Validate PSK index in pattern to avoid panic

Previously, when the handshake tokens were compiled from the parsed
Noise parameters, snow would panic by attempting to access an
out-of-bounds index.

src/error.rs

@@ -4,7 +4,7 @@ use std::fmt;
 
 /// All errors in snow will include an `ErrorKind`.
 #[allow(missing_docs)]
-#[derive(Debug)]
+#[derive(Debug, PartialEq)]
 #[non_exhaustive]
 pub enum Error {
     /// The noise pattern failed to parse.
@@ -36,7 +36,7 @@ pub enum Error {
 /// The various stages of initialization used to help identify
 /// the specific cause of an `Init` error.
 #[allow(missing_docs)]
-#[derive(Debug)]
+#[derive(Debug, PartialEq)]
 pub enum PatternProblem {
     TooFewParameters,
     UnsupportedHandshakeType,
@@ -59,7 +59,7 @@ impl From<PatternProblem> for Error {
 /// The various stages of initialization used to help identify
 /// the specific cause of an `Init` error.
 #[allow(missing_docs)]
-#[derive(Debug)]
+#[derive(Debug, PartialEq)]
 pub enum InitStage {
     ValidateKeyLengths,
     ValidatePskLengths,
@@ -81,7 +81,7 @@ impl From<InitStage> for Error {
 
 /// A prerequisite that may be missing.
 #[allow(missing_docs)]
-#[derive(Debug)]
+#[derive(Debug, PartialEq)]
 pub enum Prerequisite {
     LocalPrivateKey,
     RemotePublicKey,
@@ -95,7 +95,7 @@ impl From<Prerequisite> for Error {
 
 /// Specific errors in the state machine.
 #[allow(missing_docs)]
-#[derive(Debug)]
+#[derive(Debug, PartialEq)]
 pub enum StateProblem {
     MissingKeyMaterial,
     MissingPsk,

src/params/mod.rs

@@ -309,4 +309,14 @@ mod tests {
             _ => panic!("missing token!"),
         }
     }
+
+    #[test]
+    fn test_invalid_psk_handshake() {
+        let p: NoiseParams = "Noise_XXpsk9_25519_AESGCM_SHA256".parse().unwrap();
+
+        assert_eq!(
+            Error::Pattern(PatternProblem::InvalidPsk),
+            HandshakeTokens::try_from(&p.handshake).unwrap_err()
+        );
+    }
 }

src/params/patterns.rs

@@ -291,6 +291,7 @@ impl<'a> TryFrom<&'a HandshakeChoice> for HandshakeTokens {
     #[allow(clippy::cognitive_complexity)]
     fn try_from(handshake: &'a HandshakeChoice) -> Result<Self, Self::Error> {
         // Hfs cannot be combined with one-way handshake patterns
+        #[cfg(feature = "hfs")]
         check_hfs_and_oneway_conflict(handshake)?;
 
         #[rustfmt::skip]
@@ -489,7 +490,7 @@ impl<'a> TryFrom<&'a HandshakeChoice> for HandshakeTokens {
 
         for modifier in handshake.modifiers.list.iter() {
             match modifier {
-                HandshakeModifier::Psk(n) => apply_psk_modifier(&mut patterns, *n),
+                HandshakeModifier::Psk(n) => apply_psk_modifier(&mut patterns, *n)?,
                 #[cfg(feature = "hfs")]
                 HandshakeModifier::Hfs => apply_hfs_modifier(&mut patterns),
                 _ => return Err(PatternProblem::UnsupportedModifier.into()),
@@ -517,21 +518,18 @@ fn check_hfs_and_oneway_conflict(handshake: &HandshakeChoice) -> Result<(), Erro
     }
 }
 
-#[cfg(not(feature = "hfs"))]
-fn check_hfs_and_oneway_conflict(_: &HandshakeChoice) -> Result<(), Error> {
-    Ok(())
-}
-
-fn apply_psk_modifier(patterns: &mut Patterns, n: u8) {
-    match n {
-        0 => {
-            patterns.2[0].insert(0, Token::Psk(n));
-        },
-        _ => {
-            let i = (n as usize) - 1;
-            patterns.2[i].push(Token::Psk(n));
-        },
+/// Given our PSK modifier, we inject the token at the appropriate place.
+fn apply_psk_modifier(patterns: &mut Patterns, n: u8) -> Result<(), Error> {
+    let tokens = patterns
+        .2
+        .get_mut((n as usize).saturating_sub(1))
+        .ok_or(Error::Pattern(PatternProblem::InvalidPsk))?;
+    if n == 0 {
+        tokens.insert(0, Token::Psk(n));
+    } else {
+        tokens.push(Token::Psk(n));
     }
+    Ok(())
 }
 
 #[cfg(feature = "hfs")]