diff --git a/source/node/dataPacking.ts b/source/node/dataPacking.ts
index 4bcaa9b..90d741a 100644
--- a/source/node/dataPacking.ts
+++ b/source/node/dataPacking.ts
@@ -1,6 +1,7 @@
 import { getBinarySignature, getBinaryContentBorder } from "../shared/signature";
 import { SIZE_ENCODING_BYTES } from "../symbols";
 import {
+    DerivationMethod,
     EncryptedBinaryComponents,
     EncryptedComponentsBase,
     EncryptedPayloadFooter,
@@ -51,8 +52,9 @@ export function prepareFooter(encryptedComponents: EncryptedPayloadFooter): Buff
 
 export function prepareHeader(encryptedComponents: EncryptedPayloadHeader): Buffer {
     const signature = Buffer.from(getBinarySignature());
-    const { iv, salt, rounds, method } = encryptedComponents;
+    const { derivation, iv, salt, rounds, method } = encryptedComponents;
     const componentsPrefix = JSON.stringify({
+        derivation,
         iv,
         salt,
         rounds,
@@ -103,10 +105,17 @@ export function unpackEncryptedData(encryptedContent: Buffer): EncryptedBinaryCo
     const footerData = encryptedContent.slice(offset, offset + footerSize);
     offset += footerSize;
     // Decode
-    const { iv, salt, rounds, method } = JSON.parse(headerData.toString("utf8"));
+    const {
+        derivation = DerivationMethod.PBKDF2_SHA256,
+        iv,
+        salt,
+        rounds,
+        method
+    } = JSON.parse(headerData.toString("utf8"));
     const { auth } = JSON.parse(footerData.toString("utf8"));
     return {
         content: contentBuff,
+        derivation,
         iv,
         salt,
         auth,
diff --git a/source/node/derivation.ts b/source/node/derivation.ts
index 61607b7..8b542c2 100644
--- a/source/node/derivation.ts
+++ b/source/node/derivation.ts
@@ -1,6 +1,6 @@
 import { pbkdf2 as derivePBKDF2 } from "pbkdf2";
 import { DERIVED_KEY_ALGORITHM, HMAC_KEY_SIZE, PASSWORD_KEY_SIZE } from "../symbols";
-import { DerivedKeyInfo } from "../types";
+import { DerivationMethod, DerivedKeyInfo } from "../types";
 
 export async function deriveKeyFromPassword(
     password: string,
@@ -17,6 +17,7 @@ export async function deriveKeyFromPassword(
     if (!rounds || rounds <= 0) {
         throw new Error("Failed deriving key: Rounds must be greater than 0");
     }
+    const method = DerivationMethod.PBKDF2_SHA256;
     const bits = generateHMAC ? (PASSWORD_KEY_SIZE + HMAC_KEY_SIZE) * 8 : PASSWORD_KEY_SIZE * 8;
     const derivedKeyData = await pbkdf2(password, salt, rounds, bits);
     const derivedKeyHex = derivedKeyData.toString("hex");
@@ -25,12 +26,13 @@ export async function deriveKeyFromPassword(
         ? Buffer.from(derivedKeyHex.substr(0, dkhLength / 2), "hex")
         : Buffer.from(derivedKeyHex, "hex");
     return {
-        salt: salt,
-        key: keyBuffer,
-        rounds: rounds,
         hmac: generateHMAC
             ? Buffer.from(derivedKeyHex.substr(dkhLength / 2, dkhLength / 2), "hex")
-            : null
+            : null,
+        key: keyBuffer,
+        method,
+        rounds: rounds,
+        salt: salt
     };
 }
 
diff --git a/source/node/stream.ts b/source/node/stream.ts
index a6865a0..233051c 100644
--- a/source/node/stream.ts
+++ b/source/node/stream.ts
@@ -259,6 +259,7 @@ export function createEncryptStream(adapter: IocaneAdapter, password: string): W
         .then(({ iv, keyDerivationInfo }) => {
             const ivHex = iv.toString("hex");
             const header = prepareHeader({
+                derivation: keyDerivationInfo.method,
                 iv: ivHex,
                 salt: keyDerivationInfo.salt,
                 rounds: keyDerivationInfo.rounds,
diff --git a/source/shared/textPacking.ts b/source/shared/textPacking.ts
index 444dfe8..131d90b 100644
--- a/source/shared/textPacking.ts
+++ b/source/shared/textPacking.ts
@@ -1,25 +1,47 @@
-import { EncryptedComponents, EncryptionAlgorithm, PackedEncryptedText } from "../types";
+import {
+    DerivationMethod,
+    EncryptedComponents,
+    EncryptionAlgorithm,
+    PackedEncryptedText
+} from "../types";
 import { ALGO_DEFAULT } from "../symbols";
 
+/**
+ * This is a legacy default from LONG ago - it is not used for new encryptions, and
+ * is only applied when decrypting an old string that never specified the number of
+ * rounds.
+ */
 const PBKDF2_ROUND_DEFAULT = 1000;
 
 export function packEncryptedText(encryptedComponents: EncryptedComponents): PackedEncryptedText {
-    const { content, iv, salt, auth, rounds, method } = encryptedComponents;
-    return [content, iv, salt, auth, rounds, method].join("$");
+    const { content, derivation, iv, salt, auth, rounds, method } = encryptedComponents;
+    return [content, iv, salt, auth, rounds, method, derivation].join("$");
 }
 
 export function unpackEncryptedText(encryptedContent: PackedEncryptedText): EncryptedComponents {
-    const [content, iv, salt, auth, roundsRaw, methodRaw] = <
-        [string, string, string, string, string, EncryptionAlgorithm]
+    const [content, iv, salt, auth, roundsRaw, methodRaw, deriveMethodRaw] = <
+        [string, string, string, string, string, EncryptionAlgorithm, string]
     >encryptedContent.split("$");
     // iocane was originally part of Buttercup's core package and used defaults from that originally.
     // There will be 4 components for pre 0.15.0 archives, and 5 in newer archives. The 5th component
-    // is the pbkdf2 round count, which is optional:
+    // is the pbkdf2 round count, which was optional:
     const rounds = roundsRaw ? parseInt(roundsRaw, 10) : PBKDF2_ROUND_DEFAULT;
-    // Originally only "cbc" was supported, but GCM was added in version 1
+    // Encryption method: originally only "cbc" was supported, but GCM was added in version 1
     const method = methodRaw || ALGO_DEFAULT;
+    // Derivation method: Original iocane strings used PBKDF2+SHA256 implicitly
+    const deriveMethodNum = deriveMethodRaw
+        ? parseInt(deriveMethodRaw, 10)
+        : DerivationMethod.PBKDF2_SHA256;
+    if (Object.values(DerivationMethod).includes(deriveMethodNum) === false) {
+        throw new Error(
+            `Failed unpacking encrypted content: Invalid key derivation type: ${deriveMethodRaw}`
+        );
+    }
+    const derivation = deriveMethodNum as DerivationMethod;
+    // Return results
     return {
         content,
+        derivation,
         iv,
         salt,
         auth,
diff --git a/source/types.ts b/source/types.ts
index e50498c..276cbeb 100644
--- a/source/types.ts
+++ b/source/types.ts
@@ -4,11 +4,17 @@ export type BufferLike = Buffer | ArrayBuffer;
 
 export type DataLike = string | Buffer | ArrayBuffer;
 
+export enum DerivationMethod {
+    PBKDF2_SHA256 = 1,
+    PBKDF2_SHA512 = 2
+}
+
 export interface DerivedKeyInfo {
-    salt: string;
-    key: Buffer | ArrayBuffer;
     hmac: Buffer | ArrayBuffer | null;
+    key: Buffer | ArrayBuffer;
+    method: DerivationMethod;
     rounds: number;
+    salt: string;
 }
 
 export interface EncryptedBinaryComponents extends EncryptedComponentsBase {
@@ -26,10 +32,11 @@ export interface EncryptedPayloadFooter {
 }
 
 export interface EncryptedPayloadHeader {
+    derivation: DerivationMethod;
     iv: string;
-    salt: string;
-    rounds: number;
     method: EncryptionAlgorithm;
+    rounds: number;
+    salt: string;
 }
 
 export enum EncryptionAlgorithm {
diff --git a/source/web/dataPacking.ts b/source/web/dataPacking.ts
index f1f1967..ffce261 100644
--- a/source/web/dataPacking.ts
+++ b/source/web/dataPacking.ts
@@ -8,6 +8,7 @@ import {
 } from "./tools";
 import { SIZE_ENCODING_BYTES } from "../symbols";
 import {
+    DerivationMethod,
     EncryptedBinaryComponents,
     EncryptedComponentsBase,
     EncryptedPayloadFooter,
@@ -58,8 +59,9 @@ export function prepareFooter(encryptedComponents: EncryptedPayloadFooter): Arra
 
 export function prepareHeader(encryptedComponents: EncryptedPayloadHeader): ArrayBuffer {
     const signature = new Uint8Array(getBinarySignature()).buffer;
-    const { iv, salt, rounds, method } = encryptedComponents;
+    const { derivation, iv, salt, rounds, method } = encryptedComponents;
     const componentsPrefix = JSON.stringify({
+        derivation,
         iv,
         salt,
         rounds,
@@ -115,10 +117,17 @@ export function unpackEncryptedData(encryptedContent: ArrayBuffer): EncryptedBin
     const footerData = encryptedContent.slice(offset, offset + footerSize);
     offset += footerSize;
     // Decode
-    const { iv, salt, rounds, method } = JSON.parse(arrayBufferToString(headerData));
+    const {
+        derivation = DerivationMethod.PBKDF2_SHA256,
+        iv,
+        salt,
+        rounds,
+        method
+    } = JSON.parse(arrayBufferToString(headerData));
     const { auth } = JSON.parse(arrayBufferToString(footerData));
     return {
         content: contentBuff,
+        derivation,
         iv,
         salt,
         auth,
diff --git a/source/web/derivation.ts b/source/web/derivation.ts
index 4f56f08..359b450 100644
--- a/source/web/derivation.ts
+++ b/source/web/derivation.ts
@@ -5,7 +5,7 @@ import {
     stringToArrayBuffer
 } from "./tools";
 import { HMAC_KEY_SIZE, PASSWORD_KEY_SIZE } from "../symbols";
-import { DerivedKeyInfo } from "../types";
+import { DerivationMethod, DerivedKeyInfo } from "../types";
 
 function checkBrowserSupport() {
     if (!window.TextEncoder || !window.TextDecoder) {
@@ -28,6 +28,7 @@ export async function deriveKeyFromPassword(
     if (!rounds || rounds <= 0) {
         throw new Error("Failed deriving key: Rounds must be greater than 0");
     }
+    const method = DerivationMethod.PBKDF2_SHA256;
     const bits = generateHMAC ? (PASSWORD_KEY_SIZE + HMAC_KEY_SIZE) * 8 : PASSWORD_KEY_SIZE * 8;
     const derivedKeyData = await pbkdf2(password, salt, rounds, bits);
     const derivedKeyHex = arrayBufferToHexString(derivedKeyData);
@@ -36,12 +37,13 @@ export async function deriveKeyFromPassword(
         ? hexStringToArrayBuffer(derivedKeyHex.substr(0, dkhLength / 2))
         : hexStringToArrayBuffer(derivedKeyHex);
     return {
-        salt: salt,
-        key: keyBuffer,
-        rounds: rounds,
         hmac: generateHMAC
             ? hexStringToArrayBuffer(derivedKeyHex.substr(dkhLength / 2, dkhLength / 2))
-            : null
+            : null,
+        key: keyBuffer,
+        method,
+        rounds: rounds,
+        salt
     };
 }