DER writer: Add the possibility to abandon empty SEQUENCEs

[openssl.git] / crypto / der_writer.c

diff --git a/crypto/der_writer.c b/crypto/der_writer.c
index 26fd88592da612cedaeb8c42a43a47d613fdbea9..876278750470374c1c3577122374082729ff7aed 100644 (file)
--- a/crypto/der_writer.c
+++ b/crypto/der_writer.c
@@ -24,12 +24,24 @@ static int int_start_context(WPACKET *pkt, int tag)
 
 static int int_end_context(WPACKET *pkt, int tag)
 {
+    /*
+     * If someone set the flag WPACKET_FLAGS_ABANDON_ON_ZERO_LENGTH on this
+     * sub-packet and this sub-packet has nothing written to it, the DER length
+     * will not be written, and the total written size will be unchanged before
+     * and after WPACKET_close().  We use size1 and size2 to determine if
+     * anything was written, and only write our tag if it has.
+     *
+     */
+    size_t size1, size2;
+
     if (tag < 0)
         return 1;
     if (!ossl_assert(tag <= 30))
         return 0;
-    return WPACKET_close(pkt)
-        && WPACKET_put_bytes_u8(pkt, DER_C_CONTEXT | tag);
+    return WPACKET_get_total_written(pkt, &size1)
+        && WPACKET_close(pkt)
+        && WPACKET_get_total_written(pkt, &size2)
+        && (size1 == size2 || WPACKET_put_bytes_u8(pkt, DER_C_CONTEXT | tag));
 }
 
 int DER_w_precompiled(WPACKET *pkt, int tag,
@@ -136,7 +148,24 @@ int DER_w_begin_sequence(WPACKET *pkt, int tag)
 
 int DER_w_end_sequence(WPACKET *pkt, int tag)
 {
-    return WPACKET_close(pkt)
-        && WPACKET_put_bytes_u8(pkt, DER_F_CONSTRUCTED | DER_P_SEQUENCE)
+    /*
+     * If someone set the flag WPACKET_FLAGS_ABANDON_ON_ZERO_LENGTH on this
+     * sub-packet and this sub-packet has nothing written to it, the DER length
+     * will not be written, and the total written size will be unchanged before
+     * and after WPACKET_close().  We use size1 and size2 to determine if
+     * anything was written, and only write our tag if it has.
+     *
+     * Because we know that int_end_context() needs to do the same check,
+     * we reproduce this flag if the written length was unchanged, or we will
+     * have an erroneous context tag.
+     */
+    size_t size1, size2;
+
+    return WPACKET_get_total_written(pkt, &size1)
+        && WPACKET_close(pkt)
+        && WPACKET_get_total_written(pkt, &size2)
+        && (size1 == size2
+            ? WPACKET_set_flags(pkt, WPACKET_FLAGS_ABANDON_ON_ZERO_LENGTH)
+            : WPACKET_put_bytes_u8(pkt, DER_F_CONSTRUCTED | DER_P_SEQUENCE))
         && int_end_context(pkt, tag);
 }