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

ok miod@ jsing@

diff --git a/lib/libssl/bs_cbb.c b/lib/libssl/bs_cbb.c
index 29312e1..904edb9 100644 (file)
--- a/lib/libssl/bs_cbb.c
+++ b/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/lib/libssl/src/ssl/bs_cbb.c b/lib/libssl/src/ssl/bs_cbb.c
index 29312e1..904edb9 100644 (file)
--- a/lib/libssl/src/ssl/bs_cbb.c
+++ b/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. */