This is great!
I have finally create my function based on
CTR_ModePolicy::SeekToIteration function. Here it is if anyone want to use
it:
void SeekToIteration(unsigned long long iterationCount, char *
str_original_IV, int i_size_IV, char * str_new_IV )
{
int carry=0;
for (int i=i_size_IV-1; i>=0; i--)
{
unsigned int sum = str_original_IV[i] + byte(iterationCount) +
carry;
str_new_IV[i] = (byte) sum;
carry = sum >> 8;
iterationCount >>= 8;
}
}
I have a question regarding the chunk or block size I have to divide the
whole file. Is there any recomentación about it? Is 1 KB valid? or 4KB is
better? Or 1 MB? Or there is not any limit or better value than other?
Thank you very much to everyone who helped!
El miércoles, 21 de octubre de 2015, 14:15:21 (UTC+2), jean-pierre.muench
escribió:
>
> Am 21.10.2015 um 08:05 schrieb lasl...@gmail.com <javascript:>:
>
>
> OK, I see.
> But what if I want to get the IV of 123th block? Do I have to do it 123
> times or there is an IncrementCounterByX(byte*,unsigned int, unsignet int )
> function?
>
> The simplest solution is to just loop IncrementCounterByOne().
> The more advanced solution would be to use the function CTR uses for this
> purpose (see below). You'd replace m_register[] with your input IV ad get
> your input into m_counterArray[] (in practice you'd use two pointers here).
>
> BR
>
> JPM
>
> void CTR_ModePolicy::SeekToIteration(lword iterationCount)
>
> {
>
> int carry=0;
>
> for (int i=BlockSize()-1; i>=0; i--)
>
> {
>
> unsigned int sum = m_register[i] + byte(iterationCount) + carry;
>
> m_counterArray[i] = (byte) sum;
>
> carry = sum >> 8;
>
> iterationCount >>= 8;
>
> }
>
> }
>
>
>
> El martes, 20 de octubre de 2015, 23:11:10 (UTC+2), jean-pierre.muench
> escribió:
>
> Am 20.10.2015 um 19:11 schrieb lasl...@gmail.com:
>
> Hi:
>
> I have obviously misunderstand how the TAG works. I've been
> playing around with code and now I have it clear. Thank you for your help.
>
>
>
> About the concept of "blocking" or "chunking": my question was
> more focused to some sample code in which, starting an IV for the first
> block, shows me how to create the following IV incremented by one the
> previous IV.
>
> If your problem is only the change of the IV for the next chunk there's
> always "IncrementCounterByOne(byte*,unsigned int)" in Crypto++. You input
> your old IV IncrementCounterByOne(IV,12) and can feed IV directly into the
> next call to GCM.
>
> The first IV may be all-0 or some random value (AutoSeededRandomPool,
> AutoSeededX917RNG).
>
> BR
>
> JPM
>
>
> I'm reading the documentation and testing and thank you very much
> for your help.
>
> Thank you very much!
>
>
> El lunes, 19 de octubre de 2015, 20:27:37 (UTC+2), jean-pierre.muench
> escribió:
>
> Am 19.10.2015 um 19:05 schrieb lasl...@gmail.com:
>
> Hi Jean Pierre:
>
> Thank you very much for your answer!
>
> First of all: yes, I need read and write at any place in the stored
> file. I know this is a problem.
>
> I was reading some doc about AES-GCM, and the way you propose to work,
> and have some doubts/observations:
>
> - I understand that using 256bit AES, the GCM mode, and size of TAG =
> 96 bytes (the shortest recommended) will get a resulting file,
> 37.5% larger than the original (regardless of the header).
>
> I think you're getting something wrong here. The longest standard tag for
> GCM is 16 bytes (128 bits) and the shortest standard tag is 12 bytes (96
> bits).
>
> Some of the files we will encrypt are about 1 GB or even 2GB, so we are
> speaking about 375MB for tags. This is correct?
>
> I don't know how you calculated those 375MB. Depending on how far you want
> to chunk you can have 16 byte tag overhead (for the whole files, only one
> chunk) or more if you want smaller chunks. Reasonable would be a
> sector-size alignment, meaning Data||Tag would be as large as one sector of
> your filesystem. This would result in an increase of 3.2% (512-byte
> sectors) to 0.09% (16,384-byte sectors). Or you could just use some
> arbitrary unit like one Megabyte per chunk, resulting in a 0.0015% increase
> in storage needed.
>
>
> - You say *In your scenario I'd suggest dividing the file into
> several subsections*: Where can I find the info about how to handle
> the subsection concept for AES-GCM?
>
> Chunking and Blocking (see Jeffrey's answer) are standard concepts. What I
> basically suggest is to divide the file into
> Header||Chunk1||Chunk2||Chunk3... where each Chunk has it's own tag and is
> fully independent from the others (besides the somewhat dependent IV).
>
>
> - As I can see here
> <https://groups.google.com/forum/#%21topic/cryptopp-users/UHlnZ8r-0Gc>
> https://groups.google.com/forum/#!topic/cryptopp-users/UHlnZ8r-0Gc
> there is not option to seek in GCM. It´s true?
>
> Yes, you can't seek with AES-GCM and Crypto++, because you'd skip parts of
> the enciphered data, which you can't because the authentication would fail
> in this case.
>
> By the way, what do you think about using XTS as shown in the link? I see
> this option is not implemented in Crypto++...
>
> See Jeffrey's answer.
>
>
>
> - If I want *not *to have TAG overhead I have two options AES-CTR+HMAC
> or AES-CBC+HMAC:
>
> CTR would be the better option here as it can be parallelized (supports
> seek() in Crypto++) and has less severe IV requirements.
>
>
> - Pros:
> - File size will change only with header info.
>
> You can also get constant tag size "tag overhead" with AES-GCM by only
> using one chunk.
>
>
> - Need to change HMAC every time file is changed. A test to perform:
> how long it takes to compute HMAC of a 1GB file?
>
> With SHA-256, you'd roughly need 20 cycles / byte on such long messages
> with a C implementation on an Intel Core 2 Duo [1]. Meaning you may get
> something like 10 cycles / byte with ASM code on a modern CPU. HMAC is not
> significantly slower than plain hashing. This would mean you'd need 10^10
> cycles for this - or equivalently - 5 seconds on a 2GHz CPU. Reading the
> whole file is likely to take longer.
>
> BR
>
> JPM
>
> [1]: https://www.schneier.com/skein1.3.pdf
>
>
> - Cons:
>
>
> - More slow encryption/decryption
>
>
> Thank you very much for your help!
>
> El domingo, 18 de octubre de 2015, 21:06:59 (UTC+2), jean-pierre.muench
> escribió:
>
> Am 18.10.2015 um 18:20 schrieb lasl...@gmail.com:
>
> Hi all:
>
> Never before have I worked in cryptography. Excuse me if the question
> is very long but i´m very lost.
>
> We have a new project in my company related to cryptography and would
> appreciate you give me advice.
>
> We have many customers who are going to save many files encrypted on
> our servers. The files are now unencrypted. We'll encrypt files and, from
> now on, read and change the content without completely decrypt.
> Each file can have its own password, but we believe that most customers
> will always use the same password.
>
> I'm thinking about putting a header to the file, as is done in
> <https://www.aescrypt.com/aes_>https://www.aescrypt.com/aes_file_format.html.
> In this way we can have a new Initialization Vector ( if necessary ) for
> each compressed file and the hash of the password.
>
> The header format is actually looking surprisingly good. However I'd
> suggest only using 12 byte IVs (AES-GCM only needs a 12 byte IV) and
> replacing the header's "HMAC" with an AES-GCM tag. And I'd suggest
> authenticating the whole unencrypted header (extensions, version, ...) with
> the header's tag (GCM can handle associated data).
>
> In the header we also will keep pointers to various sections of the
> document in which we have to read for some encrypted information and write
> some encrypted information on the fly. For this reason I need a semi-ramdom
> access.
>
> This actually is really tricky. Just to make sure: You need to be able
> securely read and write at any place in the stored file?
> This will force you to re-authenticate the whole file (which can be done
> quite fast via this trick[1]) or to pull off some other trick. In your
> scenario I'd suggest dividing the file into several subsections. Each
> subsection will have it's own IV which is just the IV of the previous
> subsection incremented by one (this is safe with AES-GCM). And at the end
> of each subsection there will be an authentication tag for said section.
> This will give you a linear increase in size, but a linear decrease in
> computation time as you only need to consider a specific subsection when
> re-authenticating or verifying.
>
>
> We have quite clear that we must use AES to encrypt files. We're not
> sure whether to use 128bit or 256bit. There is really too much difference
> in safety between them (we need to consider speed) ?
>
> You need to consider two points here. a) Marketing. AES-256 with a 2^256
> key space will sound much more impressive than AES-128 with a tiny 2^128
> key space. b) Security period. If you want to keep the data confidential
> for a very long time (decades?) than you want to use AES-256. Otherwise
> AES-128 is just fine (and NSA approved for secret information) and will be
> a bit faster.
>
>
> And the most important question is what may be the most secure mode,
> even assuming that an attacker could download all the files from the server?
>
> AES-GCM is the mode of choice in modern Cryptography. It is considered
> good style to use AES-GCM and will be significantly faster than most other
> modes (like AES-CTR+HMAC or AES-CBC+HMAC). Furthermore you may be able to
> exploit the underlying construction to be faster by multi threading via [1].
>
> I hope this answered your questions, if there are any left you may ask
> here or over at Crypto.SE[2] although you may not make it too broad there.
>
> BR
>
> JPM
>
> [1]: https://crypto.stackexchange.com/a/27468/23623
> [2]: https://crypto.stackexchange.com/
>
>
>
> Thank you in advance!
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