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.
diff --git a/library/pkwrite.c b/library/pkwrite.c
index 8eabd88..fa93470 100644
--- a/library/pkwrite.c
+++ b/library/pkwrite.c
@@ -37,6 +37,7 @@
#include "mbedtls/rsa.h"
#endif
#if defined(MBEDTLS_ECP_C)
+#include "mbedtls/bignum.h"
#include "mbedtls/ecp.h"
#endif
#if defined(MBEDTLS_ECDSA_C)
@@ -54,6 +55,13 @@
#define mbedtls_free free
#endif
+#if defined(MBEDTLS_ECP_C)
+/* Implementation that should never be optimized out by the compiler */
+static void mbedtls_zeroize( void *v, size_t n ) {
+ volatile unsigned char *p = (unsigned char*)v; while( n-- ) *p++ = 0;
+}
+#endif /* MBEDTLS_ECP_C */
+
#if defined(MBEDTLS_RSA_C)
/*
* RSAPublicKey ::= SEQUENCE {
@@ -143,6 +151,26 @@
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_zeroize( tmp, byte_length );
+ return( ret );
+}
#endif /* MBEDTLS_ECP_C */
int mbedtls_pk_write_pubkey( unsigned char **p, unsigned char *start,
@@ -340,9 +368,8 @@
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 ) );