Add sntrup761.sh

This generates sntrup.c from supercop, it was missed in
440b7b5 ("Add sntrup761x25519-sha512 post-quantum key exchange")

sntrup761.sh is taken from OpenSSH, updated for most recent supercop
release, with some small adaptations for Dropbear.

src/sntrup761.sh

@@ -0,0 +1,104 @@
+#!/bin/sh
+#       $OpenBSD: sntrup761.sh,v 1.9 2024/09/16 05:37:05 djm Exp $
+#       Placed in the Public Domain.
+#
+AUTHOR="supercop-20241022/crypto_kem/sntrup761/ref/implementors"
+FILES=" supercop-20241022/cryptoint/crypto_int16.h
+	supercop-20241022/cryptoint/crypto_int32.h
+	supercop-20241022/cryptoint/crypto_int64.h
+	supercop-20241022/crypto_sort/int32/portable4/sort.c
+	supercop-20241022/crypto_sort/uint32/useint32/sort.c
+	supercop-20241022/crypto_kem/sntrup761/compact/kem.c
+"
+###
+
+set -euo pipefail
+cd $1
+echo '/*'
+echo ' * Public Domain, Authors:'
+sed -e '/Alphabetical order:/d' -e 's/^/ * - /' < $AUTHOR
+echo ' */'
+echo
+echo '#include <string.h>'
+echo '#include "sntrup761_compat.h"'
+echo
+echo '#if DROPBEAR_SNTRUP761'
+echo
+echo '#define crypto_declassify(x, y) do {} while (0)'
+echo
+# Map the types used in this code to the ones in crypto_api.h.  We use #define
+# instead of typedef since some systems have existing intXX types and do not
+# permit multiple typedefs even if they do not conflict.
+for t in int8 uint8 int16 uint16 int32 uint32 int64 uint64; do
+	echo "#define $t crypto_${t}"
+done
+
+for x in 16 32 64 ; do
+	echo "extern volatile crypto_int$x crypto_int${x}_optblocker;"
+done
+
+echo
+for i in $FILES; do
+	echo "/* from $i */"
+	# Changes to all files:
+	#  - remove all includes, we inline everything required.
+	#  - make functions not required elsewhere static.
+	#  - rename the functions we do use.
+	#  - remove unnecessary defines and externs.
+	sed -e "/#include/d" \
+	    -e "s/crypto_kem_/crypto_kem_sntrup761_/g" \
+	    -e "s/^void /static void /g" \
+	    -e "s/^int16 /static int16 /g" \
+	    -e "s/^uint16 /static uint16 /g" \
+	    -e "/^extern /d" \
+	    -e '/CRYPTO_NAMESPACE/d' \
+	    -e "/^#define int32 crypto_int32/d" \
+	    -e 's/[	 ]*$//' \
+	    $i | \
+	case "$i" in
+	*/cryptoint/crypto_int16.h)
+	    sed -e "s/static void crypto_int16_store/void crypto_int16_store/" \
+		-e "s/^[#]define crypto_int16_optblocker.*//" \
+	        -e "s/static void crypto_int16_minmax/void crypto_int16_minmax/"
+	    ;;
+	*/cryptoint/crypto_int32.h)
+	# Use int64_t for intermediate values in crypto_int32_minmax to
+	# prevent signed 32-bit integer overflow when called by
+	# crypto_sort_int32. Original code depends on -fwrapv (we set -ftrapv)
+	    sed -e "s/static void crypto_int32_store/void crypto_int32_store/" \
+		-e "s/^[#]define crypto_int32_optblocker.*//" \
+		-e "s/crypto_int32 crypto_int32_r = crypto_int32_y ^ crypto_int32_x;/crypto_int64 crypto_int32_r = (crypto_int64)crypto_int32_y ^ (crypto_int64)crypto_int32_x;/" \
+		-e "s/crypto_int32 crypto_int32_z = crypto_int32_y - crypto_int32_x;/crypto_int64 crypto_int32_z = (crypto_int64)crypto_int32_y - (crypto_int64)crypto_int32_x;/" \
+	        -e "s/static void crypto_int32_minmax/void crypto_int32_minmax/"
+	    ;;
+	*/cryptoint/crypto_int64.h)
+	    sed -e "s/static void crypto_int64_store/void crypto_int64_store/" \
+		-e "s/^[#]define crypto_int64_optblocker.*//" \
+	        -e "s/static void crypto_int64_minmax/void crypto_int64_minmax/"
+	    ;;
+	*/int32/portable4/sort.c)
+	    sed -e "s/void crypto_sort[(]/void crypto_sort_int32(/g"
+	    ;;
+	*/int32/portable5/sort.c)
+	    sed -e "s/crypto_sort_smallindices/crypto_sort_int32_smallindices/"\
+	        -e "s/void crypto_sort[(]/void crypto_sort_int32(/g"
+	    ;;
+	*/uint32/useint32/sort.c)
+	    sed -e "s/void crypto_sort/void crypto_sort_uint32/g"
+	    ;;
+	# Remove unused function to prevent warning.
+	*/crypto_kem/sntrup761/ref/int32.c)
+	    sed -e '/ int32_div_uint14/,/^}$/d'
+	    ;;
+	# Remove unused function to prevent warning.
+	*/crypto_kem/sntrup761/ref/uint32.c)
+	    sed -e '/ uint32_div_uint14/,/^}$/d'
+	    ;;
+	# Default: pass through.
+	*)
+	    cat
+	    ;;
+	esac
+	echo
+done
+echo '#endif /* DROPBEAR_SNTRUP761 */'