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Revision History
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June 3, 1991, Version
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The June 3, 1991, version is part of the initial public release of PKCS. It was published as NIST/OSI Implementors Workshop document SECSIG-91-17.
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November 1, 1993, Version
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The November 1, 1993, version incorporates several editorial changes, including the addition of a revision history. It is updated to be consistent with the following versions of the PKCS documents: PKCS #1 PKCS #3 RSA Encryption Standard. Version 1.5, November 1993. Diffie-Hellman Key-Agreement Standard. Version 1.4, November 1993.
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Appendix B
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PKCS #5 PKCS #6 PKCS #7 PKCS #8 PKCS #9 PKCS #10 Password-Based Encryption Standard. Version 1.5, November 1993. Extended-Certificate Syntax Standard. Version 1.5, November 1993. Cryptographic Message Syntax Standard. Version 1.5, November 1993. Private-Key Information Syntax Standard. Version 1.2, November 1993. Selected Attribute Types. Version 1.1, November 1993. Certification Request Syntax Standard. Version 1.0, November 1993.
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The following substantive changes were made: Section 5 Section 6 Description of T61String type is added. Names are changed, consistent with other PKCS examples.
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APPENDIX
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Further Technical Details
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In this appendix, you will find extra information not covered in the main body of the book. It is a deeper look at some of the topics described. This information is not necessary for a proper understanding of the main concepts, but should be interesting reading for those who want to explore cryptography a little further.
How Digest-Based PRNGs Work
As mentioned in 2, most PRNGs (pseudo-random number generators) are based on digest algorithms. The algorithm takes a seed and just as sowing a botanical seed produces a plant produces a virtually unlimited number of pseudo-random numbers. Here is a typical implementation using SHA-1 as the underlying digest algorithm. Suppose the user wants two 128-bit session keys. The first step is give the seed to the PRNG, which digests it using SHA-1. The seed is the message of the message digest. This produces a 20-byte internal value, commonly called the state, which must be kept secret. Next, the user asks the PRNG for 16 bytes (the data of the first 128-bit session key). The PRNG uses SHA-1 to digest the state. Now the state, rather than the seed, is the
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Appendix C
message of the message digest. The digest produces 20 bytes. The user needs only 16, so the PRNG outputs the first 16 bytes. The user then requests 16 more bytes (the data of the second 128-bit session key). The PRNG has four left over from the last call; it could return them, but it also needs 12 more to fill the second request. To get the next 12 bytes, the PRNG changes the state somehow and digests the resulting new state. Because the PRNG has changed the state, this next block of 20 bytes will be different from the first block. The PRNG now has 20 new bytes. It returns the four left over from the first digest and the first 12 from the current digest. Each time the PRNG produces output, it either returns leftovers or changes the state, digests the state, and returns as many bytes from that result as needed. How does a PRNG change the state It may simply add one to the current state. Recall that if you change a message, even if only by one bit, the resulting output will be significantly different. No matter what the input message is, the output will always pass tests of randomness. So if the PRNG takes a current state and adds one to it, digesting the new state will produce completely different, pseudo-random output. If the current state is
0xFF FF FF FF . . . FF
then adding one to it will change the state to
0x00 00 00 00 . . . 00
It s certainly possible to change the state by adding a different constant. Instead of adding 1, the PRNG could add a 20-byte number. In that way, all bytes of the state are manipulated in each operation. A simpler PRNG would not bother with an internal state. Instead, it would digest the seed to create the first block of output and then would digest the first block of output to create the second block. Such a PRNG would be horrible. Here s why. Suppose Ray (the attacker from 3) wants to read Pao-Chi s e-mail. The first thing Ray does is to get Pao-Chi to send him some encrypted e-mail that is, to send him a few digital envelopes. With this e-mail, what Ray has is several 128-bit session keys (and possibly some initialization vectors if the encryption algorithm is a block cipher with a feedback mode). These keys are a series of pseudo-random bytes, each block produced by digesting the preceding block. With a little work, Ray can figure out a block boundary. Now Ray eavesdrops on Pao-Chi s future e-mails. What is the 128-bit session key used for the next e-mail
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