Fix output size check for stream ciphers

The previous output size check presumed a block cipher. Therefore, it
enforced an unnecessary extra byte in the case of stream ciphers. This
patch ensures that our size checks don't force the caller to
overallocate for stream ciphers.

openssl/src/symm.rs

@@ -492,12 +492,16 @@ impl Crypter {
     ///
     /// # Panics
     ///
-    /// Panics if `output.len() < input.len() + block_size` where
-    /// `block_size` is the block size of the cipher (see `Cipher::block_size`),
-    /// or if `output.len() > c_int::max_value()`.
+    /// Panics for stream ciphers if `output.len() < input.len()`.
+    ///
+    /// Panics for block ciphers if `output.len() < input.len() + block_size`,
+    /// where `block_size` is the block size of the cipher (see `Cipher::block_size`).
+    ///
+    /// Panics if `output.len() > c_int::max_value()`.
     pub fn update(&mut self, input: &[u8], output: &mut [u8]) -> Result<usize, ErrorStack> {
         unsafe {
-            assert!(output.len() >= input.len() + self.block_size);
+            let block_size = if self.block_size > 1 { self.block_size } else { 0 };
+            assert!(output.len() >= input.len() + block_size);
             assert!(output.len() <= c_int::max_value() as usize);
             let mut outl = output.len() as c_int;
             let inl = input.len() as c_int;
@@ -523,10 +527,11 @@ impl Crypter {
     ///
     /// # Panics
     ///
-    /// Panics if `output` is less than the cipher's block size.
+    /// Panics for block ciphers if `output.len() < block_size`,
+    /// where `block_size` is the block size of the cipher (see `Cipher::block_size`).
     pub fn finalize(&mut self, output: &mut [u8]) -> Result<usize, ErrorStack> {
         unsafe {
-            assert!(output.len() >= self.block_size);
+            if self.block_size > 1 { assert!(output.len() >= self.block_size); }
             let mut outl = cmp::min(output.len(), c_int::max_value() as usize) as c_int;
 
             cvt(ffi::EVP_CipherFinal(
@@ -753,6 +758,22 @@ mod tests {
     use super::*;
     use hex::{self, FromHex};
 
+    #[test]
+    fn test_stream_cipher_output() {
+        let key = [0u8; 16];
+        let iv = [0u8; 16];
+        let mut c = super::Crypter::new(
+            super::Cipher::aes_128_ctr(),
+            super::Mode::Encrypt,
+            &key,
+            Some(&iv),
+        ).unwrap();
+
+        assert_eq!(c.update(&[0u8; 15], &mut [0u8; 15]).unwrap(), 15);
+        assert_eq!(c.update(&[0u8; 1], &mut [0u8; 1]).unwrap(), 1);
+        assert_eq!(c.finalize(&mut [0u8; 0]).unwrap(), 0);
+    }
+
     // Test vectors from FIPS-197:
     // http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf
     #[test]