diff --git a/mercurial/thirdparty/sha1dc/cext.c b/mercurial/thirdparty/sha1dc/cext.c
new file mode 100644
--- /dev/null
+++ b/mercurial/thirdparty/sha1dc/cext.c
@@ -0,0 +1,195 @@
+#define PY_SSIZE_T_CLEAN
+#include <Python.h>
+
+#include "lib/sha1.h"
+
+#if PY_MAJOR_VERSION >= 3
+#define IS_PY3K
+#endif
+
+/* helper to switch things like string literal depending on Python version */
+#ifdef IS_PY3K
+#define PY23(py2, py3) py3
+#else
+#define PY23(py2, py3) py2
+#endif
+
+static char sha1dc_doc[] = "Efficient detection of SHA1 collision constructs.";
+
+/* clang-format off */
+typedef struct {
+	PyObject_HEAD
+	SHA1_CTX ctx;
+} pysha1ctx;
+/* clang-format on */
+
+static int pysha1ctx_init(pysha1ctx *self, PyObject *args)
+{
+	const char *data = NULL;
+	Py_ssize_t len;
+
+	SHA1DCInit(&(self->ctx));
+	/* We don't want "safe" sha1s, wherein sha1dc can give you a
+	   different hash for something that's trying to give you a
+	   collision. We just want to detect collisions.
+	 */
+	SHA1DCSetSafeHash(&(self->ctx), 0);
+	if (!PyArg_ParseTuple(args, PY23("|s#", "|y#"), &data, &len)) {
+		return -1;
+	}
+	if (data) {
+		SHA1DCUpdate(&(self->ctx), data, len);
+	}
+	return 0;
+}
+
+static void pysha1ctx_dealloc(pysha1ctx *self)
+{
+	PyObject_Del(self);
+}
+
+static PyObject *pysha1ctx_update(pysha1ctx *self, PyObject *args)
+{
+	const char *data;
+	Py_ssize_t len;
+	if (!PyArg_ParseTuple(args, PY23("s#", "y#"), &data, &len)) {
+		return NULL;
+	}
+	SHA1DCUpdate(&(self->ctx), data, len);
+	Py_RETURN_NONE;
+}
+
+/* it is intentional that this take a ctx by value, as that clones the
+   context so we can keep using .update() without poisoning the state
+   with padding.
+*/
+static int finalize(SHA1_CTX ctx, unsigned char *hash_out) {
+	if (SHA1DCFinal(hash_out, &ctx)) {
+		PyErr_SetString(PyExc_OverflowError,
+				"sha1 collision attack detected");
+		return 0;
+	}
+	return 1;
+}
+
+static PyObject *pysha1ctx_digest(pysha1ctx *self)
+{
+	unsigned char hash[20];
+	if (!finalize(self->ctx, hash)) {
+		return NULL;
+	}
+	return PyBytes_FromStringAndSize((char *)hash, 20);
+}
+
+static PyObject *pysha1ctx_hexdigest(pysha1ctx *self)
+{
+	unsigned char hash[20];
+	if (!finalize(self->ctx, hash)) {
+		return NULL;
+	}
+	char hexhash[40];
+	static const char hexdigit[] = "0123456789abcdef";
+	for (int i = 0; i < 20; ++i) {
+		hexhash[i * 2] = hexdigit[hash[i] >> 4];
+		hexhash[i * 2 + 1] = hexdigit[hash[i] & 15];
+	}
+	return PyBytes_FromStringAndSize(hexhash, 40);
+}
+
+static PyTypeObject sha1ctxType;
+
+static PyObject *pysha1ctx_copy(pysha1ctx *self)
+{
+	pysha1ctx *clone = (pysha1ctx *)PyObject_New(pysha1ctx, &sha1ctxType);
+	if (!clone) {
+		return NULL;
+	}
+	clone->ctx = self->ctx;
+	return (PyObject *)clone;
+}
+
+static PyMethodDef pysha1ctx_methods[] = {
+    {"update", (PyCFunction)pysha1ctx_update, METH_VARARGS,
+     "Update this hash object's state with the provided bytes."},
+    {"digest", (PyCFunction)pysha1ctx_digest, METH_NOARGS,
+     "Return the digest value as a string of binary data."},
+    {"hexdigest", (PyCFunction)pysha1ctx_hexdigest, METH_NOARGS,
+     "Return the digest value as a string of hexadecimal digits."},
+    {"copy", (PyCFunction)pysha1ctx_copy, METH_NOARGS,
+     "Return a copy of the hash object."},
+    {NULL},
+};
+
+/* clang-format off */
+static PyTypeObject sha1ctxType = {
+	PyVarObject_HEAD_INIT(NULL, 0)                    /* header */
+	"sha1dc.sha1",                                    /* tp_name */
+	sizeof(pysha1ctx),                                /* tp_basicsize */
+	0,                                                /* tp_itemsize */
+	(destructor)pysha1ctx_dealloc,                    /* tp_dealloc */
+	0,                                                /* tp_print */
+	0,                                                /* tp_getattr */
+	0,                                                /* tp_setattr */
+	0,                                                /* tp_compare */
+	0,                                                /* tp_repr */
+	0,                                                /* tp_as_number */
+	0,                                                /* tp_as_sequence */
+	0,                                                /* tp_as_mapping */
+	0,                                                /* tp_hash */
+	0,                                                /* tp_call */
+	0,                                                /* tp_str */
+	0,                                                /* tp_getattro */
+	0,                                                /* tp_setattro */
+	0,                                                /* tp_as_buffer */
+	Py_TPFLAGS_DEFAULT,                               /* tp_flags */
+	"sha1 implementation that looks for collisions",  /* tp_doc */
+	0,                                                /* tp_traverse */
+	0,                                                /* tp_clear */
+	0,                                                /* tp_richcompare */
+	0,                                                /* tp_weaklistoffset */
+	0,                                                /* tp_iter */
+	0,                                                /* tp_iternext */
+	pysha1ctx_methods,                                /* tp_methods */
+	0,                                                /* tp_members */
+	0,                                                /* tp_getset */
+	0,                                                /* tp_base */
+	0,                                                /* tp_dict */
+	0,                                                /* tp_descr_get */
+	0,                                                /* tp_descr_set */
+	0,                                                /* tp_dictoffset */
+	(initproc)pysha1ctx_init,                         /* tp_init */
+	0,                                                /* tp_alloc */
+};
+/* clang-format on */
+
+static PyMethodDef methods[] = {
+    {NULL, NULL},
+};
+
+static void module_init(PyObject *mod)
+{
+	sha1ctxType.tp_new = PyType_GenericNew;
+	if (PyType_Ready(&sha1ctxType) < 0)
+		return;
+	Py_INCREF(&sha1ctxType);
+
+	PyModule_AddObject(mod, "sha1", (PyObject *)&sha1ctxType);
+}
+
+#ifdef IS_PY3K
+static struct PyModuleDef sha1dc_module = {PyModuleDef_HEAD_INIT, "sha1dc",
+                                           sha1dc_doc, -1, methods};
+
+PyMODINIT_FUNC PyInit_sha1dc(void)
+{
+	PyObject *mod = PyModule_Create(&sha1dc_module);
+	module_init(mod);
+	return mod;
+}
+#else
+PyMODINIT_FUNC initsha1dc(void)
+{
+	PyObject *mod = Py_InitModule3("sha1dc", methods, sha1dc_doc);
+	module_init(mod);
+}
+#endif
diff --git a/setup.py b/setup.py
--- a/setup.py
+++ b/setup.py
@@ -1481,6 +1481,14 @@
         ],
     ),
     Extension(
+        'mercurial.thirdparty.sha1dc',
+        [
+            'mercurial/thirdparty/sha1dc/cext.c',
+            'mercurial/thirdparty/sha1dc/lib/sha1.c',
+            'mercurial/thirdparty/sha1dc/lib/ubc_check.c',
+        ],
+    ),
+    Extension(
         'hgext.fsmonitor.pywatchman.bser', ['hgext/fsmonitor/pywatchman/bser.c']
     ),
     RustStandaloneExtension(