[Muscle] Export RSA public key out of the card

[Michael StJohns]

Sorry - I forgot that from your original posts.

I do suggest exporting the private key and doing a manual decrypt to find out how the card encrypts the data.

byte[] ciphertext (after the card encrypts your data)
byte[] privateModulus
byte[] privateExponent

BigInteger cipherBig = new BigInteger (1, ciphertext);
BigInteger modBig = new BigInteger (1, privateModulus);
BigInteger expBig = new BigInteger (1, privateExponent);

BigInteger plaintextBig = cipherBig.modpow(expBig, modBig);

The last 4 octets of plaintextBig.toByteArray() should be your 01 02 03 04 bytes.  The rest should be the padding.

Mike


At 12:27 PM 5/1/2009, jose85 wrote:

>No my initial text was only 4 bytes array, like in the first post,
>but once it is crypted by the card the length is 128 bytes .
>
>
>OK - I think I see one more possibility for the problem.  If you're using
>padding, then the plain text MUST be shorter than the key length to allow
>for the addition of padding.  Reduce the cipher text by 20 octets and try
>again.  
>
>Mike
>
>
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