diff --git a/TODO.md b/TODO.md
new file mode 100644
index 0000000..d67b776
--- /dev/null
+++ b/TODO.md
@@ -0,0 +1,9 @@
+# TODO
+
+This file tracks planned changes to the project, especially breaking changes that have to wait
+until the next major release.
+
+## Planned changes for next major release
+
+- Change argument types in `src/service.rs` for public and private keys to use `[u8; 32]` instead
+  of `[u8]`. This should remove all the `ProgrammingError`s resulting from failed `try_into()` calls.
diff --git a/src/service.rs b/src/service.rs
index 2cf34fd..26eb6ff 100644
--- a/src/service.rs
+++ b/src/service.rs
@@ -121,7 +121,6 @@ impl Account {
     pub fn signup_key(client: Client, user: &User, main_key: &[u8]) -> Result<Self> {
         super::init()?;
 
-        // TODO: change argument type to [u8; 32]
         let main_key: [u8; 32] = main_key
             .try_into()
             .map_err(|_| Error::ProgrammingError("Key should be exactly 32 bytes long."))?;
@@ -212,7 +211,6 @@ impl Account {
     pub fn login_key(client: Client, username: &str, main_key: &[u8]) -> Result<Self> {
         super::init()?;
 
-        // TODO: change argument type to [u8; 32]
         let main_key: [u8; 32] = main_key
             .try_into()
             .map_err(|_| Error::ProgrammingError("Key should be exactly 32 bytes long."))?;
@@ -424,7 +422,6 @@ impl Account {
         let version = super::CURRENT_VERSION;
 
         let encryption_key = if let Some(encryption_key) = encryption_key {
-            // TODO: change argument type to Option<&[u8; 32]>
             encryption_key.try_into().map_err(|_| {
                 Error::ProgrammingError("Encryption key must be exactly 32 bytes long")
             })?
@@ -465,7 +462,6 @@ impl Account {
         encryption_key: Option<&[u8]>,
     ) -> Result<Self> {
         let encryption_key = if let Some(encryption_key) = encryption_key {
-            // TODO: change argument type to Option<&[u8; 32]>
             encryption_key.try_into().map_err(|_| {
                 Error::ProgrammingError("Encryption key must be exactly 32 bytes long")
             })?
@@ -1184,7 +1180,6 @@ impl CollectionInvitationManager {
         pubkey: &[u8],
         access_level: CollectionAccessLevel,
     ) -> Result<()> {
-        // TODO: change argument type to &[u8; 32]
         let pubkey: &[u8; 32] = pubkey
             .try_into()
             .map_err(|_| Error::ProgrammingError("Public key should be exactly 32 bytes long"))?;