Marcel Logen in sci.crypt:

[...]

The ciphertext is a decimal number which has to be transformed
(as Richard wrote here, too, some weeks ago, IIRC) to a 16-bit
or 256-bit coded plaintext.

Sorry, a correction:

Read "16-bit" as "8-bit" (256 values, base 16) and "256-bit"
as "base 256".

- subsequently use "bc" for transformation to 16-bit:

Read "16-bit" as "base 16".

I. e. transform from base 10 to base 16:

| cl@pc-731:/tmp$ echo "obase=16;$(cat two)" | BC_LINE_LENGTH=0 bc
| A353230322F332F373220686A722E2E2E0A0A2E2E2E656E6F64206C6C65570A0A3F7468676972202C676E696874656D6F732073277461687420747562202E2E2E656772656D6520747865747265687069632065687420676E69656573206D6F72662074656720756F79206E6F6974636166736974617320666F20676E696

C6565662065687420796C6E6F207369202C73616C61202C657A6972702072756F59202E74786574726568706963206568742064657470797263656420796C6C756673736563637573206576616820756F592021534E4F4954414C55544152474E4F43

| cl@pc-731:/tmp$

Marcel (Lines: 29)
--
╭──────────╮ ╭───╮ ╭─╮ ╭─╮ ╭─╮ ╭─╮ ..67..
╮ ...3..╰───────╮ ╰───╮ ╭─╯ │ │ ╰──╯ ╰────╮ ╭──╯ ╰────╯ │ ╭─╮
│ ╭────╮ ╭─╮ ╰─╮ ╭─╯ │ ╭──╯ ╭─╯ ..40..╭───╯ ╰─╮ ╭──────╯ │ ╰─╮
╰───╯ ╰─╯ ╰────╯ ╰────╯ ╰─────╯ ..40..╰────────╯ ╰────────╯ ╰─

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 01/06/2025 07:26, Marcel Logen wrote:

- the text then has only to be reversed

Curiouser and curiouser!

I asked ChatGPT to have a look at this, and it too had to reverse
the text, but...

I didn't!

Did the entire universe change endianness while I wasn't looking?

--
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
Sig line 4 vacant - apply within

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

Richard Heathfield in sci.crypt:

On 01/06/2025 07:26, Marcel Logen wrote:

- the text then has only to be reversed

Curiouser and curiouser!

I asked ChatGPT to have a look at this, and it too had to reverse
the text, but...

I didn't!

Did the entire universe change endianness while I wasn't looking?

With "dc" the same result here:

| cl@pc-731:/tmp$ dc -e "$(cat two) P" | hexdump -Cv
| 00000000 0a 35 32 30 32 2f 33 2f 37 32 20 68 6a 72 2e 2e |.5202/3/72 hjr..|
| 00000010 2e 0a 0a 2e 2e 2e 65 6e 6f 64 20 6c 6c 65 57 0a |......enod lleW.|
| 00000020 0a 3f 74 68 67 69 72 20 2c 67 6e 69 68 74 65 6d |.?thgir ,gnihtem|
| 00000030 6f 73 20 73 27 74 61 68 74 20 74 75 62 20 2e 2e |os s'taht tub ..|
| 00000040 2e 65 67 72 65 6d 65 20 74 78 65 74 72 65 68 70 |.egreme txetrehp|
| 00000050 69 63 20 65 68 74 20 67 6e 69 65 65 73 20 6d 6f |ic eht gniees mo|
| 00000060 72 66 20 74 65 67 20 75 6f 79 20 6e 6f 69 74 63 |rf teg uoy noitc|
| 00000070 61 66 73 69 74 61 73 20 66 6f 20 67 6e 69 6c 65 |afsitas fo gnile|
| 00000080 65 66 20 65 68 74 20 79 6c 6e 6f 20 73 69 20 2c |ef eht ylno si ,|
| 00000090 73 61 6c 61 20 2c 65 7a 69 72 70 20 72 75 6f 59 |sala ,ezirp ruoY|
| 000000a0 20 2e 74 78 65 74 72 65 68 70 69 63 20 65 68 74 | .txetrehpic eht|
| 000000b0 20 64 65 74 70 79 72 63 65 64 20 79 6c 6c 75 66 | detpyrced ylluf|
| 000000c0 73 73 65 63 63 75 73 20 65 76 61 68 20 75 6f 59 |sseccus evah uoY|
| 000000d0 20 21 53 4e 4f 49 54 41 4c 55 54 41 52 47 4e 4f | !SNOITALUTARGNO|
| 000000e0 43 |C|
| 000000e1

Here the Python3 one-liner:

| cl@pc-731:/tmp$ dc -e "$(cat two) P" | python3 -c 'import sys;a1=sys.stdin.buffer.read();b2=a1[::-1];print(b2);import hashlib;print(hashlib.md5(b2).hexdigest())'
| b"CONGRATULATIONS! You have successfully decrypted the ciphertext. Your prize, alas, is only the feeling of satisfaction you get from seeing the ciphertext emerge... but that's something, right?\n\nWell done...\n\n...rjh 27/3/2025\n"
| 2f682c420d4f5cd443719f33050eac67
| cl@pc-731:/tmp$

The "[::-1]" reverses the text.

Marcel
--
Sun Jun 1 09:15:34 2025 CEST (1748762134)
pc-731
87 4ix0 c4v4
Lines: 50

--- SoupGate-Win32 v1.05
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Richard Heathfield in sci.crypt:

On 01/06/2025 08:15, Marcel Logen wrote:

Richard Heathfield in sci.crypt:

On 01/06/2025 07:26, Marcel Logen wrote:

- the text then has only to be reversed

[...]

Mystery solved.

It turns out that my bignum library (which is little-endian)
doesn't bother to switch to or from big-endian before file
operations. In 25 years, this is the first time it's ever mattered.

Hehe. Interesting.

Marcel
--
Sun Jun 1 11:14:04 2025 CEST (1748769244)
pc-731
87 34cj ome1
Lines: 22

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 01/06/2025 08:15, Marcel Logen wrote:

Richard Heathfield in sci.crypt:

On 01/06/2025 07:26, Marcel Logen wrote:

- the text then has only to be reversed

Curiouser and curiouser!

I asked ChatGPT to have a look at this, and it too had to reverse
the text, but...

I didn't!

Did the entire universe change endianness while I wasn't looking?

With "dc" the same result here:

| cl@pc-731:/tmp$ dc -e "$(cat two) P" | hexdump -Cv

Mystery solved.

It turns out that my bignum library (which is little-endian)
doesn't bother to switch to or from big-endian before file
operations. In 25 years, this is the first time it's ever mattered.

--
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
Sig line 4 vacant - apply within

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

Marcel Logen in sci.crypt:

[...]

This data has an odd number of hex digits, so prepend a "0";
then you can use the Linux tool "xxd" for transformation to
readable text:

If you want to use this method for 'encrypting' binary
data, I would recommend prefixing the data with 0xFF or
0x80 before encryption - and removing it again after
decryption.

| user15@o15:/tmp$ { echo -n 'ibase=16;' && echo '000102030405060708090A0B0C0D0E0F' ; } | BC_LINE_LENGTH=0 bc
| 5233100606242806050955395731361295

| user15@o15:/tmp$ { echo 'obase=16' && echo '5233100606242806050955395731361295' ; } | BC_LINE_LENGTH=0 bc
| 102030405060708090A0B0C0D0E0F

With 0xFF prepended:

| user15@o15:/tmp$ { echo -n 'ibase=16;' && echo 'FF000102030405060708090A0B0C0D0E0F' ; } | BC_LINE_LENGTH=0 bc
| 86772008797939914425966575850496625282575

| user15@o15:/tmp$ { echo 'obase=16' && echo '86772008797939914425966575850496625282575' ; } | BC_LINE_LENGTH=0 bc
| FF000102030405060708090A0B0C0D0E0F

Marcel (Lines: 33)
--
╭───────╮ ╭─╮ ╭───────╮ ╭──────────╮ ..67..
╰─╮ ╭─╮ │ ╭──╯ │ ╭─╮ ╰──╮ ╭─╯ ╭─╮ ╰───────╮ ╰─╮..67..
╮ ╰─╯ │ ╰─╯ │ │ ╰─────╯ ╰────╮ ╭──╮ │ │ ..50..╭─╮ │ ╭─╯ ╭──
╰──────╯ ╰─╯ ..29..╰─────╯ ╰─╯ ╰───────╯ ╰──╯ ╰─────╯

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)