
         {
@@ -1101,7 +1098,7 @@ int pk_parse_key( pk_context *pk,
         if( ( pk_info = pk_info_from_type( POLARSSL_PK_ECKEY ) ) == NULL )
             return( POLARSSL_ERR_PK_UNKNOWN_PK_ALG );
 
-        if( ( ret = pk_init_ctx( pk, pk_info                   ) ) != 0 ||
+        if( ( ret = pk_init_ctx( pk, pk_info ) ) != 0 ||
             ( ret = pk_parse_key_sec1_der( pk_ec( *pk ),
                                            pem.buf, pem.buflen ) ) != 0 )
         {
@@ -1170,12 +1167,24 @@ int pk_parse_key( pk_context *pk,
     * error
     */
 #if defined(POLARSSL_PKCS12_C) || defined(POLARSSL_PKCS5_C)
-    if( ( ret = pk_parse_key_pkcs8_encrypted_der( pk, key, keylen,
-                                                  pwd, pwdlen ) ) == 0 )
     {
-        return( 0 );
+        unsigned char *key_copy;
+
+        if( ( key_copy = polarssl_malloc( keylen ) ) == NULL )
+            return( POLARSSL_ERR_PK_MALLOC_FAILED );
+
+        memcpy( key_copy, key, keylen );
+
+        ret = pk_parse_key_pkcs8_encrypted_der( pk, key_copy, keylen,
+                                                pwd, pwdlen );
+
+        polarssl_zeroize( key_copy, keylen );
+        polarssl_free( key_copy );
     }
 
+    if( ret == 0 )
+        return( 0 );
+
     pk_free( pk );
 
     if( ret == POLARSSL_ERR_PK_PASSWORD_MISMATCH )
@@ -1193,7 +1202,7 @@ int pk_parse_key( pk_context *pk,
     if( ( pk_info = pk_info_from_type( POLARSSL_PK_RSA ) ) == NULL )
         return( POLARSSL_ERR_PK_UNKNOWN_PK_ALG );
 
-    if( ( ret = pk_init_ctx( pk, pk_info                           ) ) != 0 ||
+    if( ( ret = pk_init_ctx( pk, pk_info ) ) != 0 ||
         ( ret = pk_parse_key_pkcs1_der( pk_rsa( *pk ), key, keylen ) ) == 0 )
     {
         return( 0 );
@@ -1206,7 +1215,7 @@ int pk_parse_key( pk_context *pk,
     if( ( pk_info = pk_info_from_type( POLARSSL_PK_ECKEY ) ) == NULL )
         return( POLARSSL_ERR_PK_UNKNOWN_PK_ALG );
 
-    if( ( ret = pk_init_ctx( pk, pk_info                         ) ) != 0 ||
+    if( ( ret = pk_init_ctx( pk, pk_info ) ) != 0 ||
         ( ret = pk_parse_key_sec1_der( pk_ec( *pk ), key, keylen ) ) == 0 )
     {
         return( 0 );