> From: [email protected] On Behalf Of Jonas Schnelli > Sent: Wednesday, 30 November, 2011 09:57
> I try to pre-calculate the size of a EVP_Seal encrypted buffer > (just the seal, exkl. keys). How do i precalculate that? I assume > with some blocksize stuff (i'm a crypto novice). > I'm using AES256 as EVP_CIPHER. The 'seal' or 'envelope' operation actually produces symmetrically encrypted data PLUS PK-encrypted/wrapped (nonce) key and optionally IV, and typically some recipient-identifier information. These are meaningless and unusable separately. I assume you mean to ask about only the encrypted "bulk" data. If you use a block mode that requires padding (CBC or ECB, but don't use ECB unless you know what you're doing), the ciphertext will be up to one block longer than the plaintext, thus buffersize (if you use a single buffer instead of pieces) must be at least one block longer. In general this is EVP_CIPHER_[CTX_]block_size(ciph) and for AES it is 16 bytes. If you use a stream mode (CFB, OFB, CTR) (or a stream cipher, which AES isn't) ciphertext is same size as plaintext. ______________________________________________________________________ OpenSSL Project http://www.openssl.org User Support Mailing List [email protected] Automated List Manager [email protected]
