ssh-cipher: allows to decrypt a payload without altering state (#266)

When decrypting messages from the wire, we need to first decrypt the
first couple of bytes (4 or 16 bytes) just to get the length of packet.

Once we get the length of the packet we can decrypt the full payload. If
we just used the Decryptor api as it is, we would alter the internal
state.

ssh-cipher/src/decryptor.rs

@@ -25,6 +25,7 @@ pub struct Decryptor {
 }
 
 /// Inner decryptor enum which is deliberately kept out of the public API.
+#[derive(Clone)]
 enum Inner {
     #[cfg(feature = "aes-cbc")]
     Aes128Cbc(cbc::Decryptor<Aes128>),
@@ -42,6 +43,31 @@ enum Inner {
     TDesCbc(cbc::Decryptor<TdesEde3>),
 }
 
+impl Inner {
+    fn decrypt(&mut self, buffer: &mut [u8]) -> Result<()> {
+        #[cfg(any(feature = "aes-cbc", feature = "aes-ctr", feature = "tdes"))]
+        match self {
+            #[cfg(feature = "aes-cbc")]
+            Self::Aes128Cbc(cipher) => cbc_decrypt(cipher, buffer),
+            #[cfg(feature = "aes-cbc")]
+            Self::Aes192Cbc(cipher) => cbc_decrypt(cipher, buffer),
+            #[cfg(feature = "aes-cbc")]
+            Self::Aes256Cbc(cipher) => cbc_decrypt(cipher, buffer),
+            #[cfg(feature = "aes-ctr")]
+            Self::Aes128Ctr(cipher) => ctr_decrypt(cipher, buffer),
+            #[cfg(feature = "aes-ctr")]
+            Self::Aes192Ctr(cipher) => ctr_decrypt(cipher, buffer),
+            #[cfg(feature = "aes-ctr")]
+            Self::Aes256Ctr(cipher) => ctr_decrypt(cipher, buffer),
+            #[cfg(feature = "tdes")]
+            Self::TDesCbc(cipher) => cbc_decrypt(cipher, buffer),
+        }
+        .map_err(|_| Error::Length)?;
+
+        Ok(())
+    }
+}
+
 impl Decryptor {
     /// Create a new decryptor object with the given [`Cipher`], key, and IV.
     pub fn new(cipher: Cipher, key: &[u8], iv: &[u8]) -> Result<Self> {
@@ -94,26 +120,16 @@ impl Decryptor {
     /// Returns [`Error::Length`] in the event that `buffer` is not a multiple of the cipher's
     /// block size.
     pub fn decrypt(&mut self, buffer: &mut [u8]) -> Result<()> {
-        #[cfg(any(feature = "aes-cbc", feature = "aes-ctr", feature = "tdes"))]
-        match &mut self.inner {
-            #[cfg(feature = "aes-cbc")]
-            Inner::Aes128Cbc(cipher) => cbc_decrypt(cipher, buffer),
-            #[cfg(feature = "aes-cbc")]
-            Inner::Aes192Cbc(cipher) => cbc_decrypt(cipher, buffer),
-            #[cfg(feature = "aes-cbc")]
-            Inner::Aes256Cbc(cipher) => cbc_decrypt(cipher, buffer),
-            #[cfg(feature = "aes-ctr")]
-            Inner::Aes128Ctr(cipher) => ctr_decrypt(cipher, buffer),
-            #[cfg(feature = "aes-ctr")]
-            Inner::Aes192Ctr(cipher) => ctr_decrypt(cipher, buffer),
-            #[cfg(feature = "aes-ctr")]
-            Inner::Aes256Ctr(cipher) => ctr_decrypt(cipher, buffer),
-            #[cfg(feature = "tdes")]
-            Inner::TDesCbc(cipher) => cbc_decrypt(cipher, buffer),
-        }
-        .map_err(|_| Error::Length)?;
+        self.inner.decrypt(buffer)
+    }
 
-        Ok(())
+    /// Decrypt the given buffer in place without altering the internal state
+    ///
+    /// Returns [`Error::Length`] in the event that `buffer` is not a multiple of the cipher's
+    /// block size.
+    pub fn peek_decrypt(&self, buffer: &mut [u8]) -> Result<()> {
+        let mut inner = self.inner.clone();
+        inner.decrypt(buffer)
     }
 }