diff --git a/infix-parser-left-assoc-2.rkt b/infix-parser-left-assoc-2.rkt
new file mode 100644
index 0000000..ac0a197
--- /dev/null
+++ b/infix-parser-left-assoc-2.rkt
@@ -0,0 +1,102 @@
+;; The first three lines of this file were inserted by DrRacket. They record metadata
+;; about the language level of this file in a form that our tools can easily process.
+#reader(lib "htdp-advanced-reader.ss" "lang")((modname infix-parser-left-assoc-2) (read-case-sensitive #t) (teachpacks ((lib "gui.rkt" "teachpack" "htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "gui.rkt" "teachpack" "htdp")) #f)))
+(define (alt2 parser1 parser2)
+  (lambda (lst)
+    (cond [(not (empty? (parser1 lst))) (parser1 lst)]
+          [else (parser2 lst)])))
+
+(define (alt3 parser1 parser2 parser3)
+  (lambda (lst)
+    (cond [(not (empty? (parser1 lst))) (parser1 lst)]
+          [(not (empty? (parser2 lst))) (parser2 lst)]
+          [else (parser3 lst)])))
+
+(define (seq2 parser1 parser2)
+  (lambda (lst)
+    (cond [(empty? (parser1 lst)) empty]
+          [(empty? (parser2 (second (parser1 lst)))) empty]
+          [else (list (list (first (parser1 lst))
+                            (first (parser2 (second (parser1 lst)))))
+                      (second (parser2 (second (parser1 lst)))))])))
+
+(define (seq3 parser1 parser2 parser3)
+  (lambda (lst)
+    (cond [(empty? (parser1 lst)) empty]
+          [(empty? (parser2 (second (parser1 lst)))) empty]
+          [(empty? (parser3 (second (parser2 (second (parser1 lst)))))) empty]
+          [else (list (list (first (parser1 lst))
+                            (first (parser2 (second (parser1 lst))))
+                            (first (parser3 (second (parser2 (second (parser1 lst)))))))
+                      (second (parser3 (second (parser2 (second (parser1 lst)))))))])))
+
+(define (accept-itm itm)
+  (lambda (lst)
+    (cond [(empty? lst) empty]
+          [(equal? (first lst) itm) (list itm (rest lst))]
+          [else empty])))
+
+(define (accept-digit)
+  (lambda (lst)
+    (cond [(empty? lst) empty]
+          [(or (equal? "0" (first lst))
+               (equal? "1" (first lst))
+               (equal? "2" (first lst))
+               (equal? "3" (first lst))
+               (equal? "4" (first lst))
+               (equal? "5" (first lst))
+               (equal? "6" (first lst))
+               (equal? "7" (first lst))
+               (equal? "8" (first lst))
+               (equal? "9" (first lst)))
+           (list (first lst) (rest lst))]
+          [else empty])))
+
+(define (accept-empty)
+  (lambda (lst)
+    (cond [(empty? lst) (list empty empty)]
+          [else empty])))
+
+
+
+
+
+
+
+(define (infix->prefix str)
+  (infix->prefix-expl (explode str)))
+
+(define (infix->prefix-expl lst)
+  (cond [(empty? (parse-add-sub lst)) empty]
+        [else (tree->prefix (first (parse-add-sub lst)))]))
+
+(define (parse-add-sub lst)
+  (parse-add-sub-x (second (parse-mul-div lst))
+                   (first (parse-mul-div lst))))
+
+(define (parse-add-sub-x lst expr)
+  (cond [(not (empty? ((accept-itm "+") lst))) (parse-add-sub-x (second (parse-mul-div (second ((accept-itm "+") lst))))
+                                                                (list expr "+" (first (parse-mul-div (second ((accept-itm "+") lst))))))]
+        [(not (empty? ((accept-itm "-") lst))) (parse-add-sub-x (second (parse-mul-div (second ((accept-itm "-") lst))))
+                                                                (list expr "-" (first (parse-mul-div (second ((accept-itm "-") lst))))))]
+        [else (list expr lst)]))
+
+(define (parse-mul-div lst)
+  (parse-mul-div-x (second (parse-number lst))
+                   (first (parse-number lst))))
+
+(define (parse-mul-div-x lst expr)
+  (cond [(not (empty? ((accept-itm "*") lst))) (parse-mul-div-x (second (parse-number (second ((accept-itm "*") lst))))
+                                                                (list expr "*" (first (parse-number (second ((accept-itm "*") lst))))))]
+        [(not (empty? ((accept-itm "/") lst))) (parse-mul-div-x (second (parse-number (second ((accept-itm "/") lst))))
+                                                                (list expr "/" (first (parse-number (second ((accept-itm "/") lst))))))]
+        [else (list expr lst)]))
+
+(define parse-number (accept-digit))
+
+
+(define (tree->prefix tree)
+  (cond [(string? tree) tree]
+        [else (string-append (second tree)
+                             (tree->prefix (first tree))
+                             (tree->prefix (third tree)))]))
\ No newline at end of file
diff --git a/infix-parser.rkt b/infix-parser.rkt
new file mode 100644
index 0000000..fbe2043
--- /dev/null
+++ b/infix-parser.rkt
@@ -0,0 +1,89 @@
+;; The first three lines of this file were inserted by DrRacket. They record metadata
+;; about the language level of this file in a form that our tools can easily process.
+#reader(lib "htdp-advanced-reader.ss" "lang")((modname infix-parser) (read-case-sensitive #t) (teachpacks ((lib "gui.rkt" "teachpack" "htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "gui.rkt" "teachpack" "htdp")) #f)))
+(define (alt2 parser1 parser2)
+  (lambda (lst)
+    (cond [(not (empty? (parser1 lst))) (parser1 lst)]
+          [else (parser2 lst)])))
+
+(define (alt3 parser1 parser2 parser3)
+  (lambda (lst)
+    (cond [(not (empty? (parser1 lst))) (parser1 lst)]
+          [(not (empty? (parser2 lst))) (parser2 lst)]
+          [else (parser3 lst)])))
+
+(define (seq2 parser1 parser2)
+  (lambda (lst)
+    (cond [(empty? (parser1 lst)) empty]
+          [(empty? (parser2 (second (parser1 lst)))) empty]
+          [else (list (list (first (parser1 lst))
+                            (first (parser2 (second (parser1 lst)))))
+                      (second (parser2 (second (parser1 lst)))))])))
+
+(define (seq3 parser1 parser2 parser3)
+  (lambda (lst)
+    (cond [(empty? (parser1 lst)) empty]
+          [(empty? (parser2 (second (parser1 lst)))) empty]
+          [(empty? (parser3 (second (parser2 (second (parser1 lst)))))) empty]
+          [else (list (list (first (parser1 lst))
+                            (first (parser2 (second (parser1 lst))))
+                            (first (parser3 (second (parser2 (second (parser1 lst)))))))
+                      (second (parser3 (second (parser2 (second (parser1 lst)))))))])))
+
+(define (accept-itm itm)
+  (lambda (lst)
+    (cond [(empty? lst) empty]
+          [(equal? (first lst) itm) (list itm (rest lst))]
+          [else empty])))
+
+(define (accept-digit)
+  (lambda (lst)
+    (cond [(empty? lst) empty]
+          [(or (equal? "0" (first lst))
+               (equal? "1" (first lst))
+               (equal? "2" (first lst))
+               (equal? "3" (first lst))
+               (equal? "4" (first lst))
+               (equal? "5" (first lst))
+               (equal? "6" (first lst))
+               (equal? "7" (first lst))
+               (equal? "8" (first lst))
+               (equal? "9" (first lst)))
+           (list (first lst) (rest lst))]
+          [else empty])))
+
+(define (accept-empty)
+  (lambda (lst)
+    (cond [(empty? lst) (list empty empty)]
+          [else empty])))
+
+
+
+
+
+
+
+(define (infix->prefix str)
+  (infix->prefix-expl (explode str)))
+
+(define (infix->prefix-expl lst)
+  (cond [(empty? (parse-add-sub lst)) empty]
+        [else (tree->prefix (first (parse-add-sub lst)))]))
+
+(define (parse-add-sub lst)
+  ((alt3 (seq3 parse-mul-div (accept-itm "+") parse-add-sub)
+         (seq3 parse-mul-div (accept-itm "-") parse-add-sub)
+         parse-mul-div) lst))
+
+(define (parse-mul-div lst)
+  ((alt3 (seq3 parse-number (accept-itm "*") parse-mul-div)
+         (seq3 parse-number (accept-itm "/") parse-mul-div)
+         parse-number) lst))
+
+(define parse-number (accept-digit))
+
+(define (tree->prefix tree)
+  (cond [(string? tree) tree]
+        [else (string-append (second tree)
+                             (tree->prefix (first tree))
+                             (tree->prefix (third tree)))]))
\ No newline at end of file
diff --git a/onetimepad.rkt b/onetimepad.rkt
new file mode 100644
index 0000000..90b3af5
--- /dev/null
+++ b/onetimepad.rkt
@@ -0,0 +1,24 @@
+#lang racket
+
+(define c1 (list #x32 #x1b #x61 #x49 #x48 #x1c #x03 #x2a #x1e #x06 #x5e #x32 #x28 #x24 #x5c #x24))
+(define c2 (list #x1a #x12 #x12 #x3c #x48 #x39 #x58 #x01 #x39 #x41 #x50 #x41 #x2a #x18 #x5c #x1c))
+(define c3 (list #x1c #x1c #x22 #x2d #x47 #x5f #x29 #x29 #x37 #x78 #x02 #x26 #x2b #x3e #x08 #x1e))
+
+(define known-str (bytes->list #"js61xumi"))
+(define known-str-len (length known-str))
+
+(define key-part-1 (map bitwise-xor known-str (take c1 known-str-len)))
+(define key-part-2 (map bitwise-xor known-str (drop c2 known-str-len)))
+(define key (append key-part-1 key-part-2))
+
+(define (decrypt bytes key)
+  (map bitwise-xor bytes key))
+
+(define m1 (decrypt c1 key))
+(define m2 (decrypt c2 key))
+(define m3 (decrypt c3 key))
+
+(displayln (list->bytes m1))
+(displayln (list->bytes m2))
+(displayln (list->bytes m3))
+(displayln (list->bytes key))
\ No newline at end of file
diff --git a/parser-utils.rkt b/parser-utils.rkt
new file mode 100644
index 0000000..df871b4
--- /dev/null
+++ b/parser-utils.rkt
@@ -0,0 +1,58 @@
+;; The first three lines of this file were inserted by DrRacket. They record metadata
+;; about the language level of this file in a form that our tools can easily process.
+#reader(lib "htdp-advanced-reader.ss" "lang")((modname parser-utils) (read-case-sensitive #t) (teachpacks ((lib "gui.rkt" "teachpack" "htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "gui.rkt" "teachpack" "htdp")) #f)))
+(define (alt2 parser1 parser2)
+  (lambda (lst)
+    (cond [(not (empty? (parser1 lst))) (parser1 lst)]
+          [else (parser2 lst)])))
+
+(define (alt3 parser1 parser2 parser3)
+  (lambda (lst)
+    (cond [(not (empty? (parser1 lst))) (parser1 lst)]
+          [(not (empty? (parser2 lst))) (parser2 lst)]
+          [else (parser3 lst)])))
+
+(define (seq2 parser1 parser2)
+  (lambda (lst)
+    (cond [(empty? (parser1 lst)) empty]
+          [(empty? (parser2 (second (parser1 lst)))) empty]
+          [else (list (list (first (parser1 lst))
+                            (first (parser2 (second (parser1 lst)))))
+                      (second (parser2 (second (parser1 lst)))))])))
+
+(define (seq3 parser1 parser2 parser3)
+  (lambda (lst)
+    (cond [(empty? (parser1 lst)) empty]
+          [(empty? (parser2 (second (parser1 lst)))) empty]
+          [(empty? (parser3 (second (parser2 (second (parser1 lst)))))) empty]
+          [else (list (list (first (parser1 lst))
+                            (first (parser2 (second (parser1 lst))))
+                            (first (parser3 (second (parser2 (second (parser1 lst)))))))
+                      (second (parser3 (second (parser2 (second (parser1 lst)))))))])))
+
+(define (accept-itm itm)
+  (lambda (lst)
+    (cond [(empty? lst) empty]
+          [(equal? (first lst) itm) (list itm (rest lst))]
+          [else empty])))
+
+(define (accept-digit)
+  (lambda (lst)
+    (cond [(empty? lst) empty]
+          [(or (equal? "0" (first lst))
+               (equal? "1" (first lst))
+               (equal? "2" (first lst))
+               (equal? "3" (first lst))
+               (equal? "4" (first lst))
+               (equal? "5" (first lst))
+               (equal? "6" (first lst))
+               (equal? "7" (first lst))
+               (equal? "8" (first lst))
+               (equal? "9" (first lst)))
+           (list (first lst) (rest lst))]
+          [else empty])))
+
+(define (accept-empty)
+  (lambda (lst)
+    (cond [(empty? lst) (list empty empty)]
+          [else empty])))
\ No newline at end of file
diff --git a/rsa-sign.rkt b/rsa-sign.rkt
new file mode 100644
index 0000000..ad36f28
--- /dev/null
+++ b/rsa-sign.rkt
@@ -0,0 +1,10 @@
+#lang racket
+
+(require math)
+
+(define h 4294967295)
+(define n 67336047553)
+(define e 699863)
+(define d 2891856227)
+
+(with-modulus n (modexpt h d))
\ No newline at end of file