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Re: [soapbuilders] Re: Super-Encryption AND Digital Signatures

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  • mlong@bridgetonconsulting.com
    Allow me to try this will a little pseudo-math. Encryption for Sender: SymmetricKey1 = TripleDES(IV1,Key1) CipherValue1 = Encrypt[TripleDES(MyMessage)]
    Message 1 of 22 , Dec 4, 2003
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      Allow me to try this will a little pseudo-math.

      Encryption for Sender:

      SymmetricKey1 = TripleDES(IV1,Key1)
      CipherValue1 = Encrypt[TripleDES(MyMessage)]
      InnerEncryptedKey = RSA_Encrypt(Key1) [using sender's RSA private key]

      SymmetricKey2 = TripleDES(IV2,Key2)
      CipherValue2 = Encrypt[TripleDES(CipherValue1)]
      OuterEncryptedKey = RSA_Encrypt(Key2) [using receiver's RSA public key]


      Decryption for Receiver:

      Key2 = RSA_Decrypt[OuterEncryptedKey] [using receiver's RSA private key]
      CipherValue1 = Decrypt[TripleDES(CipherValue2)]
      Key1 = RSA_Decrypt[InnerEncryptedKey] [using sender's RSA public key]
      MyMessage = Decrypt[TripleDES(CipherValue1)]

      Naturally, IV1 and IV2 are the first 8 octets of their respective cipher
      values. (omitted for brevity).

      (1) You could decrypt MyMessage and send it two someone else.
      (a) But you couldn't impersonate the sender (the senders private key is
      not available)
      (2) You could decrypt CipherValue2 and re-encrypt CipherValue1 which something
      akin to CipherValue2' and forward the message.
      (a) But the decryption of CipherValue1 now yields a sender not from the
      source of the message sent, i.e. public key of forwarding sender cannot be
      used to decrypt the Key1 of CipherValue1.

      More thoughts and comments, please.

      Thx,

      -Matt









      Quoting Rich Salz <rsalz@...>:

      > > Hmmm...under my scenario the content can be duplicated, but not altered.
      > > Because the receiver does not have the private key of the sender, i.e., the
      >
      > > receiver cannot re-encrypt the inner cipher value identically (to spoof the
      >
      > > original sender) due to the fact that the original sender's private key is
      > not
      > > known to the receiver.
      >
      > Perhaps I don't understand. I was talking about re-using the cipher to
      > create a modified message. The adversary (compromised recipient)
      > doesn't need to generate a new inner key, he just re-uses it generate a
      > new messsage. Now the original sender can't prove the receiver wrong.
      > /r$
      >
      > --
      > Rich Salz, Chief Security Architect
      > DataPower Technology http://www.datapower.com
      > XS40 XML Security Gateway http://www.datapower.com/products/xs40.html
      > XML Security Overview http://www.datapower.com/xmldev/xmlsecurity.html
      >
      >
      >
    • Rich Salz
      Good -- a common notation. :) Now, a compromised receiver does the following. First, get the keys: Key2 = RSA_Decrypt[OuterEncryptedKey] [using receiver s RSA
      Message 2 of 22 , Dec 4, 2003
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        Good -- a common notation. :)

        Now, a compromised receiver does the following. First, get the keys:
        Key2 = RSA_Decrypt[OuterEncryptedKey] [using receiver's RSA private key]
        CipherValue1 = Decrypt[TripleDES(CipherValue2)]
        Key1 = RSA_Decrypt[InnerEncryptedKey] [using sender's RSA public key]
        Next, use those keys to create a bad message:
        BadCipher1 = Encrypt[TripleDES(*BAD MESSAGE*, Key1)]
        BadCipher2 = Encrypt[TripleDes(BadCipher1, Key2)]

        The adversary can now present present the two encrypted keys and
        BadCipher2, and nobody can prove they didn't come from the original
        sender. ("But you told me to give 1,000 pills -- here's the message")

        /r$
        --
        Rich Salz, Chief Security Architect
        DataPower Technology http://www.datapower.com
        XS40 XML Security Gateway http://www.datapower.com/products/xs40.html
        XML Security Overview http://www.datapower.com/xmldev/xmlsecurity.html
      • mlong@bridgetonconsulting.com
        ... But BadCipher1 now has RSA_Encrypt(Key1) from the receiver and not the original sender, because the receiver never had access to the sender s RSA private
        Message 3 of 22 , Dec 4, 2003
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          Quoting Rich Salz <rsalz@...>:

          > Good -- a common notation. :)
          >
          > Now, a compromised receiver does the following. First, get the keys:
          > Key2 = RSA_Decrypt[OuterEncryptedKey] [using receiver's RSA private key]
          > CipherValue1 = Decrypt[TripleDES(CipherValue2)]
          > Key1 = RSA_Decrypt[InnerEncryptedKey] [using sender's RSA public key]
          > Next, use those keys to create a bad message:
          > BadCipher1 = Encrypt[TripleDES(*BAD MESSAGE*, Key1)]
          > BadCipher2 = Encrypt[TripleDes(BadCipher1, Key2)]
          >
          > The adversary can now present present the two encrypted keys and
          > BadCipher2, and nobody can prove they didn't come from the original
          > sender. ("But you told me to give 1,000 pills -- here's the message")

          But BadCipher1 now has RSA_Encrypt(Key1) from the receiver and not the
          original sender, because the receiver never had access to the sender's RSA
          private key. Therefore, the proof of who sent the message resides with the
          decryption of Key1 (since it is always encrypted with the sender's private
          key).

          Where is this breaking down?
        • Rich Salz
          ... No, use the original encrypted Key1. ... Right, but there s no proof of what content the sender provided. More precisely, the proof is who generated
          Message 4 of 22 , Dec 4, 2003
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            >>Now, a compromised receiver does the following. First, get the keys:
            >>Key2 = RSA_Decrypt[OuterEncryptedKey] [using receiver's RSA private key]
            >>CipherValue1 = Decrypt[TripleDES(CipherValue2)]
            >>Key1 = RSA_Decrypt[InnerEncryptedKey] [using sender's RSA public key]
            >>Next, use those keys to create a bad message:
            >>BadCipher1 = Encrypt[TripleDES(*BAD MESSAGE*, Key1)]
            >>BadCipher2 = Encrypt[TripleDes(BadCipher1, Key2)]

            > But BadCipher1 now has RSA_Encrypt(Key1) from the receiver and not the
            > original sender

            No, use the original encrypted Key1.

            > Therefore, the proof of who sent the message resides with the
            > decryption of Key1

            Right, but there's no proof of what content the sender provided. More
            precisely, the proof is "who generated Key1", which is even further from
            protecting the content.

            > Where is this breaking down?

            Our terminology, I think.

            Both sender and receiver need Key1 and Key2. Since both 3DES is
            symmetric, Key1 and Key2 can both encrypt and decrypt. Once the
            adversary (compromised receiver) has both keys, he can use them to
            encrypt anthing he wants. Now take that bogus message -- and the keys
            *in their original wrappers* -- and prove the sender didn't generate
            that content.

            Does this help?
            /r$
            --
            Rich Salz, Chief Security Architect
            DataPower Technology http://www.datapower.com
            XS40 XML Security Gateway http://www.datapower.com/products/xs40.html
            XML Security Overview http://www.datapower.com/xmldev/xmlsecurity.html
          • mlong@bridgetonconsulting.com
            ... Thanks, I m clear now. Receiver decrypts both wrappers then reuses RSA_Encrypted(Key) for the outbound traffic. Now, my trusted intermediary/trust-broker
            Message 5 of 22 , Dec 4, 2003
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              Quoting Rich Salz <rsalz@...>:
              >
              > No, use the original encrypted Key1.

              Thanks, I'm clear now. Receiver decrypts both wrappers then reuses
              RSA_Encrypted(Key) for the outbound traffic.

              Now, my trusted intermediary/trust-broker scenario to combat this.
              Setup:
              (a) Subdivide users into two categories, i.e., consumers/senders and
              providers/receivers.
              (b) Consumers create endpoints within the intermediary to connect to
              providers.
              (c) Both consumers and providers share Public Key with intermediary, but not
              with each other (not that it matters).
              (d) Providers share authentication information for their respective services
              with the intermediary, but not the consumers. Likewise for consumers.

              The Message:
              (1) Encrypted message sent by consumer "C" to intermediary "I" targeted at
              provider "P".
              (2) "I" authenticates "C" and uses the auth info to determine Public Key match
              for encrypted key.
              (3) "I" fully decrypts the message, then re-encrypts the message based "I"
              Private Key, "P" symmetric algorithm, and "P's" Public Key.
              (3) "I" forwards message to "P" utilizing orthogonal authentication info
              provided by "P" and original sender identification (which doesn't need
              encryption AFAIK)
              (4) "P" authenticates "I" and such decrypts message using "I's" PK.

              Therefore, as long as the message authenticates for "I", then it must come
              from "I" where the original sender has been authenticated and message properly
              decrypted and re-encrypted. Naturally, this relies on the authentication
              information being well protected from both perspective of "C","P", and "I".

              Tell me where this break, please.

              Thx,

              -Matt
            • Rich Salz
              A simpler fix is for the sender to do SHA1(message), and then encrypt (key1+digest) with their private key. That s simpler because it s a classic digital
              Message 6 of 22 , Dec 4, 2003
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                A simpler fix is for the sender to do SHA1(message), and then
                encrypt (key1+digest) with their private key. That's simpler
                because it's a classic digital signature, and its properties are
                well understood.

                The two biggest problems with your current idea are that
                1. "I" must be online and completely trusted for every single
                message exchange. This gives up all the benefits of public-
                key crypto.
                2. There's no end-to-end security link. What prevents P from
                using his own keypair to forge a message that looks like
                I-on-behalf-of-C?

                A simpler fix for your first scheme might be for the sender to include
                RSA_Encrypt(SHA1(message)) alongside the encrypted key1. Then perhaps
                you include a timestamp, so adversaries can't capture and reply old
                messages.

                I know you think that the standard mechanisms are expensive and full
                of overhead. There's a reason: without them, you leave yourself
                open to various attacks.
                /r$

                --
                Rich Salz Chief Security Architect
                DataPower Technology http://www.datapower.com
                XS40 XML Security Gateway http://www.datapower.com/products/xs40.html
                XML Security Overview http://www.datapower.com/xmldev/xmlsecurity.html
              • mlong@bridgetonconsulting.com
                Rich, RSA_Encrypt(SHA1(message) + key1) this looks promising. Also, doesn t eliminate the need for a xml digital signature. You see what I m after, i.e., high
                Message 7 of 22 , Dec 9, 2003
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                  Rich,

                  RSA_Encrypt(SHA1(message) + key1) this looks promising. Also, doesn't
                  eliminate the need for a xml digital signature.

                  You see what I'm after, i.e., high security + scalable implementable features
                  + compact wire format.

                  Thoughts!?!

                  -Thx,

                  -Matt



                  Quoting Rich Salz <rsalz@...>:

                  > A simpler fix is for the sender to do SHA1(message), and then
                  > encrypt (key1+digest) with their private key. That's simpler
                  > because it's a classic digital signature, and its properties are
                  > well understood.
                  >
                  > The two biggest problems with your current idea are that
                  > 1. "I" must be online and completely trusted for every single
                  > message exchange. This gives up all the benefits of public-
                  > key crypto.
                  > 2. There's no end-to-end security link. What prevents P from
                  > using his own keypair to forge a message that looks like
                  > I-on-behalf-of-C?
                  >
                  > A simpler fix for your first scheme might be for the sender to include
                  > RSA_Encrypt(SHA1(message)) alongside the encrypted key1. Then perhaps
                  > you include a timestamp, so adversaries can't capture and reply old
                  > messages.
                  >
                  > I know you think that the standard mechanisms are expensive and full
                  > of overhead. There's a reason: without them, you leave yourself
                  > open to various attacks.
                  > /r$
                  >
                  > --
                  > Rich Salz Chief Security Architect
                  > DataPower Technology http://www.datapower.com
                  > XS40 XML Security Gateway http://www.datapower.com/products/xs40.html
                  > XML Security Overview http://www.datapower.com/xmldev/xmlsecurity.html
                  >
                  >
                  >
                • Rich Salz
                  ... Yeah, there s no such thing as a free lunch. :) Folks often complain about how big SSL is, or how complicated XML DSIG is, etc. Unfortunately, they are
                  Message 8 of 22 , Dec 9, 2003
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                    > You see what I'm after, i.e., high security + scalable implementable features
                    > + compact wire format.
                    >
                    > Thoughts!?!

                    Yeah, there's no such thing as a free lunch. :)

                    Folks often complain about how "big" SSL is, or how complicated
                    XML DSIG is, etc. Unfortunately, they are that way because they need
                    to be in order to be resistant to various threats. And then you have
                    to fight the deployment barriers: if SSL, PKCS#7 and/or XML DSIG are
                    already everywhere, what's the incentive to try something that hasn't
                    had the same level of analysis? Unless you're Ron Rivest (the R of RSA)
                    designing a new micro-payment protocol (www.peppercoin.com), you're
                    generally better off accepting the trade-offs of commodity security
                    mechanisms.

                    Now, RSA_PublicKey_Encrypt(SHA1(message) + key1) seems reasonable
                    to me. But it's quite possible that there's some obscure corner of
                    crypto that makes this a bad idea. I still think it's worth
                    posting it to the cryptography mailing list.

                    /r$

                    --
                    Rich Salz Chief Security Architect
                    DataPower Technology http://www.datapower.com
                    XS40 XML Security Gateway http://www.datapower.com/products/xs40.html
                    XML Security Overview http://www.datapower.com/xmldev/xmlsecurity.html
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