# title£ºEncrypt and Decrypt messages with a new way
# purpose: the code for Task2.2 & 2.3
# author: Siyu Chen
# Date: 2021/12/1
# N.B. : Some steps can be merged to one step,but I retain them to show my ideas more clearly.
#        I really hope I don't confuse you.

#Task2.2
# create matrix, define the amounts of rows and columns
Polybius<-matrix(nrow=4,ncol=10)
# input the elements of per rows
Polybius[1,]<-c(NA,'Z','M','D',NA,'K','W','Q',NA,'G')
Polybius[2,]<-c('V',NA,'A',NA,'R',NA,'E',NA,'T','O')
Polybius[3,]<-c(NA,'J','I','S','B','P','Y',NA,'F',NA)
Polybius[4,]<-c('L',NA,NA,'C',NA,'U','H','N',NA,'X')
# create numerical labels of rows and columns
rownames(Polybius)<-c(0:3)
colnames(Polybius)<-c(0:9)
# display the matrix
Polybius

# Encrypt keyword
keyword_temp<-NULL
for(k in 1:5){
  c<-substring('PEACH',k,k) # extract character one by one
  for(i in 1:4){  # i replace row
    for (j in 1:10){  # j replace column
      if(!is.na(Polybius[i,j])){  # filter the NA
        if(Polybius[i,j]==c){
          keyword_temp<-c(keyword_temp,i-1)
          keyword_temp<-c(keyword_temp,j-1)
        }
      }
    }
  }
}
keyword_temp
# convert keyword_temp to numbers of each character
keyword_temp_x=NULL
keyword_temp_y=NULL
for(i in seq(keyword_temp)){
  if(!i%%2==0){
    keyword_temp_x<-c(keyword_temp_x,keyword_temp[i])
  }else{
    keyword_temp_y<-c(keyword_temp_y,keyword_temp[i])
  }
}
keyword_encrypt<-10*keyword_temp_x+keyword_temp_y
keyword_encrypt

# Encrypt message
ciphertext_temp1<-NULL
for(k in 1:12){
  c<-substring('RAISETHEFLAG',k,k) # extract character one by one
  for(i in 1:4){  # i replace row
    for (j in 1:10){  # j replace column
      if(!is.na(Polybius[i,j])){  # filter the NA
        if(Polybius[i,j]==c){
          ciphertext_temp1<-c(ciphertext_temp1,i-1)
          ciphertext_temp1<-c(ciphertext_temp1,j-1)
        }
      }
    }
  }
}
ciphertext_temp1

# convert ciphertext_temp1 to numbers of each character
ciphertext_temp1_x=NULL
ciphertext_temp1_y=NULL
for(i in seq(ciphertext_temp1)){
  if(!i%%2==0){
    ciphertext_temp1_x<-c(ciphertext_temp1_x,ciphertext_temp1[i])
  }else{
    ciphertext_temp1_y<-c(ciphertext_temp1_y,ciphertext_temp1[i])
  }
}
ciphertext_temp2<-10*ciphertext_temp1_x+ciphertext_temp1_y
ciphertext_temp2

# encrypt with keyword_encrypt
ciphertext_temp3<-ciphertext_temp2+keyword_encrypt
ciphertext_temp3

# reconvert ciphertext_temp3 to each digit
ciphertext_temp3_x=NULL
ciphertext_temp3_y=NULL
ciphertext_temp4=NULL
for(i in seq(ciphertext_temp3)){
  ciphertext_temp3_x<-c(ciphertext_temp3_x,ciphertext_temp3[i]%/%10)
  ciphertext_temp4<-c(ciphertext_temp4,ciphertext_temp3_x[i])
  ciphertext_temp3_y<-c(ciphertext_temp3_y,ciphertext_temp3[i]%%10)
  ciphertext_temp4<-c(ciphertext_temp4,ciphertext_temp3_y[i])
}
ciphertext_temp4

# add " "
count=0 
ciphertext_temp5<-NULL
for(i in 1:length(ciphertext_temp4)){
  if(count!=0 & (count)%%5==0) ciphertext_temp5<-c(ciphertext_temp5,' ')
  ciphertext_temp5<-c(ciphertext_temp5,ciphertext_temp4[i])
  count<-count+1
}
ciphertext_temp5

# fill the final block up to 5 digits
ciphertext<-NULL
if(length(ciphertext_temp5)%%6!=0){
  num<-5-length(ciphertext_temp5)%%6
  x<-sample(0:9,num,replace=TRUE)
  ciphertext<-c(ciphertext_temp5,x)
}
# display the encrypted message
ciphertext

# convert ciphertext to a vector contains one element
len=length(ciphertext)%/%6+1
finaltext_temp=NULL
finaltext=NULL
for(h in 1:len){
  finaltext_temp<-paste(ciphertext[(6*h-5):(6*h-1)],collapse="")
  finaltext<-paste(finaltext,finaltext_temp,collapse=" ")
}
# display the finaltext
print(finaltext)

# ------the output of Task2.2 by steps (just for reading ; can be deleted)------
Polybius
keyword_temp
keyword_encrypt
ciphertext_temp1
ciphertext_temp2
ciphertext_temp3
ciphertext_temp4
ciphertext_temp5
ciphertext
finaltext

#Task2.3
# read ciphertext
ciphertext<-c(4,8,5,2,1,
              8,2,5,5,9,
              5,3,3,2,0,
              8,3,1,5,1,
              4,4,3,8,2,
              1,3,3,4,9,
              4,0,3,5,2,
              4,2,2,6,0,
              4,8,5,2,1,
              8,4,3,4,9,
              4,8,3,4,3,
              2,3,5,1,8
)
# let the length of ciphertext to be even
len=length(ciphertext)-(length(ciphertext)%%2)
# seperate row-coordinate and column-coordinate
ciphertext_x=NULL
ciphertext_y=NULL
for(i in 1:len){
  if(!i%%2==0){
    ciphertext_x<-c(ciphertext_x,ciphertext[i])
  }else{
    ciphertext_y<-c(ciphertext_y,ciphertext[i])
  }
}
ciphertext_temp1<-10*ciphertext_x+ciphertext_y
ciphertext_temp1

# decipher the keyword
keyword_temp<-NULL
for(k in 1:5){
  c<-substring('LEMON',k,k) # extract character one by one
  for(i in 1:4){  # i replace row
    for (j in 1:10){  # j replace column
      if(!is.na(Polybius[i,j])){  # filter the NA
        if(Polybius[i,j]==c){
          keyword_temp<-c(keyword_temp,i-1)
          keyword_temp<-c(keyword_temp,j-1)
        }
      }
    }
  }
}
keyword_temp
# convert keyword_temp to numbers of each character
keyword_temp_x=NULL
keyword_temp_y=NULL
for(i in seq(keyword_temp)){
  if(!i%%2==0){
    keyword_temp_x<-c(keyword_temp_x,keyword_temp[i])
  }else{
    keyword_temp_y<-c(keyword_temp_y,keyword_temp[i])
  }
}
keyword_encrypt<-10*keyword_temp_x+keyword_temp_y
keyword_encrypt

# decrypt with keyword_encrypt
ciphertext_temp2<-ciphertext_temp1-keyword_encrypt
ciphertext_temp2

# reconvert ciphertext_temp3 to each digit
ciphertext_temp2_x=NULL
ciphertext_temp2_y=NULL
ciphertext_temp3=NULL
for(i in seq(ciphertext_temp2)){
  ciphertext_temp2_x<-c(ciphertext_temp2_x,ciphertext_temp2[i]%/%10)
  ciphertext_temp3<-c(ciphertext_temp3,ciphertext_temp2_x[i])
  ciphertext_temp2_y<-c(ciphertext_temp2_y,ciphertext_temp2[i]%%10)
  ciphertext_temp3<-c(ciphertext_temp3,ciphertext_temp2_y[i])
}
ciphertext_temp3
# filter the negative and the following numbers
ciphertext_temp4=NULL
for(i in seq(ciphertext_temp3)){
  if(ciphertext_temp3[i]<0) break
  ciphertext_temp4<-c(ciphertext_temp4,ciphertext_temp3[i])
}
ciphertext_temp4

# seperate row-coordinate and column-coordinate
len=length(ciphertext_temp4)-(length(ciphertext_temp4)%%2)
ciphertext_temp4_x=NULL
ciphertext_temp4_y=NULL
for(i in 1:len){
  if(!i%%2==0){
    ciphertext_temp4_x<-c(ciphertext_temp4_x,ciphertext_temp4[i])
  }else{
    ciphertext_temp4_y<-c(ciphertext_temp4_y,ciphertext_temp4[i])
  }
}

# decipher the message
message_temp=NULL
for(i in seq(ciphertext_temp4_x)){
  message_temp<-c(message_temp,Polybius[ciphertext_temp4_x[i]+1,ciphertext_temp4_y[i]+1])
}
message_temp
# convert it to a vector which contains one element that holds the entire message
plaintext<-paste(message_temp,collapse="")
plaintext

# ------the output of Task2.3 by steps (just for reading ; can be deleted)-------
ciphertext
ciphertext_temp1
keyword_temp
keyword_encrypt
ciphertext_temp2
ciphertext_temp3
ciphertext_temp4
message_temp
plaintext