Add mbedtls_asn1_write_len() support for 3 and 4 byte lengths

As a consequence also adds coverage for reading 3 and 4 byte lengths (which were not covered before)
Mbed-TLS · Jul 14, 2016 · 9d63b77 · 9d63b77
1 parent 7d1ad31
commit 9d63b77
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Showing 3 changed files with 52 additions and 18 deletions.
diff --git a/include/mbedtls/asn1write.h b/include/mbedtls/asn1write.h
@@ -40,8 +40,6 @@ extern "C" {
  * \param start     start of the buffer (for bounds-checking)
  * \param len       the length to write
  *
- * \note            lengths over 65535 are not supported at the moment
- *
  * \return          the length written or a negative error code
  */
 int mbedtls_asn1_write_len( unsigned char **p, unsigned char *start, size_t len );

diff --git a/library/asn1write.c b/library/asn1write.c
@@ -41,11 +41,6 @@
 
 int mbedtls_asn1_write_len( unsigned char **p, unsigned char *start, size_t len )
 {
-    // We don't support lengths over 65535 for now
-    //
-    if( len > 0xFFFF )
-        return( MBEDTLS_ERR_ASN1_INVALID_LENGTH );
-
     if( len < 0x80 )
     {
         if( *p - start < 1 )
@@ -65,14 +60,43 @@ int mbedtls_asn1_write_len( unsigned char **p, unsigned char *start, size_t len
         return( 2 );
     }
 
-    if( *p - start < 3 )
-        return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
+    if( len <= 0xFFFF )
+    {
+        if( *p - start < 3 )
+            return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--(*p) = ( len       ) & 0xFF;
+        *--(*p) = ( len >>  8 ) & 0xFF;
+        *--(*p) = 0x82;
+        return( 3 );
+    }
+
+    if( len <= 0xFFFFFF )
+    {
+        if( *p - start < 4 )
+            return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--(*p) = ( len       ) & 0xFF;
+        *--(*p) = ( len >>  8 ) & 0xFF;
+        *--(*p) = ( len >> 16 ) & 0xFF;
+        *--(*p) = 0x83;
+        return( 4 );
+    }
 
-    *--(*p) = len % 256;
-    *--(*p) = ( len / 256 ) % 256;
-    *--(*p) = 0x82;
+    if( len <= 0xFFFFFFFF )
+    {
+        if( *p - start < 5 )
+            return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
+
+        *--(*p) = ( len       ) & 0xFF;
+        *--(*p) = ( len >>  8 ) & 0xFF;
+        *--(*p) = ( len >> 16 ) & 0xFF;
+        *--(*p) = ( len >> 24 ) & 0xFF;
+        *--(*p) = 0x84;
+        return( 5 );
+    }
 
-    return( 3 );
+    return( MBEDTLS_ERR_ASN1_INVALID_LENGTH );
 }
 
 int mbedtls_asn1_write_tag( unsigned char **p, unsigned char *start, unsigned char tag )

diff --git a/tests/suites/test_suite_asn1write.data b/tests/suites/test_suite_asn1write.data
@@ -67,14 +67,26 @@ mbedtls_asn1_write_len:255:"81FF":2:2
 ASN.1 Write / Read Length #5 (Len = 255, buffer too small)
 mbedtls_asn1_write_len:255:"81FF":1:MBEDTLS_ERR_ASN1_BUF_TOO_SMALL
 
-ASN.1 Write / Read Length #6 (Len = 256, long form)
-mbedtls_asn1_write_len:256:"820100":3:3
+ASN.1 Write / Read Length #6 (Len = 258, byte order)
+mbedtls_asn1_write_len:258:"820102":3:3
 
-ASN.1 Write / Read Length #7 (Len = 65535, max supported length)
+ASN.1 Write / Read Length #7 (Len = 65535, long form)
 mbedtls_asn1_write_len:65535:"82FFFF":3:3
 
 ASN.1 Write / Read Length #8 (Len = 65535, buffer too small)
 mbedtls_asn1_write_len:65535:"82FFFF":2:MBEDTLS_ERR_ASN1_BUF_TOO_SMALL
 
-ASN.1 Write / Read Length #9 (Len = 65536, not supported)
-mbedtls_asn1_write_len:65536:"":0:MBEDTLS_ERR_ASN1_INVALID_LENGTH
+ASN.1 Write / Read Length #9 (Len = 66051, byte order)
+mbedtls_asn1_write_len:66051:"83010203":4:4
+
+ASN.1 Write / Read Length #10 (Len = 16777215, long form)
+mbedtls_asn1_write_len:16777215:"83FFFFFF":4:4
+
+ASN.1 Write / Read Length #11 (Len = 16777215, buffer too small)
+mbedtls_asn1_write_len:16777215:"83FFFFFF":3:MBEDTLS_ERR_ASN1_BUF_TOO_SMALL
+
+ASN.1 Write / Read Length #12 (Len = 16909060, byte order)
+mbedtls_asn1_write_len:16909060:"8401020304":5:5
+
+ASN.1 Write / Read Length #12 (Len = 16909060, buffer too small)
+mbedtls_asn1_write_len:16909060:"8401020304":4:MBEDTLS_ERR_ASN1_BUF_TOO_SMALL