diff --git a/ext/.gitignore b/ext/.gitignore
index 836cdc2..41301d3 100644
--- a/ext/.gitignore
+++ b/ext/.gitignore
@@ -4,3 +4,4 @@
 Makefile*
 *.so
 asn1c/
+sha1collisiondetection/
diff --git a/ext/README b/ext/README
index 1285dc7..9a48db4 100644
--- a/ext/README
+++ b/ext/README
@@ -10,6 +10,9 @@ cd ..
 asn1c/asn1c/asn1c -S asn1c/skeletons -pdu=all -pdu=Certificate -fwide-types *.asn1
 rm converter-sample.c
 
+git clone https://github.com/cr-marcstevens/sha1collisiondetection.git
+cp sha1collisiondetection/lib/* .
+
 # RFC3280 has a Time type which will cause the compiler to create a Time.h
 # file. This will conflict with <time.h> on a case insensitive filesystem.
 # You can work around this problem with this hack:
diff --git a/ext/asn1validator.c b/ext/certlint_ext.c
similarity index 84%
rename from ext/asn1validator.c
rename to ext/certlint_ext.c
index b7fc4e2..f473e14 100644
--- a/ext/asn1validator.c
+++ b/ext/certlint_ext.c
@@ -1,6 +1,9 @@
 /*
- * Ruby bindings for interfacing with asn1c-generated ASN.1 PDU parsing code.
- * Copyright (c) 2016 Matt Palmer <matt@hezmatt.org>
+ * Ruby bindings for a variety of C libraries used for various parts of
+ * certlint's work.  See README in this directory for build instructions and
+ * references to the library code used.
+ *
+ * Copyright (c) 2016,2017 Matt Palmer <matt@hezmatt.org>
  *
  * Licensed under the Apache License, Version 2.0 (the "License"). You may not
  * use this file except in compliance with the License. A copy of the License
@@ -21,6 +24,7 @@
 #include <errno.h>
 
 #include <asn_application.h>
+#include "sha1collisiondetection/lib/sha1.h"
 
 extern asn_TYPE_descriptor_t *asn_pdu_collection[];
 
@@ -36,7 +40,7 @@ static asn_TYPE_descriptor_t *asn1pdu_type_descriptor(VALUE pdu_type) {
 		}
 		pdu++;
 	}
-	
+
 	rb_raise(rb_eArgError, "Unknown PDU type");
 	return NULL;  /* Unreachable, we hope */
 }
@@ -110,13 +114,13 @@ static VALUE asn1pdu_to_der(VALUE self) {
 	VALUE t = rb_iv_get(self, "@pdu_type");
 	void *pdu_structure = asn1pdu_decode_pdu(d, t);
 	asn_enc_rval_t rv;
-	
+
 	rv.encoded = -1;
 
 	while (rv.encoded == -1) {
 		if (derbuf) free(derbuf);
 		derlen *= 2;
-		
+
 		derbuf = malloc(derlen);
 		if (derbuf == NULL) {
 			rb_raise(rb_eNoMemError, "Unable to allocate memory for derbuf");
@@ -127,15 +131,35 @@ static VALUE asn1pdu_to_der(VALUE self) {
 	}
 
 	ASN_STRUCT_FREE(*asn1pdu_type_descriptor(t), pdu_structure);
-	
+
 	return rb_str_new(derbuf, rv.encoded);
 }
 
-void Init_asn1validator() {
+static VALUE sha1collision_check(VALUE self, VALUE data) {
+	SHA1_CTX ctx;
+	unsigned char hash[20];
+	SHA1DCInit(&ctx);
+	int iscoll;
+
+	SHA1DCSetSafeHash(&ctx, 0);
+	SHA1DCSetUseUBC(&ctx, 0);
+	SHA1DCUpdate(&ctx, RSTRING_PTR(data), RSTRING_LEN(data));
+	iscoll = SHA1DCFinal(hash,&ctx);
+
+	if (iscoll) {
+		return Qtrue;
+	} else {
+		return Qfalse;
+	}
+}
+
+void Init_certlint_ext() {
 	mod_certlint = rb_define_module("CertLint");
 	class_certlint_asn1pdu = rb_define_class_under(mod_certlint, "ASN1Validator", rb_cObject);
 
 	rb_define_method(class_certlint_asn1pdu, "initialize", asn1pdu_initialize, 2);
 	rb_define_method(class_certlint_asn1pdu, "check_constraints", asn1pdu_check_constraints, 0);
 	rb_define_method(class_certlint_asn1pdu, "to_der", asn1pdu_to_der, 0);
+
+	rb_define_singleton_method(mod_certlint, "sha1_collision?", sha1collision_check, 1);
 }
diff --git a/ext/extconf.rb b/ext/extconf.rb
index 44b1491..170827b 100644
--- a/ext/extconf.rb
+++ b/ext/extconf.rb
@@ -1,7 +1,7 @@
 require "mkmf"
 
-dir_config("asn1validator")
+dir_config("certlint_ext")
 
 $srcs = Dir["*.c"]
 
-create_makefile("asn1validator")
+create_makefile("certlint_ext")
diff --git a/lib/certlint.rb b/lib/certlint.rb
index 664ffcc..dcf3363 100644
--- a/lib/certlint.rb
+++ b/lib/certlint.rb
@@ -3,3 +3,4 @@
 require 'certlint/pemlint'
 require 'certlint/namelint'
 require 'certlint/generalnames'
+require 'certlint/sha1lint'
diff --git a/lib/certlint/certlint.rb b/lib/certlint/certlint.rb
index a046aab..c10c313 100755
--- a/lib/certlint/certlint.rb
+++ b/lib/certlint/certlint.rb
@@ -13,7 +13,7 @@
 # express or implied. See the License for the specific language governing
 # permissions and limitations under the License.
 require 'rubygems'
-require 'asn1validator'
+require 'certlint_ext'
 require 'openssl'
 
 require_relative 'namelint'
@@ -262,6 +262,9 @@ def self.lint(der)
       return messages
     end
 
+    # Check for SHAnanigans
+    messages += SHA1Lint.lint(der)
+
     # Check time fields
     OpenSSL::ASN1.traverse(der) do |_depth, offset, header_len, length, _constructed, tag_class, tag|
       start_c = offset + header_len
diff --git a/lib/certlint/sha1lint.rb b/lib/certlint/sha1lint.rb
new file mode 100644
index 0000000..dc4f511
--- /dev/null
+++ b/lib/certlint/sha1lint.rb
@@ -0,0 +1,28 @@
+#!/usr/bin/ruby -Eutf-8:utf-8
+# encoding: UTF-8
+# Copyright 2017 Matt Palmer <mpalmer@hezmatt.org>. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"). You may not
+# use this file except in compliance with the License. A copy of the License
+# is located at
+#
+#   http://aws.amazon.com/apache2.0/
+#
+# or in the "license" file accompanying this file. This file is distributed on
+# an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+# express or implied. See the License for the specific language governing
+# permissions and limitations under the License.
+
+module CertLint
+  class SHA1Lint
+    def self.lint(der)
+      messages = []
+
+      if CertLint.sha1_collision?(der.to_s)
+        messages << 'E: SHA1 collision attempt detected'
+      end
+
+      messages
+    end
+  end
+end