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Message-ID: <CANO7a6xE6PexC9ruEqc7tx4Rj5R849t4seoWcQ=KFaeAOaDQSg@mail.gmail.com>
Date: Thu, 28 Jun 2012 10:25:44 +0530
From: Dhiru Kholia <dhiru.kholia@...il.com>
To: john-dev@...ts.openwall.com
Subject: Re: Is saving 2 bytes per salt worth the effort?
On Thu, Jun 28, 2012 at 4:56 AM, Frank Dittrich
<frank_dittrich@...mail.com> wrote:
> diff --git a/src/episerver_fmt_plug.c b/src/episerver_fmt_plug.c
> index 4b46ffc..d89a0d7 100644
> --- a/src/episerver_fmt_plug.c
> +++ b/src/episerver_fmt_plug.c
> @@ -76,7 +76,7 @@ static ARCH_WORD_32 (*crypt_out)[BINARY_SIZE /
> sizeof(ARCH_WORD_32)];
>
> static struct custom_salt {
> int version;
"int version" can be changes to "char version". Even more savings!
> - unsigned char esalt[18]; /* base64 decoding, 24 / 4 * 3 = 18 */
> + unsigned char esalt[16];
> } *cur_salt;
>
> static void init(struct fmt_main *pFmt)
> @@ -104,12 +104,14 @@ static void *get_salt(char *ciphertext)
> char *ctcopy = strdup(ciphertext);
> char *keeptr = ctcopy;
> char *p;
> + unsigned char tmp_esalt[18]; /* base64 decoding, 24 / 4 * 3 = 18 */
> static struct custom_salt cs;
> ctcopy += 12; /* skip over "$episerver$*" */
> p = strtok(ctcopy, "*");
> cs.version = atoi(p);
> p = strtok(NULL, "*");
> - base64_decode(p, strlen(p), (char*)cs.esalt);
> + base64_decode(p, strlen(p), (char*) tmp_esalt);
> + memcpy(cs.esalt, tmp_esalt, 16);
> free(keeptr);
> return (void *)&cs;
> }
>
>
> esalt[18] is only needed temporarily, for base64 decoding.
> But the salt will always be 16 bytes.
>
> So, with an additional memcpy(cs.esalt, tmp_esalt, 16); we can get rid
> of two bytes per salt.
>
> Or should we rather use the last 2 bytes to store the version info after
> base64 decoding?
>
> (Or combine both ideas, use 17 bytes of esalt, and store the version
> info in the last byte?)
>
> For self test, this doesn't matter.
> But when processing a large number of hashes, saving 2 or even more
> bytes per salt might be a good idea.
How much does the extra memcpy call affect the benchmark?
--
Cheers,
Dhiru
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