vb.net code 39 reader Key Recovery via a Group of Trustees in Software

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Some companies and individuals do not like the idea of one person having access to all keys. In such situations, a better scheme is to break the key into parts and distribute them among several individuals. Suppose those individuals are the company s TTPs Michelle and Alexander and Gwen, the VP of sales. Now Pao-Chi s software comes preloaded with three public keys. Each of his keys is broken into three parts, and three digital envelopes are created. For example, Pao-Chi has a 128-bit symmetric key that he uses to encrypt the files on his hard drive; this key is separated into three blocks of five bytes, five bytes, and six bytes. Michelle s public key protects five of the bytes, Alexander s protects another five, and Gwen s protects the last six. Now if Pao-Chi needs to recover his key, all three trustees must gather to reconstruct the data. The advantage here is that no one individual can recover keys secretly. For keys to be recovered surreptitiously, all three trustees would have to agree to subvert the system, a scenario less likely to occur than if only one individual possessed the ability to recover keys. The scheme as described here has a problem. Because each trustee has a portion of the key, it would be possible for an individual to recover the known portion and then perform a brute force attack on the rest. Gwen has the largest portion six bytes (48 bits) so her task would be equivalent to breaking an 80-bit key. Such an attack is not likely, but it would be better if that avenue were closed. One way around this problem is to create a 384-bit value and split that into three 128-bit components. Each trustee knows 128 bits but is missing 256 bits of the total value. The 384-bit value is actually used to derive the key. That is, Pao-Chi generates a 384-bit value and uses it as a seed for a PRNG. The PRNG produces the session key. Each trustee gets a portion of the 384-bit value. To recover the key, you must put all three of the trustees components together and re-create the PRNG (see Figure 4-25). This splitting of the secret into multiple digital envelopes has the advantage of preventing one individual from wielding too much power. But it has the disadvantage of being more difficult to implement and also carries all the disadvantages of the TTP approach: If one trustee is on vacation, the key is still lost. Furthermore, if one trustee leaves the company, the key recovery process must start over from scratch, new public/private key pairs have to be generated and public keys distributed, and all employees must create new digital envelopes.
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Figure 4-25 Pao-Chi creates a 128-bit session key using a 384bit seed value and splits the 384-bit value into three portions, encrypting each portion with one trustee s public key. Recovering the session key means recovering the 384-bit value and recreating the PRNG
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Probably the most common key recovery method involves threshold schemes, also called secret sharing or secret splitting. A secret, such as a key, is split into several shares, some number of which must be combined to recover the secret. For example, a secret can be split into 6 shares, any 3 of which can be combined to reproduce the value. Or the secret can be split among 10 shares, any 4 of which can recover the item, or 12 shares with a threshold of 11, or 5 shares with a threshold of 5, or 100 shares with a threshold of 2. Almost any reasonable share and recovery count is possible (as long as the threshold is less than or equal to the share count). For key recovery, the secret is an RSA private key. If Pao-Chi s company implements a threshold scheme, it might work like this. The company decides how many shares there will be, how many are needed to implement key recovery, and who the trustees will be. Suppose the policy is for six trustees and three shares needed. The trustees are a system or network administrator, the HR director, and representatives from several departments. Say the sys admin is Warren, the HR
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