> From: owner-openssl-us...@openssl.org On Behalf Of Tage Korsdal Nielsen > Sent: Friday, 01 February, 2013 03:19
> New to OpenSSL, but designing a PC application that must encrypt a > stream of 48 bytes message blocks to a USB device with aes128.The iv's > gets generated and synchronized when the connection with the device is > established, and I would like to keep the same cipher context going > until the the device is disconnected. > > I wrote some test code <snip> > The first encryption/decryption is ok, but then it starts altering the > order of the individual cipher blocks in my message, and on the first > decryption it reports that only 32 bytes have been decrypted, although > the entire 48 byte output buffer is correct. > EVP_ for a block cipher by default uses PKCS#5 padding. This is added on the Encrypt side (only) when you do Final, and checked and removed on the Decrypt side. To do this, the Decrypt side must run 1 block "behind" the ciphertext. I'm not sure why (apparently-correct) data is written beyond the returned count in this case, but any data beyond the count is not valid. Padding is the default because it supports arbitrary-length data which is usually what people want. If you want padding, you can't "keep the context going". If you don't want padding, and if your plaintext is chosen as a multiple of the cipher blocksize likely you don't: EVP_CIPHER_CTX_set_padding (ctx, 0); Or use the low-level AES_* routines; they don't do padding (but do do most modes, like CBC). Note without padding your data *must* be a multiple of the blocksize. Your "odd_length" does do Final on each side and looks like it would work if you were using a context not already messed up by your previous code. But per below it isn't what you want. > I then tried with RC4, here everything was ok. > RC4 is a stream cipher and doesn't need padding. > Since I need the above functionallity with AES, I did the following > "hack", in evp_enc.c line 478: <snip: basically munged padding> > Then it worked for my streamed block sized aligned messages, but of > course not for the odd length test (which I will never use anyway). > > Strangely (for me anyway) RC4 code still works after the hack, both > block size aligned stream and odd length. > See above. ______________________________________________________________________ OpenSSL Project http://www.openssl.org User Support Mailing List openssl-users@openssl.org Automated List Manager majord...@openssl.org
