build: add skeleton for new silentpayments (BIP352) module

diff --git a/CMakeLists.txt b/CMakeLists.txt
index a84305c..09ab67d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -52,6 +52,7 @@ option(SECP256K1_ENABLE_MODULE_EXTRAKEYS "Enable extrakeys module." ON)
 option(SECP256K1_ENABLE_MODULE_SCHNORRSIG "Enable schnorrsig module." ON)
 option(SECP256K1_ENABLE_MODULE_MUSIG "Enable musig module." ON)
 option(SECP256K1_ENABLE_MODULE_ELLSWIFT "Enable ElligatorSwift module." ON)
+option(SECP256K1_ENABLE_MODULE_SILENTPAYMENTS "Enable Silent Payments module." ON)
 
 option(SECP256K1_USE_EXTERNAL_DEFAULT_CALLBACKS "Enable external default callback functions." OFF)
 if(SECP256K1_USE_EXTERNAL_DEFAULT_CALLBACKS)
@@ -289,6 +290,7 @@ message("  extrakeys ........................... ${SECP256K1_ENABLE_MODULE_EXTRA
 message("  schnorrsig .......................... ${SECP256K1_ENABLE_MODULE_SCHNORRSIG}")
 message("  musig ............................... ${SECP256K1_ENABLE_MODULE_MUSIG}")
 message("  ElligatorSwift ...................... ${SECP256K1_ENABLE_MODULE_ELLSWIFT}")
+message("  Silent Payments ..................... ${SECP256K1_ENABLE_MODULE_SILENTPAYMENTS}")
 message("Parameters:")
 message("  ecmult window size .................. ${SECP256K1_ECMULT_WINDOW_SIZE}")
 message("  ecmult gen table size ............... ${SECP256K1_ECMULT_GEN_KB} KiB")
diff --git a/Makefile.am b/Makefile.am
index 07d7a2b..5aa16da 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -314,3 +314,7 @@ endif
 if ENABLE_MODULE_ELLSWIFT
 include src/modules/ellswift/Makefile.am.include
 endif
+
+if ENABLE_MODULE_SILENTPAYMENTS
+include src/modules/silentpayments/Makefile.am.include
+endif
diff --git a/configure.ac b/configure.ac
index a21447c..258437f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -193,6 +193,10 @@ AC_ARG_ENABLE(module_ellswift,
     AS_HELP_STRING([--enable-module-ellswift],[enable ElligatorSwift module [default=yes]]), [],
     [SECP_SET_DEFAULT([enable_module_ellswift], [yes], [yes])])
 
+AC_ARG_ENABLE(module_silentpayments,
+    AS_HELP_STRING([--enable-module-silentpayments],[enable Silent Payments module [default=yes]]), [],
+    [SECP_SET_DEFAULT([enable_module_silentpayments], [yes], [yes])])
+
 AC_ARG_ENABLE(external_default_callbacks,
     AS_HELP_STRING([--enable-external-default-callbacks],[enable external default callback functions [default=no]]), [],
     [SECP_SET_DEFAULT([enable_external_default_callbacks], [no], [no])])
@@ -399,6 +403,14 @@ SECP_CFLAGS="$SECP_CFLAGS $WERROR_CFLAGS"
 
 # Processing must be done in a reverse topological sorting of the dependency graph
 # (dependent module first).
+if test x"$enable_module_silentpayments" = x"yes"; then
+  if test x"$enable_module_extrakeys" = x"no"; then
+    AC_MSG_ERROR([Module dependency error: You have disabled the extrakeys module explicitly, but it is required by the silentpayments module.])
+  fi
+  enable_module_extrakeys=yes
+  SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DENABLE_MODULE_SILENTPAYMENTS=1"
+fi
+
 if test x"$enable_module_ellswift" = x"yes"; then
   SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DENABLE_MODULE_ELLSWIFT=1"
 fi
@@ -472,6 +484,7 @@ AM_CONDITIONAL([ENABLE_MODULE_EXTRAKEYS], [test x"$enable_module_extrakeys" = x"
 AM_CONDITIONAL([ENABLE_MODULE_SCHNORRSIG], [test x"$enable_module_schnorrsig" = x"yes"])
 AM_CONDITIONAL([ENABLE_MODULE_MUSIG], [test x"$enable_module_musig" = x"yes"])
 AM_CONDITIONAL([ENABLE_MODULE_ELLSWIFT], [test x"$enable_module_ellswift" = x"yes"])
+AM_CONDITIONAL([ENABLE_MODULE_SILENTPAYMENTS], [test x"$enable_module_silentpayments" = x"yes"])
 AM_CONDITIONAL([USE_EXTERNAL_ASM], [test x"$enable_external_asm" = x"yes"])
 AM_CONDITIONAL([USE_ASM_ARM], [test x"$set_asm" = x"arm32"])
 AM_CONDITIONAL([BUILD_WINDOWS], [test "$build_windows" = "yes"])
@@ -496,6 +509,7 @@ echo "  module extrakeys        = $enable_module_extrakeys"
 echo "  module schnorrsig       = $enable_module_schnorrsig"
 echo "  module musig            = $enable_module_musig"
 echo "  module ellswift         = $enable_module_ellswift"
+echo "  module silentpayments   = $enable_module_silentpayments"
 echo
 echo "  asm                     = $set_asm"
 echo "  ecmult window size      = $set_ecmult_window"
diff --git a/include/secp256k1_silentpayments.h b/include/secp256k1_silentpayments.h
new file mode 100644
index 0000000..edc4609
--- /dev/null
+++ b/include/secp256k1_silentpayments.h
@@ -0,0 +1,32 @@
+#ifndef SECP256K1_SILENTPAYMENTS_H
+#define SECP256K1_SILENTPAYMENTS_H
+
+#include "secp256k1.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/** This module provides an implementation for Silent Payments, as specified in
+ *  BIP352. This particularly involves the creation of input tweak data by
+ *  summing up secret or public keys and the derivation of a shared secret using
+ *  Elliptic Curve Diffie-Hellman. Combined are either:
+ *    - spender's secret keys and recipient's public key (a * B, sender side)
+ *    - spender's public keys and recipient's secret key (A * b, recipient side)
+ *  With this result, the necessary key material for ultimately creating/scanning
+ *  or spending Silent Payments outputs can be determined.
+ *
+ *  Note that this module is _not_ a full implementation of BIP352, as it
+ *  inherently doesn't deal with higher-level concepts like addresses, output
+ *  script types or transactions. The intent is to provide a module for
+ *  abstracting away the elliptic-curve operations required for the protocol. For
+ *  any wallet software already using libsecp256k1, this API should provide all
+ *  the functions needed for a Silent Payments implementation without requiring
+ *  any further elliptic-curve operations from the wallet.
+ */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* SECP256K1_SILENTPAYMENTS_H */
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index a45eeb9..911804f 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -21,6 +21,15 @@ if(SECP256K1_ENABLE_MODULE_MUSIG)
   set_property(TARGET secp256k1 APPEND PROPERTY PUBLIC_HEADER ${PROJECT_SOURCE_DIR}/include/secp256k1_musig.h)
 endif()
 
+if(SECP256K1_ENABLE_MODULE_SILENTPAYMENTS)
+  if(DEFINED SECP256K1_ENABLE_MODULE_EXTRAKEYS AND NOT SECP256K1_ENABLE_MODULE_EXTRAKEYS)
+    message(FATAL_ERROR "Module dependency error: You have disabled the extrakeys module explicitly, but it is required by the silent payments module.")
+  endif()
+  set(SECP256K1_ENABLE_MODULE_EXTRAKEYS ON)
+  add_compile_definitions(ENABLE_MODULE_SILENTPAYMENTS=1)
+  set_property(TARGET secp256k1 APPEND PROPERTY PUBLIC_HEADER ${PROJECT_SOURCE_DIR}/include/secp256k1_silentpayments.h)
+endif()
+
 if(SECP256K1_ENABLE_MODULE_SCHNORRSIG)
   if(DEFINED SECP256K1_ENABLE_MODULE_EXTRAKEYS AND NOT SECP256K1_ENABLE_MODULE_EXTRAKEYS)
     message(FATAL_ERROR "Module dependency error: You have disabled the extrakeys module explicitly, but it is required by the schnorrsig module.")
diff --git a/src/modules/silentpayments/Makefile.am.include b/src/modules/silentpayments/Makefile.am.include
new file mode 100644
index 0000000..842a33e
--- /dev/null
+++ b/src/modules/silentpayments/Makefile.am.include
@@ -0,0 +1,2 @@
+include_HEADERS += include/secp256k1_silentpayments.h
+noinst_HEADERS += src/modules/silentpayments/main_impl.h
diff --git a/src/modules/silentpayments/main_impl.h b/src/modules/silentpayments/main_impl.h
new file mode 100644
index 0000000..f8ccdd7
--- /dev/null
+++ b/src/modules/silentpayments/main_impl.h
@@ -0,0 +1,16 @@
+/***********************************************************************
+ * Distributed under the MIT software license, see the accompanying    *
+ * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
+ ***********************************************************************/
+
+#ifndef SECP256K1_MODULE_SILENTPAYMENTS_MAIN_H
+#define SECP256K1_MODULE_SILENTPAYMENTS_MAIN_H
+
+#include "../../../include/secp256k1.h"
+#include "../../../include/secp256k1_silentpayments.h"
+
+/* TODO: implement functions for sender side. */
+
+/* TODO: implement functions for receiver side. */
+
+#endif
diff --git a/src/secp256k1.c b/src/secp256k1.c
index b216872..2668eee 100644
--- a/src/secp256k1.c
+++ b/src/secp256k1.c
@@ -849,3 +849,7 @@ int secp256k1_tagged_sha256(const secp256k1_context* ctx, unsigned char *hash32,
 #ifdef ENABLE_MODULE_ELLSWIFT
 # include "modules/ellswift/main_impl.h"
 #endif
+
+#ifdef ENABLE_MODULE_SILENTPAYMENTS
+# include "modules/silentpayments/main_impl.h"
+#endif