Extra byte at the start of a public key

[Guilherme Andrade]

On Sat, 11 Jul 2020 at 01:56, Guilherme Andrade <g@REDACTED> wrote:

> At any rate, along the way I encountered the ASN.1 concept of an "octet
> string"[1] - binary data whose length is a multiple of eight. It seems to
> fit the output of the code you've shown (the value of that extra byte, 4,
> when multiplied by 8, yields 32, and so it does look like a length prefix
> for the public key.) It also seems to be the return format of some of the
> OpenSSL functions invoked by the code in question back in OTP 21.3, like
> `EC_POINT_point2oct`[1].
>

Yeah, nevermind that, I clearly don't know what I'm talking about :-) -
ASN.1 "octet strings" are byte-based strings as opposed to bit-based
sequences, i.e. binary data as we commonly know it.

Please disregard my digression into foolishness.


>
> But this is mostly guessing and intuition - I'm very possibly talking out
> of my backside. Hopefully someone who knows what they're talking about will
> answer your question.
>
> [1]: https://www.obj-sys.com/asn1tutorial/node10.html
> [2]: https://www.openssl.org/docs/man1.0.2/man3/EC_POINT_point2oct.html
>
>
> On Fri, 10 Jul 2020 at 15:38, ludovic@REDACTED <ludovic@REDACTED>
> wrote:
>
>> Hello,
>>
>> I am trying some crypto code, mostly for fun. I want to generate an
>> Ethereum public key from a know private key.
>>
>> This is a sample private key :
>> <<22,99,59,58,153,154,229,99,217,142,170,105,82,105,204,11,55,249,101,152,109,41,109,195,18,134,228,189,83,139,28,92>>
>>
>> I run this code:
>>
>>     -module(t).
>>     -export([run/0]).
>>
>>     run() ->
>>         PrivateKey =
>> <<22,99,59,58,153,154,229,99,217,142,170,105,82,105,204,11,55,249,101,152,109,41,109,195,18,134,228,189,83,139,28,92>>,
>>         {PublicKey, NewPrivateKey} = crypto:generate_key(ecdh, secp256k1,
>> PrivateKey),
>>         dump_hex("Private key", PrivateKey),
>>         dump_hex("Public key", PublicKey).
>>
>>     dump_hex(Label, Bin) ->
>>         io:format("~s: ~s~n", [Label, [io_lib:format("~2.16.0B",[X]) ||
>> <<X:8>> <= Bin]]).
>>
>> And I get this result:
>>
>>     Private key:
>> 16633B3A999AE563D98EAA695269CC0B37F965986D296DC31286E4BD538B1C5C
>>     Public key:
>> 047D5ED8E5B1843FF0EF0F443419E3DC589D900F3C683D2D7268AC1D9FE47F7E7350A1...(cut)
>>
>> But I am expected to find this:
>> 7D5ED8E5B1843FF0EF0F443419E3DC589D900F3C683D2D7268AC1D9FE47F7E7350A...(cut)
>>
>> At first I was quite discouraged because crypto is hard to learn, but
>> after giving a closer look I found that I have the correct key actually. I
>> just have an extra leading byte. The expected length is 512 bits and I have
>> 520.
>>
>> Expected   7D5ED8E5B1843FF0EF0F443419E3DC589D900F3C683D...(cut)
>> Mine     047D5ED8E5B1843FF0EF0F443419E3DC589D900F3C683D...(cut)
>>
>> Do you know why the crypto library behaves like this ? Can I just cut the
>> extra leading byte ?
>>
>> Thank you
>>
>> - lud
>>
>>
>
> --
> Guilherme
>


-- 
Guilherme
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