RSA encrypt/decrypt failing in my own implementation in python

Question

I'm doing a work for school in which I've got to implement rsa generating of public/private keys and encryption/decryption of a binary message. I already generate public/private keys but my encrypt/decrypt functions aren't working. I don't get any execution errors but the message I encrypt isn't the same when I decrypt.

My code for encrypting a block:

def encrypt_block(block,block_size,e,n):
    number = int(block,2) #convert to decimal number
    cipher_number = pow(number,e,n) #method for fastest exponentiation number^e mod n
    cipher_size = len(bin(cipher_number)) - 2
    tmp_text = '{0:b}'.format(cipher_number) 
    while(len(tmp_text)<block_size): #add zeros to left to fill until block_size
        tmp_text = "0" + tmp_text
    return tmp_text

My encryption code:

    block_size = len(bin(n-1)) - 2 #size of encrypted blocks
    text_size = block_size - 5 #size of clear text blocks

    tmp_text = "" #variable for holding current block
    encrypted_message = ""

    for i in data:
        if(len(tmp_text)==text_size): #when a block is complete
            tmp_text = encrypt_block('1'+tmp_text,block_size,e,n) #add '1' so I don t loose left zeros
            encrypted_message += tmp_text
            tmp_text = ""
        if(i == '0' or i == '1'): #just precaution so I won t add other characters
            tmp_text += i


    if(tmp_text != ""): # in case last block isnt the clear text size
        tmp_text = encrypt_block('1'+tmp_text,block_size,e,n) #add '1' so I don t loose left zeros
            encrypted_message += tmp_text

    print encrypted_message

And my decryption method:

    block_size = len(bin(n-1)) - 2

    tmp_text = ""
    decrypted_message = ""

    for i in data:
        if(len(tmp_text) == block_size): 
            number = int(tmp_text,2) 
            plain_number = pow(number,d,n) 
            decrypted_message += '{0:b}'.format(plain_number)[1::] #remove the '1' that I added in all blocks to prevent loosing zeros
        if(i == '1' or i == '0'):
            tmp_text += i

    print decrypted_message

So for example if my message is:

11001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011110011001111001100111100110011

I get this encrypted message (with 64 or plus bits for key size):

0101110010110010100110111010111011111001001101010110010100000110011011101111101111100000011110101101010000000001010110000111010101001000111100000011110110110011111001111000111000101011000000101111000100110100100100010100000000011101111110111101100011110011010001111000000101010010100111010010001000110010100111111000101001010101100010101001010000110010001101000111001111110010110111001000100101001000100100110011010101000111100101100111010110010000101111100111001001100110111110000100100001010100100110110100100011100010010100101000111011101101111110001000010111101111110000100001011100110010101111010010001011101000111100110101110111011100100001000010100011010001010111010000011100111100001110100100011100000101011011000001010001011101011111010110111001111001011001100001010010110000

And when I decrypt I get this:

0000110010111101000001100010110000010000110110111110001010110011100010111010111001100011110101100

Does anyone have any idea why this isn't working?

One bug I noticed - if the cleartext is a multiple of the block size, when you finish the last block, it goes to the "incomplete last block" code instead of the code for a complete block. — user2357112
– user2357112, Commented Dec 3, 2013 at 2:57
I recommend factoring out parts of this code into helper functions. For example, pull out the code for encrypting and decrypting individual blocks. — user2357112
– user2357112, Commented Dec 3, 2013 at 3:01
In the incomplete last block I'm doing the same stuff I did inside my loop, it's just in case I get some "lonely" bits that doesn't have the size I defined for blocks. — patricia
– patricia, Commented Dec 3, 2013 at 3:02
Thank you I'll separate the code. :) I've already tried this methods in so many ways to make it work that I haven't organize my code better. — patricia
– patricia, Commented Dec 3, 2013 at 3:03
Can you use some existing tool and compare your encrypted/decrypted text with a "known-good" result? This should let you drill down and find out if the error in in the encryption or decryption (or both). — andrel
– andrel, Commented Dec 16, 2013 at 9:37

patricia · Accepted Answer · 2023-05-30 14:24:57Z

1

I already found the error. I got a problem with my prime generation. But in this code I've got a problem anyway.

In encryption, I verify if I've got some block to encrypt after my loop finishes. But in decryption I wasn't doing that validation and my last block was never decrypted. I could put the

if(i == '1' or i == '0'):
    tmp_text += i

In the beginning of my for or I'll just put an if in the end so I won't lose my last block.

edited May 30, 2023 at 14:24

answered Dec 26, 2013 at 12:46

patricia

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