From https://github.com/pratikd650/Teensy_U2F/blob/master/Teensy_U2F.cpp line 292

Instructions to generate attestation certificate using open ssl
https://wiki.openssl.org/index.php/Command_Line_Elliptic_Curve_Operations
https://www.guyrutenberg.com/2013/12/28/creating-self-signed-ecdsa-ssl-certificate-using-openssl/
 P-256 (also secp256r1)  EC key pair is    W = dG   (Note secp256k1 is Koblitz curve - not P256)
 d = private key is it 256 bits (32 bytes)
 G = generator point - it is part of the curve definition
 W = public key point - it is a (256, 256) bits  - 64 bytes
1) generate a key pair - the private key will be saved in PKCS8 format in ecprivkey.pem
openssl ecparam -name prime256v1 -genkey -noout -out ecprivkey.pem
2) dump out the private key in hex format - it will be a 32 byte key
 openssl asn1parse  -in ecprivkey.pem
3) compute the public key from the private key and the curve
openssl ec -in ecprivkey.pem -pubout -out ecpubkey.pem
4) dump out the public key in hex format - it will be 66 byte - the first two bytes are 00 04,
  openssl ec -in ecprivkey.pem -pubout -text
  after that is the point W - 32 byte + 32 byte
5) generate a self signed certificate
  openssl req -new -x509 -key ecprivkey.pem -out server.pem -days 3650
For the Certificate name give a unique certificate name. There is a 128 bit unique identification number burned into every
Teensy chip  - see http://cache.freescale.com/files/32bit/doc/data_sheet/K20P64M72SF1.pdf
You can print out the number from your Teensy using this simple program given below
6) Display the certificate
  openssl x509 -in server.pem -text -noout

7) Convert PEM certificate to DER
openssl x509 -outform der -in server.pem -out certificate.der

8) Generate a usable c-array for source code
xxd --include certificate.pem

Copy output into appropriate array in 'Certificates.cpp', overwriting existing values

9) Repeat steps 7 & 8 for public key and private key

So:
`
openssl asn1parse -in ecprivkey.pem 2>/dev/null | grep 'HEX DUMP' | perl -pe 's/^.*\[HEX DUMP\]:(.+)$/$1/' 2>/dev/null | xxd -r -p > privkey.der && xxd --include privkey.der

openssl ec -in ecprivkey.pem -pubout -text 2>/dev/null | perl -0777 -ne 'print /pub:.+ASN1/sg' 2>/dev/null | sed -e '/pub:/d;/ASN1/d' | perl -pe 's/^\s+(.+):?$/$1/gm' 2>/dev/null | perl -pe 's/\n//' 2>/dev/null | perl -pe 's/(.{2}):?/$1/g' 2>/dev/null | xxd -r -p > pubkey.der && xxd --include pubkey.der
`
and copy the arrays into the correct arrays in Certificates.cpp

If any arrays have different lengths than shown in Certificates.hpp, update these too.