diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 509ecbd..19b9d6b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -20,7 +20,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        sdk: [2.18.0, dev]
+        sdk: [3.0.0, dev]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/setup-dart@v1.3
@@ -47,7 +47,7 @@ jobs:
       matrix:
         # Add macos-latest and/or windows-latest if relevant for this package.
         os: [ubuntu-latest]
-        sdk: [2.18.0, dev]
+        sdk: [3.0.0, dev]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/setup-dart@v1.3
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6096264..3b5f944 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,12 @@
 # Changelog
 
+### 1.5.0
+
+- Make a number of fields `final` that should not be changed.
+- Add types to a few members.
+- Remove nullability from a field fields.
+- Require Dart 3.0
+
 ### 1.4.1
 
 - Fix null assertion when accessing valueBytes before encodedBytes.
@@ -20,6 +27,7 @@
 ### 1.2.2
 
 - Fix #62. Allow context specific tags to be cast to an asn1 sequence.
+
 ### 1.2.1
 
 - Fix #57
diff --git a/analysis_options.yaml b/analysis_options.yaml
index 1790435..0702f62 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -1,7 +1,9 @@
-include: package:lints/recommended.yaml
+include: package:dart_flutter_team_lints/analysis_options.yaml
 
 # This package was created before these constant rules were best practice.
 linter:
   rules:
     constant_identifier_names: false
+    lines_longer_than_80_chars: false
     non_constant_identifier_names: false
+    use_super_parameters: true
diff --git a/example/main.dart b/example/main.dart
index 71e3481..f8d51c5 100644
--- a/example/main.dart
+++ b/example/main.dart
@@ -1,5 +1,6 @@
 import 'dart:convert';
 import 'dart:typed_data';
+
 import 'package:asn1lib/asn1lib.dart';
 
 final certificateDER = decodePEM('''-----BEGIN CERTIFICATE-----
@@ -44,7 +45,7 @@ void main() {
   print(seq.valueBytes().length);
 }
 
-Uint8List decodePEM(pem) {
+Uint8List decodePEM(String pem) {
   var startsWith = [
     '-----BEGIN PUBLIC KEY-----',
     '-----BEGIN PRIVATE KEY-----',
diff --git a/lib/asn1lib.dart b/lib/asn1lib.dart
index 49202fb..c1ac8ca 100644
--- a/lib/asn1lib.dart
+++ b/lib/asn1lib.dart
@@ -1,34 +1,28 @@
-library asn1lib;
-
-import 'dart:typed_data';
 import 'dart:convert';
+import 'dart:typed_data';
 
-part 'asn1integer.dart';
-part 'asn1enumerated.dart';
-
-part 'asn1object.dart';
-part 'asn1application.dart';
-
-part 'asn1util.dart';
-part 'asn1octetstring.dart';
-part 'asn1exception.dart';
-part 'asn1sequence.dart';
-part 'asn1length.dart';
-part 'asn1parser.dart';
-part 'asn1constants.dart';
-part 'asn1null.dart';
-part 'asn1boolean.dart';
-part 'asn1set.dart';
-
-part 'asn1objectidentifier.dart';
-part 'asn1bitstring.dart';
-part 'asn1printablestring.dart';
-part 'asn1numericstring.dart';
-part 'asn1utf8string.dart';
-part 'asn1ia5string.dart';
-part 'asn1utctime.dart';
-part 'asn1bmpstring.dart';
-part 'asn1generalizedtime.dart';
-part 'asn1teletextstring.dart';
-
-part 'asn1ipaddress.dart';
+part 'src/asn1application.dart';
+part 'src/asn1bitstring.dart';
+part 'src/asn1bmpstring.dart';
+part 'src/asn1boolean.dart';
+part 'src/asn1constants.dart';
+part 'src/asn1enumerated.dart';
+part 'src/asn1exception.dart';
+part 'src/asn1generalizedtime.dart';
+part 'src/asn1ia5string.dart';
+part 'src/asn1integer.dart';
+part 'src/asn1ipaddress.dart';
+part 'src/asn1length.dart';
+part 'src/asn1null.dart';
+part 'src/asn1numericstring.dart';
+part 'src/asn1object.dart';
+part 'src/asn1objectidentifier.dart';
+part 'src/asn1octetstring.dart';
+part 'src/asn1parser.dart';
+part 'src/asn1printablestring.dart';
+part 'src/asn1sequence.dart';
+part 'src/asn1set.dart';
+part 'src/asn1teletextstring.dart';
+part 'src/asn1utctime.dart';
+part 'src/asn1utf8string.dart';
+part 'src/asn1util.dart';
diff --git a/lib/asn1application.dart b/lib/src/asn1application.dart
similarity index 70%
rename from lib/asn1application.dart
rename to lib/src/asn1application.dart
index 9e63146..3501a1c 100644
--- a/lib/asn1application.dart
+++ b/lib/src/asn1application.dart
@@ -1,12 +1,12 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 // Represent an ASN1 APPLICATION type. An Application is a
 // custom ASN1 object that delegates the interpretation to the
 // consumer.
 class ASN1Application extends ASN1Object {
-  ASN1Application({int tag = APPLICATION_CLASS}) : super(tag: tag);
+  ASN1Application({super.tag = APPLICATION_CLASS});
 
-  ASN1Application.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1Application.fromBytes(super.bytes) : super.fromBytes() {
     // check that this really is an application type
     if (!isApplication(tag)) {
       throw ASN1Exception('tag $tag is not an ASN1 Application class');
@@ -18,9 +18,9 @@ class ASN1Application extends ASN1Object {
 // custom ASN1 object that delegates the interpretation to the
 // consumer.
 class ASN1Private extends ASN1Object {
-  ASN1Private({int tag = PRIVATE_CLASS}) : super(tag: tag);
+  ASN1Private({super.tag = PRIVATE_CLASS});
 
-  ASN1Private.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1Private.fromBytes(super.bytes) : super.fromBytes() {
     // check that this really is an Private type
     if (!isPrivate(tag)) {
       throw ASN1Exception('tag $tag is not an ASN1 Private class');
@@ -32,9 +32,9 @@ class ASN1Private extends ASN1Object {
 // custom ASN1 object that delegates the interpretation to the
 // consumer.
 class ASN1ContextSpecific extends ASN1Object {
-  ASN1ContextSpecific({int tag = PRIVATE_CLASS}) : super(tag: tag);
+  ASN1ContextSpecific({super.tag = PRIVATE_CLASS});
 
-  ASN1ContextSpecific.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1ContextSpecific.fromBytes(super.bytes) : super.fromBytes() {
     // check that this really is an Private type
     if (!isContextSpecific(tag)) {
       throw ASN1Exception('tag $tag is not an ASN1 Context specific class');
diff --git a/lib/asn1bitstring.dart b/lib/src/asn1bitstring.dart
similarity index 75%
rename from lib/asn1bitstring.dart
rename to lib/src/asn1bitstring.dart
index 82279c0..ce96dbf 100644
--- a/lib/asn1bitstring.dart
+++ b/lib/src/asn1bitstring.dart
@@ -1,12 +1,12 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 Octet String. This is an array of character codes.
 ///
 class ASN1BitString extends ASN1Object {
   /// The decoded string value
-  late List<int> stringValue;
-  int unusedbits = 0;
+  late final List<int> stringValue;
+  late final int unusedbits;
 
   @override
   Uint8List contentBytes() => Uint8List.fromList(stringValue);
@@ -15,25 +15,24 @@ class ASN1BitString extends ASN1Object {
   /// Create an [ASN1BitString] initialized with String value. Optionally override the tag.
   ///
   ASN1BitString(this.stringValue,
-      {this.unusedbits = 0, int tag = BIT_STRING_TYPE})
-      : super(tag: tag);
+      {this.unusedbits = 0, super.tag = BIT_STRING_TYPE});
 
   ///
   /// Create an [ASN1OctetString] from an encoded list of bytes
   ///
-  ASN1BitString.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1BitString.fromBytes(super.bytes) : super.fromBytes() {
     unusedbits = valueBytes()[0];
     stringValue = valueBytes().sublist(1);
   }
 
   @override
-  Uint8List? _encode() {
+  Uint8List _encode() {
     var valBytes = [unusedbits];
     valBytes.addAll(stringValue);
     _valueByteLength = valBytes.length;
     _encodeHeader();
     _setValueBytes(valBytes);
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   static String decodeOctetString(Uint8List bytes) =>
diff --git a/lib/asn1bmpstring.dart b/lib/src/asn1bmpstring.dart
similarity index 80%
rename from lib/asn1bmpstring.dart
rename to lib/src/asn1bmpstring.dart
index 7e7df22..25e33a9 100644
--- a/lib/asn1bmpstring.dart
+++ b/lib/src/asn1bmpstring.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 BMP String.
@@ -7,19 +7,18 @@ part of asn1lib;
 ///
 class ASN1BMPString extends ASN1Object {
   /// The decoded string value
-  late String stringValue;
+  late final String stringValue;
 
   ///
   /// Create an [ASN1BMPString] initialized with String value.
   /// Optionally override the tag
   ///
-  ASN1BMPString(this.stringValue, {int tag = BMP_STRING_TYPE})
-      : super(tag: tag);
+  ASN1BMPString(this.stringValue, {super.tag = BMP_STRING_TYPE});
 
   ///
   /// Create an [ASN1BMPString] from an encoded list of bytes
   ///
-  ASN1BMPString.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1BMPString.fromBytes(super.bytes) : super.fromBytes() {
     var octets = valueBytes();
     var mergedOctets = <int>[];
 
@@ -33,7 +32,7 @@ class ASN1BMPString extends ASN1Object {
   }
 
   @override
-  Uint8List? _encode() {
+  Uint8List _encode() {
     var octets = utf8.encode(stringValue);
     var doubleOctets = <int>[];
 
@@ -45,7 +44,7 @@ class ASN1BMPString extends ASN1Object {
     _valueByteLength = doubleOctets.length;
     _encodeHeader();
     _setValueBytes(doubleOctets);
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   @override
diff --git a/lib/asn1boolean.dart b/lib/src/asn1boolean.dart
similarity index 60%
rename from lib/asn1boolean.dart
rename to lib/src/asn1boolean.dart
index 46236b0..f69a70f 100644
--- a/lib/asn1boolean.dart
+++ b/lib/src/asn1boolean.dart
@@ -1,31 +1,29 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 Boolean
 ///
 class ASN1Boolean extends ASN1Object {
-  bool? _boolValue;
+  late final bool booleanValue;
 
   static final ASN1Boolean ASN1TrueBoolean = ASN1Boolean(true);
   static final ASN1Boolean ASN1FalseBoolean = ASN1Boolean(false);
 
-  bool? get booleanValue => _boolValue;
-
   // ASN1Boolean(this._boolValue,{tag: BOOLEAN_TYPE}):super(tag:BOOLEAN_TYPE) {
-  ASN1Boolean(this._boolValue, {tag = BOOLEAN_TYPE}) : super(tag: tag) {
+  ASN1Boolean(this.booleanValue, {super.tag = BOOLEAN_TYPE}) {
     _valueByteLength = 1;
   }
 
   ASN1Boolean.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
     var b = bytes[_valueStartPosition];
-    _boolValue = (b == BOOLEAN_TRUE_VALUE);
+    booleanValue = (b == BOOLEAN_TRUE_VALUE);
   }
 
   @override
-  Uint8List? _encode() {
+  Uint8List _encode() {
     super._encodeHeader();
     super._setValueBytes(
-        [_boolValue == true ? BOOLEAN_TRUE_VALUE : BOOLEAN_FALSE_VALUE]);
-    return _encodedBytes;
+        [booleanValue ? BOOLEAN_TRUE_VALUE : BOOLEAN_FALSE_VALUE]);
+    return _encodedBytes!;
   }
 }
diff --git a/lib/asn1constants.dart b/lib/src/asn1constants.dart
similarity index 98%
rename from lib/asn1constants.dart
rename to lib/src/asn1constants.dart
index d04daba..7fd3554 100644
--- a/lib/asn1constants.dart
+++ b/lib/src/asn1constants.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 // Sere https://luca.ntop.org/Teaching/Appunti/asn1.html
 // tag bytes for various ASN1 BER objects
diff --git a/lib/asn1enumerated.dart b/lib/src/asn1enumerated.dart
similarity index 62%
rename from lib/asn1enumerated.dart
rename to lib/src/asn1enumerated.dart
index bdcc817..ba83d0f 100644
--- a/lib/asn1enumerated.dart
+++ b/lib/src/asn1enumerated.dart
@@ -1,9 +1,9 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An enum is encoded as an Integer.
 ///
 class ASN1Enumerated extends ASN1Integer {
-  ASN1Enumerated(int i, {tag = ENUMERATED_TYPE})
+  ASN1Enumerated(int i, {int tag = ENUMERATED_TYPE})
       : super(BigInt.from(i), tag: tag);
 }
diff --git a/lib/asn1exception.dart b/lib/src/asn1exception.dart
similarity index 82%
rename from lib/asn1exception.dart
rename to lib/src/asn1exception.dart
index 7019a8f..575f9b5 100644
--- a/lib/asn1exception.dart
+++ b/lib/src/asn1exception.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 Exception
diff --git a/lib/asn1generalizedtime.dart b/lib/src/asn1generalizedtime.dart
similarity index 87%
rename from lib/asn1generalizedtime.dart
rename to lib/src/asn1generalizedtime.dart
index 47e032c..bbca3a1 100644
--- a/lib/asn1generalizedtime.dart
+++ b/lib/src/asn1generalizedtime.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 GeneralizedTime value.
@@ -10,20 +10,19 @@ part of asn1lib;
 ///
 class ASN1GeneralizedTime extends ASN1Object {
   // The decoded date value
-  late DateTime dateTimeValue;
+  late final DateTime dateTimeValue;
 
   ///
   /// Create an [ASN1GeneralizedTime] initialized with DateTime value.
   ///
   /// Optionally override the tag
   ///
-  ASN1GeneralizedTime(this.dateTimeValue, {int tag = GENERALIZED_TIME})
-      : super(tag: tag);
+  ASN1GeneralizedTime(this.dateTimeValue, {super.tag = GENERALIZED_TIME});
 
   ///
   /// Create an [ASN1GeneralizedTime] from an encoded list of bytes
   ///
-  ASN1GeneralizedTime.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1GeneralizedTime.fromBytes(super.bytes) : super.fromBytes() {
     var octets = valueBytes();
     var stringValue = ascii.decode(octets);
     var year = stringValue.substring(0, 4);
@@ -42,7 +41,7 @@ class ASN1GeneralizedTime extends ASN1Object {
   }
 
   @override
-  Uint8List? _encode() {
+  Uint8List _encode() {
     var utc = dateTimeValue.toUtc();
     var year = utc.year.toString();
     var month = utc.month.toString();
@@ -57,7 +56,7 @@ class ASN1GeneralizedTime extends ASN1Object {
     _valueByteLength = valBytes.length;
     _encodeHeader();
     _setValueBytes(valBytes);
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   @override
diff --git a/lib/asn1ia5string.dart b/lib/src/asn1ia5string.dart
similarity index 73%
rename from lib/asn1ia5string.dart
rename to lib/src/asn1ia5string.dart
index 6a52a78..49309e8 100644
--- a/lib/asn1ia5string.dart
+++ b/lib/src/asn1ia5string.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 IA5 String.
@@ -7,30 +7,29 @@ part of asn1lib;
 ///
 class ASN1IA5String extends ASN1Object {
   /// The decoded string value
-  late String stringValue;
+  late final String stringValue;
 
   ///
   /// Create an [ASN1IA5String] initialized with String value.
   /// Optionally override the tag
   ///
-  ASN1IA5String(this.stringValue, {int tag = IA5_STRING_TYPE})
-      : super(tag: tag);
+  ASN1IA5String(this.stringValue, {super.tag = IA5_STRING_TYPE});
 
   ///
   /// Create an [ASN1IA5String] from an encoded list of bytes
   ///
-  ASN1IA5String.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1IA5String.fromBytes(super.bytes) : super.fromBytes() {
     var octets = valueBytes();
     stringValue = ascii.decode(octets);
   }
 
   @override
-  Uint8List? _encode() {
+  Uint8List _encode() {
     var octets = ascii.encode(stringValue);
     _valueByteLength = octets.length;
     _encodeHeader();
     _setValueBytes(octets);
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   @override
diff --git a/lib/asn1integer.dart b/lib/src/asn1integer.dart
similarity index 83%
rename from lib/asn1integer.dart
rename to lib/src/asn1integer.dart
index d293ac7..6abb64b 100644
--- a/lib/asn1integer.dart
+++ b/lib/src/asn1integer.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// ASN1Integer encoding / decoding.
@@ -8,30 +8,28 @@ part of asn1lib;
 /// int or BigInt
 ///
 class ASN1Integer extends ASN1Object {
-  late BigInt _intValue;
+  late final BigInt valueAsBigInteger;
 
-  ASN1Integer(this._intValue, {tag = INTEGER_TYPE}) : super(tag: tag);
+  ASN1Integer(this.valueAsBigInteger, {super.tag = INTEGER_TYPE});
 
-  ASN1Integer.fromInt(int x, {tag = INTEGER_TYPE}) : super(tag: tag) {
-    _intValue = BigInt.from(x);
+  ASN1Integer.fromInt(int x, {super.tag = INTEGER_TYPE}) {
+    valueAsBigInteger = BigInt.from(x);
     _encode();
   }
 
-  ASN1Integer.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
-    _intValue = decodeBigInt(valueBytes());
+  ASN1Integer.fromBytes(super.bytes) : super.fromBytes() {
+    valueAsBigInteger = decodeBigInt(valueBytes());
   }
 
-  int get intValue => _intValue.toInt();
-
-  BigInt? get valueAsBigInteger => _intValue;
+  int get intValue => valueAsBigInteger.toInt();
 
   @override
-  Uint8List? _encode() {
-    var t = encodeBigInt(_intValue);
+  Uint8List _encode() {
+    var t = encodeBigInt(valueAsBigInteger);
     _valueByteLength = t.length;
     super._encodeHeader();
     _setValueBytes(t);
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   static Uint8List encodeInt(int x) => encodeBigInt(BigInt.from(x));
@@ -90,7 +88,7 @@ class ASN1Integer extends ASN1Object {
     var result = Uint8List(bytes);
 
     number = number.abs();
-    for (var i = 0, j = bytes - 1; i < (bytes); i++, --j) {
+    for (var i = 0, j = bytes - 1; i < bytes; i++, --j) {
       var x = number.remainder(_b256).toInt();
       result[j] = x;
       number = number >> 8;
@@ -137,8 +135,8 @@ class ASN1Integer extends ASN1Object {
       identical(this, other) ||
       other is ASN1Integer &&
           runtimeType == other.runtimeType &&
-          _intValue == other._intValue;
+          valueAsBigInteger == other.valueAsBigInteger;
 
   @override
-  int get hashCode => _intValue.hashCode;
+  int get hashCode => valueAsBigInteger.hashCode;
 }
diff --git a/lib/asn1ipaddress.dart b/lib/src/asn1ipaddress.dart
similarity index 80%
rename from lib/asn1ipaddress.dart
rename to lib/src/asn1ipaddress.dart
index 3f7ad30..5a8c6a4 100644
--- a/lib/asn1ipaddress.dart
+++ b/lib/src/asn1ipaddress.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 IP Address. This is length-4 array of character codes.
@@ -21,24 +21,23 @@ class ASN1IpAddress extends ASN1OctetString {
   ///
   /// Create an [ASN1IpAddress] from an encoded list of bytes.
   ///
-  ASN1IpAddress.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
-    octets = valueBytes();
+  ASN1IpAddress.fromBytes(super.bytes) : super.fromBytes() {
     _assertValidLength(octets);
   }
 
   ///
   /// Create an [ASN1IpAddress] from an IP Address String such as '192.168.1.1'
   ///
-  static ASN1IpAddress fromComponentString(String path, {tag = IP_ADDRESS}) =>
-      fromComponents(path.split('.').map((v) => int.parse(v)).toList(),
-          tag: tag);
+  static ASN1IpAddress fromComponentString(String path,
+          {int tag = IP_ADDRESS}) =>
+      fromComponents(path.split('.').map(int.parse).toList(), tag: tag);
 
   ///
   /// Create an [ASN1IpAddress] from a list of int IP Address octets
   /// e.g. [192, 168, 1, 1]
   ///
   static ASN1IpAddress fromComponents(List<int> components,
-          {tag = IP_ADDRESS}) =>
+          {int tag = IP_ADDRESS}) =>
       ASN1IpAddress(components, tag: tag);
 
   /// Ensure there are no more or less than 4 octets for an IPv4 address
diff --git a/lib/asn1length.dart b/lib/src/asn1length.dart
similarity index 95%
rename from lib/asn1length.dart
rename to lib/src/asn1length.dart
index 29d9a0f..9224bca 100644
--- a/lib/asn1length.dart
+++ b/lib/src/asn1length.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// Class to encode /decode ASN1 length bytes.
@@ -65,14 +65,14 @@ class ASN1Length {
   ///
   static ASN1Length decodeLength(Uint8List encodedBytes) {
     var valueStartPosition = 2; //default
-    var length = (encodedBytes[1] & 0x7F);
+    var length = encodedBytes[1] & 0x7F;
     if (length != encodedBytes[1]) {
       var numLengthBytes = length;
 
       length = 0;
       for (var i = 0; i < numLengthBytes; i++) {
         length <<= 8;
-        length |= (encodedBytes[valueStartPosition++] & 0xFF);
+        length |= encodedBytes[valueStartPosition++] & 0xFF;
       }
     }
     /*
diff --git a/lib/asn1null.dart b/lib/src/asn1null.dart
similarity index 63%
rename from lib/asn1null.dart
rename to lib/src/asn1null.dart
index 8ae71c6..4685bdd 100644
--- a/lib/asn1null.dart
+++ b/lib/src/asn1null.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 Null Object
@@ -7,9 +7,9 @@ class ASN1Null extends ASN1Object {
   @override
   Uint8List get _encodedBytes => Uint8List.fromList([tag, 0x00]);
 
-  ASN1Null({tag = NULL_TYPE}) : super(tag: tag);
+  ASN1Null({super.tag = NULL_TYPE});
 
-  ASN1Null.fromBytes(Uint8List bytes) : super.fromBytes(bytes);
+  ASN1Null.fromBytes(super.bytes) : super.fromBytes();
 
   @override
   Uint8List _encode() => Uint8List.fromList([tag, 0x00]);
diff --git a/lib/asn1numericstring.dart b/lib/src/asn1numericstring.dart
similarity index 77%
rename from lib/asn1numericstring.dart
rename to lib/src/asn1numericstring.dart
index d63e2aa..5dd3428 100644
--- a/lib/asn1numericstring.dart
+++ b/lib/src/asn1numericstring.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 Numeric String
@@ -8,15 +8,14 @@ part of asn1lib;
 ///
 class ASN1NumericString extends ASN1Object {
   // The decoded string value
-  late String stringValue;
+  late final String stringValue;
 
   ///
   /// Create an [ASN1NumericString] initialized with String value.
   ///
   /// Optionally override the tag
   ///
-  ASN1NumericString(this.stringValue, {int tag = NUMERIC_STRING_TYPE})
-      : super(tag: tag) {
+  ASN1NumericString(this.stringValue, {super.tag = NUMERIC_STRING_TYPE}) {
     if (!RegExp(r'^[\d\s]*$').hasMatch(stringValue)) {
       throw ASN1Exception(
           'ASN1 NumericString should only include digits or spaces');
@@ -26,18 +25,18 @@ class ASN1NumericString extends ASN1Object {
   ///
   /// Create an [ASN1NumericString] from an encoded list of bytes.
   ///
-  ASN1NumericString.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1NumericString.fromBytes(super.bytes) : super.fromBytes() {
     var octets = valueBytes();
     stringValue = ascii.decode(octets);
   }
 
   @override
-  Uint8List? _encode() {
+  Uint8List _encode() {
     var octets = ascii.encode(stringValue);
     _valueByteLength = octets.length;
     _encodeHeader();
     _setValueBytes(octets);
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   @override
diff --git a/lib/asn1object.dart b/lib/src/asn1object.dart
similarity index 91%
rename from lib/asn1object.dart
rename to lib/src/asn1object.dart
index b966330..465b743 100644
--- a/lib/asn1object.dart
+++ b/lib/src/asn1object.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// Holds the encoded and decoded representation of an ASN1 object.
@@ -12,8 +12,7 @@ part of asn1lib;
 ///
 class ASN1Object {
   /// The BER tag representing this object
-  final int _tag;
-  int get tag => _tag;
+  final int tag;
 
   ///
   /// The ASN1 encoded bytes.
@@ -44,7 +43,7 @@ class ASN1Object {
   bool get isEncoded => _encodedBytes != null;
 
   /// Create an ASN1Object. Optionally set the tag
-  ASN1Object({int tag = 0}) : _tag = tag;
+  ASN1Object({this.tag = 0});
 
   ///
   /// Create an object that encapsulates a set of value bytes that are already encoded.
@@ -52,7 +51,7 @@ class ASN1Object {
   /// This is used in LDAP (for example) to encode a CHOICE type
   /// The supplied valBytes is the encoded value of the choice element
   ///
-  ASN1Object.preEncoded(int tag, Uint8List valBytes) : _tag = tag {
+  ASN1Object.preEncoded(this.tag, Uint8List valBytes) {
     _valueByteLength = valBytes.length;
     _encodeHeader();
     _encodedBytes!.setRange(
@@ -69,7 +68,7 @@ class ASN1Object {
   /// byte stream we dont always know how long an object is
   /// until we complete parsing it).
   ///
-  ASN1Object.fromBytes(Uint8List bytes) : _tag = bytes[0] {
+  ASN1Object.fromBytes(Uint8List bytes) : tag = bytes[0] {
     _encodedBytes = bytes;
     _initFromBytes();
   }
@@ -92,13 +91,13 @@ class ASN1Object {
   /// next object starts in the stream.
   ///
   ////
-  int get totalEncodedByteLength => _valueStartPosition + _valueByteLength!;
+  int get totalEncodedByteLength => _valueStartPosition + _valueByteLength;
 
   ///
   /// Length of the encoded value bytes. This does not include the length of
   /// the tag or length fields. See [totalEncodedByteLength].
   ///
-  int? _valueByteLength;
+  late int _valueByteLength;
 
   ///
   /// The index where the value bytes start. This is the position after the tag + length bytes.
@@ -116,15 +115,15 @@ class ASN1Object {
   /// calling this. We need this know how big to make the encoded object array. Subclasses are
   /// responsible for encoding their value representations using [encodeBigInt]
   ///
-  Uint8List? _encodeHeader() {
+  Uint8List _encodeHeader() {
     if (_encodedBytes == null) {
-      var lenEnc = ASN1Length.encodeLength(_valueByteLength!);
-      _encodedBytes = Uint8List(1 + lenEnc.length + _valueByteLength!);
+      var lenEnc = ASN1Length.encodeLength(_valueByteLength);
+      _encodedBytes = Uint8List(1 + lenEnc.length + _valueByteLength);
       _encodedBytes![0] = tag;
       _encodedBytes!.setRange(1, 1 + lenEnc.length, lenEnc, 0);
       _valueStartPosition = 1 + lenEnc.length;
     }
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   ///
@@ -132,7 +131,7 @@ class ASN1Object {
   ///
   /// Subclasses will need to override this.
   ///
-  Uint8List? _encode() => _encodeHeader();
+  Uint8List _encode() => _encodeHeader();
 
   ///
   /// Return just the value bytes.
@@ -150,7 +149,7 @@ class ASN1Object {
   /// some other tags like BitString include padding in their valueBytes.
   /// This method always returns the unpadded contentBytes.
   ///
-  Uint8List? contentBytes() => valueBytes();
+  Uint8List contentBytes() => valueBytes();
 
   ///
   /// Subclasses can call this to set the value bytes
diff --git a/lib/asn1objectidentifier.dart b/lib/src/asn1objectidentifier.dart
similarity index 91%
rename from lib/asn1objectidentifier.dart
rename to lib/src/asn1objectidentifier.dart
index 5556b57..854105e 100644
--- a/lib/asn1objectidentifier.dart
+++ b/lib/src/asn1objectidentifier.dart
@@ -1,10 +1,10 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 Object Identifier
 ///
 class ASN1ObjectIdentifier extends ASN1Object {
-  static final Map<String, String> DN = {
+  static const Map<String, String> DN = {
     'cn': '2.5.4.3',
     'sn': '2.5.4.4',
     'c': '2.5.4.6',
@@ -58,12 +58,12 @@ class ASN1ObjectIdentifier extends ASN1Object {
     'ecdsaWithSHA256': '1.2.840.10045.4.3.2'
   };
 
-  late List<int> oi;
+  late final List<int> oi;
 
   String? identifier;
 
-  ASN1ObjectIdentifier(this.oi, {this.identifier, tag = OBJECT_IDENTIFIER})
-      : super(tag: tag);
+  ASN1ObjectIdentifier(this.oi,
+      {this.identifier, super.tag = OBJECT_IDENTIFIER});
 
   ///
   /// Instantiate a [ASN1ObjectIdentifier] from the given [bytes].
@@ -123,7 +123,7 @@ class ASN1ObjectIdentifier extends ASN1Object {
   }
 
   @override
-  Uint8List? _encode() {
+  Uint8List _encode() {
     var valBytes = <int>[oi[0] * 40 + oi[1]];
 
     for (var ci = 2; ci < oi.length; ci++) {
@@ -146,16 +146,15 @@ class ASN1ObjectIdentifier extends ASN1Object {
     _valueByteLength = valBytes.length;
     super._encodeHeader();
     _setValueBytes(valBytes);
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   static ASN1ObjectIdentifier fromComponentString(String path,
-          {tag = OBJECT_IDENTIFIER}) =>
-      fromComponents(path.split('.').map((v) => int.parse(v)).toList(),
-          tag: tag);
+          {int tag = OBJECT_IDENTIFIER}) =>
+      fromComponents(path.split('.').map(int.parse).toList(), tag: tag);
 
   static ASN1ObjectIdentifier fromComponents(List<int> components,
-      {tag = OBJECT_IDENTIFIER}) {
+      {int tag = OBJECT_IDENTIFIER}) {
     assert(components.length >= 2);
     assert(components[0] < 3);
     assert(components[1] < 39);
@@ -166,7 +165,8 @@ class ASN1ObjectIdentifier extends ASN1Object {
 
   static final _names = <String, ASN1ObjectIdentifier>{};
 
-  static ASN1ObjectIdentifier fromName(String name, {tag = OBJECT_IDENTIFIER}) {
+  static ASN1ObjectIdentifier fromName(String name,
+      {int tag = OBJECT_IDENTIFIER}) {
     name = name.toLowerCase();
 
     for (var entry in _names.entries) {
diff --git a/lib/asn1octetstring.dart b/lib/src/asn1octetstring.dart
similarity index 79%
rename from lib/asn1octetstring.dart
rename to lib/src/asn1octetstring.dart
index 138f38a..f4ab13b 100644
--- a/lib/asn1octetstring.dart
+++ b/lib/src/asn1octetstring.dart
@@ -1,21 +1,20 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 Octet String. This is an array of character codes.
 ///
 class ASN1OctetString extends ASN1Object {
   /// The decoded string value
-  late Uint8List octets;
+  late final Uint8List octets;
 
   @override
-  Uint8List? contentBytes() => octets;
+  Uint8List contentBytes() => octets;
 
   ///
   /// Create an [ASN1OctetString] initialized with a [String] or a [List<int>].
   /// Optionally override the tag
   ///
-  ASN1OctetString(dynamic octets, {int tag = OCTET_STRING_TYPE})
-      : super(tag: tag) {
+  ASN1OctetString(dynamic octets, {super.tag = OCTET_STRING_TYPE}) {
     if (octets is String) {
       this.octets = Uint8List.fromList(octets.codeUnits);
     } else if (octets is Uint8List) {
@@ -31,7 +30,7 @@ class ASN1OctetString extends ASN1Object {
   ///
   /// Create an [ASN1OctetString] from an encoded list of bytes.
   ///
-  ASN1OctetString.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1OctetString.fromBytes(super.bytes) : super.fromBytes() {
     octets = valueBytes();
   }
 
@@ -41,13 +40,13 @@ class ASN1OctetString extends ASN1Object {
   String get stringValue => String.fromCharCodes(octets);
 
   @override
-  Uint8List? _encode() {
+  Uint8List _encode() {
     _valueByteLength = octets.length;
     _encodeHeader();
     _setValueBytes(octets);
     //this.encodedBytes.setRange(valueStartPosition,
     //  valueStartPosition + valBytes.length, valBytes);
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   @override
diff --git a/lib/asn1parser.dart b/lib/src/asn1parser.dart
similarity index 97%
rename from lib/asn1parser.dart
rename to lib/src/asn1parser.dart
index 7dd3276..6690d42 100644
--- a/lib/asn1parser.dart
+++ b/lib/src/asn1parser.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// Parses ASN1 BER Encoded bytes to create ASN1 Objects
@@ -38,7 +38,7 @@ class ASN1Parser {
     var subBytes = Uint8List.view(_bytes.buffer, offset, len);
     //print("parser _bytes=${_bytes} position=${_position} len=$l  bytes=${hex(subBytes)}");
 
-    late ASN1Object obj;
+    final ASN1Object obj;
 
     switch (tag & 0xc0) {
       // get highest 2 bits - these are the type
@@ -59,6 +59,8 @@ class ASN1Parser {
       case CONTEXT_SPECIFIC_CLASS:
         obj = ASN1Object.fromBytes(subBytes);
         break;
+      default:
+        throw UnimplementedError();
     }
 
     _position = _position + obj.totalEncodedByteLength;
diff --git a/lib/asn1printablestring.dart b/lib/src/asn1printablestring.dart
similarity index 75%
rename from lib/asn1printablestring.dart
rename to lib/src/asn1printablestring.dart
index a94ba91..83bd1ee 100644
--- a/lib/asn1printablestring.dart
+++ b/lib/src/asn1printablestring.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 Printable String.
@@ -8,31 +8,30 @@ part of asn1lib;
 ///
 class ASN1PrintableString extends ASN1Object {
   // The decoded string value
-  late String stringValue;
+  late final String stringValue;
 
   ///
   /// Create an [ASN1PrintableString] initialized with String value.
   ///
   /// Optionally override the tag
   ///
-  ASN1PrintableString(this.stringValue, {int tag = PRINTABLE_STRING_TYPE})
-      : super(tag: tag);
+  ASN1PrintableString(this.stringValue, {super.tag = PRINTABLE_STRING_TYPE});
 
   ///
   /// Create an [ASN1PrintableString] from an encoded list of bytes.
   ///
-  ASN1PrintableString.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1PrintableString.fromBytes(super.bytes) : super.fromBytes() {
     var octets = valueBytes();
     stringValue = ascii.decode(octets);
   }
 
   @override
-  Uint8List? _encode() {
+  Uint8List _encode() {
     var octets = ascii.encode(stringValue);
     _valueByteLength = octets.length;
     _encodeHeader();
     _setValueBytes(octets);
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   @override
diff --git a/lib/asn1sequence.dart b/lib/src/asn1sequence.dart
similarity index 88%
rename from lib/asn1sequence.dart
rename to lib/src/asn1sequence.dart
index 543a2a8..cc36c50 100644
--- a/lib/asn1sequence.dart
+++ b/lib/src/asn1sequence.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// Represents a ASN1 BER encoded sequence.
@@ -15,7 +15,7 @@ class ASN1Sequence extends ASN1Object {
   /// Note that bytes array b could be longer than the actual encoded sequence - in which case
   /// we ignore any remaining bytes.
   ///
-  ASN1Sequence.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1Sequence.fromBytes(super.bytes) : super.fromBytes() {
     if (isUniversal(tag) && !isSequence(tag)) {
       throw ASN1Exception('The tag $tag does not look like a sequence type');
     }
@@ -25,7 +25,7 @@ class ASN1Sequence extends ASN1Object {
   ///
   /// Create a new empty ASN1 Sequence. Optionally override the default tag
   ///
-  ASN1Sequence({int tag = CONSTRUCTED_SEQUENCE_TYPE}) : super(tag: tag);
+  ASN1Sequence({super.tag = CONSTRUCTED_SEQUENCE_TYPE});
 
   ///
   /// Add an [ASN1Object] to the sequence. Objects will be serialized to BER in the order they were added
@@ -35,7 +35,7 @@ class ASN1Sequence extends ASN1Object {
   }
 
   @override
-  Uint8List? _encode() {
+  Uint8List _encode() {
     _valueByteLength = _childLength();
     super._encodeHeader();
     var i = _valueStartPosition;
@@ -45,7 +45,7 @@ class ASN1Sequence extends ASN1Object {
       encodedBytes.setRange(i, i + b.length, b);
       i += b.length;
     }
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   ///
@@ -54,7 +54,7 @@ class ASN1Sequence extends ASN1Object {
   int _childLength() {
     var l = 0;
     for (var obj in elements) {
-      l += obj._encode()!.length;
+      l += obj._encode().length;
     }
     return l;
   }
diff --git a/lib/asn1set.dart b/lib/src/asn1set.dart
similarity index 88%
rename from lib/asn1set.dart
rename to lib/src/asn1set.dart
index 8b23e99..06b17e8 100644
--- a/lib/asn1set.dart
+++ b/lib/src/asn1set.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1Set.
@@ -11,14 +11,14 @@ class ASN1Set extends ASN1Object {
   ///
   /// Note that bytes could be longer than the actual sequence - in which case we would ignore any remaining bytes
   ///
-  ASN1Set.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1Set.fromBytes(super.bytes) : super.fromBytes() {
     if (!isSet(tag)) {
       throw ASN1Exception('The tag $tag does not look like a set type');
     }
     _decodeSet();
   }
 
-  ASN1Set({int tag = CONSTRUCTED_SET_TYPE}) : super(tag: tag);
+  ASN1Set({super.tag = CONSTRUCTED_SET_TYPE});
 
   ///
   /// Add an element to the set
@@ -28,7 +28,7 @@ class ASN1Set extends ASN1Object {
   }
 
   @override
-  Uint8List? _encode() {
+  Uint8List _encode() {
     _valueByteLength = _childLength();
     //super._encode();
 
@@ -39,7 +39,7 @@ class ASN1Set extends ASN1Object {
       encodedBytes.setRange(i, i + b.length, b);
       i += b.length;
     }
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   ///
diff --git a/lib/asn1teletextstring.dart b/lib/src/asn1teletextstring.dart
similarity index 69%
rename from lib/asn1teletextstring.dart
rename to lib/src/asn1teletextstring.dart
index 82f97f9..2d11c94 100644
--- a/lib/asn1teletextstring.dart
+++ b/lib/src/asn1teletextstring.dart
@@ -1,35 +1,34 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 Teletext String.
 ///
 class ASN1TeletextString extends ASN1Object {
   // The decoded string value
-  late String stringValue;
+  late final String stringValue;
 
   ///
   /// Create an [ASN1TeletextString] initialized with String value.
   ///
   /// Optionally override the tag
   ///
-  ASN1TeletextString(this.stringValue, {int tag = UTF8_STRING_TYPE})
-      : super(tag: tag);
+  ASN1TeletextString(this.stringValue, {super.tag = UTF8_STRING_TYPE});
 
   ///
   /// Create an [ASN1TeletextString] from an encoded list of bytes
   ///
-  ASN1TeletextString.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1TeletextString.fromBytes(super.bytes) : super.fromBytes() {
     var octets = valueBytes();
     stringValue = ascii.decode(octets);
   }
 
   @override
-  Uint8List? _encode() {
+  Uint8List _encode() {
     var octets = ascii.encode(stringValue);
     _valueByteLength = octets.length;
     _encodeHeader();
     _setValueBytes(octets);
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   @override
diff --git a/lib/asn1utctime.dart b/lib/src/asn1utctime.dart
similarity index 88%
rename from lib/asn1utctime.dart
rename to lib/src/asn1utctime.dart
index ad66290..0691d00 100644
--- a/lib/asn1utctime.dart
+++ b/lib/src/asn1utctime.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 UtcTime value.
@@ -10,19 +10,19 @@ part of asn1lib;
 ///
 class ASN1UtcTime extends ASN1Object {
   // The decoded date value
-  late DateTime dateTimeValue;
+  late final DateTime dateTimeValue;
 
   ///
   /// Create an [ASN1UtcTime] initialized with DateTime value.
   ///
   /// Optionally override the tag
   ///
-  ASN1UtcTime(this.dateTimeValue, {int tag = UTC_TIME_TYPE}) : super(tag: tag);
+  ASN1UtcTime(this.dateTimeValue, {super.tag = UTC_TIME_TYPE});
 
   ///
   /// Create an [ASN1UtcTime] from an encoded list of bytes
   ///
-  ASN1UtcTime.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1UtcTime.fromBytes(super.bytes) : super.fromBytes() {
     // The DateTime.parse() function wants:
     // * Either T or space as separator between date and time.
     // * Full year with 4 digits (the UtcTime in ASN.1 has only two digits for year).
@@ -41,7 +41,7 @@ class ASN1UtcTime extends ASN1Object {
   }
 
   @override
-  Uint8List? _encode() {
+  Uint8List _encode() {
     var utc = dateTimeValue.toUtc();
     var year = utc.year.toString().substring(2).padLeft(2, '0');
     var month = utc.month.toString().padLeft(2, '0');
@@ -56,7 +56,7 @@ class ASN1UtcTime extends ASN1Object {
     _valueByteLength = valBytes.length;
     _encodeHeader();
     _setValueBytes(valBytes);
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   @override
diff --git a/lib/asn1utf8string.dart b/lib/src/asn1utf8string.dart
similarity index 69%
rename from lib/asn1utf8string.dart
rename to lib/src/asn1utf8string.dart
index b3773e3..8db153c 100644
--- a/lib/asn1utf8string.dart
+++ b/lib/src/asn1utf8string.dart
@@ -1,35 +1,34 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 ///
 /// An ASN1 UTF8 String.
 ///
 class ASN1UTF8String extends ASN1Object {
   // The decoded string value
-  late String utf8StringValue;
+  late final String utf8StringValue;
 
   ///
   /// Create an [ASN1UTF8String] initialized with String value.
   ///
   /// Optionally override the tag
   ///
-  ASN1UTF8String(this.utf8StringValue, {int tag = UTF8_STRING_TYPE})
-      : super(tag: tag);
+  ASN1UTF8String(this.utf8StringValue, {super.tag = UTF8_STRING_TYPE});
 
   ///
   /// Create an [ASN1UTF8String] from an encoded list of bytes
   ///
-  ASN1UTF8String.fromBytes(Uint8List bytes) : super.fromBytes(bytes) {
+  ASN1UTF8String.fromBytes(super.bytes) : super.fromBytes() {
     var octets = valueBytes();
     utf8StringValue = utf8.decode(octets);
   }
 
   @override
-  Uint8List? _encode() {
+  Uint8List _encode() {
     var octets = utf8.encode(utf8StringValue);
     _valueByteLength = octets.length;
     _encodeHeader();
     _setValueBytes(octets);
-    return _encodedBytes;
+    return _encodedBytes!;
   }
 
   @override
diff --git a/lib/asn1util.dart b/lib/src/asn1util.dart
similarity index 92%
rename from lib/asn1util.dart
rename to lib/src/asn1util.dart
index 47992a1..4518b67 100644
--- a/lib/asn1util.dart
+++ b/lib/src/asn1util.dart
@@ -1,4 +1,4 @@
-part of asn1lib;
+part of '../asn1lib.dart';
 
 class ASN1Util {
   // convert a list to a hex string. Used for debugging ASN1 output
@@ -18,7 +18,7 @@ class ASN1Util {
     if (x is List) {
       return listToString(x as List<int>);
     } else {
-      return x.toRadixString(16);
+      return (x as int).toRadixString(16);
     }
   }
 
diff --git a/pubspec.yaml b/pubspec.yaml
index b0170b1..561c8d3 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,13 +1,13 @@
 name: asn1lib
-version: 1.4.1
+version: 1.5.0
 description: An ASN1 parser library for Dart. Encodes / decodes from ASN1 Objects to BER bytes
 homepage: https://github.com/wstrange/asn1lib
 environment:
- sdk: '>=2.12.0 <4.0.0'
+ sdk: ^3.0.0
 
 dev_dependencies:
+  dart_flutter_team_lints: ^1.0.0
   test: ^1.22.0
-  lints: ^2.0.0
 
 # PEM certs are for testing - and are OK to publish
 false_secrets:
diff --git a/test/app_test.dart b/test/app_test.dart
index dca7e3d..5f4e9ae 100644
--- a/test/app_test.dart
+++ b/test/app_test.dart
@@ -1,7 +1,8 @@
 import 'dart:convert';
 import 'dart:typed_data';
-import 'package:test/test.dart';
+
 import 'package:asn1lib/asn1lib.dart';
+import 'package:test/test.dart';
 
 void main() {
   // https://github.com/wstrange/asn1lib/issues/57
diff --git a/test/asn1_generalized_time_test.dart b/test/asn1_generalized_time_test.dart
index 470ec98..41406cd 100644
--- a/test/asn1_generalized_time_test.dart
+++ b/test/asn1_generalized_time_test.dart
@@ -1,7 +1,7 @@
 import 'dart:typed_data';
 
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
+import 'package:test/test.dart';
 
 void main() {
   test('encode', () {
diff --git a/test/asn1_integer_test.dart b/test/asn1_integer_test.dart
index 0476188..6383472 100644
--- a/test/asn1_integer_test.dart
+++ b/test/asn1_integer_test.dart
@@ -1,9 +1,9 @@
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
+import 'package:test/test.dart';
 
 // list of test value pairs. The first value is the integer to encode, the
 // second is expected length of the content bytes (excludes type and length).
-final List<int> intTestValues = [
+const List<int> intTestValues = [
   0x00000000,
   1,
   0x00000001,
@@ -189,7 +189,7 @@ void testPair(int x, int expectedLength) {
   //print(
   //    'Value 0x${x.toRadixString(16)},$expectedLength. Encoded Bytes =  ${_printBytes(int1.encodedBytes.sublist(2))}');
 
-  var contentLength = int1.contentBytes()!.length;
+  var contentLength = int1.contentBytes().length;
   expect(contentLength, expectedLength,
       reason: 'expected number of content bytes is wrong');
   var int2 = ASN1Integer.fromBytes(int1.encodedBytes);
diff --git a/test/asn1_object_prencoded_test.dart b/test/asn1_object_prencoded_test.dart
index 500d4ff..c6c8080 100644
--- a/test/asn1_object_prencoded_test.dart
+++ b/test/asn1_object_prencoded_test.dart
@@ -1,7 +1,5 @@
-library asn1test;
-
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
+import 'package:test/test.dart';
 
 void main() {
   test('support optional tagging', () {
diff --git a/test/asn1_parser_test.dart b/test/asn1_parser_test.dart
index 7fd4fc8..ec552f9 100644
--- a/test/asn1_parser_test.dart
+++ b/test/asn1_parser_test.dart
@@ -1,9 +1,7 @@
-library asn1test;
-
 import 'dart:typed_data';
 
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
+import 'package:test/test.dart';
 
 void main() {
   test('ASN1Parser Encode/Decode Test', () {
diff --git a/test/asn1_teletext_string_test.dart b/test/asn1_teletext_string_test.dart
index 7be68b1..08a4213 100644
--- a/test/asn1_teletext_string_test.dart
+++ b/test/asn1_teletext_string_test.dart
@@ -1,7 +1,7 @@
 import 'dart:typed_data';
 
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
+import 'package:test/test.dart';
 
 void main() {
   test('encode', () {
diff --git a/test/asn1bit_string.dart b/test/asn1bit_string.dart
index 5b3cdc8..a50cc5d 100644
--- a/test/asn1bit_string.dart
+++ b/test/asn1bit_string.dart
@@ -1,8 +1,7 @@
-library asn1test;
+import 'dart:typed_data';
 
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
-import 'dart:typed_data';
+import 'package:test/test.dart';
 
 void main() {
   test('Decode 0 unused bits', () {
diff --git a/test/asn1element_test.dart b/test/asn1element_test.dart
index 598ed67..7e5f21a 100644
--- a/test/asn1element_test.dart
+++ b/test/asn1element_test.dart
@@ -1,15 +1,13 @@
-library asn1test;
+import 'dart:math';
+import 'dart:typed_data';
 
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
-
-import 'dart:typed_data';
-import 'dart:math';
+import 'package:test/test.dart';
 
 void main() {
   test('Test integer encoding', () {
     // key = integer to encode, val = expected encoding bytes
-    var m = {};
+    var m = <int, List<int>>{};
     m[0] = [0x0];
     m[127] = [0x7f];
     m[128] = [0x0, 0x80];
diff --git a/test/asn1ia5string_test.dart b/test/asn1ia5string_test.dart
index 7315eca..276538c 100644
--- a/test/asn1ia5string_test.dart
+++ b/test/asn1ia5string_test.dart
@@ -1,9 +1,7 @@
-library asn1test;
+import 'dart:typed_data';
 
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
-
-import 'dart:typed_data';
+import 'package:test/test.dart';
 
 void main() {
   test('encode', () {
diff --git a/test/asn1ipaddress_test.dart b/test/asn1ipaddress_test.dart
index 418004a..fb7c947 100644
--- a/test/asn1ipaddress_test.dart
+++ b/test/asn1ipaddress_test.dart
@@ -1,9 +1,7 @@
-library asn1test;
+import 'dart:typed_data';
 
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
-
-import 'dart:typed_data';
+import 'package:test/test.dart';
 
 void main() {
   test('IpAddress default constructor', () {
diff --git a/test/asn1null_test.dart b/test/asn1null_test.dart
index 0cb8d34..634c958 100644
--- a/test/asn1null_test.dart
+++ b/test/asn1null_test.dart
@@ -1,7 +1,5 @@
-library asn1test;
-
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
+import 'package:test/test.dart';
 
 void main() {
   test('tagless', () {
diff --git a/test/asn1numericstring_test.dart b/test/asn1numericstring_test.dart
index 9452d94..eab42f4 100644
--- a/test/asn1numericstring_test.dart
+++ b/test/asn1numericstring_test.dart
@@ -1,9 +1,7 @@
-library asn1test;
+import 'dart:typed_data';
 
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
-
-import 'dart:typed_data';
+import 'package:test/test.dart';
 
 void main() {
   test('encode', () {
diff --git a/test/asn1objectidentifier_test.dart b/test/asn1objectidentifier_test.dart
index 7291038..b3222df 100644
--- a/test/asn1objectidentifier_test.dart
+++ b/test/asn1objectidentifier_test.dart
@@ -1,9 +1,7 @@
-library asn1test;
+import 'dart:typed_data';
 
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
-
-import 'dart:typed_data';
+import 'package:test/test.dart';
 
 // many tests from - http://luca.ntop.org/Teaching/Appunti/asn1.html
 // print(a.oi.map((x) => '0x' + x.toRadixString(16).padLeft(2, '0')));
diff --git a/test/asn1utf8string_test.dart b/test/asn1utf8string_test.dart
index e9d1777..22fa1d8 100644
--- a/test/asn1utf8string_test.dart
+++ b/test/asn1utf8string_test.dart
@@ -1,9 +1,7 @@
-library asn1test;
+import 'dart:typed_data';
 
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
-
-import 'dart:typed_data';
+import 'package:test/test.dart';
 
 void main() {
   test('encode', () {
diff --git a/test/bigint_test.dart b/test/bigint_test.dart
index f35468f..66ab359 100644
--- a/test/bigint_test.dart
+++ b/test/bigint_test.dart
@@ -1,11 +1,9 @@
-library asn1test;
-
-import 'package:test/test.dart';
-import 'package:asn1lib/asn1lib.dart';
-
 import 'dart:convert';
-import 'dart:typed_data';
 import 'dart:io';
+import 'dart:typed_data';
+
+import 'package:asn1lib/asn1lib.dart';
+import 'package:test/test.dart';
 
 void main() {
   test('rsa private key', () {
diff --git a/test/der_encoding_test.dart b/test/der_encoding_test.dart
index 9ff670a..9033cc0 100644
--- a/test/der_encoding_test.dart
+++ b/test/der_encoding_test.dart
@@ -1,7 +1,5 @@
-library asn1test.der;
-
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
+import 'package:test/test.dart';
 
 void main() {
   group('der-encoding', () {
diff --git a/test/issue_61.dart b/test/issue_61.dart
index ddfafff..d2bb9dc 100644
--- a/test/issue_61.dart
+++ b/test/issue_61.dart
@@ -1,7 +1,9 @@
+import 'dart:convert';
 import 'dart:typed_data';
+
 import 'package:asn1lib/asn1lib.dart';
 import 'package:test/test.dart';
-import 'dart:convert';
+
 import '../example/main.dart'; // for decodePEM
 
 // Test cases for https://github.com/wstrange/asn1lib/issues/61
diff --git a/test/issue_62_test.dart b/test/issue_62_test.dart
index 378e866..babe62e 100644
--- a/test/issue_62_test.dart
+++ b/test/issue_62_test.dart
@@ -1,7 +1,7 @@
 import 'dart:typed_data';
 
-import 'package:test/test.dart';
 import 'package:asn1lib/asn1lib.dart';
+import 'package:test/test.dart';
 
 // test for issue https://github.com/wstrange/asn1lib/issues/62
 void main() {
diff --git a/test/pem_encoded_rsa_keys_test.dart b/test/pem_encoded_rsa_keys_test.dart
index 872f257..04c7747 100644
--- a/test/pem_encoded_rsa_keys_test.dart
+++ b/test/pem_encoded_rsa_keys_test.dart
@@ -1,8 +1,9 @@
 import 'dart:convert';
-import 'package:test/test.dart';
-import 'package:asn1lib/asn1lib.dart';
 import 'dart:typed_data';
 
+import 'package:asn1lib/asn1lib.dart';
+import 'package:test/test.dart';
+
 void main() {
   var publicKeyDER = decodePEM('''-----BEGIN PUBLIC KEY-----
 MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA1W25XAj9GvCvSY7JzVAh
@@ -202,7 +203,7 @@ L2fTYScBC9dHB+QBDm/c/oYpIj9tsKuxNJO0Io+b1cIziWqOytwlHnzAx9X/KGeB
 
     expect(publicKeyBitString.valueBytes().length, 271);
 
-    asn1Parser = ASN1Parser(publicKeyBitString.contentBytes()!);
+    asn1Parser = ASN1Parser(publicKeyBitString.contentBytes());
     var pkSeq = asn1Parser.nextObject() as ASN1Sequence;
 
     var expected = BigInt.from(65537);
@@ -250,7 +251,7 @@ c0 f1 cc 0e 1a 2c a6 52-b1 ee 6e a3 fe 21 cb e5
     var privateKey = topLevelSeq.elements[2];
     expect(privateKey.valueBytes().length, 1193);
 
-    asn1Parser = ASN1Parser(privateKey.contentBytes()!);
+    asn1Parser = ASN1Parser(privateKey.contentBytes());
     var pkSeq = asn1Parser.nextObject() as ASN1Sequence;
     version = pkSeq.elements[0];
     var modulus = pkSeq.elements[1] as ASN1Integer;
@@ -376,7 +377,7 @@ f0:57:25:e7:78:12:17:5d:f5
   });
 }
 
-Uint8List decodePEM(pem) {
+Uint8List decodePEM(String pem) {
   var startsWith = [
     '-----BEGIN PUBLIC KEY-----',
     '-----BEGIN PRIVATE KEY-----',