secp256r1 65 byte key size in packet capture

[Matt Caswell]

On 19/06/2024 12:14, Lokesh Chakka wrote:
> Now I need to explore C APIs for getting those keys as hex array.
> Could you please suggest any good references for beginners.

You would need to first load the key from the file to create an EVP_PKEY 
object. For example you could use the PEM_read_PUBKEY() function for 
this. See:

https://www.openssl.org/docs/man3.3/man3/PEM_read_PUBKEY.html

Once you have the key as an EVP_PKEY object, you can get the raw 
encoding as a char array in a format suitable for TLS using the 
EVP_PKEY_get1_encoded_public_key() function. See:

https://www.openssl.org/docs/man3.3/man3/EVP_PKEY_get1_encoded_public_key.html

Matt



> 
> Regards
> --
> Lokesh Chakka.
> 
> 
> On Wed, Jun 19, 2024 at 4:21 PM Matt Caswell <matt at openssl.org 
> <mailto:matt at openssl.org>> wrote:
> 
> 
> 
>     On 19/06/2024 09:15, Lokesh Chakka wrote:
>      > hello,
>      >
>      > I'm trying to generate public/private keys with following commands:
>      >
>      > openssl ecparam -name secp256r1 -genkey -out pvtkey.pem
>      > openssl ec -in pvtkey.pem -pubout
>      >
>      > I'm seeing the sizeof private key as 164 bytes and public key as
>     124 bytes.
>      >
>      > In a wireshark capture( attached ), I'm seeing key length as 65
>     bytes.
> 
>     What you are doing is confusing. You have generated public/private key
>     pair for secp256r1 - but the wireshark capture you show seems to be the
>     key share from a TLSv1.3 handshake. TLSv1.3 key shares are ephemeral so
>     - you'll get a different key share every time. You don't need to create
>     a public/private key for this. OpenSSL does it for you.
> 
>     Anyway. Taking the key that you generated:
> 
>     -----BEGIN PUBLIC KEY-----
>     MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEVSmp4UnlQbzbe6eopByeEUzkmYHP
>     GgaKvSt/xdAgvDp7FXKTpST8UM9LpF8f4JETOXgDDGvNlIDqVFo+T0hdtQ==
>     -----END PUBLIC KEY-----
> 
>     This is just a PEM encoding of the real key (base 64 encoding of DER
>     structured data in PEM headers). Not sure where you get 124 bytes from,
>     but you can look take a look at the actual key data like this:
> 
>     $ openssl pkey -in /tmp/key.pem -pubin -noout -text
>     Public-Key: (256 bit)
>     pub:
>           04:55:29:a9:e1:49:e5:41:bc:db:7b:a7:a8:a4:1c:
>           9e:11:4c:e4:99:81:cf:1a:06:8a:bd:2b:7f:c5:d0:
>           20:bc:3a:7b:15:72:93:a5:24:fc:50:cf:4b:a4:5f:
>           1f:e0:91:13:39:78:03:0c:6b:cd:94:80:ea:54:5a:
>           3e:4f:48:5d:b5
>     ASN1 OID: prime256v1
>     NIST CURVE: P-256
> 
>     This shows you the 65 bytes of raw public key data contained within the
>     key file.
> 
>     This key is in "uncompressed" format (the 04 byte at the start
>     indicates
>     this). Since it is uncompressed we then get an x and a y value to
>     indicate the point on the curve. Each of these are 32 bytes long (256
>     bits) - so this gives you 65 bytes in total.
> 
>     Matt
> 
> 
> 
>      >
>      > Can someone help me understand why the difference?
>      >
>      > Thanks & Regards
>      > --
>      > Lokesh Chakka.
>