diff --git a/Makefile b/Makefile
index f2bb7f2f63..68948dfbf3 100644
--- a/Makefile
+++ b/Makefile
@@ -162,6 +162,11 @@ all::
 # algorithm. This is slower, but may detect attempted collision attacks.
 # Takes priority over other *_SHA1 knobs.
 #
+# Define DC_SHA1_EXTERNAL in addition to DC_SHA1 if you want to build / link
+# git with the external SHA1 collision-detect library.
+# Without this option, i.e. the default behavior is to build git with its
+# own built-in code (or submodule).
+#
 # Define DC_SHA1_SUBMODULE in addition to DC_SHA1 to use the
 # sha1collisiondetection shipped as a submodule instead of the
 # non-submodule copy in sha1dc/. This is an experimental option used
@@ -1475,6 +1480,15 @@ ifdef APPLE_COMMON_CRYPTO
 	BASIC_CFLAGS += -DSHA1_APPLE
 else
 	DC_SHA1 := YesPlease
+	BASIC_CFLAGS += -DSHA1_DC
+	LIB_OBJS += sha1dc_git.o
+ifdef DC_SHA1_EXTERNAL
+	ifdef DC_SHA1_SUBMODULE
+$(error Only set DC_SHA1_EXTERNAL or DC_SHA1_SUBMODULE, not both)
+	endif
+	BASIC_CFLAGS += -DDC_SHA1_EXTERNAL
+	EXTLIBS += -lsha1detectcoll
+else
 ifdef DC_SHA1_SUBMODULE
 	LIB_OBJS += sha1collisiondetection/lib/sha1.o
 	LIB_OBJS += sha1collisiondetection/lib/ubc_check.o
@@ -1484,17 +1498,15 @@ else
 	LIB_OBJS += sha1dc/ubc_check.o
 endif
 	BASIC_CFLAGS += \
-		-DSHA1_DC \
 		-DSHA1DC_NO_STANDARD_INCLUDES \
 		-DSHA1DC_INIT_SAFE_HASH_DEFAULT=0 \
 		-DSHA1DC_CUSTOM_INCLUDE_SHA1_C="\"cache.h\"" \
-		-DSHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_C="\"sha1dc_git.c\"" \
-		-DSHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_H="\"sha1dc_git.h\"" \
 		-DSHA1DC_CUSTOM_INCLUDE_UBC_CHECK_C="\"git-compat-util.h\""
 endif
 endif
 endif
 endif
+endif
 
 ifdef SHA1_MAX_BLOCK_SIZE
 	LIB_OBJS += compat/sha1-chunked.o
diff --git a/hash.h b/hash.h
index bef3e630a0..024d0d3d50 100644
--- a/hash.h
+++ b/hash.h
@@ -8,11 +8,7 @@
 #elif defined(SHA1_OPENSSL)
 #include <openssl/sha.h>
 #elif defined(SHA1_DC)
-#ifdef DC_SHA1_SUBMODULE
-#include "sha1collisiondetection/lib/sha1.h"
-#else
-#include "sha1dc/sha1.h"
-#endif
+#include "sha1dc_git.h"
 #else /* SHA1_BLK */
 #include "block-sha1/sha1.h"
 #endif
diff --git a/sha1dc_git.c b/sha1dc_git.c
index 4d32b4f77e..e0cc9d988c 100644
--- a/sha1dc_git.c
+++ b/sha1dc_git.c
@@ -1,8 +1,19 @@
-/*
- * This code is included at the end of sha1dc/sha1.c with the
- * SHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_C macro.
- */
+#include "cache.h"
 
+#ifdef DC_SHA1_EXTERNAL
+/*
+ * Same as SHA1DCInit, but with default save_hash=0
+ */
+void git_SHA1DCInit(SHA1_CTX *ctx)
+{
+	SHA1DCInit(ctx);
+	SHA1DCSetSafeHash(ctx, 0);
+}
+#endif
+
+/*
+ * Same as SHA1DCFinal, but convert collision attack case into a verbose die().
+ */
 void git_SHA1DCFinal(unsigned char hash[20], SHA1_CTX *ctx)
 {
 	if (!SHA1DCFinal(hash, ctx))
@@ -11,6 +22,9 @@ void git_SHA1DCFinal(unsigned char hash[20], SHA1_CTX *ctx)
 	    sha1_to_hex(hash));
 }
 
+/*
+ * Same as SHA1DCUpdate, but adjust types to match git's usual interface.
+ */
 void git_SHA1DCUpdate(SHA1_CTX *ctx, const void *vdata, unsigned long len)
 {
 	const char *data = vdata;
diff --git a/sha1dc_git.h b/sha1dc_git.h
index a8a5c1da16..a8c2729278 100644
--- a/sha1dc_git.h
+++ b/sha1dc_git.h
@@ -1,19 +1,23 @@
-/*
- * This code is included at the end of sha1dc/sha1.h with the
- * SHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_H macro.
- */
+/* Plumbing with collition-detecting SHA1 code */
+
+#ifdef DC_SHA1_SUBMODULE
+#include "sha1collisiondetection/lib/sha1.h"
+#elif defined(DC_SHA1_EXTERNAL)
+#include <sha1dc/sha1.h>
+#else
+#include "sha1dc/sha1.h"
+#endif
+
+#ifdef DC_SHA1_EXTERNAL
+void git_SHA1DCInit(SHA1_CTX *);
+#else
+#define git_SHA1DCInit	SHA1DCInit
+#endif
 
-/*
- * Same as SHA1DCFinal, but convert collision attack case into a verbose die().
- */
 void git_SHA1DCFinal(unsigned char [20], SHA1_CTX *);
-
-/*
- * Same as SHA1DCUpdate, but adjust types to match git's usual interface.
- */
 void git_SHA1DCUpdate(SHA1_CTX *ctx, const void *data, unsigned long len);
 
 #define platform_SHA_CTX SHA1_CTX
-#define platform_SHA1_Init SHA1DCInit
+#define platform_SHA1_Init git_SHA1DCInit
 #define platform_SHA1_Update git_SHA1DCUpdate
 #define platform_SHA1_Final git_SHA1DCFinal