'''[https://tools.ietf.org/id/draft-msporny-base58-02.txt%|%base58]''' is like [base64], but without these characters: `+/0OIl`



** See Also **

   [ycl%|%ycl string base encode/decode]:   Encodes and decodes [binary%|%bytes] using the given list of encoding characters.



** Description **

base58 is used in the bitcoin system, and there is apparently a different
version of base58 used by Flickr.

The rationale behind the use of base58, at least for bitcoin, is:

   * Characters like `0` and `O`, or `I` and `l` are too similar in some fonts and could be used to create account numbers that are hard to distinguish visually.
   * A string with non-alphanumeric characters is not as easily accepted as an account number.
   * E-mail content isn't broken into lines where there's no punctuation to break at.
   * Doubleclicking selects the whole number as one word if it's all alphanumeric.

The alphabet used is:  `123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz`.

----

[JMN] 2011-06-28:  Does anyone know if there are existing Tcl implementations of base58 as used by bitcoin?

----
'''[AK] - 2011-06-28 17:41:27'''

I do not know of any implementation. If one is made by whoever I would strongly recommend to submit the code to Tcllib for inclusion, as it already has base64, base32, ascii85, uu, yEnc, etc. I.e. a base58 implementation would fit right in.

----

[aspect]:  this came up in the [Tcl Chatroom] so I did a quick implementation.  base58 is not so good for encoding binary chunks, as it doesn't correspond neatly to a length in bits.  This version base58-encodes an integer:

======
proc base58 {hex} {
  set alphabet 123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz
  set result {}; scan $hex %llx int; set alen [string length $alphabet]
  while {$int} {
    set result [string index $alphabet [expr {$int%$alen}]]$result
    set int [expr {$int/$alen}]
  }
  set result
}
======

[AvL]: I changed the name of the previous example to "base58", fixed a reversal bug and made it
take a hexadecimal string to match later usage samples on this page.

For bitcoin-related use, here's another snippet that extracts the address from a scriptPub:

======
package require sha256
set scriptPubKey "76a9146ccf14fa32539e2d148a3b39d87ae0e7c6f17b5988ac"

if {[regexp {^76a914([0-9a-f]{40})88ac$} $scriptPubKey _ xpub]} {
   set xpub 00$xpub
   set bpub [binary format H* $xpub]
   set sha [sha2::sha256 [sha2::sha256 -bin $bpub]]
   set num 1[base58 $xpub[string range $sha 0 7]]
}

======

Maybe we ought to open a separate page for bitcoin-stuff in Tcl, once it
becomes more than just base58 and a usage sample.


An example usage to generate unique identifiers a la youtube might be:

======
package require md5
string range [base58 [md5::md5 -hex myidentifier]] 0 11
======

Whether your identifiers are sufficiently unique is a matter for you to decide ..  Note that the only rationale for including [md5] is to make the distribution more uniform and obscure the source of identifiers.  The md5 output is also being truncated to a number between 0 and 58**12, or:

======
tclsh8.5 [~]list [expr log(58**12)/log(2)] bits
70.29577194153087 bits
======


<<categories>> Cryptography | Internet | Bitcoin