Add tool to extract RSA public key from a DcsrSpec.

This tool allows the caller to select the format (pkcs#1 vs spki) and
encoding (der vs pem).

Cargo.toml

@@ -38,3 +38,4 @@ yubihsm = { git = "https://github.com/oxidecomputer/yubihsm.rs", branch = "sessi
 zeroize = "1.8.1"
 zeroize_derive = "1.4.2"
 glob = "0.3.2"
+rsa = "0.9.3"

src/bin/dcsr-pubkey.rs

@@ -0,0 +1,87 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+use anyhow::{Context, Result};
+use clap::{Parser, ValueEnum};
+use oks::config::DcsrSpec;
+use rsa::{
+    pkcs1::{EncodeRsaPublicKey, LineEnding},
+    pkcs8::EncodePublicKey,
+};
+use std::{
+    fs,
+    io::{self, Write},
+    path::PathBuf,
+};
+
+#[derive(ValueEnum, Copy, Clone, Debug, Default)]
+enum Format {
+    Pkcs1,
+    #[default]
+    Spki,
+}
+
+#[derive(ValueEnum, Copy, Clone, Debug, Default)]
+enum Encoding {
+    Der,
+    #[default]
+    Pem,
+}
+
+/// Extract the RSA public key from a DcsrSpec and dump it to stdout in the
+/// requested format and encoding.
+#[derive(Parser, Debug)]
+struct Args {
+    /// The encoding used to serialize the public key.
+    #[clap(default_value_t, long, value_enum)]
+    encoding: Encoding,
+
+    /// The format use to represent the public key.
+    #[clap(default_value_t, long, value_enum)]
+    format: Format,
+
+    /// Path to a DcsrSpec file.
+    dcsr_file: PathBuf,
+}
+
+fn main() -> Result<()> {
+    let args = Args::parse();
+
+    let json = fs::read_to_string(&args.dcsr_file)
+        .context("Reading file name argument to string")?;
+
+    let spec: DcsrSpec = serde_json::from_str(&json)
+        .context("Failed to deserialize DcsrSpec from json")?;
+
+    let pub_key = spec.dcsr.debug_public_key;
+
+    match args.encoding {
+        Encoding::Der => {
+            let der = match args.format {
+                Format::Pkcs1 => pub_key.to_pkcs1_der().context(
+                    "Get DER encoded, PKCS#1 formatted RSA public key",
+                )?,
+                Format::Spki => pub_key.to_public_key_der().context(
+                    "Get DER encoded, SPKI formatted RSA public key",
+                )?,
+            };
+            io::stdout()
+                .write_all(der.as_bytes())
+                .context("write encoded public key to stdout")
+        }
+        Encoding::Pem => {
+            let pem = match args.format {
+                Format::Pkcs1 => pub_key
+                    .to_pkcs1_pem(LineEnding::default())
+                    .context("Get PEM encoded PKCS#1 from RSA public key")?,
+                Format::Spki => pub_key
+                    .to_public_key_pem(LineEnding::default())
+                    .context("Get SPKI PEM from RSA public key")?,
+            };
+            io::stdout()
+                .write_all(pem.as_bytes())
+                .context("write encoded public key to stdout")
+        }
+    }
+}