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

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  • Rich Salz
    ... 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
    Message 1 of 22 , Dec 4, 2003
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      > 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
    • 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 2 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 3 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 4 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 5 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 6 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 7 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 8 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 9 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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