From: Klaus Sembritzki Date: Sun, 25 Jan 2026 09:47:42 GMT Subject: Re: The SHA256 of "xy\n" (ASCII, no CRLF) contains 1337, ACBAD in za, and I am 1aa Message-ID: In-Reply-To: On Sat, Jan 24, 2026 at 10:25 AM Klaus Sembritzki wrote: > > On Sat, Jan 24, 2026 at 8:28 AM Jeff King wrote: > > > > On Fri, Jan 23, 2026 at 02:36:41PM -0800, Junio C Hamano wrote: > > > > > Jeff King 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 > > #include > > #include > > > > 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 > #include > #include > #include > > #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 I have to admit there is a bug in my previous code, it only works correctly for even numbers. It should have been: #define REVERTED_BIT(n, position, width) (((n & (1 << position)) >> position) << (width - position - 2 + (width & 1))) 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; } The actual performance measurements are: counter width: 32 | n: 103832253 | i: 1588397616 counter width: 31 | n: 103832253 | i: 1588397616 counter width: 30 | n: 166340413 | i: 396135154 counter width: 29 | n: 166340413 | i: 396135154 counter width: 28 | n: 268041599 | i: 2281045247 # 2**28 is close to the first finding when counting by incrementing the LSB, so this is slow. counter width: 27: This does not find anything, because 2**27 is lower than the first finding when counting by incrementing the LSB. counter width: 0 | n: 171728193 | i: 171728193 # math.log2(171728193) = 27.35555166834193