Re: The SHA256 of "xy\n" (ASCII, no CRLF) contains 1337, ACBAD in za, and I am 1aa

From: Klaus Sembritzki <klausem@gmail.com>
Date: Jan 24, 2026, 09:25 UTC
Message-ID: <CADMnYXA6_uCZU42NR2vFKM9uhfaWOdu0tkzPi6Ya8WW2rzknGg@mail.gmail.com>
In-Reply-To: <20260124072814.GA3455597@coredump.intra.peff.net>

On Sat, Jan 24, 2026 at 8:28 AM Jeff King <peff@peff.net> wrote:

Show 67 quoted lines

>
> On Fri, Jan 23, 2026 at 02:36:41PM -0800, Junio C Hamano wrote:
>
> > Jeff King <peff@peff.net> writes:
> >
> > > On Fri, Jan 23, 2026 at 09:16:46PM +0100, Klaus Sembritzki wrote:
> > >
> > >> $ # My initials (ks): 1aa
> > >> $ echo ks | sha256sum
> > >> $ 1aa44e718d5bc9b7ff2003dbbb6f154e16636d5c2128ffce4751af5124b65337
> > >>
> > >> $ # 50566750337
> > >> $ echo thinking | sha256sum
> > >> $ 50566750337beb9e98e553fd9196d10576f9eb0cbc6b66e2586b9d73af4f352f
> > >
> > > Oh man, I've got deadbeef!
> > >
> > >   $ echo jk35252822 | sha256sum
> > >   33f1a74529870456c56ad97c59cfed6bdeadbeef9b9bc3f4ff49bb203e36f96b
> > >
> > > What could it all mean?
> >
> > Sorry, but I have to admit that I completely lack humor receptor
> > cells.
>
> Probably because it was not that funny. :)
>
> The original message seemed to be looking for Numerology-style meanings
> in random data. I wasn't sure if it was serious or not, but I could not
> resist either playing along (if not) or trolling (if so).
>
> But here's my deadbeef brute-force program for fun.
>
> -Peff
>
> -- >8 --
> #include <stdio.h>
> #include <string.h>
> #include <openssl/evp.h>
>
> int main(int argc, const char **argv)
> {
>         const char needle[] = { 0xde, 0xad, 0xbe, 0xef };
>         const EVP_MD *algo = EVP_sha256();
>         EVP_MD_CTX *ctx = EVP_MD_CTX_new();
>
>         EVP_DigestInit_ex(ctx, algo, NULL);
>         while (*++argv)
>                 EVP_DigestUpdate(ctx, *argv, strlen(*argv));
>
>         for (unsigned i = 0; ; i++) {
>                 char buf[16];
>                 char *p;
>                 unsigned char digest[32];
>                 EVP_MD_CTX *copy = EVP_MD_CTX_dup(ctx);
>
>                 p = buf + sizeof(buf);
>                 for (unsigned v = i; v; v /= 10)
>                         *--p = '0' + (v % 10);
>                 EVP_DigestUpdate(copy, p, buf + sizeof(buf) - p);
>                 EVP_DigestUpdate(copy, "\n", 1);
>                 EVP_DigestFinal_ex(copy, digest, NULL);
>                 EVP_MD_CTX_free(copy);
>                 if (memmem(digest, sizeof(digest), needle, sizeof(needle)))
>                         printf("%d\n", i);
>         }
> }

Incrementing the MSB instead of the LSB (indexing naturally starts with 1 in that case) seems to improve the performance, and it finds different solutions, if the program is terminated early. The rationale is that there is autocorrelation in the observations, though I cannot judge what that means in this concrete example. The speedup is not that dramatic here, so SHA256 seems to be pretty random.

#include <stdio.h> #include <string.h> #include <stdint.h> #include <openssl/evp.h>

#ifdef POLYFILL
EVP_MD_CTX *EVP_MD_CTX_dup(const EVP_MD_CTX *in)
{
    EVP_MD_CTX *out = EVP_MD_CTX_new();

    if (out != NULL && !EVP_MD_CTX_copy_ex(out, in)) {
        EVP_MD_CTX_free(out);
        out = NULL;
    }
    return out;
}
#endif

#define REVERTED_BIT(n, position, width) (((n & (1 << position)) >> position) << (width - position - 2))

uint32_t revert_bits(uint32_t n, uint32_t width) {
    uint32_t result = 0;
    for (uint32_t i = 0; i < width; ++i) {
        result |= REVERTED_BIT(n, i, width);
    }
    return result;
}

int main(int argc, const char **argv)
{
    const char needle[] = { 0xde, 0xad, 0xbe, 0xef };
    const EVP_MD *algo = EVP_sha256();
    EVP_MD_CTX *ctx = EVP_MD_CTX_new();

    EVP_DigestInit_ex(ctx, algo, NULL);
    while (*++argv)
        EVP_DigestUpdate(ctx, *argv, strlen(*argv));

    for (uint32_t n = 1; ; n++) {
#if (COUNTER_WIDTH != 0)
        uint32_t i = revert_bits(n, COUNTER_WIDTH);
#else
        uint32_t i = n;
#endif
        // printf("%u %u\n", n, i);
        char buf[16];
        char *p;
        unsigned char digest[32];
        EVP_MD_CTX *copy = EVP_MD_CTX_dup(ctx);

        p = buf + sizeof(buf);
        for (uint32_t v = i; v; v /= 10)
            *--p = '0' + (v % 10);
        EVP_DigestUpdate(copy, p, buf + sizeof(buf) - p);
        EVP_DigestUpdate(copy, "\n", 1);
        EVP_DigestFinal_ex(copy, digest, NULL);
        EVP_MD_CTX_free(copy);
        if (memmem(digest, sizeof(digest), needle, sizeof(needle)))
        {
            printf("counter width: %2u | n: %10u | i: %10u\n",
COUNTER_WIDTH, n, i);
            return 0;
        }
    }
    return 1;
}

cc -DPOLYFILL -DCOUNTER_WIDTH=32 jk_evp.c -lssl -lcrypto -o jk_evp_msb_32 cc -DPOLYFILL -DCOUNTER_WIDTH=31 jk_evp.c -lssl -lcrypto -o jk_evp_msb_31 cc -DPOLYFILL -DCOUNTER_WIDTH=30 jk_evp.c -lssl -lcrypto -o jk_evp_msb_30 cc -DPOLYFILL -DCOUNTER_WIDTH=29 jk_evp.c -lssl -lcrypto -o jk_evp_msb_29 cc -DPOLYFILL -DCOUNTER_WIDTH=0 jk_evp.c -lssl -lcrypto -o jk_evp_0 counter width: 32 | n: 103832253 | i: 1588397616 counter width: 31 | n: 62413559 | i: 1003915120 counter width: 30 | n: 166340413 | i: 396135154 counter width: 29 | n: 137077701 | i: 171728193 counter width: 0 | n: 171728193 | i: 171728193