Re: secp256r1 65 byte key size in packet capture

You need to do base64 decoding to find out the real size of the ASN.1
encoded data.

Tomas Mraz, OpenSSL

On Wed, 2024-06-19 at 14:58 +0530, Lokesh Chakka wrote:
> hi,
> 
> please check the following :
> 
> =====================================================================
> =====================
> $ openssl ecparam -name secp256r1 -genkey -out pvtkey.pem
> using curve name prime256v1 instead of secp256r1
> $ cat pvtkey.pem 
> -----BEGIN EC PARAMETERS-----
> BggqhkjOPQMBBw==
> -----END EC PARAMETERS-----
> -----BEGIN EC PRIVATE KEY-----
> MHcCAQEEIAXXAWUj/cUQT8pDLKp5r269mw58aTzr/hYAEXQZVQqUoAoGCCqGSM49
> AwEHoUQDQgAEVSmp4UnlQbzbe6eopByeEUzkmYHPGgaKvSt/xdAgvDp7FXKTpST8
> UM9LpF8f4JETOXgDDGvNlIDqVFo+T0hdtQ==
> -----END EC PRIVATE KEY-----
> $ openssl ec -in pvtkey.pem -pubout
> read EC key
> writing EC key
> -----BEGIN PUBLIC KEY-----
> MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEVSmp4UnlQbzbe6eopByeEUzkmYHP
> GgaKvSt/xdAgvDp7FXKTpST8UM9LpF8f4JETOXgDDGvNlIDqVFo+T0hdtQ==
> -----END PUBLIC KEY-----
> =====================================================================
> =====================
> 
> sizeof private key is 164 bytes and the public key is 124 bytes.
> 
> 
> Thanks & Regards
> --
> Lokesh Chakka.
> 
> 
> On Wed, Jun 19, 2024 at 2:28 PM Tomas Mraz <[email protected]> wrote:
> > Hi Lokesh,
> > 
> > I am not sure how do you count the sizes of 164 bytes and 124 bytes
> > for
> > the pem files.
> > 
> > If I use -outform DER (and use -noout with the ecparam to avoid
> > outputting the params because the private key already contains info
> > about the params used) I see the following sizes for the DER
> > encoded
> > data:
> > 
> > private key: 121 bytes
> > public key: 91 bytes
> > 
> > Given both files contain information about the group used and other
> > ASN.1 encoding related stuff, and that the private key file
> > contains 32
> > bytes of the private key but also the encoded uncompressed public
> > key
> > of 65 bytes, this is fully expected.
> > 
> > Tomas Mraz, OpenSSL
> > 
> > On Wed, 2024-06-19 at 13:45 +0530, 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.
> > > 
> > > Can someone help me understand why the difference?
> > > 
> > > Thanks & Regards
> > > --
> > > Lokesh Chakka.
> > 

-- 
Tomáš Mráz, OpenSSL