rage: Detect PowerShell corruption during decryption

This matches the detection done by `age`.

Closes #287.

rage/i18n/en-US/rage.ftl

@@ -114,6 +114,9 @@ err-enc-plugin-name-flag = {-flag-plugin-name} can't be used with {-flag-encrypt
 
 ## Decryption errors
 
+err-detected-powershell-corruption = It looks like this file was corrupted by PowerShell redirection.
+rec-detected-powershell-corruption = Consider using -o or -a to encrypt files in PowerShell.
+
 rec-dec-excessive-work = To decrypt, retry with {-flag-max-work-factor} {$wf}
 
 err-dec-armor-flag = {-flag-armor} can't be used with {-flag-decrypt}.

rage/src/bin/rage/error.rs

@@ -134,6 +134,18 @@ impl fmt::Display for EncryptError {
     }
 }
 
+#[derive(Debug)]
+pub(crate) struct DetectedPowerShellCorruptionError;
+
+impl fmt::Display for DetectedPowerShellCorruptionError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        wlnfl!(f, "err-detected-powershell-corruption")?;
+        wfl!(f, "rec-detected-powershell-corruption")
+    }
+}
+
+impl std::error::Error for DetectedPowerShellCorruptionError {}
+
 pub(crate) enum DecryptError {
     Age(age::DecryptError),
     ArmorFlag,

rage/src/bin/rage/main.rs

@@ -450,6 +450,27 @@ fn decrypt(opts: AgeOptions) -> Result<(), error::DecryptError> {
 
     let (input, output) = set_up_io(opts.input, opts.output, file_io::OutputFormat::Unknown)?;
 
+    // CRLF_MANGLED_INTRO and UTF16_MANGLED_INTRO are the intro lines of the age format after
+    // mangling by various versions of PowerShell redirection, truncated to the length of the
+    // correct intro line. See https://github.com/FiloSottile/age/issues/290 for more info.
+    const CRLF_MANGLED_INTRO: &[u8] = b"age-encryption.org/v1\r";
+    const UTF16_MANGLED_INTRO: &[u8] =
+        b"\xff\xfea\x00g\x00e\x00-\x00e\x00n\x00c\x00r\x00y\x00p\x00";
+    let err_powershell_corruption = Box::new(|| {
+        Err(io::Error::new(
+            io::ErrorKind::InvalidInput,
+            error::DetectedPowerShellCorruptionError,
+        ))
+    });
+
+    let input = ReadChecker::new(
+        input,
+        [
+            (CRLF_MANGLED_INTRO, err_powershell_corruption.clone()),
+            (UTF16_MANGLED_INTRO, err_powershell_corruption),
+        ],
+    );
+
     match age::Decryptor::new(ArmoredReader::new(input))? {
         age::Decryptor::Passphrase(decryptor) => {
             // The `rpassword` crate opens `/dev/tty` directly on Unix, so we don't have