From a7cfdad82e5bb9e94fc001bab2f6a71b8f49234f Mon Sep 17 00:00:00 2001
From: Gilles Peskine <Gilles.Peskine@arm.com>
Date: Sat, 11 Aug 2018 00:48:44 +0200
Subject: [PATCH] Fix pk_write with an EC key to write a constant-length
 private value

When writing a private EC key, use a constant size for the private
value, as specified in RFC 5915. Previously, the value was written
as an ASN.1 INTEGER, which caused the size of the key to leak
about 1 bit of information on average, and could cause the value to be
1 byte too large for the output buffer.
---
 library/pkwrite.c | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/library/pkwrite.c b/library/pkwrite.c
index 8eabd889b..4ae38af0a 100644
--- a/library/pkwrite.c
+++ b/library/pkwrite.c
@@ -37,7 +37,9 @@
 #include "mbedtls/rsa.h"
 #endif
 #if defined(MBEDTLS_ECP_C)
+#include "mbedtls/bignum.h"
 #include "mbedtls/ecp.h"
+#include "mbedtls/platform_util.h"
 #endif
 #if defined(MBEDTLS_ECDSA_C)
 #include "mbedtls/ecdsa.h"
@@ -143,6 +145,26 @@ static int pk_write_ec_param( unsigned char **p, unsigned char *start,
 
     return( (int) len );
 }
+
+/*
+ * privateKey  OCTET STRING -- always of length ceil(log2(n)/8)
+ */
+static int pk_write_ec_private( unsigned char **p, unsigned char *start,
+                                mbedtls_ecp_keypair *ec )
+{
+    int ret;
+    size_t byte_length = ( ec->grp.pbits + 7 ) / 8;
+    unsigned char tmp[MBEDTLS_ECP_MAX_BYTES];
+
+    ret = mbedtls_mpi_write_binary( &ec->d, tmp, byte_length );
+    if( ret != 0 )
+        goto exit;
+    ret = mbedtls_asn1_write_octet_string( p, start, tmp, byte_length );
+
+exit:
+    mbedtls_platform_zeroize( tmp, byte_length );
+    return( ret );
+}
 #endif /* MBEDTLS_ECP_C */
 
 int mbedtls_pk_write_pubkey( unsigned char **p, unsigned char *start,
@@ -340,9 +362,8 @@ int mbedtls_pk_write_key_der( mbedtls_pk_context *key, unsigned char *buf, size_
                             MBEDTLS_ASN1_CONTEXT_SPECIFIC | MBEDTLS_ASN1_CONSTRUCTED | 0 ) );
         len += par_len;
 
-        /* privateKey: write as MPI then fix tag */
-        MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_mpi( &c, buf, &ec->d ) );
-        *c = MBEDTLS_ASN1_OCTET_STRING;
+        /* privateKey */
+        MBEDTLS_ASN1_CHK_ADD( len, pk_write_ec_private( &c, buf, ec ) );
 
         /* version */
         MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_int( &c, buf, 1 ) );