diff --git a/Cargo.toml b/Cargo.toml
index da9cf4d..f8215db 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -29,7 +29,7 @@ littlefs2 = "0.2.1"
 rand_core = { version = "0.5.1", features = ["getrandom"] }
 
 [features]
-default = ["applet"]
+default = []
 applet = ["apdu-dispatch"]
 strict-pin = []
 
diff --git a/src/commands.rs b/src/commands.rs
index e8290b5..6c72c4a 100644
--- a/src/commands.rs
+++ b/src/commands.rs
@@ -6,8 +6,8 @@
 use core::convert::{TryFrom, TryInto};
 
 // use flexiber::Decodable;
+use heapless::ArrayLength;
 use iso7816::{Instruction, Status};
-use apdu_dispatch::{Command as IsoCommand, command::Data};
 
 pub use crate::{container as containers, piv_types, Pin, Puk};
 
@@ -43,7 +43,7 @@ impl<'l> Command<'l> {
     /// Core method, constructs a PIV command, if the iso7816::Command is valid.
     ///
     /// Inherent method re-exposing the `TryFrom` implementation.
-    pub fn try_from(command: &'l IsoCommand) -> Result<Self, Status> {
+    pub fn try_from<C: ArrayLength<u8>>(command: &'l iso7816::Command<C>) -> Result<Self, Status> {
         command.try_into()
     }
 }
@@ -54,12 +54,12 @@ pub struct Select<'l> {
     pub aid: &'l [u8],
 }
 
-impl<'l> TryFrom<&'l Data> for Select<'l> {
+impl<'l> TryFrom<&'l [u8]> for Select<'l> {
     type Error = Status;
     /// We allow ourselves the option of answering to more than just the official PIV AID.
     /// For instance, to offer additional functionality, under our own RID.
-    fn try_from(data: &'l Data) -> Result<Self, Self::Error> {
-        Ok(match data.as_slice() {
+    fn try_from(data: &'l [u8]) -> Result<Self, Self::Error> {
+        Ok(match data {
             crate::constants::PIV_AID => Self { aid: data },
             _ => return Err(Status::NotFound),
         })
@@ -70,9 +70,9 @@ impl<'l> TryFrom<&'l Data> for Select<'l> {
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub struct GetData(containers::Container);
 
-impl TryFrom<&Data> for GetData {
+impl TryFrom<&[u8]> for GetData {
     type Error = Status;
-    fn try_from(data: &Data) -> Result<Self, Self::Error> {
+    fn try_from(data: &[u8]) -> Result<Self, Self::Error> {
         let mut decoder = flexiber::Decoder::new(data);
         let tagged_slice: flexiber::TaggedSlice = decoder.decode().map_err(|_| Status::IncorrectDataParameter)?;
         if tagged_slice.tag() != flexiber::Tag::application(0x1C) {
@@ -135,7 +135,7 @@ impl TryFrom<u8> for VerifyLogout {
 pub struct VerifyArguments<'l> {
     pub key_reference: VerifyKeyReference,
     pub logout: VerifyLogout,
-    pub data: &'l Data
+    pub data: &'l [u8],
 }
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
@@ -161,7 +161,7 @@ impl TryFrom<VerifyArguments<'_>> for Verify {
         }
         Ok(match (logout.0, data.len()) {
             (false, 0) => Verify::Status(key_reference),
-            (false, 8) => Verify::Login(VerifyLogin::PivPin(data.as_slice().try_into().map_err(|_| Status::IncorrectDataParameter)?)),
+            (false, 8) => Verify::Login(VerifyLogin::PivPin(data.try_into().map_err(|_| Status::IncorrectDataParameter)?)),
             (false, _) => return Err(Status::IncorrectDataParameter),
             (true, 0) => Verify::Logout(key_reference),
             (true, _) => return Err(Status::IncorrectDataParameter),
@@ -192,7 +192,7 @@ impl TryFrom<u8> for ChangeReferenceKeyReference {
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub struct ChangeReferenceArguments<'l> {
     pub key_reference: ChangeReferenceKeyReference,
-    pub data: &'l Data
+    pub data: &'l [u8],
 }
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
@@ -232,9 +232,9 @@ pub struct ResetPinRetries {
     pub puk: [u8;  8],
 }
 
-impl TryFrom<&Data> for ResetPinRetries {
+impl TryFrom<&[u8]> for ResetPinRetries {
     type Error = Status;
-    fn try_from(data: &Data) -> Result<Self, Self::Error> {
+    fn try_from(data: &[u8]) -> Result<Self, Self::Error> {
         if data.len() != 16 {
             return Err(Status::IncorrectDataParameter);
         }
@@ -317,7 +317,7 @@ pub struct AuthenticateArguments<'l> {
     /// this is passed through as-is.
     pub unparsed_algorithm: u8,
     pub key_reference: AuthenticateKeyReference,
-    pub data: &'l Data
+    pub data: &'l [u8],
 }
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
@@ -326,7 +326,7 @@ pub enum Authenticate {
 
 impl TryFrom<AuthenticateArguments<'_>> for Authenticate {
     type Error = Status;
-    fn try_from(arguments: AuthenticateArguments<'_>) -> Result<Self, Self::Error> {
+    fn try_from(_arguments: AuthenticateArguments<'_>) -> Result<Self, Self::Error> {
         todo!();
     }
 }
@@ -335,9 +335,9 @@ impl TryFrom<AuthenticateArguments<'_>> for Authenticate {
 pub struct PutData {
 }
 
-impl TryFrom<&Data> for PutData {
+impl TryFrom<&[u8]> for PutData {
     type Error = Status;
-    fn try_from(data: &Data) -> Result<Self, Self::Error> {
+    fn try_from(_data: &[u8]) -> Result<Self, Self::Error> {
         todo!();
     }
 }
@@ -369,7 +369,7 @@ impl TryFrom<u8> for GenerateAsymmetricKeyReference {
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub struct GenerateAsymmetricArguments<'l> {
     pub key_reference: GenerateAsymmetricKeyReference,
-    pub data: &'l Data
+    pub data: &'l [u8],
 }
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
@@ -378,12 +378,12 @@ pub enum GenerateAsymmetric {
 
 impl TryFrom<GenerateAsymmetricArguments<'_>> for GenerateAsymmetric {
     type Error = Status;
-    fn try_from(arguments: GenerateAsymmetricArguments<'_>) -> Result<Self, Self::Error> {
+    fn try_from(_arguments: GenerateAsymmetricArguments<'_>) -> Result<Self, Self::Error> {
         todo!();
     }
 }
 
-impl<'l> TryFrom<&'l IsoCommand> for Command<'l> {
+impl<'l, C: ArrayLength<u8>> TryFrom<&'l iso7816::Command<C>> for Command<'l> {
     type Error = Status;
     /// The first layer of unraveling the iso7816::Command onion.
     ///
@@ -391,7 +391,7 @@ impl<'l> TryFrom<&'l IsoCommand> for Command<'l> {
     /// in the "Command Syntax" boxes of NIST SP 800-73-4, and return early errors.
     ///
     /// The individual piv::Command TryFroms then further interpret these validated parameters.
-    fn try_from(command: &'l IsoCommand) -> Result<Self, Self::Error> {
+    fn try_from(command: &'l iso7816::Command<C>) -> Result<Self, Self::Error> {
         let (class, instruction, p1, p2) = (command.class(), command.instruction(), command.p1, command.p2);
         let data = command.data();
 
@@ -408,11 +408,11 @@ impl<'l> TryFrom<&'l IsoCommand> for Command<'l> {
         Ok(match (class.into_inner(), instruction, p1, p2) {
 
             (0x00, Instruction::Select, 0x04, 0x00) => {
-                Self::Select(Select::try_from(data)?)
+                Self::Select(Select::try_from(data.as_slice())?)
             }
 
             (0x00, Instruction::GetData, 0x3F, 0xFF) => {
-                Self::GetData(GetData::try_from(data)?.0)
+                Self::GetData(GetData::try_from(data.as_slice())?.0)
             }
 
             (0x00, Instruction::Verify, p1, p2) => {
@@ -427,7 +427,7 @@ impl<'l> TryFrom<&'l IsoCommand> for Command<'l> {
             }
 
             (0x00, Instruction::ResetRetryCounter, 0x00, 0x80) => {
-                Self::ResetPinRetries(ResetPinRetries::try_from(data)?)
+                Self::ResetPinRetries(ResetPinRetries::try_from(data.as_slice())?)
             }
 
             (0x00, Instruction::GeneralAuthenticate, p1, p2) => {
@@ -437,7 +437,7 @@ impl<'l> TryFrom<&'l IsoCommand> for Command<'l> {
             }
 
             (0x00, Instruction::PutData, 0x3F, 0xFF) => {
-                Self::PutData(PutData::try_from(data)?)
+                Self::PutData(PutData::try_from(data.as_slice())?)
             }
 
             (0x00, Instruction::GenerateAsymmetricKeyPair, 0x00, p2) => {
diff --git a/src/lib.rs b/src/lib.rs
index 9bb7780..6a24781 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -19,8 +19,8 @@ pub use piv_types::{Pin, Puk};
 use core::convert::TryInto;
 
 use flexiber::EncodableHeapless;
-use iso7816::Status;
-use apdu_dispatch::{Command as IsoCommand, response};
+use heapless::ArrayLength;
+use iso7816::{Data, Status};
 use trussed::client;
 use trussed::{syscall, try_syscall};
 
@@ -28,15 +28,23 @@ use constants::*;
 
 pub type Result = iso7816::Result<()>;
 
-pub struct Authenticator<T>
+/// PIV authenticator Trussed app.
+///
+/// The `C` parameter is necessary, as PIV includes command sequences,
+/// where we need to store the previous command, so we need to know how
+/// much space to allocate.
+pub struct Authenticator<C, T>
+where
+    C: ArrayLength<u8>,
 {
-    state: state::State,
+    state: state::State<C>,
     trussed: T,
     // trussed: RefCell<Trussed>,
 }
 
-impl<T> Authenticator<T>
+impl<C, T> Authenticator<C, T>
 where
+    C: ArrayLength<u8>,
     T: client::Client + client::Ed255 + client::Tdes,
 {
     pub fn new(
@@ -59,7 +67,11 @@ where
     pub fn deselect(&mut self) {
     }
 
-    pub fn select(&mut self, _apdu: &IsoCommand, reply: &mut response::Data) -> Result {
+    pub fn select<R>(&mut self, _apdu: &iso7816::Command<C>, reply: &mut Data<R>) -> Result
+    where
+        R: ArrayLength<u8>,
+
+    {
         use piv_types::Algorithms::*;
         info_now!("selecting PIV maybe");
 
@@ -79,8 +91,12 @@ where
         Ok(())
     }
 
-    pub fn respond(&mut self, command: &IsoCommand, reply: &mut response::Data) -> Result {
-        info_now!("PIV responding to {:?}", command);
+    pub fn respond<R>(&mut self, command: &iso7816::Command<C>, reply: &mut Data<R>) -> Result
+    where
+        R: ArrayLength<u8>,
+    {
+        // need to implement Debug on iso7816::Command
+        // info_now!("PIV responding to {:?}", command);
         let last_or_only = command.class().chain().last_or_only();
 
         // TODO: avoid owned copy?
@@ -100,7 +116,7 @@ where
 
             None => {
                 if last_or_only {
-                    // IsoCommand
+                    // iso7816::Command<C>
                     command.clone()
                 } else {
                     self.state.runtime.chained_command = Some(command.clone());
@@ -216,7 +232,7 @@ where
         return Ok(Default::default());
     }
 
-    // pub fn old_respond(&mut self, command: &IsoCommand, reply: &mut response::Data) -> Result {
+    // pub fn old_respond(&mut self, command: &iso7816::Command<C>, reply: &mut Data<R>) -> Result {
 
     //     // TEMP
     //     // blocking::dbg!(self.state.persistent(&mut self.trussed).timestamp(&mut self.trussed));
@@ -246,7 +262,7 @@ where
 
     //         None => {
     //             if last_or_only {
-    //                 // IsoCommand
+    //                 // iso7816::Command<C>
     //                 command.clone()
     //             } else {
     //                 self.state.runtime.chained_command = Some(command.clone());
@@ -325,7 +341,10 @@ where
     // - 9000, 61XX for success
     // - 6982 security status
     // - 6A80, 6A86 for data, P1/P2 issue
-    fn general_authenticate(&mut self, command: &IsoCommand, reply: &mut response::Data) -> Result {
+    fn general_authenticate<R>(&mut self, command: &iso7816::Command<C>, reply: &mut Data<R>) -> Result
+    where
+        R: ArrayLength<u8>,
+    {
 
         // For "SSH", we need implement A.4.2 in SP-800-73-4 Part 2, ECDSA signatures
         //
@@ -435,7 +454,10 @@ where
         Ok(())
     }
 
-    fn request_for_challenge(&mut self, command: &IsoCommand, remaining_data: &[u8], reply: &mut response::Data) -> Result {
+    fn request_for_challenge<R>(&mut self, command: &iso7816::Command<C>, remaining_data: &[u8], reply: &mut Data<R>) -> Result
+    where
+        R: ArrayLength<u8>,
+    {
         // - data is of the form
         //     00 87 03 9B 16 7C 14 80 08 99 6D 71 40 E7 05 DF 7F 81 08 6E EF 9C 02 00 69 73 E8
         // - remaining data contains <decrypted challenge> 81 08 <encrypted counter challenge>
@@ -486,7 +508,10 @@ where
         Ok(())
     }
 
-    fn request_for_witness(&mut self, command: &IsoCommand, remaining_data: &[u8], reply: &mut response::Data) -> Result {
+    fn request_for_witness<R>(&mut self, command: &iso7816::Command<C>, remaining_data: &[u8], reply: &mut Data<R>) -> Result
+    where
+        R: ArrayLength<u8>,
+    {
         // invariants: parsed data was '7C L1 80 00' + remaining_data
 
         if command.p1 != 0x03 || command.p2 != 0x9b {
@@ -661,7 +686,10 @@ where
     //    }
     //}
 
-    fn generate_asymmetric_keypair(&mut self, command: &IsoCommand, reply: &mut response::Data) -> Result {
+    fn generate_asymmetric_keypair<R>(&mut self, command: &iso7816::Command<C>, reply: &mut Data<R>) -> Result
+    where
+        R: ArrayLength<u8>,
+    {
         if !self.state.runtime.app_security_status.management_verified {
             return Err(Status::SecurityStatusNotSatisfied);
         }
@@ -784,7 +812,7 @@ where
         Ok(())
     }
 
-    fn put_data(&mut self, command: &IsoCommand) -> Result {
+    fn put_data(&mut self, command: &iso7816::Command<C>) -> Result {
         info_now!("PutData");
         if command.p1 != 0x3f || command.p2 != 0xff {
             return Err(Status::IncorrectP1OrP2Parameter);
@@ -878,7 +906,10 @@ where
         // }
         // todo!();
 
-    fn get_data(&mut self, container: container::Container, reply: &mut response::Data) -> Result {
+    fn get_data<R>(&mut self, container: container::Container, reply: &mut Data<R>) -> Result
+    where
+        R: ArrayLength<u8>,
+    {
 
         // TODO: check security status, else return Status::SecurityStatusNotSatisfied
 
@@ -888,8 +919,7 @@ where
         match container {
             Container::DiscoveryObject => {
                 // Err(Status::InstructionNotSupportedOrInvalid)
-                let data = response::Data::try_from_slice(DISCOVERY_OBJECT).unwrap();
-                reply.extend_from_slice(&data).ok();
+                reply.extend_from_slice(DISCOVERY_OBJECT).ok();
                 // todo!("discovery object"),
             }
 
@@ -934,7 +964,7 @@ where
 
             // // '5F FF01' (754B)
             // YubicoObjects::AttestationCertificate => {
-            //     let data = response::Data::try_from_slice(YUBICO_ATTESTATION_CERTIFICATE).unwrap();
+            //     let data = Data<R>::try_from_slice(YUBICO_ATTESTATION_CERTIFICATE).unwrap();
             //     reply.extend_from_slice(&data).ok();
             // }
 
@@ -943,21 +973,22 @@ where
         Ok(())
     }
 
-    fn yubico_piv_extension(&mut self, command: &IsoCommand, instruction: YubicoPivExtension, reply: &mut response::Data) -> Result {
+    fn yubico_piv_extension<R>(&mut self, command: &iso7816::Command<C>, instruction: YubicoPivExtension, reply: &mut Data<R>) -> Result
+    where
+        R: ArrayLength<u8>,
+    {
         info_now!("yubico extension: {:?}", &instruction);
         match instruction {
             YubicoPivExtension::GetSerial => {
                 // make up a 4-byte serial
-                let data = response::Data::try_from_slice(
-                    &[0x00, 0x52, 0xf7, 0x43]).unwrap();
-                reply.extend_from_slice(&data).ok();
+                reply.extend_from_slice(
+                    &[0x00, 0x52, 0xf7, 0x43]).ok();
             }
 
             YubicoPivExtension::GetVersion => {
                 // make up a version, be >= 5.0.0
-                let data = response::Data::try_from_slice(
-                    &[0x06, 0x06, 0x06]).unwrap();
-                reply.extend_from_slice(&data).ok();
+                reply.extend_from_slice(
+                    &[0x06, 0x06, 0x06]).ok();
             }
 
             YubicoPivExtension::Attest => {
@@ -968,8 +999,7 @@ where
                 let slot = command.p1;
 
                 if slot == 0x9a {
-                    let data = response::Data::try_from_slice(YUBICO_ATTESTATION_CERTIFICATE_FOR_9A).unwrap();
-                    reply.extend_from_slice(&data).ok();
+                    reply.extend_from_slice(YUBICO_ATTESTATION_CERTIFICATE_FOR_9A).ok();
                 } else {
 
                     return Err(Status::FunctionNotSupported)
@@ -1040,7 +1070,7 @@ where
 
 
 #[cfg(feature = "apdu-dispatch")]
-impl<T> apdu_dispatch::app::Aid for Authenticator<T> {
+impl<T> apdu_dispatch::app::Aid for Authenticator<apdu_dispatch::command::Size, T> {
 
     fn aid(&self) -> &'static [u8] {
         &constants::PIV_AID
@@ -1053,17 +1083,17 @@ impl<T> apdu_dispatch::app::Aid for Authenticator<T> {
 
 
 #[cfg(feature = "apdu-dispatch")]
-impl<T> apdu_dispatch::app::App<apdu_dispatch::command::Size, apdu_dispatch::response::Size> for Authenticator<T>
+impl<T> apdu_dispatch::app::App<apdu_dispatch::command::Size, apdu_dispatch::response::Size> for Authenticator<apdu_dispatch::command::Size, T>
 where
     T: client::Client + client::Ed255 + client::Tdes
 {
-    fn select(&mut self, apdu: &IsoCommand, reply: &mut response::Data) -> Result {
+    fn select(&mut self, apdu: &apdu_dispatch::Command, reply: &mut apdu_dispatch::response::Data) -> Result {
         self.select(apdu, reply)
     }
 
     fn deselect(&mut self) { self.deselect() }
 
-    fn call(&mut self, _: iso7816::Interface, apdu: &IsoCommand, reply: &mut response::Data) -> Result {
+    fn call(&mut self, _: iso7816::Interface, apdu: &apdu_dispatch::Command, reply: &mut apdu_dispatch::response::Data) -> Result {
         self.respond(apdu, reply)
     }
 }
diff --git a/src/state.rs b/src/state.rs
index b793dd9..5e4762b 100644
--- a/src/state.rs
+++ b/src/state.rs
@@ -1,4 +1,5 @@
 use core::convert::{TryFrom, TryInto};
+use heapless::ArrayLength;
 
 use trussed::{
     block,
@@ -139,14 +140,14 @@ pub struct Keys {
 
 
 #[derive(Clone, Debug, Default, Eq, PartialEq)]
-pub struct State {
-    pub runtime: Runtime,
+pub struct State<C: ArrayLength<u8>> {
+    pub runtime: Runtime<C>,
     // temporary "state", to be removed again
     // pub hack: Hack,
     // trussed: RefCell<Trussed<S>>,
 }
 
-impl State {
+impl<C: ArrayLength<u8>> State<C> {
     pub fn new() -> Self {
         Default::default()
     }
@@ -234,7 +235,7 @@ impl<T> AsRef<PersistentState> for Persistent<'_, T> {
 }
 
 #[derive(Clone, Debug, Default, Eq, PartialEq)]
-pub struct Runtime {
+pub struct Runtime<C: ArrayLength<u8>> {
     // aid: Option<
     // consecutive_pin_mismatches: u8,
 
@@ -242,7 +243,7 @@ pub struct Runtime {
     pub currently_selected_application: SelectableAid,
     pub app_security_status: AppSecurityStatus,
     pub command_cache: Option<CommandCache>,
-    pub chained_command: Option<apdu_dispatch::Command>,
+    pub chained_command: Option<iso7816::Command<C>>,
 }
 
 pub trait Aid {
@@ -551,6 +552,3 @@ where
 
 }
 
-impl Runtime {
-}
-