See below...
Pawe� Krawczyk wrote:
>
> Hello, I'm having problems with the EVP_Encrypt/Decrypt interface.
> The following program doesn't work as expected. It does simple
> data->encrypt->decrypt->in. Specifically, I get some garbage mixed with
> the plaintext in the final buffer.
>
> It seems like the EVP_DncryptUpdate decrypts (n-1) blocks of 8 bytes,
> which is OK, but the EVP_DncryptFinal doesn't decrypt the remaining
> block and returns `bad_decrypt' error - everything is in the comments
> below. What am I doing wrong?
>
....
>
> /* Encrypt from data to out */
> /* Length of the data is 36 */
> EVP_EncryptUpdate( &ectx, out, &outl, data, strlen(data));
After this call 'outl' bytes of encrypted data have been written to
'out' thereform you *must* update the pointer passed to
EVP_EncrytpFinal() so it doesn't overwrite the data:
> toutl = outl; /* Now outl is 32 */
> EVP_EncryptFinal( &ectx, out, &outl);
i.e. instead use something like:
EVP_EncryptFinal(&ectx, out + outl, &outl);
> toutl += outl; /* Now outl is 8 */
>
> /* Clean up and reinitialize */
> EVP_CIPHER_CTX_cleanup(&ectx);
> EVP_DecryptInit( &ectx, EVP_des_ede3_cbc(), enckey, NULL );
>
> /* Decrypt from out to in */
> /* toutl is 40 */
> EVP_DecryptUpdate( &ectx, in, &inl, out, toutl);
> /* inl is 32 - correct, but the result in `in' is garbage,
> * and then some plaintext (starting with `qrst...'
> */
> ret = EVP_DecryptFinal( &ectx, in, &inl);
Similarly this should be:
ret = EVP_DecryptFinal(&ectx, in + inl, &inl);
> /* This fails, inl is 0 and error `bad_decrypt' is set */
>
> if (ret != 1)
> {
> ERR_print_errors(bio_err);
> }
>
> return 0;
> }
Steve.
--
Dr Stephen N. Henson. http://www.drh-consultancy.demon.co.uk/
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