diff --git a/CMakeLists.txt b/CMakeLists.txt
@@ -37,6 +37,7 @@ OPTION( PROFILE				"Generate profiling information"		OFF )
 OPTION( ENABLE_TRACE		"Enables tracing support"				OFF )
 OPTION( LIBGIT2_FILENAME	"Name of the produced binary"			OFF )
 
+OPTION( USE_SHA1DC			"Use SHA-1 with collision detection"	OFF )
 OPTION( USE_ICONV			"Link with and use iconv library" 		OFF )
 OPTION( USE_SSH				"Link with libssh to enable SSH support" ON )
 OPTION( USE_GSSAPI			"Link with libgssapi for SPNEGO auth"   OFF )
@@ -292,13 +293,16 @@ ELSE ()
 ENDIF()
 
 # Specify sha1 implementation
-IF (WIN32 AND NOT MINGW AND NOT SHA1_TYPE STREQUAL "builtin")
-	ADD_DEFINITIONS(-DWIN32_SHA1)
+IF (USE_SHA1DC)
+	ADD_DEFINITIONS(-DGIT_SHA1_COLLISIONDETECT)
+	FILE(GLOB SRC_SHA1 src/hash/hash_collisiondetect.c src/hash/sha1dc/*)
+ELSEIF (WIN32 AND NOT MINGW)
+	ADD_DEFINITIONS(-DGIT_SHA1_WIN32)
 	FILE(GLOB SRC_SHA1 src/hash/hash_win32.c)
 ELSEIF (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
-	ADD_DEFINITIONS(-DGIT_COMMON_CRYPTO)
-ELSEIF (OPENSSL_FOUND AND NOT SHA1_TYPE STREQUAL "builtin")
-	ADD_DEFINITIONS(-DOPENSSL_SHA1)
+	ADD_DEFINITIONS(-DGIT_SHA1_COMMON_CRYPTO)
+ELSEIF (OPENSSL_FOUND)
+	ADD_DEFINITIONS(-DGIT_SHA1_OPENSSL)
 	IF (CMAKE_SYSTEM_NAME MATCHES "FreeBSD")
 		LIST(APPEND LIBGIT2_PC_LIBS "-lssl")
 	ELSE()

diff --git a/COPYING b/COPYING
@@ -958,3 +958,36 @@ necessary.  Here is a sample; alter the names:
 That's all there is to it!
 
 ----------------------------------------------------------------------
+
+The bundled SHA1 collision detection code is licensed under the MIT license:
+
+MIT License
+
+Copyright (c) 2017:
+    Marc Stevens
+    Cryptology Group
+    Centrum Wiskunde & Informatica
+    P.O. Box 94079, 1090 GB Amsterdam, Netherlands
+    marc@marc-stevens.nl
+
+    Dan Shumow
+    Microsoft Research
+    danshu@microsoft.com
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/include/git2/errors.h b/include/git2/errors.h
@@ -100,7 +100,8 @@ typedef enum {
 	GITERR_REBASE,
 	GITERR_FILESYSTEM,
 	GITERR_PATCH,
-	GITERR_WORKTREE
+	GITERR_WORKTREE,
+	GITERR_SHA1
 } git_error_t;
 
 /**

diff --git a/src/hash.h b/src/hash.h
@@ -16,11 +16,13 @@ int git_hash_global_init(void);
 int git_hash_ctx_init(git_hash_ctx *ctx);
 void git_hash_ctx_cleanup(git_hash_ctx *ctx);
 
-#if defined(GIT_COMMON_CRYPTO)
+#if defined(GIT_SHA1_COLLISIONDETECT)
+# include "hash/hash_collisiondetect.h"
+#elif defined(GIT_SHA1_COMMON_CRYPTO)
 # include "hash/hash_common_crypto.h"
-#elif defined(OPENSSL_SHA1)
+#elif defined(GIT_SHA1_OPENSSL)
 # include "hash/hash_openssl.h"
-#elif defined(WIN32_SHA1)
+#elif defined(GIT_SHA1_WIN32)
 # include "hash/hash_win32.h"
 #else
 # include "hash/hash_generic.h"

diff --git a/src/hash/hash_collisiondetect.h b/src/hash/hash_collisiondetect.h
@@ -0,0 +1,57 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
+#ifndef INCLUDE_hash_collisiondetect_h__
+#define INCLUDE_hash_collisiondetect_h__
+
+#include "hash.h"
+#include "sha1dc/sha1.h"
+
+struct git_hash_ctx {
+	SHA1_CTX c;
+};
+
+#define git_hash_global_init() 0
+#define git_hash_ctx_init(ctx) git_hash_init(ctx)
+#define git_hash_ctx_cleanup(ctx)
+
+GIT_INLINE(int) git_hash_init(git_hash_ctx *ctx)
+{
+	assert(ctx);
+	SHA1DCInit(&ctx->c);
+	return 0;
+}
+
+GIT_INLINE(int) git_hash_update(git_hash_ctx *ctx, const void *data, size_t len)
+{
+    const char *p = data;
+
+	assert(ctx);
+
+	/* We expect a size_t, but sha1dc only takes an int */
+	while (len > INT_MAX) {
+		SHA1DCUpdate(&ctx->c, p, INT_MAX);
+		p += INT_MAX;
+		len -= INT_MAX;
+	}
+
+	SHA1DCUpdate(&ctx->c, p, len);
+	return 0;
+}
+
+GIT_INLINE(int) git_hash_final(git_oid *out, git_hash_ctx *ctx)
+{
+	assert(ctx);
+	if (SHA1DCFinal(out->id, &ctx->c)) {
+		giterr_set(GITERR_SHA1, "SHA1 collision attack detected");
+		return -1;
+	}
+
+	return 0;
+}
+
+#endif /* INCLUDE_hash_collisiondetect_h__ */