diff --git a/CMakeLists.txt b/CMakeLists.txt
index a8b6570..d2db36f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -149,6 +149,7 @@ endif()
 
 include_directories(src)
 include_directories(external/src)
+include_directories(external/src/crypto)
 include_directories(external/src/cryptonote)
 include_directories(${UV_INCLUDE_DIR})
 include_directories(external/src/cppzmq)
diff --git a/external/src/crypto/sha256.h b/external/src/crypto/sha256.h
index 38f7c82..00fb1e4 100644
--- a/external/src/crypto/sha256.h
+++ b/external/src/crypto/sha256.h
@@ -9,6 +9,10 @@
 #ifndef SHA256_H
 #define SHA256_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*************************** HEADER FILES ***************************/
 #include <stdint.h>
 
@@ -30,4 +34,8 @@ void sha256_final(SHA256_CTX *ctx, uint8_t* hash);
 
 void sha256(const void* data, uint32_t len, uint8_t* hash);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif   // SHA256_H
diff --git a/src/merkle.cpp b/src/merkle.cpp
index fec00a2..2a77966 100644
--- a/src/merkle.cpp
+++ b/src/merkle.cpp
@@ -19,6 +19,7 @@
 #include "keccak.h"
 #include "merkle.h"
 #include "keccak.h"
+#include "sha256.h"
 
 namespace p2pool {
 
@@ -271,4 +272,24 @@ bool verify_merkle_proof(hash h, const std::vector<hash>& proof, size_t index, s
 	return (h == root);
 }
 
+uint32_t get_aux_slot(const hash &id, uint32_t nonce, uint32_t n_aux_chains)
+{
+	if (n_aux_chains <= 1) {
+		return 0;
+	}
+
+	constexpr uint8_t HASH_KEY_MM_SLOT = 'm';
+
+	uint8_t buf[HASH_SIZE + sizeof(uint32_t) + 1];
+
+	memcpy(buf, &id, HASH_SIZE);
+	memcpy(buf + HASH_SIZE, &nonce, sizeof(uint32_t));
+	buf[HASH_SIZE + sizeof(uint32_t)] = HASH_KEY_MM_SLOT;
+
+	hash res;
+	sha256(buf, sizeof(buf), res.h);
+
+	return *reinterpret_cast<uint32_t*>(res.h) % n_aux_chains;
+}
+
 } // namespace p2pool
diff --git a/src/merkle.h b/src/merkle.h
index a9c6cbb..8c36a65 100644
--- a/src/merkle.h
+++ b/src/merkle.h
@@ -21,8 +21,11 @@ namespace p2pool {
 
 void merkle_hash(const std::vector<hash>& hashes, hash& root);
 void merkle_hash_full_tree(const std::vector<hash>& hashes, std::vector<std::vector<hash>>& tree);
+
 bool get_merkle_proof(const std::vector<std::vector<hash>>& tree, const hash& h, std::vector<std::pair<bool, hash>>& proof);
 bool verify_merkle_proof(hash h, const std::vector<std::pair<bool, hash>>& proof, const hash& root);
 bool verify_merkle_proof(hash h, const std::vector<hash>& proof, size_t index, size_t count, const hash& root);
 
+uint32_t get_aux_slot(const hash &id, uint32_t nonce, uint32_t n_aux_chains);
+
 } // namespace p2pool
diff --git a/tests/src/check_win7.py b/tests/src/check_win7.py
index 7a7371d..675c33e 100644
--- a/tests/src/check_win7.py
+++ b/tests/src/check_win7.py
@@ -272,7 +272,7 @@ for line in lines:
 	count += 1
 	api = line[29:].strip()
 	if api not in allowedAPIs:
-		print('API call "{}" is not checked for Windows 7 compatibility. Check it and then add it to the list in tests/src/check_imports.py'.format(api))
+		print('API call "{}" is not checked for Windows 7 compatibility. Check it and then add it to the list in tests/src/check_win7.py'.format(api))
 		exit_code = 1
 
 if exit_code == 0:
diff --git a/tests/src/merkle_tests.cpp b/tests/src/merkle_tests.cpp
index 3d0cf3e..f02c08d 100644
--- a/tests/src/merkle_tests.cpp
+++ b/tests/src/merkle_tests.cpp
@@ -224,4 +224,23 @@ TEST(merkle, tree)
 	check_full_tree();
 }
 
+TEST(merkle, aux_slot)
+{
+	hash id;
+
+	ASSERT_EQ(get_aux_slot(id, 0, 0), 0U);
+	ASSERT_EQ(get_aux_slot(id, 0, 1), 0U);
+	ASSERT_EQ(get_aux_slot(id, 0, 2), 0U);
+	ASSERT_EQ(get_aux_slot(id, 0, 3), 0U);
+	ASSERT_EQ(get_aux_slot(id, 0, 4), 0U);
+	ASSERT_EQ(get_aux_slot(id, 0, 5), 1U);
+	ASSERT_EQ(get_aux_slot(id, 0, 6), 0U);
+	ASSERT_EQ(get_aux_slot(id, 0, 7), 5U);
+	ASSERT_EQ(get_aux_slot(id, 0, 8), 0U);
+	ASSERT_EQ(get_aux_slot(id, 0, 9), 6U);
+
+	ASSERT_EQ(get_aux_slot(id, 0, std::numeric_limits<uint32_t>::max()), 2389612776U);
+	ASSERT_EQ(get_aux_slot(id, 1, std::numeric_limits<uint32_t>::max()), 1080669337U);
+}
+
 }
diff --git a/tests/src/sha256_tests.cpp b/tests/src/sha256_tests.cpp
index c63ae9a..e29e805 100644
--- a/tests/src/sha256_tests.cpp
+++ b/tests/src/sha256_tests.cpp
@@ -16,11 +16,8 @@
  */
 
 #include "common.h"
-#include "gtest/gtest.h"
-
-extern "C" {
 #include "sha256.h"
-}
+#include "gtest/gtest.h"
 
 namespace p2pool {
 
@@ -28,7 +25,7 @@ TEST(sha256, hashing)
 {
 	auto check = [](const char* input, const char* output) {
 		hash h;
-		sha256(input, strlen(input), h.h);
+		sha256(input, static_cast<uint32_t>(strlen(input)), h.h);
 
 		char buf[128];
 		log::Stream s(buf);