Re: secp256r1 65 byte key size in packet capture

[Lokesh Chakka]

Is there a way to have all those man pages installed in my system.
I'm using Ubuntu 24.

On Wed, Jun 19, 2024, 17:49 Matt Caswell  wrote:

>
>
> 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  > > 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.
> >
>

Re: 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 > 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.

Re: secp256r1 65 byte key size in packet capture

[Lokesh Chakka]

Hi Matt,

I'm trying to craft a client hello packet using a C program. I'm learning
about these keys, openssl, TLS etc.

So

openssl ecparam -name secp256r1 -genkey -out pvtkey.pem
openssl ec -in pvtkey.pem -pubout -out pubkey.pem
openssl pkey -in pubkey.pem -pubin -noout -text

will give me the 65 bytes. Understood. Thanks alot.

Now I need to explore C APIs for getting those keys as hex array.
Could you please suggest any good references for beginners.

Regards
--
Lokesh Chakka.


On Wed, Jun 19, 2024 at 4:21 PM Matt Caswell  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.
>

Re: secp256r1 65 byte key size in packet capture

[Matt Caswell]

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.

Re: secp256r1 65 byte key size in packet capture

[Lokesh Chakka]

Understood. Thanks alot.
But I'm still Not able to understand why it is 65 bytes in the key value.


Thanks & Regards
--
Lokesh Chakka.


On Wed, Jun 19, 2024 at 3:03 PM Tomas Mraz  wrote:

> 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  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
>
>

Re: secp256r1 65 byte key size in packet capture

[Tomas Mraz]

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  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

Re: secp256r1 65 byte key size in packet capture

[Lokesh Chakka]

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  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
>
>

Re: secp256r1 65 byte key size in packet capture

[Tomas Mraz]

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