From 41c305f6cde8418f9475474d761a3163d973efe6 Mon Sep 17 00:00:00 2001
From: Andrew Kozlik <andrew.kozlik@gmail.com>
Date: Thu, 30 Nov 2023 20:41:21 +0100
Subject: [PATCH] feat(crypto): Use entire DER encoding in DER_ITEM.

---
 crypto/buffer.c | 10 ++++++++++
 crypto/buffer.h |  1 +
 crypto/der.c    | 17 ++++++++++++-----
 crypto/der.h    |  7 ++++++-
 4 files changed, 29 insertions(+), 6 deletions(-)

diff --git a/crypto/buffer.c b/crypto/buffer.c
index 829aad1d2..9b6d34eac 100644
--- a/crypto/buffer.c
+++ b/crypto/buffer.c
@@ -64,6 +64,16 @@ bool buffer_get(BUFFER_READER *buf, uint8_t *byte) {
   return true;
 }
 
+bool buffer_seek(BUFFER_READER *buf, size_t pos) {
+  if ((buf->data == NULL) || (pos > buf->size)) {
+    return false;
+  }
+
+  buf->pos = pos;
+
+  return true;
+}
+
 bool buffer_read_buffer(BUFFER_READER *src, BUFFER_READER *dest, size_t size) {
   if ((src->data == NULL) || (src->pos + size > src->size)) {
     return false;
diff --git a/crypto/buffer.h b/crypto/buffer.h
index 4c2f4e608..6be46dcf2 100644
--- a/crypto/buffer.h
+++ b/crypto/buffer.h
@@ -48,6 +48,7 @@ void buffer_writer_init(BUFFER_WRITER *buf, uint8_t *data, size_t size);
 size_t __wur buffer_remaining(BUFFER_READER *buf);
 bool __wur buffer_peek(const BUFFER_READER *buf, uint8_t *byte);
 bool __wur buffer_get(BUFFER_READER *buf, uint8_t *byte);
+bool __wur buffer_seek(BUFFER_READER *buf, size_t pos);
 bool __wur buffer_read_buffer(BUFFER_READER *src, BUFFER_READER *dest,
                               size_t size);
 void buffer_lstrip(BUFFER_READER *buf, uint8_t byte);
diff --git a/crypto/der.c b/crypto/der.c
index 2813978ae..e863ef85e 100644
--- a/crypto/der.c
+++ b/crypto/der.c
@@ -87,6 +87,7 @@ bool der_write_length(BUFFER_WRITER *buf, size_t len) {
 }
 
 bool der_read_item(BUFFER_READER *buf, DER_ITEM *item) {
+  size_t begin_pos = buf->pos;
   if (!buffer_get(buf, &item->id) || ((item->id & 0x1f) == 0x1f)) {
     // Multi-byte identifiers not supported.
     return false;
@@ -97,7 +98,13 @@ bool der_read_item(BUFFER_READER *buf, DER_ITEM *item) {
     return false;
   }
 
-  return buffer_read_buffer(buf, &item->cont, len);
+  size_t header_size = buf->pos - begin_pos;
+  if (!buffer_seek(buf, begin_pos) ||
+      !buffer_read_buffer(buf, &item->buf, header_size + len)) {
+    return false;
+  }
+
+  return buffer_seek(&item->buf, header_size);
 }
 
 // Reencode a positive integer which violates the encoding rules in Rec. ITU-T
@@ -111,13 +118,13 @@ bool der_reencode_int(BUFFER_READER *reader, BUFFER_WRITER *writer) {
   }
 
   // Strip any leading 0x00 bytes.
-  buffer_lstrip(&item.cont, 0x00);
-  size_t len = buffer_remaining(&item.cont);
+  buffer_lstrip(&item.buf, 0x00);
+  size_t len = buffer_remaining(&item.buf);
 
   // Positive integers should start with one 0x00 byte if and only if the most
   // significant byte is >= 0x80.
   uint8_t msb = 0;
-  bool prepend_null = (!buffer_peek(&item.cont, &msb) || msb >= 0x80);
+  bool prepend_null = (!buffer_peek(&item.buf, &msb) || msb >= 0x80);
   if (prepend_null) {
     len += 1;
   }
@@ -132,5 +139,5 @@ bool der_reencode_int(BUFFER_READER *reader, BUFFER_WRITER *writer) {
     }
   }
 
-  return buffer_write_buffer(writer, &item.cont);
+  return buffer_write_buffer(writer, &item.buf);
 }
diff --git a/crypto/der.h b/crypto/der.h
index 4d249c287..b6c84033c 100644
--- a/crypto/der.h
+++ b/crypto/der.h
@@ -33,9 +33,14 @@
 #define DER_SEQUENCE 0x30
 #define DER_INTEGER 0x02
 
+// Struct representing a DER-encoded ASN.1 data value.
 typedef struct {
+  // Single-octet identifier encoding the ASN.1 class, type and tag number.
   uint8_t id;
-  BUFFER_READER cont;
+  // A buffer containing the entire DER encoding of the data value including the
+  // tag and length, but with the position indicator initialized to the offset
+  // of the contents octets.
+  BUFFER_READER buf;
 } DER_ITEM;
 
 bool __wur der_read_length(BUFFER_READER *buf, size_t *len);