Fix output buffer check introduced in sfackler#1733

Sadly the condition used to relax output buffer checks that depended
on the `num` parameter does not really hold so this change effectively
reverts PR sfackler#1733.

As clarified on the OpenSSL mailing list [0] and during integration tests
the `num` parameter does not reflect the internal buffer cache size thus
one needs to pessimistically assume that each call to `cipher_update`
will need sufficient size to contain one additional block. Streaming
ciphers are not affected by this revert.

[0]: https://mta.openssl.org/pipermail/openssl-users/2022-December/015727.html

openssl/src/cipher_ctx.rs

@@ -379,31 +379,12 @@ impl CipherCtxRef {
         unsafe { ffi::EVP_CIPHER_CTX_num(self.as_ptr()) as usize }
     }
 
-    /// Returns number of bytes cached in partial block update.
-    #[cfg(ossl110)]
-    fn used_block_size(&self) -> usize {
-        self.num()
-    }
-
-    /// Returns maximum number of bytes that could be cached.
-    #[cfg(not(ossl110))]
-    fn used_block_size(&self) -> usize {
-        self.block_size()
-    }
-
     /// Calculate the minimal size of the output buffer given the
     /// input buffer size.
     ///
     /// For streaming ciphers the minimal output size is the same as
     /// the input size. For block ciphers the minimal output size
-    /// additionally depends on the partial blocks that might have
-    /// been written in previous calls to [`Self::cipher_update`].
-    ///
-    /// This function takes into account the number of partially
-    /// written blocks for block ciphers for supported targets
-    /// (OpenSSL >= 1.1). For unsupported targets the number of
-    /// partially written bytes is assumed to contain one full block
-    /// (pessimistic case).
+    /// must be big enough to hold one additional block.
     ///
     /// # Panics
     ///
@@ -412,8 +393,7 @@ impl CipherCtxRef {
         let block_size = self.block_size();
         if block_size > 1 {
             // block cipher
-            let num = self.used_block_size();
-            let total_size = inlen + num;
+            let total_size = inlen + block_size;
             let num_blocks = total_size / block_size;
             num_blocks * block_size
         } else {
@@ -588,16 +568,13 @@ impl CipherCtxRef {
     ///
     /// This function is the same as [`Self::cipher_update`] but with the
     /// output size check removed. It can be used when the exact
-    /// buffer size control is maintained by the caller and the
-    /// underlying cryptographic library doesn't expose exact block
-    /// cache data (e.g. OpenSSL < 1.1, BoringSSL, LibreSSL).
+    /// buffer size control is maintained by the caller.
     ///
     /// SAFETY: The caller is expected to provide `output` buffer
     /// large enough to contain correct number of bytes. For streaming
     /// ciphers the output buffer size should be at least as big as
     /// the input buffer. For block ciphers the size of the output
-    /// buffer depends on the state of partially updated blocks (see
-    /// [`Self::minimal_output_size`]).
+    /// buffer depends on the state of partially updated blocks.
     #[corresponds(EVP_CipherUpdate)]
     pub unsafe fn cipher_update_unchecked(
         &mut self,
@@ -757,75 +734,6 @@ mod test {
         aes_128_cbc(cipher);
     }
 
-    #[test]
-    #[cfg(ossl110)]
-    fn partial_block_updates() {
-        test_block_cipher_for_partial_block_updates(Cipher::aes_128_cbc());
-        test_block_cipher_for_partial_block_updates(Cipher::aes_256_cbc());
-        test_block_cipher_for_partial_block_updates(Cipher::des_ede3_cbc());
-    }
-
-    #[cfg(ossl110)]
-    fn test_block_cipher_for_partial_block_updates(cipher: &'static CipherRef) {
-        let mut key = vec![0; cipher.key_length()];
-        rand_bytes(&mut key).unwrap();
-        let mut iv = vec![0; cipher.iv_length()];
-        rand_bytes(&mut iv).unwrap();
-
-        let mut ctx = CipherCtx::new().unwrap();
-
-        ctx.encrypt_init(Some(cipher), Some(&key), Some(&iv))
-            .unwrap();
-        ctx.set_padding(false);
-
-        let block_size = cipher.block_size();
-        assert!(block_size > 1, "Need a block cipher, not a stream cipher");
-
-        // update cipher with non-full block
-        // expect no output until a block is complete
-        let outlen = ctx
-            .cipher_update(&vec![0; block_size - 1], Some(&mut [0; 0]))
-            .unwrap();
-        assert_eq!(0, outlen);
-
-        // update cipher with missing bytes from the previous block
-        // and one additional block, output should contain two blocks
-        let mut two_blocks = vec![0; block_size * 2];
-        let outlen = ctx
-            .cipher_update(&vec![0; block_size + 1], Some(&mut two_blocks))
-            .unwrap();
-        assert_eq!(block_size * 2, outlen);
-
-        ctx.cipher_final_vec(&mut vec![0; 0]).unwrap();
-
-        // try to decrypt
-        ctx.decrypt_init(Some(cipher), Some(&key), Some(&iv))
-            .unwrap();
-        ctx.set_padding(false);
-
-        // update cipher with non-full block
-        // expect no output until a block is complete
-        let outlen = ctx
-            .cipher_update(&two_blocks[0..block_size - 1], Some(&mut [0; 0]))
-            .unwrap();
-        assert_eq!(0, outlen);
-
-        // update cipher with missing bytes from the previous block
-        // and one additional block, output should contain two blocks
-        let mut two_blocks_decrypted = vec![0; block_size * 2];
-        let outlen = ctx
-            .cipher_update(
-                &two_blocks[block_size - 1..],
-                Some(&mut two_blocks_decrypted),
-            )
-            .unwrap();
-        assert_eq!(block_size * 2, outlen);
-
-        ctx.cipher_final_vec(&mut vec![0; 0]).unwrap();
-        // check if the decrypted blocks are the same as input (all zeros)
-        assert_eq!(two_blocks_decrypted, vec![0; block_size * 2]);
-    }
-
     #[test]
     fn test_stream_ciphers() {
         test_stream_cipher(Cipher::aes_192_ctr());
@@ -893,44 +801,20 @@ mod test {
         assert_eq!(output_decrypted, vec![1; 32]);
     }
 
-    #[test]
-    #[should_panic(expected = "Output buffer size should be at least 16 bytes.")]
-    #[cfg(ossl110)]
-    fn full_block_updates_aes_128() {
-        output_buffer_too_small(Cipher::aes_128_cbc());
-    }
-
-    #[test]
-    #[should_panic(expected = "Output buffer size should be at least 16 bytes.")]
-    #[cfg(ossl110)]
-    fn full_block_updates_aes_256() {
-        output_buffer_too_small(Cipher::aes_256_cbc());
-    }
-
-    #[test]
-    #[should_panic(expected = "Output buffer size should be at least 8 bytes.")]
-    #[cfg(ossl110)]
-    fn full_block_updates_3des() {
-        output_buffer_too_small(Cipher::des_ede3_cbc());
-    }
-
     #[test]
     #[should_panic(expected = "Output buffer size should be at least 32 bytes.")]
-    #[cfg(not(ossl110))]
     fn full_block_updates_aes_128() {
         output_buffer_too_small(Cipher::aes_128_cbc());
     }
 
     #[test]
     #[should_panic(expected = "Output buffer size should be at least 32 bytes.")]
-    #[cfg(not(ossl110))]
     fn full_block_updates_aes_256() {
         output_buffer_too_small(Cipher::aes_256_cbc());
     }
 
     #[test]
     #[should_panic(expected = "Output buffer size should be at least 16 bytes.")]
-    #[cfg(not(ossl110))]
     fn full_block_updates_3des() {
         output_buffer_too_small(Cipher::des_ede3_cbc());
     }