PROV: Add the beginning of a DER writing library

This library is meant to be small and quick.  It's based on WPACKET,
which was extended to support DER writing.  The way it's used is a
bit unusual, as it's used to write the structures backward into a
given buffer.  A typical quick call looks like this:

    /*
     * Fill in this structure:
     *
     * something ::= SEQUENCE {
     *     id OBJECT IDENTIFIER,
     *     x [0] INTEGER OPTIONAL,
     *     y [1] BOOLEAN OPTIONAL,
     *     n INTEGER
     * }
     */
    unsigned char buf[nnnn], *p = NULL;
    size_t encoded_len = 0;
    WPACKET pkt;
    int ok;

    ok =   WPACKET_init_der(&pkt, buf, sizeof(buf)
        && DER_w_start_sequence(&pkt, -1)
        && DER_w_bn(&pkt, -1, bn)
        && DER_w_boolean(&pkt, 1, bool)
        && DER_w_precompiled(&pkt, -1, OID, sizeof(OID))
        && DER_w_end_sequence(&pkt, -1)
        && WPACKET_finish(&pkt)
        && WPACKET_get_total_written(&pkt, &encoded_len)
        && (p = WPACKET_get_curr(&pkt)) != NULL;

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11450)

2 files changed, 143 insertions, 1 deletions

diff --git a/crypto/build.info b/crypto/build.info
index baa31ee8e1..864506a18d 100644
--- a/crypto/build.info
+++ b/crypto/build.info
@@ -71,7 +71,7 @@ $UTIL_COMMON=\
         cryptlib.c params.c params_from_text.c bsearch.c ex_data.c o_str.c \
         ctype.c threads_pthread.c threads_win.c threads_none.c initthread.c \
         context.c sparse_array.c asn1_dsa.c packet.c param_build.c $CPUIDASM \
-        param_build_set.c
+        param_build_set.c der_writer.c
 $UTIL_DEFINE=$CPUIDDEF
 
 SOURCE[../libcrypto]=$UTIL_COMMON \
diff --git a/crypto/der_writer.c b/crypto/der_writer.c
new file mode 100644
index 0000000000..26fd88592d
--- /dev/null
+++ b/crypto/der_writer.c
@@ -0,0 +1,142 @@
+/*
+ * Copyright 2020 The OpenSSL Project Authors. All Rights Reserved.
+ *
+ * Licensed under the Apache License 2.0 (the "License").  You may not use
+ * this file except in compliance with the License.  You can obtain a copy
+ * in the file LICENSE in the source distribution or at
+ * https://www.openssl.org/source/license.html
+ */
+
+#include <stdlib.h>
+#include <string.h>
+#include "internal/cryptlib.h"
+#include "internal/der.h"
+#include "crypto/bn.h"
+
+static int int_start_context(WPACKET *pkt, int tag)
+{
+    if (tag < 0)
+        return 1;
+    if (!ossl_assert(tag <= 30))
+        return 0;
+    return WPACKET_start_sub_packet(pkt);
+}
+
+static int int_end_context(WPACKET *pkt, int tag)
+{
+    if (tag < 0)
+        return 1;
+    if (!ossl_assert(tag <= 30))
+        return 0;
+    return WPACKET_close(pkt)
+        && WPACKET_put_bytes_u8(pkt, DER_C_CONTEXT | tag);
+}
+
+int DER_w_precompiled(WPACKET *pkt, int tag,
+                      const unsigned char *precompiled, size_t precompiled_n)
+{
+    return int_start_context(pkt, tag)
+        && WPACKET_memcpy(pkt, precompiled, precompiled_n)
+        && int_end_context(pkt, tag);
+}
+
+int DER_w_boolean(WPACKET *pkt, int tag, int b)
+{
+    return int_start_context(pkt, tag)
+        && WPACKET_start_sub_packet(pkt)
+        && (!b || WPACKET_put_bytes_u8(pkt, 0xFF))
+        && !WPACKET_close(pkt)
+        && !WPACKET_put_bytes_u8(pkt, DER_P_BOOLEAN)
+        && int_end_context(pkt, tag);
+}
+
+static int int_der_w_integer(WPACKET *pkt, int tag,
+                             int (*put_bytes)(WPACKET *pkt, const void *v,
+                                              unsigned int *top_byte),
+                             const void *v)
+{
+    unsigned int top_byte = 0;
+
+    return int_start_context(pkt, tag)
+        && WPACKET_start_sub_packet(pkt)
+        && put_bytes(pkt, v, &top_byte)
+        && ((top_byte & 0x80) == 0 || WPACKET_put_bytes_u8(pkt, 0))
+        && WPACKET_close(pkt)
+        && WPACKET_put_bytes_u8(pkt, DER_P_INTEGER)
+        && int_end_context(pkt, tag);
+}
+
+static int int_put_bytes_ulong(WPACKET *pkt, const void *v,
+                               unsigned int *top_byte)
+{
+    const unsigned long *value = v;
+    unsigned long tmp = *value;
+    size_t n = 0;
+
+    while (tmp != 0) {
+        n++;
+        *top_byte = (tmp & 0xFF);
+        tmp >>= 8;
+    }
+    if (n == 0)
+        n = 1;
+
+    return WPACKET_put_bytes__(pkt, *value, n);
+}
+
+/* For integers, we only support unsigned values for now */
+int DER_w_ulong(WPACKET *pkt, int tag, unsigned long v)
+{
+    return int_der_w_integer(pkt, tag, int_put_bytes_ulong, &v);
+}
+
+static int int_put_bytes_bn(WPACKET *pkt, const void *v,
+                            unsigned int *top_byte)
+{
+    unsigned char *p = NULL;
+    size_t n = BN_num_bytes(v);
+
+    /* The BIGNUM limbs are in LE order */
+    *top_byte =
+        ((bn_get_words(v) [(n - 1) / BN_BYTES]) >> (8 * ((n - 1) % BN_BYTES)))
+        & 0xFF;
+
+    if (!WPACKET_allocate_bytes(pkt, n, &p))
+        return 0;
+    if (p != NULL)
+        BN_bn2bin(v, p);
+    return 1;
+}
+
+int DER_w_bn(WPACKET *pkt, int tag, const BIGNUM *v)
+{
+    if (v == NULL || BN_is_negative(v))
+        return 0;
+    if (BN_is_zero(v))
+        return DER_w_ulong(pkt, tag, 0);
+
+    return int_der_w_integer(pkt, tag, int_put_bytes_bn, v);
+}
+
+int DER_w_null(WPACKET *pkt, int tag)
+{
+    return int_start_context(pkt, tag)
+        && WPACKET_start_sub_packet(pkt)
+        && WPACKET_close(pkt)
+        && WPACKET_put_bytes_u8(pkt, DER_P_NULL)
+        && int_end_context(pkt, tag);
+}
+
+/* Constructed things need a start and an end */
+int DER_w_begin_sequence(WPACKET *pkt, int tag)
+{
+    return int_start_context(pkt, tag)
+        && WPACKET_start_sub_packet(pkt);
+}
+
+int DER_w_end_sequence(WPACKET *pkt, int tag)
+{
+    return WPACKET_close(pkt)
+        && WPACKET_put_bytes_u8(pkt, DER_F_CONSTRUCTED | DER_P_SEQUENCE)
+        && int_end_context(pkt, tag);
+}