ECDSA: Add mbedtls_raw_ecdsa_signature_to_asn1 without MPI usage

Refactor mbedtls_ecdsa_signature_to_raw to allow overlapping buffers

include/mbedtls/ecdsa.h

@@ -280,6 +280,29 @@ int mbedtls_ecdsa_signature_to_asn1( const mbedtls_mpi *r,
                              const mbedtls_mpi *s, unsigned char *sig,
                              size_t *slen, size_t ssize );
 
+/**
+ * \brief           Convert a signature from a raw representation to ASN.1
+ *
+ * \param r         First number of the signature
+ * \param s         Second number of the signature
+ * \param num_len   Length of each number in bytes
+ * \param sig       Buffer that will hold the signature
+ * \param slen      Length of the signature written
+ * \param ssize     Size of the sig buffer
+ *
+ * \note            The size of the buffer \c ssize should be at least
+ *                  `MBEDTLS_ECDSA_MAX_SIG_LEN(grp->pbits)` bytes long if
+ *                  the signature was produced from curve \c grp,
+ *                  otherwise this function will return an error.
+ *
+ * \return          0 if successful,
+ *                  or a MBEDTLS_ERR_MPI_XXX or MBEDTLS_ERR_ASN1_XXX error code
+ *
+ */
+int mbedtls_raw_ecdsa_signature_to_asn1(const unsigned char *r,
+                             const unsigned char *s, uint16_t num_len,
+                             unsigned char *sig, size_t *slen, size_t ssize );
+
 /**
  * \brief           Read and verify an ECDSA signature
  *

library/ecdsa.c

@@ -298,7 +298,7 @@ int mbedtls_ecdsa_signature_to_raw( const unsigned char *sig,
     unsigned char *p = (unsigned char *) sig;
     unsigned char *buf_ptr;
     const unsigned char *end = sig + ssize;
-    size_t len, bytes_skipped, i;
+    size_t len, bytes_skipped;
 
     if( 2 * byte_len > bufsize )
     {
@@ -335,10 +335,7 @@ int mbedtls_ecdsa_signature_to_raw( const unsigned char *sig,
     }
     *buflen = len - bytes_skipped;
 
-    for( i = bytes_skipped; i < len; i++ )
+    memmove(buf_ptr, &p[bytes_skipped], *buflen);
-    {
-        buf_ptr[i - bytes_skipped] = p[i];
-    }
     p += len;
     buf_ptr += *buflen;
 
@@ -358,11 +355,7 @@ int mbedtls_ecdsa_signature_to_raw( const unsigned char *sig,
     }
 
     *buflen += len - bytes_skipped;
-
+    memmove(buf_ptr, &p[bytes_skipped], len - bytes_skipped);
-    for( i = bytes_skipped; i < len; i++ )
-    {
-        buf_ptr[i - bytes_skipped] = p[i];
-    }
 
     return( ret );
 }
@@ -388,6 +381,76 @@ int mbedtls_ecdsa_signature_to_asn1( const mbedtls_mpi *r, const mbedtls_mpi *s,
     memset( sig + len, 0, ssize - len );
     *slen = len;
 
+    return( 0 );
+ }
+
+int mbedtls_raw_ecdsa_signature_to_asn1( const unsigned char *r,
+                                 const unsigned char *s, uint16_t num_len,
+                                 unsigned char *sig, size_t *slen, size_t ssize )
+{
+    int ret;
+    unsigned char *p = sig + ssize;
+    size_t total_len = 0;
+    size_t padding_len = 0;
+
+    /*
+     * Step 1: write S
+     */
+    memmove( p - num_len, s, num_len );
+    p -= num_len;
+    total_len += num_len;
+    if( *p & 0x80 )
+    {
+        if( p - sig < 1 )
+            return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--p = 0x00;
+        padding_len += 1;
+    }
+    total_len += padding_len;
+
+    MBEDTLS_ASN1_CHK_ADD( total_len, mbedtls_asn1_write_len( &p, sig,
+                                             num_len + padding_len ) );
+    MBEDTLS_ASN1_CHK_ADD( total_len, mbedtls_asn1_write_tag( &p, sig,
+                                             MBEDTLS_ASN1_INTEGER ) );
+
+    padding_len = 0;
+
+    /*
+     * Step 2: write R
+     */
+    memmove( p - num_len, r, num_len );
+    p -= num_len;
+    total_len += num_len;
+    if( *p & 0x80 )
+    {
+        if( p - sig < 1 )
+            return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--p = 0x00;
+        padding_len += 1;
+    }
+    total_len += padding_len;
+
+    MBEDTLS_ASN1_CHK_ADD( total_len, mbedtls_asn1_write_len( &p, sig,
+                                             num_len + padding_len ) );
+    MBEDTLS_ASN1_CHK_ADD( total_len, mbedtls_asn1_write_tag( &p, sig,
+                                             MBEDTLS_ASN1_INTEGER ) );
+
+    /*
+    * Step 3: write rest of the data
+    */
+    MBEDTLS_ASN1_CHK_ADD( total_len, mbedtls_asn1_write_len( &p, sig, total_len ) );
+    MBEDTLS_ASN1_CHK_ADD( total_len, mbedtls_asn1_write_tag( &p, sig,
+                                             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) );
+
+    /*
+    * Step 4: move to the beginning of the buffer, zeroize the rest
+    */
+    memmove( sig, p, total_len );
+    memset( sig + total_len, 0, ssize - total_len );
+    *slen = total_len;
+
     return( 0 );
 }
 

library/pkcs11_client.c

@@ -191,9 +191,7 @@ static int pkcs11_sign( void *ctx_arg,
          */
         uint16_t byte_len = ( ( ctx->bit_length + 7 ) / 8 );
         size_t sig_size = MBEDTLS_ECDSA_MAX_SIG_LEN( ctx->bit_length );
-        mbedtls_mpi r, s;
+
-        mbedtls_mpi_init( &r );
-        mbedtls_mpi_init( &s );
         rv = CKR_OK;
         if( ck_sig_len != 2 * byte_len )
         {
@@ -201,22 +199,15 @@ static int pkcs11_sign( void *ctx_arg,
             rv = CKR_GENERAL_ERROR;
             goto ecdsa_exit;
         }
-        if( mbedtls_mpi_read_binary( &r, sig, byte_len ) != 0 ||
+
-            mbedtls_mpi_read_binary( &s, sig + byte_len, byte_len ) != 0 )
-        {
-            rv = CKR_HOST_MEMORY;
-            goto ecdsa_exit;
-        }
         /* The signature buffer is guaranteed to have enough room for
            the encoded signature by the pk_sign interface. */
-        if( mbedtls_ecdsa_signature_to_asn1( &r, &s, sig, sig_len, sig_size ) != 0 )
+        if( mbedtls_raw_ecdsa_signature_to_asn1( sig, sig + byte_len, byte_len, sig, sig_len, sig_size ) != 0 )
         {
             rv = CKR_GENERAL_ERROR;
             goto ecdsa_exit;
         }
     ecdsa_exit:
-        mbedtls_mpi_free( &r );
-        mbedtls_mpi_free( &s );
         if( rv != CKR_OK )
             goto exit;
     }