openssl error "assertion failed" during EVP_EncryptFinal_ex, delphi

Question

While working with openSSL library, I met a problem with EVP_EncryptFinal_ex. Concretely, it fails with fatal error ./crypto/evp/evp_enc.c(348) OpenSSL internal error, assertion failed: b <= sizeof ctx -> buf every time, not depending on algorithm (aes or des).

Here is my code. It is simplified as much as possible.

procedure AESTest;
var
  key                       : TBytes;
  keyLen                    : Integer;
  dataIn                    : string;
  dataOut                   : TBytes;
  inLen, outLen, resLen     : integer;
  // Context of an algorithm pointer
  e_ctx                     : Pointer;
begin
  // 256 bit key
  keyLen := 32;
  setlength(key, KeyLen);
  RAND_bytes(@(key[0]), KeyLen);

  // Input data to encrypt
  dataIn := 'Simple data of 29 bits length';
  inLen := length(dataIn);    

  // Init ctx
  e_ctx := EVP_CIPHER_CTX_new();
  EVP_CIPHER_CTX_init(e_ctx);
  EVP_EncryptInit_ex(e_ctx, EVP_aes_256_cbc, nil, @key[0], nil);

  // Prepare ouput buf in order to openSSL docs
  outLen := inLen + EVP_CIPHER_CTX_block_size(e_ctx) - 1;
  setlength(dataOut, outLen);

  EVP_EncryptUpdate(e_ctx, @dataOut[0], outLen, @dataIn[1], inLen);

  EVP_EncryptFinal_ex(e_ctx, @dataOut[outLen], resLen);

  outLen := outLen + resLen;
  setlength(dataOut, outLen);
  // ... here goes decryption part but it does not matter now
end;    

Just to be precise, imports used:

const
   LIB_DLL_NAME = 'libeay32.dll';
type
   PEVP_CIPHER_CTX : Pointer;
   PEVP_CIPHER : Pointer;

function EVP_CIPHER_CTX_new : PEVP_CIPHER_CTX; cdecl; external LIB_DLL_NAME;
procedure EVP_CIPHER_CTX_init(a: PEVP_CIPHER_CTX); cdecl; external LIB_DLL_NAME;
function EVP_aes_256_cbc : PEVP_CIPHER_CTX; cdecl; external LIB_DLL_NAME;
function RAND_bytes(Arr : PByte; ArrLen : integer) : integer; cdecl; external LIB_DLL_NAME;
function EVP_CIPHER_CTX_block_size(ctx: PEVP_CIPHER_CTX): integer; cdecl; external LIB_DLL_NAME;
function EVP_EncryptInit_ex(ctx: PEVP_CIPHER_CTX; cipher_type: PEVP_CIPHER; Engine : Pointer; key: PByte; iv: PByte): integer; cdecl; external LIB_DLL_NAME;
function EVP_EncryptUpdate(ctx: PEVP_CIPHER_CTX; data_out: PByte; var outl: integer; data_in: PByte; inl: integer): integer; cdecl; external LIB_DLL_NAME;
function EVP_EncryptFinal_ex(ctx: PEVP_CIPHER_CTX; data_out: PByte; var outl: integer): integer; external LIB_DLL_NAME;

I actually tried to read source codes (evp_enc.c) and found the assertion:

    OPENSSL_assert(b <= sizeof ctx->buf);

Here b is size of a block for current cypher. This assertion makes sense, but still I can't figure out how it could be failed in my code.

I am trying to beat this problem for a couple of days already, and I would be grateful for any advices.

UPDATE: Here are two lines from evp_enc.c:

    b=ctx->cipher->block_size;
    OPENSSL_assert(b <= sizeof ctx->buf);

according to the code, b is a size of block for current cipher, for aes_256_cbc it is 16 bit long.

Answer 1

The problem is in your declaration of the function EVP_EncryptFinal_ex. You shoud add cdecl directive (like in all other functions). So, the new declaration will be:

function EVP_EncryptFinal_ex(ctx: PEVP_CIPHER_CTX; data_out: PByte; var outl: integer): integer; cdecl; external LIB_DLL_NAME;