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

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  • Rich Salz
    In talking it over with a colleague here, we did find one weakness in your scheme. Once the recipient has unwrapped the outer part, and then unwrapped the
    Message 1 of 22 , Dec 3, 2003
      In talking it over with a colleague here, we did find one weakness in
      your scheme. Once the recipient has unwrapped the outer part, and then
      unwrapped the internal key, they can forge any message and make it
      appear as if it came from the sender. I don't know if you're worried
      about that or not. "But you promised me $10,000. not $1,000. See, I have
      your original message."

      One way to fix this might be to include a signed hash of the original
      document.

      You should look at PKCS#7 (sorry I wasn't clear, when I said what's
      wrong with a standard I meant any standard, not just XML DSIG).

      As for your intermediary approach.. you know about Kerberos, right?
      /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
      ... You are correct, but sender-2-recipient is secured AFAIK, e.g., using SSL to send credit card info to a processor doesn t guarantee the processor isn t
      Message 2 of 22 , Dec 3, 2003
        Quoting Rich Salz <rsalz@...>:

        > In talking it over with a colleague here, we did find one weakness in
        > your scheme. Once the recipient has unwrapped the outer part, and then
        > unwrapped the internal key, they can forge any message and make it
        > appear as if it came from the sender. I don't know if you're worried
        > about that or not. "But you promised me $10,000. not $1,000. See, I have
        > your original message."

        You are correct, but sender-2-recipient is secured AFAIK, e.g., using SSL to
        send credit card info to a processor doesn't guarantee the processor isn't
        publishing the information to a chat room, but you inherently trust that VISA
        isn't doing that. Only the sender and the intended recipient can see/decrypt
        the information. Right!?!
        >
        > One way to fix this might be to include a signed hash of the original
        > document.
        >
        > You should look at PKCS#7 (sorry I wasn't clear, when I said what's
        > wrong with a standard I meant any standard, not just XML DSIG).
        >
        > As for your intermediary approach.. you know about Kerberos, right?

        I have doubts about Kerberos in the short-term, because of the overhead of
        ticket exchange and the decentralization of KDCs. One can certainly utilize
        Kerberos intra-enterprise effectively as generally you are working with a
        single KDC, but once you start scaling with reckless abandon outside the
        enterprise engineering issues compond rather quickly.

        This is a good discussion.

        -Matt
      • mlong@bridgetonconsulting.com
        ... Isn t the security equally as strong as sending CC info over SSL, or not? -Matt
        Message 3 of 22 , Dec 3, 2003
          Quoting Rich Salz <rsalz@...>:
          > Anyhow, when I posted the weakness I said it's something you *might*
          > care about. As long as the sender is is not concerned about the
          > possibility of receiver fraud (even if the receive has been compromised
          > by an adversary), than it doesn't matter. Whether or not that is an
          > issue depends on the application. But it certainly makes the technique
          > inapplicable for widepsread general use.

          Isn't the security equally as strong as sending CC info over SSL, or not?

          -Matt
        • Rich Salz
          ... Since VISA is liable for any fraud if they publish your ccard number, there is strong incentive for them to not do that kind of thing. Similarly, there are
          Message 4 of 22 , Dec 3, 2003
            > You are correct, but sender-2-recipient is secured AFAIK, e.g., using SSL to
            > send credit card info to a processor doesn't guarantee the processor isn't
            > publishing the information to a chat room, but you inherently trust that VISA
            > isn't doing that. Only the sender and the intended recipient can see/decrypt
            > the information. Right!?!

            Since VISA is liable for any fraud if they publish your ccard number,
            there is strong incentive for them to not do that kind of thing.
            Similarly, there are strict rules and penalties for disclosure of
            personal information. Now, SET was worried about this, and they made
            sure that things were encrypted every-which-way, so that, e.g., the
            Merchant never saw your CCard number, but passed it along to the
            CreditCard clearinghouse, who *could* see the number, but couldn't see
            some merchant info, etc. etc. It all turned out to be too heavyweight,
            and the existing legal framework (plus incentives from the CCard
            companies to merchants) was enough to support "just use SSL."

            Anyhow, when I posted the weakness I said it's something you *might*
            care about. As long as the sender is is not concerned about the
            possibility of receiver fraud (even if the receive has been compromised
            by an adversary), than it doesn't matter. Whether or not that is an
            issue depends on the application. But it certainly makes the technique
            inapplicable for widepsread general use.

            > This is a good discussion.

            Yup. Hope we're not boring everyone else. :)
            /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
            ... Yes; the weaknesses are pretty much the same: with a symmetric key either side can forge content. But for Ccards over the web, the out-of-band framework
            Message 5 of 22 , Dec 3, 2003
              > Isn't the security equally as strong as sending CC info over SSL, or not?

              Yes; the weaknesses are pretty much the same: with a symmetric key
              either side can forge content.

              But for Ccards over the web, the out-of-band framework makes it
              reasonable to do accept this risk. Neither the merchant nor the payment
              house want to expose the information they receive, nor do any other kind
              of "defraud the sender" thing because there are financial and legal
              incentives to not do that. SSL gives hop-by-hop privacy, with some
              information about the entity at the other end. It does *not* give
              signature-style content integrity, and it's not a end-to-end
              authenticated signed content. It's transport-level security (that's why
              the IETF renamed it to TLS). For retail CCard transactions over the Web,
              TLS is okay. For many other things -- where there are outside
              agreements, risk management, or other factors influencing things -- it
              is not okay.

              For example, a doctor should not be able to use either SSL or your
              technique to do online perscriptions. A corrupt pharmacist (perhaps
              he's a friend of Rush Limbaugh :), could turn the scrip from 100 pills
              of Vicodin to 500 pills of vicodin, and *the doctor can't prove
              otherwise.* That's why I said what you want to do might be okay for
              your application, but not for general purpose.

              /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
              ... 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
              Message 6 of 22 , Dec 4, 2003
                Quoting Rich Salz <rsalz@...>:

                > > Isn't the security equally as strong as sending CC info over SSL, or not?
                >
                > Yes; the weaknesses are pretty much the same: with a symmetric key
                > either side can forge content.
                >
                > For example, a doctor should not be able to use either SSL or your
                > technique to do online perscriptions. A corrupt pharmacist (perhaps
                > he's a friend of Rush Limbaugh :), could turn the scrip from 100 pills
                > of Vicodin to 500 pills of vicodin, and *the doctor can't prove
                > otherwise.* That's why I said what you want to do might be okay for
                > your application, but not for general purpose.


                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.

                However, the receiver could create a new outer cipher value (assuming the
                receiver is the intended recipient) and forward the message. Then the
                recipient of the forwarded message would then receive an inner cipher value !=
                to the authenticated sender. Which tells the sender something is wrong
                assuming a point-2-point processing model.

                -Matt
              • 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 7 of 22 , Dec 4, 2003
                  > 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 8 of 22 , Dec 4, 2003
                    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 9 of 22 , Dec 4, 2003
                      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 10 of 22 , Dec 4, 2003
                        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 11 of 22 , Dec 4, 2003
                          >>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 12 of 22 , Dec 4, 2003
                            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 13 of 22 , Dec 4, 2003
                              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 14 of 22 , Dec 9, 2003
                                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 15 of 22 , Dec 9, 2003
                                  > 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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