如何正确处理C到Ada的字节缓冲区？AES解密输出异常排查

我正在编写基于OpenSSL libcrypto库的简单Ada文件加解密程序，目前遇到输出流处理问题，解密后的内容出现乱码。参考OpenSSL官方的EVP_CIPHER_fetch示例实现，但解密输出如下：

The zeitgeist contends that their hand was, in this moment, a stagey mouth. Some plumbic chives are thought of simply as sideboar (3072 worth of ^@) correct text again

以下是使用AES-128-CTR算法的Ada代码：

with EVP;                   use EVP;
with Types;                 use Types;
with Text_IO;
with System;
with Interfaces.C;          use Interfaces.C;
with Ada.Streams.Stream_IO; use Ada.Streams.Stream_IO;
with Ada.Streams;           use Ada.Streams;
with Ada.Command_Line;      use Ada.Command_Line;

procedure Ada_Aes is
   ctx       : EVP_CIPHER_CTX_PTR;
   cipher    : EVP_CIPHER_PTR;
   Junk      : Integer;
   Algo      : char_array         := To_C ("AES-128-CTR", False);
   Prop      : char_array         := To_C ("", True);
   ossl_ctx  : OSSL_LIB_CTX_PTR;
   engine_st : ENGINE_ST_PTR;
   Key       : aliased char_array := To_C ("abcdefghabcdefgh");
   IV        : aliased char_array := To_C ("abcdefghabcdefgh");

   InputF        : File_Type;
   OutputF       : File_Type;
   InputS        : Stream_Access;
   OutputS       : Stream_Access;
   Inlen, Outlen : Integer;

   Mode : String := Argument (1); -- Operation ('encrypt' or 'decrypt')
   Input_File_Name  : String := Argument (2);
   Output_File_Name : String := Argument (3);

   Test       : Byte;
   Inbuf      : Byte_Array (1 .. BUFSIZE)      := (others => 0);
   Outbuf     : Byte_Array (1 .. BUFSIZE + 32) := (others => 0);
   Outbuf_Len : aliased Integer;
   Inbuf_Len  : aliased Integer                := BUFSIZE;

   Op : Operation;
begin
   if Mode = "encrypt" then
      Op := Encrypt;
   elsif Mode = "decrypt" then
      Op := Decrypt;
   else
      Text_IO.Put_Line ("Invalid mode. Use 'encrypt' or 'decrypt'");
      return;
   end if;

   Open (InputF, In_File, Input_File_Name);
   Open (OutputF, Out_File, Output_File_Name);
   InputS  := Stream (InputF);
   OutputS := Stream (OutputF);

   ctx    := EVP_CIPHER_CTX_new;
   cipher := EVP_aes_128_ctr;
   if EVP_CipherInit
       (Ctx      => ctx, Cipher => cipher, CipherKey => Key'Access,
        CipherIV => IV'Access, Op => Op) /= 
     1
   then
      raise Constraint_Error;
   end if;
   Text_IO.Put_Line (Size (InputF)'Image);
   Text_IO.Put_Line (Index (InputF)'Image);

   while (Index (InputF) < Size (InputF)) loop
      Byte_Array'Read (InputS, Inbuf);
      if EVP_CipherUpdate
          (ctx, Outbuf (1)'Access, Outbuf_Len'Access, Inbuf (1)'Access,
           Inbuf_Len) /= 
        1
      then
         raise Constraint_Error;
      end if;
      Byte_Array'Write (OutputS, Outbuf (1 .. Outbuf_Len));
   end loop;
   if EVP_CipherFinal_ex (ctx, Outbuf (1)'Access, Outbuf_Len'Access) /= 1 then
      raise Constraint_Error;
   end if;
   Byte_Array'Write (OutputS, Outbuf (1 .. Outbuf_Len));

   EVP_CIPHER_CTX_free (ctx);

   Close (InputF);
   Close (OutputF);
exception
   when End_Error =>
      if EVP_CipherFinal_ex (ctx, Outbuf (1)'Access, Outbuf_Len'Access) /= 1
      then
         raise Constraint_Error;
      end if;
      Byte_Array'Write (OutputS, Outbuf (1 .. Outbuf_Len));
      Text_IO.Put_Line (Outbuf_Len'Image);
      Text_IO.Put_Line (Outbuf_Len'Image);

      EVP_CIPHER_CTX_free (ctx);
end Ada_Aes;

问题核心在于缓冲区实际读取长度的错误处理，修复点如下：

1. 修正输入缓冲区的实际读取长度
   原代码固定用BUFSIZE作为EVP_CipherUpdate的输入长度，但最后一次读取文件时，缓冲区大概率无法填满，此时传入完整BUFSIZE会让OpenSSL处理多余的空字节，导致解密乱码。需改为每次读取后记录实际读取的字节数：

   -- 替换原Byte_Array'Read逻辑
   declare
      Actual_Read : Integer := 0;
   begin
      while Actual_Read < Inbuf'Length and then not End_Of_File(InputF) loop
         Byte'Read(InputS, Inbuf(Actual_Read + 1));
         Actual_Read := Actual_Read + 1;
      end loop;
      Inbuf_Len := Actual_Read;
      if Inbuf_Len = 0 then
         exit;
      end if;
   end;
   

2. 重构文件读取循环逻辑
   原循环用Index(InputF) < Size(InputF)作为条件，无法准确处理最后一次不完整读取。改为直接检查文件是否结束，结合实际读取长度处理：

   -- 替换原while循环
   while not End_Of_File(InputF) loop
      declare
         Actual_Read : Integer := 0;
      begin
         -- 读取实际可用的字节数
         while Actual_Read < Inbuf'Length and then not End_Of_File(InputF) loop
            Byte'Read(InputS, Inbuf(Actual_Read + 1));
            Actual_Read := Actual_Read + 1;
         end loop;
         Inbuf_Len := Actual_Read;
         if Inbuf_Len = 0 then
            exit;
         end if;
   
         -- 调用加解密更新函数
         if EVP_CipherUpdate
             (ctx, Outbuf (1)'Access, Outbuf_Len'Access, Inbuf (1)'Access,
              Inbuf_Len) /= 1
         then
            raise Constraint_Error;
         end if;
         -- 写入实际处理后的输出内容
         Byte_Array'Write (OutputS, Outbuf (1 .. Outbuf_Len));
      end;
   end loop;
   

3. 简化异常处理逻辑
   重构后的循环已正确处理文件结束场景，无需额外捕获End_Error。保留资源释放逻辑即可，避免重复调用EVP_CipherFinal_ex。

4. CTR模式特殊处理
   AES-CTR属于流密码模式，EVP_CipherFinal_ex不会产生额外输出（返回长度为0），但仍需调用以完成上下文清理，可添加日志确认这一点，避免写入空内容。

内容的提问来源于stack exchange，提问作者Adam