CODEWITHSUNDEEP
bouncycastlejavacardelliptic-curvediffie-hellman
TajnosAgentos
TajnosAgentos

Reputation: 167

Javacard KeyAgreement differs from BouncyCastle KeyAgreement

My problem looks like this. I have generated keys on the card and the terminal sides. I have on the terminal side the card public and private keys and the terminals public and private keys, and the same on the card side (i'm doing tests so thats why i have all of them on the terminal and on the card). When i generate KeyAgreement (terminal side) for the card as private and for the terminal as private the secters are the same, so the generation is OK and i get a 24 bytes (192 bit) secret. When i generate the the secrets on the card (2 cases like on the terminal) the secrets are also the same, but they ale shorter - 20 bytes (160 bit). Here are the generation codes. the terminal:

ECPublicKey publicKey;
ECPrivateKey privateKey;

...

KeyAgreement aKeyAgree = KeyAgreement.getInstance("ECDH", "BC");
aKeyAgree.init(privateKey);
aKeyAgree.doPhase(publicKey, true);
byte[] aSecret = aKeyAgree.generateSecret();

and the card side:

eyAgreement = KeyAgreement.getInstance(KeyAgreement.ALG_EC_SVDP_DH, false);
short length = terminalEcPublicKey.getW(array, (short) 0);

keyAgreement.init(cardEcPrivateKey);
short secretlength = keyAgreement.generateSecret(array, (short)0, length, buffer, (short)0);

Upvotes: 4

Views: 973

Answers (1)

Chooch
Chooch

Reputation: 583

There is a problem in your implementation of KeyAgreement.ALG_EC_SVDP_DH in the terminal side. The correct length of output of the this method of key agreement should always be 20 bytes since SHA-1 is being performed on the derived output.

So in your terminal side, you should perform SHA-1 after generating the secret data.

Upvotes: 6

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