openssl error "assertion failed" during EVP_EncryptFinal_ex, delphi - 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;