moth/packages/net-re/2/index.mdwn

ASCII
=====

ASCII is a map of 7-bit values--0x00 through 0x7F--to glyphs (printable
characters).  Here is the ASCII chart from the Linux ASCII(7) man page:

          0 1 2 3 4 5 6 7
        -----------------
       0: ␀ ␐   0 @ P ` p
       1: ␁ ␑ ! 1 A Q a q
       2: ␂ ␒ " 2 B R b r
       3: ␃ ␓ # 3 C S c s
       4: ␄ ␔ $ 4 D T d t
       5: ␅ ␕ % 5 E U e u
       6: ␆ ␖ & 6 F V f v
       7: ␇ ␗ ' 7 G W g w
       8: ␈ ␘ ( 8 H X h x
       9: ␉ ␙ ) 9 I Y i y
       A: ␊ ␚ * : J Z j z
       B: ␋ ␛ + ; K [ k {
       C: ␌ ␜ , < L \ l |
       D: ␍ ␝ - = M ] m }
       E: ␎ ␞ . > N ^ n ~
       F: ␏ ␟ / ? O _ o ␡

Characters 0x00 through 0x1F and character 0x7F are the so-called
"control characters" and are typically not displayed.

So, the following sequence of hex values:

    68 65 6C 6C 6F 20 77 6F 72 6C 64 2E

when decoded as ASCII yields the string:

    hello world


Base 64
=======

Base 64 is a map of 6-bit values--0x00 through 0x3F--to ASCII
characters.  Here is the Base 64 alphabet:

          0 1 2 3
        ---------
       0: A Q g w
       1: B R h x
       2: C S i y
       3: D T j z
       4: E U k 0
       5: F V l 1
       6: G W m 2
       7: H X n 3
       8: I Y o 4
       9: J Z p 5
       A: K a q 6
       B: L b r 7
       C: M c s 8
       D: N d t 9
       E: O e u +
       F: P f v /

The following sequence of hex values:

    00 00 12 2F 3C 07

when encoded as Base 64 yields the ASCII string:

    AASv8H


6-bit values in an 8-bit world
==============================

Modern computers, almost universally, use an 8-bit byte.  Base64 is used
to map an 8-bit byte stream to a set of 64 universally printable
characters.

In order to do this, the 8-bit *byte* stream must be converted into a
*bit* stream.  This bit stream must then be converted into a stream of
6-bit bytes, as in the following figure:

    |   00    |   04    |   AF    |   F0    |   74    |   14    |
    |         |         |         |         |         |         |
    |000000 00|0000 0100|10 101111|111100 00|0111 0100|00 010100|
    |                                                           |
    |000000|00 0000|0100 10|101111|111100|00 0111|0100 00|010100|
    |      |       |       |      |      |       |       |      |
    |  00  |  00   |  12   |  2F  |  3C  |  07   |  10   |  14  |

The resulting 6-bit byte stream is then mapped into the ASCII alphabet
given in the previous section, resulting in a "Base64-encoded string":

    AASv8HQU


Question
========

The key for this page is the decode of this Base64-encoded string:

    Z3JlYXQgam9i