added simple code example of mceliece cryptosystem

1 files changed, 325 insertions, 0 deletions

diff --git a/buch/papers/mceliece/example_code/mceliece_simple.py b/buch/papers/mceliece/example_code/mceliece_simple.py
new file mode 100644
index 0000000..3f17504
--- /dev/null
+++ b/buch/papers/mceliece/example_code/mceliece_simple.py
@@ -0,0 +1,325 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+"""
+Created on Sun Apr 18 08:58:30 2021
+
+@author: terminator
+"""
+import numpy as np
+from numpy.polynomial import Polynomial as Poly
+
+
+def gen_perm_M(n):
+    '''
+    generating random binary permutation matrix: https://en.wikipedia.org/wiki/Permutation_matrix 
+
+    Parameters
+    ----------
+    n : int
+        size of output matrix.
+
+    Returns
+    -------
+    perm_M : np.ndarray
+        binary permutation matrix.
+
+    '''
+    perm_M=np.zeros((n, n), dtype=int)
+    perm_V=np.random.default_rng().permutation(n)
+    perm_M[perm_V, np.r_[0:n]]=1
+    return perm_M
+
+
+
+def create_linear_code_matrix(n, k, g):
+    '''
+    create generator matrix for linear encoding
+    use this matrix to create code_vector: matrix @ data_vector=code_vector
+
+    Parameters
+    ----------
+    n : int
+        len of code_vector (matrix @ data_vector).
+    k : int
+        len of data_vector.
+    g : numpy.Polynominal
+        polynomina wich defines the the constellations of .
+
+    Returns
+    -------
+    np.ndarray
+        Generator matrix
+
+    '''
+    if n != k+len(g)-1:
+        raise Exception("n, k not compatible with degree of g")
+    rows=[]
+    for i in range(k): #create potences of p
+        row=np.r_[np.zeros(i), g.coef, np.zeros(n-k-i)]
+        rows.append(row)
+    return np.array(rows, dtype=int).T
+
+
+def gf2_inv(M, get_full=False):
+    '''
+    create inverse of matrix M in gf2
+
+    Parameters
+    ----------
+    M : numpy.ndarray
+        input Matrix.
+    get_full : Bool, optional
+        if Ture, return inverse of G with I on the left (if gaussian inversion successful) If True, vaidity is not proven. The default is False.
+
+    Returns
+    -------
+    np.ndarray or None
+        returns inverse if M was not singular in gf2, else None.
+
+    '''
+    size=len(M)
+    G=np.hstack((M, np.eye(size)))
+    G=np.array(G, dtype=int)
+    for n in range(size): #forward reduction
+        if G[n, n] == 0:    #swap line if necessary
+            for i in range(n+1, size):
+                if G[i, n]:
+                    G[i, :], G[n, :] = G[n, :].copy(), G[i, :].copy()   #swap
+            
+        for i in range(n+1, size): #downward reduction
+            if G[i, n]:
+                G[i, :] = G[i, :] ^ G[n, :]
+    #reached buttom_right with pivo 
+    for n in range(size-1, -1, -1):  #backwards
+        for i in range(n):
+            if G[i, n]:
+                G[i, :]= G[i, :] ^ G[n, :]
+                
+
+                    
+    if get_full:
+        return G
+    else:
+        valid=np.sum(np.abs(G[:, :size]-np.eye(size))) == 0  #reduction successfull when eye left on the left of G        
+        if not valid:
+            return None
+        else:
+            return G[:, size:]
+
+def create_rand_bin_M(n, with_inverse=False):
+    '''
+    create random binary matrix
+
+    Parameters
+    ----------
+    n : int
+        size.
+    with_inverse : bool, optional
+        if False, return only random binary matrix.
+        if True, return also inverse of random martix. for this purpose, random matrix will not be singular.
+        The default is False.
+
+    Returns
+    -------
+    M : TYPE
+        if with_inverse is True: return tuple(random_matrix, inverse_of_random_matrix)
+        else: return random_matrix
+
+    '''
+    inv=None
+    while type(inv) == type(None):  #do it until inversion of m is successful wich means det(M)%2 != 0
+        M=np.random.randint(0,2, (n,n))
+        inv=gf2_inv(M)
+    if with_inverse:
+        return(M, inv)
+    else:
+        return M
+    
+def create_syndrome_table(n, g):
+    '''
+    create syndrome table for correcting errors in linear code
+
+    Parameters
+    ----------
+    n : int
+        len of linear-code code_vector.
+    g : numpy.Polinominal
+        generator polynominal used by linear-code-encoder.
+
+    Returns
+    -------
+    list of error_vectors, one per syndrome. 
+        get the corresponding error_vector by using the value represented by your syndrome as index of this list.
+
+    '''
+    zeros=np.zeros(n, dtype=int)
+    syndrome_table=[0 for i in range(n+1)]
+    syndrome_table[0]=zeros #when syndrome = 0, no bit-error to correct
+    for i in range(n):
+        faulty_vector=zeros.copy()
+        faulty_vector[i]=1
+        q, r=divmod(Poly(faulty_vector), g)
+        r=np.r_[r.coef%2, np.zeros(len(g)-len(r))]
+        index=np.sum([int(a*2**i) for i, a in enumerate(r)])
+        syndrome_table[index]=faulty_vector
+    return np.array(syndrome_table)
+        
+    
+def decode_linear_code(c, g, syndrome_table):
+    '''
+    function to decode codeword encoded with linear_code
+
+    Parameters
+    ----------
+    c : list or np.ndarray
+        code_vector.
+    g : numpy.Polynominal
+        generator polynominal, used to create code_vector.
+    syndrome_table : list of error_vectors
+        if codeword contains an error, syndrome_table is used to correct wrong bit.
+
+    Returns
+    -------
+    numpy.ndarray
+        data_vector.
+
+    '''
+    q, r=divmod(Poly(c), g)
+    q=np.r_[q.coef%2, np.zeros(len(c)-len(q)-len(g)+1)]
+    r=np.r_[r.coef%2, np.zeros(len(g)-len(r))]
+    syndrome_index=np.sum([int(a*2**i) for i, a in enumerate(r)])
+    while syndrome_index > 0:
+        c=c ^ syndrome_table[syndrome_index]
+        q, r=divmod(Poly(c), g)
+        q=np.r_[q.coef%2, np.zeros(len(c)-len(q)-len(g)+1)]
+        r=np.r_[r.coef%2, np.zeros(len(g)-len(r))]
+        syndrome_index=np.sum([int(a*2**i) for i, a in enumerate(r)])
+    return np.array(q, dtype=int)
+    
+def encode_linear_code(d, G):
+    '''
+    uses generator matrix G to encode d
+
+    Parameters
+    ----------
+    d : numpy.ndarray
+        data_vector.
+    G : numpy.ndarray
+        generator matrix.
+
+    Returns
+    -------
+    c : numpy.ndarray
+        G @ d.
+
+    '''
+    c=(G @ d)%2
+    return c
+
+def encrypt(d, pub_key, t):
+    '''
+    encrypt data with public key, adding t bit-errors
+
+    Parameters
+    ----------
+    d : numpy.ndarray or list of numpy.ndarray
+        data_vector or list of data vectors to encrypt.
+    pub_key : numpy.ndarray
+        public key matrix used to encrypt data.
+    t : int
+        number of random errors to add to code_vector.
+
+    Returns
+    -------
+    numpy.ndarray or list of numpy.ndarray (depending on d)
+        encrypted data.
+
+    '''
+    if type(d) in (list,):
+        encrypted_list=[encrypt(data, pub_key, t) for data in d]
+        return encrypted_list
+    else:
+        c = np.array((pub_key @ d)%2, dtype=int)
+        
+        indexes_of_errors=np.random.default_rng().permutation(pub_key.shape[0])[:t] #add t random errors to codeword
+        e=np.zeros(pub_key.shape[0], dtype=int)
+        e[indexes_of_errors]=1
+        c=c ^ e
+        return np.array(c, dtype=int)
+
+def decrypt(c, P_inv, linear_code_decoder, S_inv):
+    '''
+    decrypt encrypted message
+
+    Parameters
+    ----------
+    c : numpy.ndarray or list of numpy.ndarray
+        code_vector or list of code_vectors to decrypt.
+    P_inv : numpy.ndarray
+        inverted permutation matrix.
+    linear_code_decoder : function(x)
+        function to use to decode linear code.
+    S_inv : numpy.ndarray
+        inverted random binary matrix.
+
+    Returns
+    -------
+    numpy.ndarray or list of numpy.ndarray (depending on d)
+        decrypted data.
+
+    '''
+    if type(c) in (list,):
+        decrypted_list=[decrypt(codew, P_inv, linear_code_decoder, S_inv) for codew in c]
+        return decrypted_list
+    else:
+        c=np.array(P_inv @ c, dtype=int)%2
+        d=linear_code_decoder(c)
+        d=(S_inv @ d)%2
+        return np.array(d, dtype=int)
+
+def str_to_blocks4(string):
+    blocks=[]
+    for char in string:
+        bits=[int(value) for value in np.binary_repr(ord(char), 8)[::-1]] #lsb @ index 0
+        blocks.append(np.array(bits[:4], dtype=int)) #lower nibble first
+        blocks.append(np.array(bits[4:], dtype=int))
+    return blocks
+
+def blocks4_to_str(blocks):
+    string=''
+    for i in range(0, len(blocks), 2):
+        char=np.sum([b*2**i for i, b in enumerate(blocks[i])]) + \
+            np.sum([b*2**(i+4) for i, b in enumerate(blocks[i+1])])
+        string+=chr(char)
+    return string
+
+#shared attributes:
+n=7
+k=4
+t=1
+
+#private key(s):
+g=Poly([1,1,0,1])  #generator polynom for 7/4 linear code (from table, 1.0 + 1.0·x¹ + 0.0·x² + 1.0·x³)
+P_M=gen_perm_M(n)    #create permutation matrix
+G_M=create_linear_code_matrix(n, k, g)  #linear code generator matrix
+S_M, S_inv=create_rand_bin_M(k, True) #random binary matrix and its inverse
+P_M_inv=P_M.T #inverse permutation matrix
+
+syndrome_table=create_syndrome_table(n, g) #part of linear-code decoder
+linear_code_decoder=lambda c:decode_linear_code(c, g, syndrome_table)
+
+#public key:
+pub_key=(P_M @ G_M @ S_M)%2
+
+
+msg_tx='Hello World?'
+
+blocks_tx=str_to_blocks4(msg_tx)
+encrypted=encrypt(blocks_tx, pub_key, t)
+
+blocks_rx=decrypt(encrypted, P_M_inv, linear_code_decoder, S_inv)
+msg_rx=blocks4_to_str(blocks_rx)
+
+print(f'msg_rx: {msg_rx}')
+        
+        
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