Changes to 05-SEP-2013: PKI and Tcl between r6 and r15
'''Trust me.'''
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Public Key Infrastructure (PKI) is, as the name implies, all of the infrastructure required to operate a public key cryptography system. The name also implies that PKI is a complex system with lots of mandatory parts however this is not the case. A simple PKI implementation is fairly straight-forward.
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The goal of PKI is to provide a mechanism to trust an unknown party. This is done using certificates, which certify that another entity (which you may trust) has verified that this entity is who it claims to be. This certificate is presented by the unknown party as a means of identifying itself to you.
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So what is to stop someone from providing false information in a certificate ? Well, that's where the public key aspect of PKI comes into play. Certificates are digitally signed by the issuer (also called a "Certificate Authority") of the certificate.
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To explain how digital signatures work we must first explain public key cryptography. Public key cryptography uses the properties of certain mathematical operations to perform operations on numerical values that can only be reversed or verified with the "opposite" key. That is, if something is encrypted with the private key it can only be decrypted or verified with the public key and conversely if something is encrypted with the public key it can only be decrypted or verified with the private key (and not the public key).
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The most common public key cryptography algorithm in use today is RSA [[citation needed]], and it is also one of the easiest to demonstrate. RSA is named for Ron Rivest, Adi Shamir, and Leonard Adleman who made the algorithm public and are thus widely credited as the inventors of the algorithm.
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RSA is described in PKCS #1 [http://www.emc.com/collateral/white-papers/h11300-pkcs-1v2-2-rsa-cryptography-standard-wp.pdf] which was later published as RFC 3447 [http://www.ietf.org/rfc/rfc3447.txt] but a simple definition of applying RSA is "modular exponentation". That is modular arithmetic (as in the modulus operator) applied to values that have had the the exponentiation operator applied to them. For example
26729 (plain-text) ^ 65537 (public-exponent) mod 37837 (public-modulus) = 36784 (cipher-text)
is modular exponentiation of `26729` raised the power of `65537` all modulo `37837`, which results in `36784`.
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Looking closer at this example of modular exponentiation we can see some of the public key cryptography properties that we need starting to emerge. Specifically we cannot take the `cipher-text` and convert it back to the `plain-text` with just the information that we know. We must know the `private-exponent` (or be able to derive it by factoring the `public-modulus`, but more on that later).
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Thus the above example is also an example of encrypting the value `26729` (which is 0x6869, or "hi" in ASCII) with an RSA key whose `public-exponent` (often simply referred to as the exponent) is `65537` and whose public-modulus (often simply referred to as the modulus) is `37837` resulting in the encrypted value of `36784`.
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However, if we know the private key we can reverse the operation:
36784 (cipher-text) ^ 40193 (private-exponent) mod 37837 (public-modulus) = 26729 (plain-text)
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This is not too useful for digital signatures because it requires the `private-exponent` to do anything meaningful. With a digital signature we want to perform an operation on some plain-text using our private data (for RSA `private-exponent`) that can be verified using only public information (such as `public-exponent` and `public-modulus`). Fortunately RSA allows us to do that by simply swapping the values around. That is:
12345 (plain-text) ^ 40193 (private-exponent) mod 37837 (public-modulus) = 3293 (cipher-text)
which CAN be reversed using only public information, as in:
3293 (cipher-text) ^ 65537 (public-exponent) mod 37837 (public-modulus) = 12345 (plain-text)
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RSA guarantees that the chance of there being another value of `private-exponent` which generates the same `cipher-text` for a given `plain-text` is extremely low relative to the size of the key. In this way we can assert that if a given `cipher-text` can be decrypted to a `plain-text` using a given `public-exponent` and `public-modulus` then it must have been encrypted with the secretly held `private-exponent` and thus as long as `private-exponent` is protected as a secret only the holder could have generated this message.
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So to generate a digital signature one just encrypts the data with the "private key" (`private-exponent` in RSA) and the message is valid when decrypted with the "public key" (`public-exponent` in RSA) then it must have been written by the holder of the private key.
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This is certainly a valid and workable solution for some messages, however there are some limitations of RSA that make it not a desirable general solution. Specifically RSA cannot encrypt messages larger than the size of the key. That is, if the key is 16 bits then the `plain-text` message can be no larger than 16 bits.
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Instead what is done is a cryptographically secure message digest algorithm such as MD5 (defined in RFC ???), SHA1 (defined in RFC ???), SHA256 (defined in RFC ???), etc is used to compute a digest of the message which is much smaller than the message itself. This digest is then encrypted with the private key and verified with the public key. In this way arbitrarily long messages can be digitally signed using RSA.
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This property of digital signatures means that if the certificate is altered or forged after being signed it will be detected.
Only in r6
Only in r15 --> Modified slightly between r6 and r15