diff --git a/src/crypto/sha1/sha1.go b/src/crypto/sha1/sha1.go
index db70b7d09a..8c48042b1c 100644
--- a/src/crypto/sha1/sha1.go
+++ b/src/crypto/sha1/sha1.go
@@ -75,7 +75,7 @@ func (d *digest) UnmarshalBinary(b []byte) error {
 	b, d.h[4] = consumeUint32(b)
 	b = b[copy(d.x[:], b):]
 	b, d.len = consumeUint64(b)
-	d.nx = int(d.len) % chunk
+	d.nx = int(d.len % chunk)
 	return nil
 }
 
diff --git a/src/crypto/sha1/sha1_test.go b/src/crypto/sha1/sha1_test.go
index 4f229262ad..c047204bf3 100644
--- a/src/crypto/sha1/sha1_test.go
+++ b/src/crypto/sha1/sha1_test.go
@@ -11,6 +11,7 @@ import (
 	"crypto/rand"
 	"encoding"
 	"fmt"
+	"hash"
 	"io"
 	"testing"
 )
@@ -152,6 +153,63 @@ func TestBlockGeneric(t *testing.T) {
 	}
 }
 
+// Tests for unmarshaling hashes that have hashed a large amount of data
+// The initial hash generation is omitted from the test, because it takes a long time.
+// The test contains some already-generated states, and their expected sums
+// Tests a problem that is outlined in Github issue #29543
+// The problem is triggered when an amount of data has been hashed for which
+// the data length has a 1 in the 32nd bit. When casted to int, this changes
+// the sign of the value, and causes the modulus operation to return a
+// different result.
+type unmarshalTest struct {
+	state string
+	sum   string
+}
+
+var largeUnmarshalTests = []unmarshalTest{
+	// Data length: 7_102_415_735
+	unmarshalTest{
+		state: "sha\x01\x13\xbc\xfe\x83\x8c\xbd\xdfP\x1f\xd8ڿ<\x9eji8t\xe1\xa5@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuv\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\xa7VCw",
+		sum:   "bc6245c9959cc33e1c2592e5c9ea9b5d0431246c",
+	},
+	// Data length: 6_565_544_823
+	unmarshalTest{
+		state: "sha\x01m;\x16\xa6R\xbe@\xa9nĈ\xf9S\x03\x00B\xc2\xdcv\xcf@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuv\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x87VCw",
+		sum:   "8f2d1c0e4271768f35feb918bfe21ea1387a2072",
+	},
+}
+
+func safeSum(h hash.Hash) (sum []byte, err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			err = fmt.Errorf("sum panic: %v", r)
+		}
+	}()
+
+	return h.Sum(nil), nil
+}
+
+func TestLargeHashes(t *testing.T) {
+	for i, test := range largeUnmarshalTests {
+
+		h := New()
+		if err := h.(encoding.BinaryUnmarshaler).UnmarshalBinary([]byte(test.state)); err != nil {
+			t.Errorf("test %d could not unmarshal: %v", i, err)
+			continue
+		}
+
+		sum, err := safeSum(h)
+		if err != nil {
+			t.Errorf("test %d could not sum: %v", i, err)
+			continue
+		}
+
+		if fmt.Sprintf("%x", sum) != test.sum {
+			t.Errorf("test %d sum mismatch: expect %s got %x", i, test.sum, sum)
+		}
+	}
+}
+
 var bench = New()
 var buf = make([]byte, 8192)