Getting the EdDSA public key

[rgmhtt]

I have played around with
https://pycryptodome.readthedocs.io/en/v3.15.0/src/public_key/ecc.html

And can get the private key as in the example. But I cannot grok from this page how to get the public key into a pem file and into a variable for use (as in rfc9374).

Can I please have some pointers for this?

[rgmhtt]

So now I am looking at

https://github.com/Legrandin/pycryptodome/blob/master/lib/Crypto/PublicKey/ECC.py

And I see the comment for "generate" that it only makes the private key. So how to get the matching public key?

Adding to the example:

pbkey = key.public_key()
print("EdDSA: ", pbkey)

I get

EdDSA: EccKey(curve='Ed25519', point_x=38407921114805875551575778217610815520550461829205596500984344243185069659061, point_y=26785287844728461117960275388743876036711181504507927201470807081658546111647)

How do I get the 32 byte value for this and the PEM output?

[Legrandin]

The ECC key object has a public_key() method and an export_key() method.

So, you can combine them with:

>> from Crypto.PublicKey import ECC
>> key = ECC.generate(curve="ed25519")
>> key.public_key().export_key(format="PEM")
'-----BEGIN PUBLIC KEY-----\nMCowBQYDK2VwAyEAi1/8fWnNVkSDwXzs2l7jt7sdmycVryOW4IBuGYVrNBs=\n-----END PUBLIC KEY-----'

[rgmhtt]

Ah! So I can get the 32 byte raw public key to use with:

pbraw = key.public_key().export_key(format="raw")

then feed that into the ORCHID calculation in sec 3.5.2 of rfc9374
where "HOST_ID" is the raw public key
...

[rgmhtt]

Now in a new script I want to read in the pem file and use it.

I start with

from Cryptodome.PublicKey import ECC

then open the pem file with the private key with:

f = open(prkeyname,'rt')
prkey = ECC.import_key(f.read())

And using nacl I can sign an object with:

sk = SigningKey(prkey.seed)
mysig = sk.sign(bytes.fromhex(pleasesign)).signature

But I cannot get the public 32-byte value:

pbraw = key.public_key().export_key(format="raw")

gets the error

NameError: name 'key' is not defined

this is the same object that worked in the script that generated the key.

If I open the pem public key file:

f = open(pbkeyname,'rt')
pbkey = ECC.import_key(f.read())
print("pubkey: ", pbkey)

pubkey: EccKey(curve='Ed25519', point_x=45951565200278136569478977242469963253965153691604191946226602838686630660802, point_y=12693269240344607886295298306705897841232144188110415929470094619404945768176)

How do I get the 32-byte public key from this?

thank you

[rgmhtt]

Got it. I think:

f = open(pbkeyname,'rt')
pbkey = ECC.import_key(f.read())
pbraw = pbkey.public_key().export_key(format="raw").hex()
print("HI: ", pbraw)

HI: f0beded7c542fcefd620f5f3f3b5f95627f50a308e1dacfe507adc62c322101c

ARGH!!!

This object stuff does not come easy for me. I way predate it...