diff --git a/cryptobyte/asn1.go b/cryptobyte/asn1.go
index 528b9bf..f930f7e 100644
--- a/cryptobyte/asn1.go
+++ b/cryptobyte/asn1.go
@@ -470,7 +470,8 @@ func (s *String) ReadASN1GeneralizedTime(out *time.Time) bool {
 // It reports whether the read was successful.
 func (s *String) ReadASN1BitString(out *encoding_asn1.BitString) bool {
 	var bytes String
-	if !s.ReadASN1(&bytes, asn1.BIT_STRING) || len(bytes) == 0 {
+	if !s.ReadASN1(&bytes, asn1.BIT_STRING) || len(bytes) == 0 ||
+		len(bytes)*8/8 != len(bytes) {
 		return false
 	}
 
@@ -740,7 +741,7 @@ func (s *String) readASN1(out *String, outTag *asn1.Tag, skipHeader bool) bool {
 		length = headerLen + len32
 	}
 
-	if uint32(int(length)) != length || !s.ReadBytes((*[]byte)(out), int(length)) {
+	if int(length) < 0 || !s.ReadBytes((*[]byte)(out), int(length)) {
 		return false
 	}
 	if skipHeader && !out.Skip(int(headerLen)) {
diff --git a/cryptobyte/asn1_test.go b/cryptobyte/asn1_test.go
index 9f6c952..f6bb96d 100644
--- a/cryptobyte/asn1_test.go
+++ b/cryptobyte/asn1_test.go
@@ -31,6 +31,10 @@ var readASN1TestData = []readASN1Test{
 	{"non-minimal length", append([]byte{0x30, 0x82, 0, 0x80}, make([]byte, 0x80)...), 0x30, false, nil},
 	{"invalid tag", []byte{0xa1, 3, 0x4, 1, 1}, 31, false, nil},
 	{"high tag", []byte{0x1f, 0x81, 0x80, 0x01, 2, 1, 2}, 0xff /* actually 0x4001, but tag is uint8 */, false, nil},
+	{"2**31 - 1 length", []byte{0x30, 0x84, 0x7f, 0xff, 0xff, 0xff}, 0x30, false, nil},
+	{"2**32 - 1 length", []byte{0x30, 0x84, 0xff, 0xff, 0xff, 0xff}, 0x30, false, nil},
+	{"2**63 - 1 length", []byte{0x30, 0x88, 0x7f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff}, 0x30, false, nil},
+	{"2**64 - 1 length", []byte{0x30, 0x88, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff}, 0x30, false, nil},
 }
 
 func TestReadASN1(t *testing.T) {
diff --git a/cryptobyte/string.go b/cryptobyte/string.go
index 39bf98a..589d297 100644
--- a/cryptobyte/string.go
+++ b/cryptobyte/string.go
@@ -24,7 +24,7 @@ type String []byte
 // read advances a String by n bytes and returns them. If less than n bytes
 // remain, it returns nil.
 func (s *String) read(n int) []byte {
-	if len(*s) < n {
+	if len(*s) < n || n < 0 {
 		return nil
 	}
 	v := (*s)[:n]
@@ -105,11 +105,6 @@ func (s *String) readLengthPrefixed(lenLen int, outChild *String) bool {
 		length = length << 8
 		length = length | uint32(b)
 	}
-	if int(length) < 0 {
-		// This currently cannot overflow because we read uint24 at most, but check
-		// anyway in case that changes in the future.
-		return false
-	}
 	v := s.read(int(length))
 	if v == nil {
 		return false
diff --git a/internal/wycheproof/wycheproof_test.go b/internal/wycheproof/wycheproof_test.go
index d57a88c..49d6626 100644
--- a/internal/wycheproof/wycheproof_test.go
+++ b/internal/wycheproof/wycheproof_test.go
@@ -17,7 +17,6 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
-	"runtime"
 	"testing"
 
 	_ "crypto/sha1"
@@ -34,9 +33,6 @@ func TestMain(m *testing.M) {
 		log.Printf("skipping test because 'go' command is unavailable: %v", err)
 		os.Exit(0)
 	}
-	if runtime.GOARCH == "386" || runtime.GOARCH == "arm" {
-		os.Exit(0) // skip tests
-	}
 
 	// Download the JSON test files from github.com/google/wycheproof
 	// using `go mod download -json` so the cached source of the testdata