>Hi,
>
>I need to generate a 6 char hash of a string where each character of the hash can be an upper letter or a number. I'd like to use Craig Boyd's vfpencryption.fll, but don't know how to go about it. Any suggestions?

One possibility:

SET LIBRARY TO VFPEncryption.FLL
?STRCONV( Hash( "This is a test", {some hash function choice} ), 15 )

* Above creates a hex-encoded string of varying lengths depending on your choice of hash function
* e.g. setting {some hash function choice} = 5 (MD5), creates a 32-character string
* Online documentation at http://www.sweetpotatosoftware.com/SPSBlog/2009/08/09/MajorVFPEncryptionUpdate.aspx

* You can then take any 6-character substring of that result

This is a simple approach that avoids base-36 (A-Z plus 0-9) calculations.

A couple of drawbacks to this approach:

- the return string uses only 0 - 9 and A - F (just the hex characters), it does not use G - Z. So, a 6-character hash would have only 16^6 distinct values (16,777,216). If you choose some other method that does include G - Z, then that increases the distinct hashes to 36^6 (2,176,782,336)

- depending on how critical the hash is for verification purposes, you need to be aware of so-called "hash collisions", i.e. the possibility that two different strings could yield the same hash value. With the code I provided above, the chance is about 1 in 17 million. If you include G - Z, it drops to about 1 in 2 billion. This may be enough for casual comparisons of relatively unimportant strings but it's definitely not good enough for serious security/encryption. For example:

- suppose you're storing a hash of a password (good practice), rather than the password itself
- if someone can read your table and read the 6-character hash value, it's not difficult with modern computing hardware to generate another password that yields the same hash value and would allow access to your program

To give you an idea of the current state of cryptography, MD5 is considered "broken" these days i.e. it's not too difficult for someone with serious computing power to generate a plaintext that yields any desired MD5 hash value. MD5 gives a 32-character hex string, which is far more robust than just a 6-character hex string.