commit 0d1dcef0700764e8143ac6c8f7bd5eb0bd68d976
parent d0743de8b8cac73cc4a977b2d3a9b44a33caf37a
Author: Jared Tobin <jared@jtobin.io>
Date: Sat, 17 Nov 2018 22:47:13 +1300
Update docs.
Diffstat:
| M | docs/s1.md | | | 36 | +++++++++++++++++++----------------- |
1 file changed, 19 insertions(+), 17 deletions(-)
diff --git a/docs/s1.md b/docs/s1.md
@@ -5,12 +5,12 @@
We want to go from hex (i.e., base 16) to base64 (i.e., base.. uh, 64). So
we'll change from the representation:
- .. + a2 * 16^2 + a1 * 16^1 + a0 * 16^0
+ .. + a2 * 16^2 + a1 * 16^1 + a0 * 16^0
for some (decimal-equivalent) coefficients {a0, a1, .. } in the alphabet
0-9a-f, to the representation:
- .. + b2 * 64^2 + b1 * 64^1 + b0 * 64^0
+ .. + b2 * 64^2 + b1 * 64^1 + b0 * 64^0
for other coefficients {b0, b1, .. } in the alphabet A-Za-z0-9\+/.
@@ -27,40 +27,42 @@ equality:
#### 1.2
- $ SOLUTION=$(./bin/fixed_xor $(< data/s1/q2_input.txt) $(< data/s1/q2_against.txt))
- 746865206b696420646f6e277420706c6179
-
-ASCII-encoded output is fun:
-
- $ echo $SOLUTION | xxd -r -p
- the kid don't play
-
-**Background**:
-
Fixed-xor just encrypts by XOR'ing every bit with some corresponding bit.
-Example:
+An example, using `xxd` to output bits this time:
- echo -n 'FAB' | xxd -b && echo -n 'ICE' | xxd -b
+ $ parallel -k 'echo -n {} | xxd -b' ::: FAB ICE
00000000: 01000110 01000001 01000010 FAB
00000000: 01001001 01000011 01000101 ICE
-So XOR-ing 'FAB' with 'ICE' can be done manually:
+So XOR-ing 'FAB' with 'ICE' can be done manually -- one just computes the
+bitwise XOR:
0100 0110 0100 0001 0100 0010
0100 1001 0100 0011 0100 0101
0000 1111 0000 0010 0000 0111
-So in hex that's '0f0207':
+In hex that's '0f0207'; we can use `bc` to calculate it; note it's not
+zero-padded:
$ echo 'obase=16; ibase=2; 000011110000001000000111' | bc
F0207
-You can play things back like so to confirm:
+The `fixed_xor` binary in `./bin` will perform the reverse task here:
$ ./bin/fixed_xor '0f0207' $(echo -n ICE | xxd -p) | xxd -r -p
FAB
+Running `fixed_xor` on the question input:
+
+ $ SOLUTION=$(./bin/fixed_xor $(< data/s1/q2_input.txt) $(< data/s1/q2_against.txt))
+ 746865206b696420646f6e277420706c6179
+
+Note that the ASCII-encoded output is fun:
+
+ $ echo $SOLUTION | xxd -r -p
+ the kid don't play
+
#### 1.3
Fun fact: it's easy to memorize the (approximate) ranking of the most commonly
