Like [base64], but without these characters: `+/0OIl`

This is the definition of base58 as used by the open source decentralized cryptographic peer-to-peer currency system bitcoin.
(there is apparently a different version of base58 used by Flickr)

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

   * Don't want 0OIl characters that look the same in some fonts and could be used to create visually identical looking account numbers.
   * A string with non-alphanumeric characters is not as easily accepted as an account number.
   * E-mail usually won't line-break if 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 modified the previous sample to name "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