diff --git a/src/packet/secret_key.js b/src/packet/secret_key.js
index d260d12d..8cdd7db6 100644
--- a/src/packet/secret_key.js
+++ b/src/packet/secret_key.js
@@ -134,7 +134,7 @@ class SecretKeyPacket extends PublicKeyPacket {
         this.s2k = newS2KFromType(s2kType);
         i += this.s2k.read(bytes.subarray(i, bytes.length));
 
-        if (this.s2k.type === 'gnu-dummy') {
+        if (this.s2k.gnuType === 'gnu-dummy') {
           return;
         }
       } else if (this.s2kUsage) {
@@ -253,7 +253,7 @@ class SecretKeyPacket extends PublicKeyPacket {
     // - [Optional] If secret data is encrypted (string-to-key usage octet
     //   not zero), an Initial Vector (IV) of the same length as the
     //   cipher's block size.
-    if (this.s2kUsage && this.s2k.type !== 'gnu-dummy') {
+    if (this.s2kUsage && this.s2k.gnuType !== 'gnu-dummy') {
       optionalFieldsArr.push(...this.iv);
     }
 
@@ -306,7 +306,7 @@ class SecretKeyPacket extends PublicKeyPacket {
    * @returns {Boolean}
    */
   isDummy() {
-    return !!(this.s2k && this.s2k.type === 'gnu-dummy');
+    return !!(this.s2k && this.s2k.gnuType === 'gnu-dummy');
   }
 
   /**
@@ -327,7 +327,8 @@ class SecretKeyPacket extends PublicKeyPacket {
     this.s2k = newS2KFromType(enums.s2k.gnu, config);
     this.s2k.algorithm = 0;
     this.s2k.c = 0;
-    this.s2k.type = 'gnu-dummy';
+    this.s2k.type = 'gnu';
+    this.s2k.gnuType = 'gnu-dummy';
     this.s2kUsage = 254;
     this.symmetric = enums.symmetric.aes256;
   }
diff --git a/src/type/s2k/argon2.ts b/src/type/s2k/argon2.ts
index 5636d608..f8421b57 100644
--- a/src/type/s2k/argon2.ts
+++ b/src/type/s2k/argon2.ts
@@ -2,6 +2,8 @@ import defaultConfig from '../../config';
 import enums from '../../enums';
 import util from '../../util';
 import crypto from '../../crypto';
+import type { default as loadArgonWasmModuleType } from 'argon2id';
+import { Config } from '../../../openpgp';
 
 const ARGON2_TYPE = 0x02; // id
 const ARGON2_VERSION = 0x13;
@@ -20,21 +22,21 @@ export class Argon2OutOfMemoryError extends Error {
 }
 
 // cache argon wasm module
-let loadArgonWasmModule: Function;
-let argon2Promise: Promise<Function>;
+let loadArgonWasmModule: typeof loadArgonWasmModuleType;
+let argon2Promise: ReturnType<typeof loadArgonWasmModuleType>;
 // reload wasm module above this treshold, to deallocated used memory
 const ARGON2_WASM_MEMORY_THRESHOLD_RELOAD = 2 << 19;
 
 class Argon2S2K {
-    type: string;
-    salt: Uint8Array | null;
-    t: number;
-    p: number;
-    encodedM: number;
+    type: 'argon2';
+    private salt: Uint8Array | null;
+    private t: number;
+    private p: number;
+    private encodedM: number;
   /**
   * @param {Object} [config] - Full configuration, defaults to openpgp.config
   */
-  constructor(config = defaultConfig) {
+  constructor(config: Config) {
     
     const { passes, parallelism, memoryExponent } = config.s2kArgon2Params;
 
diff --git a/src/type/s2k/generic.ts b/src/type/s2k/generic.ts
index df6bc8ea..36b9b41b 100644
--- a/src/type/s2k/generic.ts
+++ b/src/type/s2k/generic.ts
@@ -27,9 +27,10 @@
  * @module type/s2k
  */
 
-import defaultConfig from '../../config';
-import crypto from '../../crypto';
+import type { Config, enums as enumsType } from '../../../openpgp';
 import enums from '../../enums';
+
+import crypto from '../../crypto';
 import { UnsupportedError } from '../../packet/packet';
 import util from '../../util';
 
@@ -41,7 +42,7 @@ class GenericS2K {
   /**
    * @param {Object} [config] - Full configuration, defaults to openpgp.config
    */
-  constructor(s2kType: number, config = defaultConfig) {
+  constructor(s2kType: enumsType.s2k.simple | enumsType.s2k.salted | enumsType.s2k.iterated, config: Config) {
     /**
      * Hash function identifier, or 0 for gnu-dummy keys
      * @type {module:enums.hash | 0}
diff --git a/src/type/s2k/gnu.ts b/src/type/s2k/gnu.ts
index cdbe3154..2fbceb10 100644
--- a/src/type/s2k/gnu.ts
+++ b/src/type/s2k/gnu.ts
@@ -15,46 +15,14 @@
 // License along with this library; if not, write to the Free Software
 // Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 
-/**
- * Implementation of the String-to-key specifier
- *
- * {@link https://tools.ietf.org/html/rfc4880#section-3.7|RFC4880 3.7}:
- * String-to-key (S2K) specifiers are used to convert passphrase strings
- * into symmetric-key encryption/decryption keys.  They are used in two
- * places, currently: to encrypt the secret part of private keys in the
- * private keyring, and to convert passphrases to encryption keys for
- * symmetrically encrypted messages.
- * @module type/s2k
- */
-
-import defaultConfig from '../../config';
 import enums from '../../enums';
 import { UnsupportedError } from '../../packet/packet';
 import util from '../../util';
 
-class GnuS2k {
-  algorithm: number;
-  type: string;
-  c: number;
-  /**
-   * @param {Object} [config] - Full configuration, defaults to openpgp.config
-   */
-  constructor(config = defaultConfig) {
-    /**
-     * Hash function identifier, or 0 for gnu-dummy keys
-     * @type {module:enums.hash | 0}
-     */
-    this.algorithm = enums.hash.sha256;
-    /**
-     * enums.s2k identifier or 'gnu-dummy'
-     * @type {String}
-     */
-    this.type = 'gnu';
-    /** 
-     * @type {Integer}  
-     */
-    this.c = config.s2kIterationCountByte;
-  }
+class GnuS2K {
+  type: 'gnu' = 'gnu';
+  gnuType?: 'gnu-dummy' = undefined;
+  algorithm: number = enums.hash.sha256
 
   /**
    * Parsing function for a string-to-key specifier ({@link https://tools.ietf.org/html/rfc4880#section-3.7|RFC 4880 3.7}).
@@ -68,7 +36,7 @@ class GnuS2k {
         i += 3; // GNU
         const gnuExtType = 1000 + bytes[i++];
         if (gnuExtType === 1001) {
-          this.type = 'gnu-dummy';
+          this.gnuType = 'gnu-dummy';
           // GnuPG extension mode 1001 -- don't write secret key at all
         } else {
           throw new UnsupportedError('Unknown s2k gnu protection mode.');
@@ -85,7 +53,7 @@ class GnuS2k {
    * @returns {Uint8Array} Binary representation of s2k.
    */
   write(): Uint8Array {
-    if (this.type === 'gnu-dummy') {
+    if (this.gnuType === 'gnu-dummy') {
       return new Uint8Array([101, 0, ...util.stringToUint8Array('GNU'), 1]);
     } else {
       throw new Error('GNU s2k type not supported.');
@@ -101,19 +69,8 @@ class GnuS2k {
    * @async
    */
   async produceKey(passphrase: string, numBytes: number): Promise<Uint8Array> {
-    const arr: number[] = [];
-    let rlength = 0;
-
-    while (rlength < numBytes) {
-      if (this.type !== 'gnu') {
-        throw new Error('Unknown s2k type.');
-      } else {
-        throw new Error('GNU s2k type not supported.');
-      }
-    }
-
-    return util.concatUint8Array(arr).subarray(0, numBytes);
+    throw new Error('Gnu S2K does not support producing keys');
   }
 }
 
-export default GnuS2k;
+export default GnuS2K;
diff --git a/src/type/s2k/index.ts b/src/type/s2k/index.ts
index 9b5d23e4..7fd8059a 100644
--- a/src/type/s2k/index.ts
+++ b/src/type/s2k/index.ts
@@ -1,9 +1,9 @@
-import defaultConfig from '../../config';
 import Argon2S2K, { Argon2OutOfMemoryError } from './argon2';
 import GenericS2K from './generic';
 import enums from '../../enums';
 import { UnsupportedError } from '../../packet/packet';
 import GnuS2K from './gnu';
+import { Config } from '../../../openpgp';
 
 const allowedS2KTypesForEncryption = new Set([enums.s2k.argon2, enums.s2k.iterated]);
 
@@ -14,10 +14,10 @@ const allowedS2KTypesForEncryption = new Set([enums.s2k.argon2, enums.s2k.iterat
  * @returns {Object} New s2k object
  * @throws {Error} for unknown or unsupported types
  */
-export function newS2KFromType (type: number, config = defaultConfig): Argon2S2K | GenericS2K | GnuS2K {
+export function newS2KFromType (type: number, config: Config): Argon2S2K | GenericS2K | GnuS2K {
   switch (type) {
     case enums.s2k.gnu:
-      return new GnuS2K(config);
+      return new GnuS2K();
     case enums.s2k.argon2:
       return new Argon2S2K(config);
     case enums.s2k.iterated:
@@ -35,7 +35,7 @@ export function newS2KFromType (type: number, config = defaultConfig): Argon2S2K
  * @returns {Object} New s2k object
  * @throws {Error} for unknown or unsupported types
  */
-export function newS2KFromConfig(config = defaultConfig) {
+export function newS2KFromConfig(config: Config) {
   const { s2kType } = config;
 
   if (!allowedS2KTypesForEncryption.has(s2kType)) {