Rewrite ASN.1 identifier/length parsing in CBS.

Provide internal asn1_get_identifier_cbs() and asn1_get_length_cbs()
functions that are called from asn1_get_object_cbs(). Convert the existing
ASN1_get_object() function so that it calls asn1_get_object_cbs(), before
mapping the result into the API that it implements.

ok tb@

1 files changed, 171 insertions, 0 deletions

diff --git a/src/lib/libcrypto/asn1/asn1_lib.c b/src/lib/libcrypto/asn1/asn1_lib.c
new file mode 100644
index 0000000000..542a72f6f1
--- /dev/null
+++ b/src/lib/libcrypto/asn1/asn1_lib.c
@@ -0,0 +1,171 @@
+/* $OpenBSD: asn1_lib.c,v 1.51 2021/12/25 07:04:03 jsing Exp $ */
+/*
+ * Copyright (c) 2021 Joel Sing <jsing@openbsd.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <limits.h>
+
+#include "bytestring.h"
+
+static int
+asn1_get_identifier_cbs(CBS *cbs, int der_mode, uint8_t *out_class,
+    int *out_constructed, uint32_t *out_tag_number)
+{
+        uint8_t tag_class, tag_val;
+        int tag_constructed;
+        uint32_t tag_number;
+
+        /*
+         * Decode ASN.1 identifier octets - see ITU-T X.690 section 8.1.2.
+         */
+
+        *out_class = 0;
+        *out_constructed = 0;
+        *out_tag_number = 0;
+
+        if (!CBS_get_u8(cbs, &tag_val))
+                return 0;
+
+        /*
+         * ASN.1 tag class, encoding (primitive or constructed) and tag number
+         * are encoded in one or more identifier octets - the first octet
+         * contains the 2 bit tag class, the 1 bit encoding type and 5 bits
+         * of tag number.
+         *
+         * For tag numbers larger than 30 (0x1e) the 5 bit tag number in the
+         * first octet is set to all ones (0x1f) - the tag number is then
+         * encoded in subsequent octets - each of which have a one bit
+         * continuation flag and 7 bits of tag number in big-endian form.
+         * The encoding should not contain leading zeros but can for BER.
+         */
+        tag_class = (tag_val >> 6) & 0x3;
+        tag_constructed = (tag_val >> 5) & 0x1;
+        tag_number = tag_val & 0x1f;
+
+        /* Long form. */
+        if (tag_number == 0x1f) {
+                tag_number = 0;
+                do {
+                        if (!CBS_get_u8(cbs, &tag_val))
+                                return 0;
+                        if (der_mode && tag_number == 0 && tag_val == 0x80)
+                                return 0;
+                        if (tag_number > (UINT32_MAX >> 7))
+                                return 0;
+                        tag_number = tag_number << 7 | (tag_val & 0x7f);
+                } while ((tag_val & 0x80) != 0);
+        }
+
+        *out_class = tag_class;
+        *out_constructed = tag_constructed;
+        *out_tag_number = tag_number;
+
+        return 1;
+}
+
+static int
+asn1_get_length_cbs(CBS *cbs, int der_mode, int *out_indefinite,
+    uint32_t *out_length)
+{
+        uint8_t len_bytes;
+        uint32_t length;
+        uint8_t val;
+
+        /*
+         * Decode ASN.1 length octets - see ITU-T X.690 section 8.1.3.
+         */
+
+        *out_length = 0;
+        *out_indefinite = 0;
+
+        if (!CBS_get_u8(cbs, &val))
+                return 0;
+
+        /*
+         * Short form - length is encoded in the lower 7 bits of a single byte.
+         */
+        if (val < 0x80) {
+                *out_length = val;
+                return 1;
+        }
+
+        /*
+         * Indefinite length - content continues until an End of Content (EOC)
+         * marker is reached. Must be used with constructed encoding.
+         */
+        if (val == 0x80) {
+                *out_indefinite = 1;
+                return 1;
+        }
+
+        /*
+         * Long form - the lower 7 bits of the first byte specifies the number
+         * of bytes used to encode the length, the following bytes specify the
+         * length in big-endian form. The encoding should not contain leading
+         * zeros but can for BER. A length value of 0x7f is invalid.
+         */
+        if ((len_bytes = val & 0x7f) == 0x7f)
+                return 0;
+
+        length = 0;
+
+        while (len_bytes-- > 0) {
+                if (!CBS_get_u8(cbs, &val))
+                        return 0;
+                if (der_mode && length == 0 && val == 0)
+                        return 0;
+                if (length > (UINT32_MAX >> 8))
+                        return 0;
+                length = (length << 8) | val;
+        }
+
+        *out_length = length;
+
+        return 1;
+}
+
+int
+asn1_get_object_cbs(CBS *cbs, int der_mode, uint8_t *out_tag_class,
+    int *out_constructed, uint32_t *out_tag_number, int *out_indefinite,
+    uint32_t *out_length)
+{
+        int constructed, indefinite;
+        uint32_t tag_number, length;
+        uint8_t tag_class;
+
+        *out_tag_class = 0;
+        *out_constructed = 0;
+        *out_tag_number = 0;
+        *out_indefinite = 0;
+        *out_length = 0;
+
+        if (!asn1_get_identifier_cbs(cbs, der_mode, &tag_class, &constructed,
+            &tag_number))
+                return 0;
+        if (!asn1_get_length_cbs(cbs, der_mode, &indefinite, &length))
+                return 0;
+
+        /* Indefinite length can only be used with constructed encoding. */
+        if (indefinite && !constructed)
+                return 0;
+
+        *out_tag_class = tag_class;
+        *out_constructed = constructed;
+        *out_tag_number = tag_number;
+        *out_indefinite = indefinite;
+        *out_length = length;
+
+        return 1;
+}