[openssl-users] Decrypting an OpenSSL encrypt AES256-CBC data

[Ertan Küçükoglu]

Hello,

Windows program does not know length of data. I would like to use some kind
of standard method and use exact method on Windows to decrypt.

I think my problem is really that I do not know what "padding" is used by
default. I have found below function. However, there is no detailed
explanation about it in here:
https://www.openssl.org/docs/man1.0.2/crypto/EVP_CIPHER_CTX_set_padding.html
int EVP_CIPHER_CTX_set_padding(EVP_CIPHER_CTX *x, int padding);

I wanted to learn what values can "padding" parameter be. I understand I
can set it to zero ( 0 ) for disabling padding. This is not what I want
because my plain text length is not confirmed  to be multiply of 16 bytes.

I can use PKCS#7 to decrypt on Windows so I would like to encrypt using
that padding. Just do not know what value to pass in above function now.

Thanks & regards,
Ertan Küçükoğlu




Michael Wojcik <Michael.Wojcik at microfocus.com>, 28 Ara 2018 Cum, 21:16
tarihinde şunu yazdı:

> > From: openssl-users [mailto:openssl-users-bounces at openssl.org] On
> Behalf Of Ertan Küçükoglu
> > Sent: Thursday, December 27, 2018 16:03
>
> > A- I tried to directly decrypt (no padding applied) and I get my plain
> text plus
> > some additional invisible characters at the end. I am told it maybe a
> "padding"
> > issue, my problem, during decryption.
>
> How does the Windows program know how long the decrypted data is?
>
> It sounds to me like the problem is simply that your Windows code is
> decrypting the data correctly, then reading past it into garbage left at
> the end of the buffer.
>
> If the messages are of fixed length, only use that many bytes from the
> decryption output. If they're of variable length, then the sender will have
> to tell the receiver how long they are. There are many ways of doing that;
> you haven't told us enough about your protocol to know which would be
> appropriate in your case.
>
> --
> Michael Wojcik
> Distinguished Engineer, Micro Focus
>
>
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>
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