# Shorten SHA1-hash

**URL:** <https://www.ruby-forum.com/t/shorten-sha1-hash/112253>\
**Category:** Ruby\
**Created:** [August 16, 2007, 9:21pm UTC](https://www.ruby-forum.com/t/shorten-sha1-hash/112253 "2007-08-16T21:21:21Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![splattael](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/858c86/32.png) [@splattael](https://www.ruby-forum.com/u/splattael)\
**Post date:** [August 16, 2007, 9:21pm UTC](https://www.ruby-forum.com/t/shorten-sha1-hash/112253/1 "2007-08-16T21:21:21Z")

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In one of my railsapplications I have system for up/downloading files.  
Since I don’t want people to browse all the files, but anyone should  
be able to link to his/her files each file has it’s own unique URL -  
all pretty standard.

Right now the URL just is /download/sha-sum-of-the-file (since I have  
the SHA anyway), but the length of the url is bugging me.

Now - the problem: The SHA1-sum is, as usually, just represented in  
base16 which makes it 40 chars long. However, in an url we have at  
least a-zA-Z0-9 (and also some special characters) which gives us the  
opportunity to represent the SHA-sum in at least base62 which should  
make it about half the size.

I know that I in this case just could store an extra little random  
string in my database and link to /download/little-string, but that’s  
not the point here 🙂 I would just like to hear you: How few  
(printable) characters can you shorten a SHA-sum down to?

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<div class="post-metadata">

**Author:** ![splattael](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/858c86/32.png) [@splattael](https://www.ruby-forum.com/u/splattael)\
**Post date:** [August 16, 2007, 9:35pm UTC](https://www.ruby-forum.com/t/shorten-sha1-hash/112253/2 "2007-08-16T21:35:01Z")

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You could use String#hash instead. For strings more than 6 characters  
(or  
something), it’ll return a negative number, so it’s a little hacky, but:

file\_contents.hash.to\_s.tr(’-’, ‘0’)

I’m not saying this is a nice approach, but it is pretty short 😉

Anyone with more knowledge of String#hash care to chime in?

Also, I doubt this is very cross platform friendly, just take a look at  
all  
the ifdefs in the C hash implementation.

---

<div class="post-metadata">

**Author:** ![splattael](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/858c86/32.png) [@splattael](https://www.ruby-forum.com/u/splattael)\
**Post date:** [August 16, 2007, 10:22pm UTC](https://www.ruby-forum.com/t/shorten-sha1-hash/112253/3 "2007-08-16T22:22:08Z")

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Peter Skovgaard wrote:

> least a-zA-Z0-9 (and also some special characters) which gives us the  
> opportunity to represent the SHA-sum in at least base62 which should  
> make it about half the size.
> 
> I know that I in this case just could store an extra little random  
> string in my database and link to /download/little-string, but that’s  
> not the point here 🙂 I would just like to hear you: How few  
> (printable) characters can you shorten a SHA-sum down to?

Modified Base64 for URLs is probably your best and easiest route.

Wikipedia ([Base64 - Wikipedia](http://en.wikipedia.org/wiki/Base64)) explains it well:

Base64 encoding can be helpful when fairly lengthy identifying  
information is used in an HTTP environment. Hibernate  
[http://en.wikipedia.org/wiki/Hibernate\_(Java)](http://en.wikipedia.org/wiki/Hibernate_%28Java%29), a database  
persistence framework for Java  
[http://en.wikipedia.org/wiki/Java\_(programming\_language)](http://en.wikipedia.org/wiki/Java_%28programming_language%29) objects,  
uses Base64 encoding to encode a relatively large unique id (generally  
128-bit UUIDs [http://en.wikipedia.org/wiki/UUID](http://en.wikipedia.org/wiki/UUID)) into a string for use  
as an HTTP parameter in HTTP forms or HTTP GET URLs  
[http://en.wikipedia.org/wiki/URL](http://en.wikipedia.org/wiki/URL). Also, many applications need to  
encode binary data in a way that is convenient for inclusion in URLs,  
including in hidden web form fields, and Base64 is a convenient encoding  
to render them in not only a compact way, but in a relatively unreadable  
one when trying to obscure the nature of data from a casual human  
observer.

Using a URL-encoder on standard Base64, however, is inconvenient as it  
will translate the ‘+’ and ‘/’ characters into special ‘%XX’ hexadecimal  
sequences (‘+’ = ‘%2B’ and ‘/’ = ‘%2F’). When this is later used with  
database storage or across heterogeneous systems, they will themselves  
choke on the ‘%’ character generated by URL-encoders (because the ‘%’  
character is also used in ANSI SQL as a wildcard).

For this reason, a _modified Base64 for URL_ variant exists, where /no/  
padding ‘=’ will be used, and the ‘+’ and ‘/’ characters of standard  
Base64 are respectively replaced by ‘\*’ and ‘-’, so that using URL  
encoders/decoders is no longer necessary and has no impact on the length  
of the encoded value, leaving the same encoded form intact for use in  
relational databases, web forms, and object identifiers in general.

Tom