Provide various CBS_peek_* functions.

These will be used in libcrypto.

With input from and ok tb@

2 files changed, 109 insertions, 2 deletions

diff --git a/src/lib/libssl/bs_cbs.c b/src/lib/libssl/bs_cbs.c
index 97b0163f3f..63c078c9b0 100644
--- a/src/lib/libssl/bs_cbs.c
+++ b/src/lib/libssl/bs_cbs.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: bs_cbs.c,v 1.23 2021/12/15 17:30:20 jsing Exp $       */
+/*      $OpenBSD: bs_cbs.c,v 1.24 2021/12/15 17:36:49 jsing Exp $       */
 /*
  * Copyright (c) 2014, Google Inc.
  *
@@ -47,6 +47,16 @@ cbs_get(CBS *cbs, const uint8_t **p, size_t n)
         return 1;
 }
 
+static int
+cbs_peek(CBS *cbs, const uint8_t **p, size_t n)
+{
+        if (cbs->len < n)
+                return 0;
+
+        *p = cbs->data;
+        return 1;
+}
+
 size_t
 CBS_offset(const CBS *cbs)
 {
@@ -259,6 +269,73 @@ CBS_get_u24_length_prefixed(CBS *cbs, CBS *out)
         return cbs_get_length_prefixed(cbs, out, 3);
 }
 
+static int
+cbs_peek_u(CBS *cbs, uint32_t *out, size_t len)
+{
+        uint32_t result = 0;
+        size_t i;
+        const uint8_t *data;
+
+        if (len < 1 || len > 4)
+                return 0;
+
+        if (!cbs_peek(cbs, &data, len))
+                return 0;
+
+        for (i = 0; i < len; i++) {
+                result <<= 8;
+                result |= data[i];
+        }
+        *out = result;
+        return 1;
+}
+
+int
+CBS_peek_u8(CBS *cbs, uint8_t *out)
+{
+        const uint8_t *v;
+
+        if (!cbs_peek(cbs, &v, 1))
+                return 0;
+
+        *out = *v;
+        return 1;
+}
+
+int
+CBS_peek_u16(CBS *cbs, uint16_t *out)
+{
+        uint32_t v;
+
+        if (!cbs_peek_u(cbs, &v, 2))
+                return 0;
+
+        *out = v;
+        return 1;
+}
+
+int
+CBS_peek_u24(CBS *cbs, uint32_t *out)
+{
+        return cbs_peek_u(cbs, out, 3);
+}
+
+int
+CBS_peek_u32(CBS *cbs, uint32_t *out)
+{
+        return cbs_peek_u(cbs, out, 4);
+}
+
+int
+CBS_peek_last_u8(CBS *cbs, uint8_t *out)
+{
+        if (cbs->len == 0)
+                return 0;
+
+        *out = cbs->data[cbs->len - 1];
+        return 1;
+}
+
 int
 CBS_get_any_asn1_element(CBS *cbs, CBS *out, unsigned int *out_tag,
     size_t *out_header_len)
diff --git a/src/lib/libssl/bytestring.h b/src/lib/libssl/bytestring.h
index fa5e05fa27..ce933f3f7b 100644
--- a/src/lib/libssl/bytestring.h
+++ b/src/lib/libssl/bytestring.h
@@ -1,4 +1,4 @@
-/*      $OpenBSD: bytestring.h,v 1.21 2021/12/15 17:30:20 jsing Exp $   */
+/*      $OpenBSD: bytestring.h,v 1.22 2021/12/15 17:36:49 jsing Exp $   */
 /*
  * Copyright (c) 2014, Google Inc.
  *
@@ -172,6 +172,36 @@ int CBS_get_u16_length_prefixed(CBS *cbs, CBS *out);
  */
 int CBS_get_u24_length_prefixed(CBS *cbs, CBS *out);
 
+/*
+ * CBS_peek_u8 sets |*out| to the next uint8_t from |cbs|, but does not advance
+ * |cbs|. It returns one on success and zero on error.
+ */
+int CBS_peek_u8(CBS *cbs, uint8_t *out);
+
+/*
+ * CBS_peek_u16 sets |*out| to the next, big-endian uint16_t from |cbs|, but
+ * does not advance |cbs|. It returns one on success and zero on error.
+ */
+int CBS_peek_u16(CBS *cbs, uint16_t *out);
+
+/*
+ * CBS_peek_u24 sets |*out| to the next, big-endian 24-bit value from |cbs|, but
+ * does not advance |cbs|. It returns one on success and zero on error.
+ */
+int CBS_peek_u24(CBS *cbs, uint32_t *out);
+
+/*
+ * CBS_peek_u32 sets |*out| to the next, big-endian uint32_t value from |cbs|,
+ * but does not advance |cbs|. It returns one on success and zero on error.
+ */
+int CBS_peek_u32(CBS *cbs, uint32_t *out);
+
+/*
+ * CBS_peek_last_u8 sets |*out| to the last uint8_t from |cbs|, but does not
+ * shorten |cbs|. It returns one on success and zero on error.
+ */
+int CBS_peek_last_u8(CBS *cbs, uint8_t *out);
+
 
 /* Parsing ASN.1 */