Is there a good two way hash to convert an email address to a predictable, readable, unix username? - bash

Question

We are working with a number of unix based filesystems, all of which share a similar set of restrictions on that certain characters can't be used in the username fields. One of those restrictions is no "#" , "_", or "." in the names. Being unix there are a number of other restrictions.
So the question is if there is a good known algorithm that can take an email address and turn that into a predictable unix filename. We would need to reverse this at some point to get the email.
I've considered doing thing like "."->"DOT", "#"->"AT", etc. But there are size limitations and other things that are generally problematic. I could also optimize by being able to map the #xyz.com part of the email to a special char or something. Each implementation would only have at most 3 domains it would need to support. I'm hoping someone has found a solution without a huge number of tradeoffs.
UPDATE:
-The two target filesystems are AFS and NFS.
-Base64 doesn't work as it has not compatible characters. "/"
-Readable is preferable.
Seems like the best answer would be to replace the #xyz.com domain to a single non-standard character, and then have a function that could shrink the first part of a name to something that fits in the username length restrictions of the various filesystems. But what is a good function for that?

Answer 1

You could try a modified version of the URL percent (%) encoding scheme used on for URIs.
If the percent symbol isn't allowed on your particular filesystem(s), simply replace it with a different, allowed character (and remember to encode any occurrences of that character properly).
Using this method:
mail.address#server.com
Would become:
mail%2Eaddress%40server%2Ecom
Or, if you had to substitute (for example), the letter a instead of the % symbol:
ma61ila2Ea61ddressa40servera2Ecom
Not exactly humanly-readable perhaps, but easily enough processed through an encoding algorithm. For the best space efficiency, your escape character should be a character allowed by the filesystem, yet one that is not likely to appear frequently in an address.
This encoding scheme has the advantage that there is no size increase for most normal characters. The string length will ONLY go up for characters not supported by the filesystem.

Answer 2

Check out base64. Encoding and decoding is well defined.
I'd prefer this over rolling my own format any day.

Answer 3

Hmm, from your question I'm not totally clear on this point, but since you wanted some conversion I'm assuming that you want something that is at least human readable?
Each OS may have different restrictions, but are you close enough to the platforms that you would be able to find out/test what is acceptable in a username? If you could find three 'special' characters that you could use just to do a replace on '#', '.', '_' you would be good to go. (Is that comprehensive? if not you would need to make sure you know all of them otherwise you could clash.) I searched a bit trying to find whether there was a POSIX standard, but wasn't able to find anything, so that's why I think if you can just test what's valid that would be the most direct route.
With even one special character, you could do URL encoding, either with '%' if it's available, or whatever you choose if not, say '!", then { '#'->'!40", '_'->'!5F', '.'-> '!2E' }. (The spec [RFC1738] http://www.rfc-editor.org/rfc/rfc1738.txt) defines the characters as US-ASCII so you can just find a table, e.g. in wikipedia's ASCII article and look up the correct hex digits there.) Or, you could just do your own simple mapping since you don't need the whole ASCII set, you could just do a map with two characters per escaped character and have, say, '!a','!u','!p' for at, underscore, period.
If you have two special characters, say, '%', and '!', you could delimit text that represents the character, say, %at!, &us!, and '&pd!'. (This is pretty much html-style encoding, but instead of '&' and ';' you are using the available ones, and you're making up your own mnemonics.) Another idea is that you could use runs of a symbol to determine the translated character, where each new character flops which symbol is being used. (This conveniently stops the run if we need to put two of the disallowed characters next to each other.) So assume '%' and '!', with period being 1, underscore 2, and at-sign being three, 'mickey._sample_#fake.out' would become 'mickey%!!sample%%!!!fake%out'. There are other variations but this one is easy to code.
If none of this is an option (e.g. no symbols at all, just [a-zA-Z0-9]), then really I think the Base64 answer sounds about right. Really once we're getting to anything other than a simple replacement (and even that) it's already getting hard to type if that's the goal. But if you really need to try to keep the email mostly readable, what you do is implement some sort of escaping. I'm thinking use '0' as your escape character, so now '0' becomes '00', '#' becomes '01', '.' becomes '02', and '_' becomes '03'. So now, 'mickey01._sample_#fake.out'would become 'mickey0010203sample0301fake02out'. Not beautiful but it should work; since we escaped any raw 0's, just always make sure you define a mapping for whatever you choose as your escape char and you should be fine..
That's all I can think of atm. :) Definitely if there's no need for these usernames to be readable in the raw it seems like apparently Base64 won't work, since it can produce slashes. Heck, ok, just the 2-digit US-ASCII hex value for each character and you're done...] is a good way to go; there's lots of nice debugged, heavily field-tested code out there for it and it solves your problem quite handily. :)

Answer 4

Given...
- the limited set of characters allowed in various file systems
- the desire to keep the encoded email address short (both for human readability and for possible concerns with file system limitations)
...a possible approach may be a two steps encoding logic whereby the email is
first compressed using a lossless compression algorithm such as Lempel-Ziv, effectively turning it into a "binary" form, stored in a shorter array of bytes
then this array of bytes is encoded using a Base64-like algorithm
The idea is to minimize the size of the binary representation, so that the expansion associated with the storage inefficiency of the encoding -which can only store roughly 6 bits (and probably a bit less) per character-, doesn't cause the encoded string to be too long.
Without getting overly sophisticated for the compression nor the encoding, such a system would likely produce encoded strings that are maybe 4/5 of the input string size (the email address): the compression should easily half the size, but the encoding, say Base32, would grow the binary form size by 8/5.
Efforts in improving the compression ratio may allow the selection of more "wasteful" encoding schemes (with smaller character sets) and this may help making the output more human-readable and also more broadly safe on various flavors of file systems. For example whereby a Base64 seems optimal. space-wise, using only uppercase letter (base 26) may ensure portability of the underlying scheme to file systems where the file names are not case sensitive.
Another benefit of the initial generic compression is that few, if any, assumptions need to be made about the syntax of valid input key (email addresses here).
Ideas for compression:
LZ seems like a good choice, 'though one may consider primin its initial buffer with common patterns found in email addresses (example ".com" or even "a.com", "b.com" etc.). This initial buffer would ensure several instances of "citations" per compressed email address, hence a better compression ratio overall). To further squeeze a few bytes, maybe LZH or other LZ-variations could be used.
Aside from the priming of the buffer mentioned above, another customization may be to use a shorter buffer than typical LZ algorithms, since the string we have to compress (email address instances) are themselves very short and would not benefit from say a 512 bytes buffer. (Shorter buffer sizes allow shorter codes for the citations)
Ideas for encoding:
Base64 is not suitable as-is because of the slash (/), plus (+) and equal (=) characters. Alternate characters could be used to replace these; dash (-) comes to mind, but finding three charcters, allowed by all "flavors" of the targeted file systems may be a stretch.
Never the less, Base64 and its 4 output characters per 3 payload bytes ratio provide what is probably the barely achievable upper limit of storage efficiency [for an acceptable character set].
At the lower end of this efficiency, is maybe an ASCII representation of the Hexadeciamal values of the bytes in the array. This format with a doubling of the payload bytes may be acceptable, length-wise, and is interesting because of its simplicity (there is a direct and simple relation between each nibble (4 bits) in the input and characters in the encoded string.
Base32 whereby A thru Z encode 0 thru 25 and 0 thru 5 encode 26 thru 31, respectively, essentially variation of Base64 with an 8 output characters per 5 payload bytes ratio may be a very viable compromise.