Merge branch 'master' of gitlab.com:shockrah/csnotes

312/notes/ciphers.md

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+# Block Ciphers
+
+The main concept here is twofold :
+
+* we take _blocks_ of data and cipher the _blocks_
+* A given key is actually used to generate recursive keys to be further used on the data itself
+
+
+_bs example ahead_
+
+Say we have a key 7 and some data 123456.
+We take the whole data set and chunk it into blocks(for example): 12 34 56.
+
+Let's say our function here is to just add 7 to each block so we do the first step: 
+
+```
+12 + 7 = 19
+Unlike other ciphers we don't reuse 7; instead we use the new thing as both the new key and part of our cipher text
+
+19 + 34 = 53
+Cipher: 1953..
+
+53 + 56 = 109 <= let's pretend that this rolls over 99 and back to 00
+          09  <= like this
+
+Final cipher: 195309
+```
+
+_It should be noted that in practice these functions usually take in huge keys and blocks_.
+
+# Feistal Cipher
+
+Two main components:
+
+1. each _thing_ in the data to cipher is replaced by a _ciphered thing_
+
+2. nothing is added or deleted or replaced in sequence, instead the order of _things_ is changed.
+
+Basically imagine that every _type of thing_ in our data maps to some other _type of thing/thing_ in the data and thus become swapped/reordered.
+
+# DES - Data Encryption Standard
+
+Widely used until about 2001 when AES surpassed it as the newer(ish(kinda)) standard.
+
+DEA was the actual algorithm tho:
+
+* 64 bit blocks
+* 56 bit keys
+* turns a 64-bit input into a 64-bit output (wew)
+* Steps in reverse also reverse the encryption itself

334/homework/3/chap5.txt

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+#
+# Problem lines begin with "#@".
+# For each problem line, enter your answer on the following line.
+# Do not modify this file in any way except to enter your answers.
+#
+
+#@ 1 -- enter 'a', 'b', or 'c' (don't use quotes in any answer)
+b
+#@ 2 -- enter 'a', 'b', or 'c' 
+a
+#@ 3 -- enter 'a', 'b', or 'c' 
+b
+#@ 4 -- enter 'a', 'b', or 'c' 
+a

334/homework/3/chap6.txt

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+#
+# Problem lines begin with "#@".
+# For each problem line, enter your answer on the following line.
+# Do not modify this file in any way except to enter your answers.
+#
+
+#@ 1 -- enter 'a' or 'b'  (don't use quotes in any answer)
+b
+#@ 2 -- enter 'a', 'b', or 'c' 
+a
+#@ 3 -- enter 'T' or 'F'
+F
+#@ 4 -- enter 'T' or 'F'
+F

334/homework/3/msh.c

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+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/wait.h>
+#include <time.h>
+#include <sys/types.h>
+
+#define PROMPT "msh> "
+#define EXIT_CMD "exit"
+#define MAX_BUF 121
+
+// Statuses from builtin responses
+#define RESP_HELP 1
+#define RESP_DATE 2
+#define RESP_ECHO 3
+#define RESP_SHELL 4
+
+void echo(const char*);
+void remove_newline(char*);
+void exit_branch(const char*);
+int  builtin_response(char*);
+
+
+int main(void) {
+	char buffer[MAX_BUF];
+	while(1) {
+		printf("%s", PROMPT);
+		fgets(buffer, MAX_BUF, stdin);
+		// Deal with EOF input
+		if(!strlen(buffer)) {
+			exit(0);
+		}
+		exit_branch(buffer);
+
+		remove_newline(buffer);
+		int status = builtin_response(buffer);
+		// the switch in case we need to add more response flags later
+		switch(status) {
+			case RESP_ECHO: 
+				echo(buffer);
+			case RESP_SHELL: // shell command handler was used
+				continue;
+		}
+		memset(buffer, 0x00, MAX_BUF); // reset buffer after each usage
+	}
+	return 0;
+}
+
+void echo(const char* buf) {
+	printf("%s\n", buf);
+}
+
+
+void remove_newline(char* buf) {
+	char* c = buf;
+	while(*c != '\n') { c++; }
+	*c='\0';
+}
+
+void exit_branch(const char* buf) {
+	if(!strcmp(buf, EXIT_CMD)) {
+		exit(0);
+	}
+}
+
+int builtin_response(char* buffer) {
+#define STRING_DELIMITER " "
+#define TIME_FMT "%D"
+	exit_branch(buffer);	// exit is builtin so here we are
+	if(!strcmp(buffer, "help")) {
+		echo("enter Linux commands, or ‘exit’ to exit");
+		return RESP_HELP;
+	}
+
+	// Printout the date today
+	if(!strcmp(buffer, "date")) {
+		static char date[64];
+		time_t now = time(0);
+		strftime(date, sizeof(date), TIME_FMT, localtime(&now));
+		echo(date);
+		return RESP_DATE;
+	}
+
+	char *tokens[64];
+	int idx = 0;
+	for(char* cur = strtok(buffer, STRING_DELIMITER); cur != NULL; cur=strtok(NULL, STRING_DELIMITER)) { 
+#ifdef DBG_LINE
+		printf("%s ", cur);
+#endif
+		tokens[idx] = cur;
+		idx++;
+	}
+
+	// Execute child process
+	pid_t child = fork();
+	if(child != 0) {
+		int status_child = wait(NULL);
+		if(status_child == 127) {
+			printf("msh: %s: %d", tokens[0], status_child);
+		}
+	}
+	else {
+		execvp(tokens[0], tokens);
+	}
+	// Cleanup pointers
+	for(int i = 0; i< 64; i++) {
+		tokens[i] = NULL;
+	}
+
+}