Hello, Stephen, Thomas,
From: "Thomas BERNARD" <bernard-...@stg-interactive.com>
To my understanding :
With ECB, the order in which the blocks are crypted/decrypted doesn't
matter.
With CBC and most block modes, it DOES matter !
So if block 1 is encrypted first it MUST be decrypted first.
/* decrypt second block */
EVP_DecryptUpdate(&dec_ctx, block2, &outlen, block2, 4096);
/* decrypt first block */
EVP_DecryptUpdate(&dec_ctx, block1, &outlen, block1, 4096);
is WRONG
EVP_DecryptUpdate(&dec_ctx, block1, &outlen, block1, 4096);
EVP_DecryptUpdate(&dec_ctx, block2, &outlen, block2, 4096);
should work better
Thank you very much for your quick responses. I'm glad about the answers to
Q1-Q3 as I can easily minimize the overhead in encryption/decryption.
As for Q4, yes, decrypting blocks in the same order as encrypting them
certainly produced correct data. owever, I cannot do that because the
application needs to encrypt/decrypt blocks/records in random order.
In addition, I feel I'd better use CBC rather than ECB at least for file
type 1 (collection of 4KB blocks), so that others don't know the pattern of
blocks such as "this 4KB block consists of the same 16-byte sequences." On
the other hand, it is okay for others to know "this and that 4KB blocks have
identical data" or "this and that records are identical." (is this thinking
reasonable in cryptography world?)
However, I'd like to avoid the overhead of calling
EVP_EncryptInit_ex/EVP_DecryptInit_ex for each block/record, because I
wonder if those functions have relatively high overhead especially when
AES-NI decrease the overhead of encryption/decryption. That is, I'm afraid
that repeated init calls reduces the benefits gained by AES-NI. Is there any
way?
if (ctx->cipher->ctx_size)
{
ctx->cipher_data=OPENSSL_malloc(ctx->cipher->ctx_size);
...
case EVP_CIPH_CBC_MODE:
OPENSSL_assert(EVP_CIPHER_CTX_iv_length(ctx) <=
(int)sizeof(ctx->iv));
if(iv) memcpy(ctx->oiv, iv, EVP_CIPHER_CTX_iv_length(ctx));
memcpy(ctx->iv, ctx->oiv, EVP_CIPHER_CTX_iv_length(ctx));
break;
...
if(key || (ctx->cipher->flags & EVP_CIPH_ALWAYS_CALL_INIT)) {
if(!ctx->cipher->init(ctx,key,iv,enc)) return 0;
}
Regards
MauMau
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