Explain the ASN.1 restriction that requires extra logic for encoding.

ok miod@ jsing@

2 files changed, 36 insertions, 4 deletions

diff --git a/src/lib/libssl/bs_cbb.c b/src/lib/libssl/bs_cbb.c
index 29312e104b..904edb9fb1 100644
--- a/src/lib/libssl/bs_cbb.c
+++ b/src/lib/libssl/bs_cbb.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: bs_cbb.c,v 1.9 2015/06/13 09:11:57 doug Exp $ */
+/*      $OpenBSD: bs_cbb.c,v 1.10 2015/06/13 09:16:42 doug Exp $        */
 /*
  * Copyright (c) 2014, Google Inc.
  *
@@ -370,7 +370,23 @@ CBB_add_asn1_uint64(CBB *cbb, uint64_t value)
                 return 0;
 
         for (i = 0; i < 8; i++) {
-                uint8_t byte = (value >> 8*(7-i)) & 0xff;
+                uint8_t byte = (value >> 8 * (7 - i)) & 0xff;
+
+                /*
+                 * ASN.1 restriction: first 9 bits cannot be all zeroes or
+                 * all ones.  Since this function only encodes unsigned
+                 * integers, the only concerns are not encoding leading
+                 * zeros and adding a padding byte if necessary.
+                 *
+                 * In practice, this means:
+                 * 1) Skip leading octets of all zero bits in the value
+                 * 2) After skipping the leading zero octets, if the next 9
+                 *    bits are all ones, add an all zero prefix octet (and
+                 *    set the high bit of the prefix octet if negative).
+                 *
+                 * Additionally, for an unsigned value, add an all zero
+                 * prefix if the high bit of the first octet would be one.
+                 */
                 if (!started) {
                         if (byte == 0)
                                 /* Don't encode leading zeros. */
diff --git a/src/lib/libssl/src/ssl/bs_cbb.c b/src/lib/libssl/src/ssl/bs_cbb.c
index 29312e104b..904edb9fb1 100644
--- a/src/lib/libssl/src/ssl/bs_cbb.c
+++ b/src/lib/libssl/src/ssl/bs_cbb.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: bs_cbb.c,v 1.9 2015/06/13 09:11:57 doug Exp $ */
+/*      $OpenBSD: bs_cbb.c,v 1.10 2015/06/13 09:16:42 doug Exp $        */
 /*
  * Copyright (c) 2014, Google Inc.
  *
@@ -370,7 +370,23 @@ CBB_add_asn1_uint64(CBB *cbb, uint64_t value)
                 return 0;
 
         for (i = 0; i < 8; i++) {
-                uint8_t byte = (value >> 8*(7-i)) & 0xff;
+                uint8_t byte = (value >> 8 * (7 - i)) & 0xff;
+
+                /*
+                 * ASN.1 restriction: first 9 bits cannot be all zeroes or
+                 * all ones.  Since this function only encodes unsigned
+                 * integers, the only concerns are not encoding leading
+                 * zeros and adding a padding byte if necessary.
+                 *
+                 * In practice, this means:
+                 * 1) Skip leading octets of all zero bits in the value
+                 * 2) After skipping the leading zero octets, if the next 9
+                 *    bits are all ones, add an all zero prefix octet (and
+                 *    set the high bit of the prefix octet if negative).
+                 *
+                 * Additionally, for an unsigned value, add an all zero
+                 * prefix if the high bit of the first octet would be one.
+                 */
                 if (!started) {
                         if (byte == 0)
                                 /* Don't encode leading zeros. */