Add ShortTag type to model tags that are guaranteed to be 1 or 2 bytes long.

openpgp-card/src/apdu/commands.rs

@@ -4,7 +4,7 @@
 //! Pre-defined `Command` values for the OpenPGP card application
 
 use crate::apdu::command::Command;
-use crate::{Tag, Tags};
+use crate::{ShortTag, Tags};
 
 /// 7.2.1 SELECT
 /// (select the OpenPGP application on the card)
@@ -19,11 +19,10 @@ pub(crate) fn select_openpgp() -> Command {
 }
 
 /// 7.2.6 GET DATA
-fn get_data<T: Into<Tag>>(tag: T) -> Command {
-    match *tag.into().get() {
-        [tag0] => Command::new(0x00, 0xCA, 0, tag0, vec![]),
-        [tag0, tag1] => Command::new(0x00, 0xCA, tag0, tag1, vec![]),
-        _ => panic!("this should never happen"), // FIXME
+fn get_data<T: Into<ShortTag>>(tag: T) -> Command {
+    match tag.into() {
+        ShortTag::One(tag0) => Command::new(0x00, 0xCA, 0, tag0, vec![]),
+        ShortTag::Two(tag0, tag1) => Command::new(0x00, 0xCA, tag0, tag1, vec![]),
     }
 }
 
@@ -104,11 +103,10 @@ pub(crate) fn verify_pw3(pin: Vec<u8>) -> Command {
 }
 
 /// 7.2.8 PUT DATA,
-pub(crate) fn put_data<T: Into<Tag>>(tag: T, data: Vec<u8>) -> Command {
-    match *tag.into().get() {
-        [tag0] => Command::new(0x00, 0xda, 0, tag0, data),
-        [tag0, tag1] => Command::new(0x00, 0xda, tag0, tag1, data),
-        _ => panic!("this should never happen"), // FIXME
+pub(crate) fn put_data<T: Into<ShortTag>>(tag: T, data: Vec<u8>) -> Command {
+    match tag.into() {
+        ShortTag::One(tag0) => Command::new(0x00, 0xda, 0, tag0, data),
+        ShortTag::Two(tag0, tag1) => Command::new(0x00, 0xda, tag0, tag1, data),
     }
 }
 

openpgp-card/src/lib.rs

@@ -330,6 +330,12 @@ pub(crate) enum Tags {
 }
 
 impl From<Tags> for Tag {
+    fn from(t: Tags) -> Self {
+        ShortTag::from(t).into()
+    }
+}
+
+impl From<Tags> for ShortTag {
     fn from(t: Tags) -> Self {
         match t {
             // BER identifiers https://en.wikipedia.org/wiki/X.690#BER_encoding
@@ -405,6 +411,39 @@ impl From<Tags> for Tag {
     }
 }
 
+/// A ShortTag is a Tlv tag that is guaranteed to be either 1 or 2 bytes long.
+///
+/// This covers any tag that can be used in the OpenPGP card context (the spec doesn't describe how
+/// longer tags might be used.)
+///
+/// (The type tlv::Tag will usually/always contain 1 or 2 byte long tags, in this library.
+/// But its length is not guaranteed by the type system)
+#[derive(Debug, Clone, Copy, Eq, PartialEq)]
+enum ShortTag {
+    One(u8),
+    Two(u8, u8),
+}
+
+impl From<ShortTag> for Tag {
+    fn from(n: ShortTag) -> Self {
+        match n {
+            ShortTag::One(t0) => [t0].into(),
+            ShortTag::Two(t0, t1) => [t0, t1].into(),
+        }
+    }
+}
+
+impl From<[u8; 1]> for ShortTag {
+    fn from(v: [u8; 1]) -> Self {
+        ShortTag::One(v[0])
+    }
+}
+impl From<[u8; 2]> for ShortTag {
+    fn from(v: [u8; 2]) -> Self {
+        ShortTag::Two(v[0], v[1])
+    }
+}
+
 #[derive(Debug, Clone, Copy, Eq, PartialEq)]
 pub enum PinType {
     Sign,
@@ -434,7 +473,7 @@ pub enum KeyType {
 
 impl KeyType {
     /// Get C1/C2/C3/DA values for this KeyTypes, to use as Tag
-    fn algorithm_tag(&self) -> Tag {
+    fn algorithm_tag(&self) -> ShortTag {
         match self {
             Self::Signing => Tags::AlgorithmAttributesSignature,
             Self::Decryption => Tags::AlgorithmAttributesDecryption,
@@ -448,7 +487,7 @@ impl KeyType {
     ///
     /// (NOTE: these Tags are only used for "PUT DO", but GETting
     /// fingerprint information from the card uses the combined Tag C5)
-    fn fingerprint_put_tag(&self) -> Tag {
+    fn fingerprint_put_tag(&self) -> ShortTag {
         match self {
             Self::Signing => Tags::FingerprintSignature,
             Self::Decryption => Tags::FingerprintDecryption,
@@ -462,7 +501,7 @@ impl KeyType {
     ///
     /// (NOTE: these Tags are only used for "PUT DO", but GETting
     /// timestamp information from the card uses the combined Tag CD)
-    fn timestamp_put_tag(&self) -> Tag {
+    fn timestamp_put_tag(&self) -> ShortTag {
         match self {
             Self::Signing => Tags::GenerationTimeSignature,
             Self::Decryption => Tags::GenerationTimeDecryption,

openpgp-card/src/tlv/tag.rs

@@ -1,7 +1,14 @@
 // SPDX-FileCopyrightText: 2021 Heiko Schaefer <heiko@schaefer.name>
 // SPDX-License-Identifier: MIT OR Apache-2.0
 
-//! Tag in a TLV data structure
+//! Tag in a TLV data structure.
+//!
+//! A Tag can span multiple octets, in the OpenPGP card context, only Tags spanning 1 or 2 octets
+//! should come up. However, this type can deal with arbitrary length Tags.
+//!
+//! (The `ShortTag` type models tags that are exactly 1 or 2 octets long)
+//!
+//! https://en.wikipedia.org/wiki/X.690#Encoding
 
 use nom::{branch, bytes::complete as bytes, combinator, number::complete as number, sequence};