CODEWITHSUNDEEP
certificatesmartcardpublic-key-encryptionapduca
Oscar K
Oscar K

Reputation: 305

Smart Card Retrieve public key from CA certificate

I'm trying to read out a tachograph company smart card. I can read all data just fine except the identification part. Which is the part I actually need. When I select that DF the security environment is reset and I have to re-authenticate. This process is described in sub appendix-11 of ECE/TRANS/SC.1/2006/2/Add.1. Although this document is a bit hard to understand for me.

In this picture you see the data structure of a tachograph company card. The "AUT" behind the ID part tells you that you need to authenticate. Company card memory map

After asking another question on SO and doing a lot of research on how public/private keys are used I think I have some basic understanding on how I should do the authentication. In the documentation, there is also a pretty detailed flow chart on how to get the authentication done. It's to big unfortunately to place here. But I have a question about this part: Part of authentication steps

Now, I'm counting from the top, downwards. So the first top left square is step one, the last bottom left is step 7. The middle section arrows are APDU commands that need to be send to the card, and the right section is the smart card. PK means public key. CA means certificate authority.

If you do not know the public keys, you see you need to get both the card and the CA certificate. I've done that and I can read them from the card. The part I don't understand are step 6 and 7. You see I need to verify the Card CA.C (which is some part of the certificate) with the European Public key. Where do I get the European public key and what algorithm is used to decrypt it?

EDIT: Is this the Verification process? And if so, it says to open the sign with the CA public key, How do I get this?

enter image description here

EDIT 2: I've found the European public key from this link. The CAR part of the CA certificate on the card matches the first 8 bytes from the public key. Meaning it is the correct public key. Now If I understand correctly, I need to Open the sign following step three from CSM_019 from the picture above. To open the sign, I need the correct algorithm using the public key I quess? Does anyone know what algorithm is used?

Upvotes: 1

Views: 1007

Answers (1)

guidot
guidot

Reputation: 5333

Step 6: Nothing to decrypt here: You verify the signature, also part of the certificate (Card.CA.C), and if it is correct the contained key (public key of card CA) may be extracted and used for the next step.

Step 7: You verify the signature of card certificate (made with the card.ca key just retrieved) and if it is correct you now have the public key of the card (with the certainty, that it is correct, otherwise signature would have mismatched).

The scheme uses this two-step approach, so that only the Eur.PK public key is needed instead of the keys of all card CAs.

Upvotes: 1

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