Fix possible panic in internals::left_pad (#262)

Co-authored-by: Diego Fabregat Traver <diego.fabregat@rwth-aachen.de>

src/errors.rs

@@ -60,6 +60,9 @@ pub enum Error {
 
     /// Label too long.
     LabelTooLong,
+
+    /// Invalid padding length.
+    InvalidPadLen,
 }
 
 #[cfg(feature = "std")]
@@ -87,6 +90,7 @@ impl core::fmt::Display for Error {
             Error::Pkcs8(err) => write!(f, "{}", err),
             Error::Internal => write!(f, "internal error"),
             Error::LabelTooLong => write!(f, "label too long"),
+            Error::InvalidPadLen => write!(f, "invalid padding length"),
         }
     }
 }

src/internals.rs

@@ -174,14 +174,34 @@ pub fn unblind(key: &impl PublicKeyParts, m: &BigUint, unblinder: &BigUint) -> B
 
 /// Returns a new vector of the given length, with 0s left padded.
 #[inline]
-pub fn left_pad(input: &[u8], size: usize) -> Vec<u8> {
-    let n = if input.len() > size {
-        size
-    } else {
-        input.len()
-    };
+pub fn left_pad(input: &[u8], padded_len: usize) -> Result<Vec<u8>> {
+    if input.len() > padded_len {
+        return Err(Error::InvalidPadLen);
+    }
 
-    let mut out = vec![0u8; size];
-    out[size - n..].copy_from_slice(input);
-    out
+    let mut out = vec![0u8; padded_len];
+    out[padded_len - input.len()..].copy_from_slice(input);
+    Ok(out)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_left_pad() {
+        const INPUT_LEN: usize = 3;
+        let input = vec![0u8; INPUT_LEN];
+
+        // input len < padded len
+        let padded = left_pad(&input, INPUT_LEN + 1).unwrap();
+        assert_eq!(padded.len(), INPUT_LEN + 1);
+
+        // input len == padded len
+        let padded = left_pad(&input, INPUT_LEN).unwrap();
+        assert_eq!(padded.len(), INPUT_LEN);
+
+        // input len > padded len
+        let padded = left_pad(&input, INPUT_LEN - 1);
+        assert!(padded.is_err());
+    }
 }

src/raw.rs

@@ -3,7 +3,7 @@ use num_bigint::BigUint;
 use rand_core::CryptoRngCore;
 use zeroize::Zeroize;
 
-use crate::errors::{Error, Result};
+use crate::errors::Result;
 use crate::internals;
 use crate::key::{RsaPrivateKey, RsaPublicKey};
 
@@ -29,16 +29,12 @@ impl EncryptionPrimitive for RsaPublicKey {
         let mut c_bytes = c.to_bytes_be();
         let ciphertext = internals::left_pad(&c_bytes, pad_size);
 
-        if pad_size < ciphertext.len() {
-            return Err(Error::Verification);
-        }
-
         // clear out tmp values
         m.zeroize();
         c.zeroize();
         c_bytes.zeroize();
 
-        Ok(ciphertext)
+        ciphertext
     }
 }
 
@@ -59,6 +55,6 @@ impl DecryptionPrimitive for RsaPrivateKey {
         m.zeroize();
         m_bytes.zeroize();
 
-        Ok(plaintext)
+        plaintext
     }
 }