Add cluelessh-key debug create-fake-privkey

This command creates a fake private key that looks like it's the real
private key for a corresponding public key.
Even `ssh-keygen -y` gets confused.

bin/cluelessh-key/src/main.rs

@@ -6,8 +6,9 @@ use std::{
 
 use base64::Engine;
 use clap::Parser;
-use cluelessh_keys::private::{
-    EncryptedPrivateKeys, KeyEncryptionParams, PlaintextPrivateKey, PrivateKey,
+use cluelessh_keys::{
+    private::{EncryptedPrivateKeys, KeyEncryptionParams, PlaintextPrivateKey, PrivateKey},
+    public::{PublicKey, PublicKeyWithComment},
 };
 use eyre::{bail, Context};
 
@@ -51,6 +52,8 @@ enum DebugCommand {
     Unpem { id_file: PathBuf },
     /// Extract the encrypted part of the private key
     ExtractEncrypted { id_file: PathBuf },
+    /// Create a fake private key that contains random private key bytes and the actual public key bytes.
+    CreateFakePrivkey { public_key: PathBuf },
 }
 
 #[derive(clap::ValueEnum, Clone)]
@@ -98,6 +101,53 @@ fn main() -> eyre::Result<()> {
             std::io::stdout().lock().write_all(&data)?;
             Ok(())
         }
+        Subcommand::Debug {
+            cmd: DebugCommand::CreateFakePrivkey { public_key },
+        } => {
+            let file = std::fs::read_to_string(&public_key)
+                .wrap_err_with(|| format!("reading file {}", public_key.display()))?;
+            let public_key = file
+                .parse::<PublicKeyWithComment>()
+                .wrap_err("failed to parse public key")?;
+
+            let mut fake_private_key = PlaintextPrivateKey::generate(
+                "".into(),
+                cluelessh_keys::KeyGenerationParams {
+                    key_type: match public_key.key {
+                        PublicKey::Ed25519 { .. } => cluelessh_keys::KeyType::Ed25519,
+                        PublicKey::EcdsaSha2NistP256 { .. } => cluelessh_keys::KeyType::Ecdsa,
+                    },
+                },
+            );
+
+            match public_key.key {
+                PublicKey::Ed25519 { public_key } => {
+                    let PrivateKey::Ed25519 {
+                        public_key: fake_public_key,
+                        ..
+                    } = &mut fake_private_key.private_key
+                    else {
+                        panic!()
+                    };
+                    *fake_public_key = public_key;
+                }
+                PublicKey::EcdsaSha2NistP256 { public_key } => {
+                    let PrivateKey::EcdsaSha2NistP256 {
+                        public_key: fake_public_key,
+                        ..
+                    } = &mut fake_private_key.private_key
+                    else {
+                        panic!()
+                    };
+                    *fake_public_key = public_key;
+                }
+            }
+
+            let fake_private_key = fake_private_key.encrypt(KeyEncryptionParams::plaintext())?;
+
+            println!("{}", fake_private_key.to_bytes_armored());
+            Ok(())
+        }
         Subcommand::Info {
             id_file,
             decrypt,

bin/cluelesshd/src/main.rs

@@ -25,7 +25,7 @@ use eyre::{bail, eyre, Context, Result};
 use rustix::fs::MemfdFlags;
 use serde::{Deserialize, Serialize};
 use tokio::net::{TcpListener, TcpStream};
-use tracing::{ error, info, warn};
+use tracing::{error, info, warn};
 
 use tracing_subscriber::EnvFilter;
 

lib/cluelessh-keys/src/authorized_keys.rs

@@ -1,5 +1,3 @@
-use base64::Engine;
-
 use crate::public::{PublicKey, PublicKeyWithComment};
 
 pub struct AuthorizedKeys {
@@ -16,33 +14,10 @@ impl AuthorizedKeys {
         let mut keys: Vec<PublicKeyWithComment> = Vec::new();
 
         for line in lines {
-            let mut parts = line.split_whitespace();
-            let alg = parts
-                .next()
-                .ok_or_else(|| Error("missing algorithm on line".to_owned()))?;
-            let key_blob = parts
-                .next()
-                .ok_or_else(|| Error("missing key on line".to_owned()))?;
-            let key_blob = base64::prelude::BASE64_STANDARD
-                .decode(key_blob)
-                .map_err(|err| Error(format!("invalid base64 encoding for key: {err}")))?;
-            let comment = parts.next().unwrap_or_default();
-
-            let public_key = PublicKey::from_wire_encoding(&key_blob)
-                .map_err(|err| Error(format!("unsupported key: {err}")))?;
-
-            if public_key.algorithm_name() != alg {
-                return Err(Error(format!(
-                    "algorithm name mismatch: {} != {}",
-                    public_key.algorithm_name(),
-                    alg
-                )));
-            }
-
-            keys.push(PublicKeyWithComment {
-                key: public_key,
-                comment: comment.to_owned(),
-            });
+            let key = line
+                .parse::<PublicKeyWithComment>()
+                .map_err(|err| Error(err.0))?;
+            keys.push(key);
         }
 
         Ok(Self { keys })

lib/cluelessh-keys/src/private.rs

@@ -108,7 +108,6 @@ impl EncryptedPrivateKeys {
         p.array(*MAGIC);
         p.string(self.cipher.name().as_bytes());
         p.string(self.kdf.name().as_bytes());
-        dbg!(self.kdf.options());
         p.string(self.kdf.options());
 
         p.u32(self.public_keys.len() as u32);
@@ -302,9 +301,13 @@ impl KeyEncryptionParams {
 impl PlaintextPrivateKey {
     pub fn generate(comment: String, params: KeyGenerationParams) -> Self {
         let keytype = crypto::generate_private_key(params);
+        Self::new(comment, keytype)
+    }
+
+    pub fn new(comment: String, private_key: PrivateKey) -> Self {
         Self {
             comment,
-            private_key: keytype,
+            private_key,
             checkint: rand::random(),
         }
     }

lib/cluelessh-keys/src/public.rs

@@ -2,7 +2,10 @@
 
 // <https://datatracker.ietf.org/doc/html/rfc4716> exists but is kinda weird
 
-use std::fmt::{Debug, Display};
+use std::{
+    fmt::{Debug, Display},
+    str::FromStr,
+};
 
 use base64::Engine;
 
@@ -26,6 +29,39 @@ pub struct PublicKeyWithComment {
     pub comment: String,
 }
 
+impl FromStr for PublicKeyWithComment {
+    type Err = ParseError;
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let mut parts = s.split_ascii_whitespace();
+        let alg = parts
+            .next()
+            .ok_or_else(|| ParseError("missing algorithm on line".to_owned()))?;
+        let key_blob = parts
+            .next()
+            .ok_or_else(|| ParseError("missing key on line".to_owned()))?;
+        let key_blob = base64::prelude::BASE64_STANDARD
+            .decode(key_blob)
+            .map_err(|err| ParseError(format!("invalid base64 encoding for key: {err}")))?;
+        let comment = parts.next().unwrap_or_default();
+
+        let public_key = PublicKey::from_wire_encoding(&key_blob)
+            .map_err(|err| ParseError(format!("unsupported key: {err}")))?;
+
+        if public_key.algorithm_name() != alg {
+            return Err(ParseError(format!(
+                "algorithm name mismatch: {} != {}",
+                public_key.algorithm_name(),
+                alg
+            )));
+        }
+
+        Ok(Self {
+            key: public_key,
+            comment: comment.to_owned(),
+        })
+    }
+}
+
 impl PublicKey {
     /// Parses an SSH public key from its wire encoding as specified in
     /// RFC4253, RFC5656, and RFC8709.