Re: Another RSACryptoServiceProvider question

From: Pieter Philippaerts (Pieter_at_nospam.mentalis.org)
Date: 10/29/03

  • Next message: Daryn Kiely: "Re: Another RSACryptoServiceProvider question" 
    Date: Wed, 29 Oct 2003 20:49:43 +0100

    "Daryn Kiely" <anonymous@discussions.microsoft.com> wrote in message
    > This works great if the fOAEP flag is
    > false (use PKCS padding) but fails if I use OAEP
    > padding. In both cases the encryption works, but in the
    > latter case the code throws a Cryptographic Exception
    > with the message "Error occured while decoding OAEP
    > padding".

    There are two problems with your code. The first is that you use the
    rsaEncrypt object to both encrypt and decrypt. rsaEncrypt.Decrypt will
    obviously not work because it doesn't have access to the private key.
    However I assume it was an error in your post and not an error in the code
    you're using because you said PKCS#1 worked.

    Secondly, you're trying to encrypt/decrypt too many bytes. If you change the
    number of bytes you're trying to encrypt to 86, everything works perfectly.
    This is in accordance with the MSDN documentation, that states "Modulus
    size -2 -2*hLen, where hLen is the size of the hash". If you take modulus
    size to be 128 bytes [or 1024 bits] and hLen 20 bytes [ie. SHA-1], you get
    128 - 2 - 2 * 20 = 86.
    It's strange that the encrypt method works and the decrypt doesn't. The
    encrypt method should have thrown an exception.

    Regards,
    Pieter Philippaerts
    Managed SSL/TLS: http://www.mentalis.org/go.php?sl

  • Next message: Daryn Kiely: "Re: Another RSACryptoServiceProvider question"

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