Re: Reverse usage of public/private RSA encryption keys for licensing?

charismatic_evangelist_at_yahoo.com
Date: 11/24/04

  • Next message: William Stacey [MVP]: "Re: Reverse usage of public/private RSA encryption keys for licensing?" 
    Date: 23 Nov 2004 15:22:13 -0800

    Thanks everybody for all your expertise! I (believe that I) have a
    solution:

    #1. Here at our office, I generate a RSA private and public key via:

        System.Security.Cryptography.RSACryptoServiceProvider RSA;
        RSA = new RSACryptoServiceProvider();

    and save the keys via:

        string privateParameters = RSA.ToXmlString(true));
        string publicParameters = RSA.ToXmlString(false));

    I then hash my data that I want to ensure non-tampering via:

        byte[] rawData, hash;
        System.Security.Cryptography.HashAlgorithm HashMan;
        HashMan = new System.Security.Cryptography.SHA1CryptoServiceProvider();
        hash = HashMan.ComputeHash(rawData);

    I then make a signature of this hash with the RSA private Crypto:

        signatureFormatter = new RSAPKCS1SignatureFormatter(RSA);
        signatureFormatter.SetHashAlgorithm("SHA1");
        signature = signatureFormatter.CreateSignature(hash);

    ... and prepend the 128-byte signature and the 20-byte hash in front
    of the raw data and write back out to a new file, which becomes my
    signed license file.

    #2. At the destination computers, I instantiate a RSA Crypto provider:

        RSAPublic = new RSACryptoServiceProvider();

    and load it ONLY WITH THE PUBLIC KEY:

        RSAPublic.FromXmlString(publicParameters);

    I extract from the raw data the claimed signature and the claimed
    hash:

        byte[] rawSignature; // bytes 0 - 127 of raw data
        byte[] rawHash; // bytes 128 - 147 of raw data

    I skip over the first 148 (128 + 20) bytes of data to be validated and
    compute a SHA1 hash of the bytes to be validated. If my hash equals
    the hash stored in the file at location 128 for 20 bytes, then I
    proceed, else error.

    Then, I instantiate a signatureDeformatter with the public RSA Crypto:

       signatureDeformatter = new RSAPKCS1SignatureDeformatter(RSAPublic);
       signatureDeformatter.SetHashAlgorithm("SHA1");

    and, finally, verify the signature with:

        signatureDeformatter.VerifySignature(rawHash, rawSignature);

    It seems to work with the few files I've tried. I thank all of you
    for your valuabe input.

    "Valery Pryamikov" <Valery@nospam.harper.no> wrote in message news:<#bENQWY0EHA.1264@TK2MSFTNGP12.phx.gbl>...
    > In other words, assuming special hash properties, there is a theorem that
    > proves that forging of RSA signature is at least as difficult as RSA problem
    > it-self. And without hash, no such proof exists - ie. without hash it may be
    > simpler to forger than to solve RSA problem.
    >
    > -Valery.
    > http://www.harper.no/valery
    >
    > "Valery Pryamikov" <Valery@nospam.harper.no> wrote in message
    > news:eIRHvEY0EHA.2012@TK2MSFTNGP15.phx.gbl...
    > > It's just that there is a theorem that proves that if we use random oracle
    > > as hash (highest level of collision resistance), than existence of an
    > > algorithm to forge signature (RSA private key encryption of random oracle
    > > hash) gives ability to solve RSA problem with sufficient amount of signing
    > > requests (non-tight reduction means that amount of signing requests could
    > > be quite high). Hash is essential part of this theorem - ie. no hash - no
    > > security prove. SHA1 doesn't provide the same level of collision
    > > resistance, but having really good collision resistances properties (none
    > > has managed to find SHA1 collision so far) it presents good practical
    > > substitution to the random oracle. Using private exponent for encrypting
    > > arbitrary data doesn't have any security prove.
    > >
    > > -Valery.
    > > http://www.harper.no/valery
    > >
    > > "William Stacey [MVP]" <staceywREMOVE@mvps.org> wrote in message
    > > news:OQhm1KX0EHA.1296@TK2MSFTNGP10.phx.gbl...
    > >>> Use RSA singing. RSA signature with strong hash (ie. SHA1) provides much
    > >>> better security than encryption of data with RSA private key (ie. what
    > >>> you
    > >>> asked about).
    > >>
    > >> Hi Valery. That seems to suggest that RSA signature entails more then
    > >> just
    > >> encrypting the hash bytes? Is there more going on? TIA
    > >>
    > >> --
    > >> William Stacey, MVP
    > >> http://mvp.support.microsoft.com
    > >>
    > >>
    > >
    > >

  • Next message: William Stacey [MVP]: "Re: Reverse usage of public/private RSA encryption keys for licensing?"

    Relevant Pages

    • Re: How to verify a SignedData with Crypto++
      ... When I said CAPICOM does not support some of the algorithms, ... My problem is that I can't get the Crypto++ to verify the signature ... signature (RSA) using the public key and get the digest. ...
      (sci.crypt)
    • Re: Reverse usage of public/private RSA encryption keys for licensing?
      ... I generate a RSA private and public key via: ... I then make a signature of this hash with the RSA private Crypto: ...
      (microsoft.public.dotnet.framework)
    • Re: Encrypt data
      ... the signature ... bytes exactly for a 1024 bit RSA key). ... public key. ... 3DES key) to encrypt any amount of data. ...
      (microsoft.public.platformsdk.security)
    • Re: RSA Decryption with public key?
      ... application knows the private key in a public key, ... The thingy that with RSA often referred as ... "Decryption with public key" is actually a *Signature Verification ...
      (microsoft.public.dotnet.security)
    • Re: Encrypt data
      ... You don't 'decode' a signature. ... bytes exactly for a 1024 bit RSA key). ... public key. ... 3DES key) to encrypt any amount of data. ...
      (microsoft.public.platformsdk.security)