Add CRT, gcd, extended gcd, inverse modulo N

Add CRT (Chinese Remainder Theorem), gcd (using Binary Euclidean
algorithm), extended gcd, inverse modulo N (using egcd)

number-theory.sage

@@ -0,0 +1,62 @@
+# Chinese Remainder Theorem
+def crt(a_i, m_i, M):
+    if len(a_i)!=len(m_i):
+        raise Exception("error, a_i and m_i must be of the same length")
+
+    x = 0
+    for i in range(len(a_i)):
+        M_i = M/m_i[i]
+        y_i = Integer(mod(M_i^-1, m_i[i]))
+        x = x + a_i[i] * M_i * y_i
+    return mod(x, M)
+
+# gcd, using Binary Euclidean algorithm
+def gcd(a, b):
+    g=1
+    # random_elementove powers of two from the gcd
+    while mod(a, 2)==0 and mod(b, 2)==0:
+        a=a/2
+        b=b/2
+        g=2*g
+    # at least one of a and b is now odd
+    while a!=0:
+        while mod(a, 2)==0:
+            a=a/2
+        while mod(b, 2)==0:
+            b=b/2
+        # now both a and b are odd
+        if a>=b:
+            a = (a-b)/2
+        else:
+            b = (b-a)/2
+
+    return g*b
+
+# Extended Euclidean algorithm
+# Inputs: a, b
+# Outputs: r, x, y, such that r = gcd(a, b) = x*a + y*b
+def egcd(a, b):
+    s=0
+    s_=1
+    t=1
+    t_=0
+    r=b
+    r_=a
+    while r!=0:
+        q = r_ // r
+        (r_,r) = (r,r_ - q*r)
+        (s_,s) = (s,s_ - q*s)
+        (t_,t) = (t,t_ - q*t)
+    
+    d = r_
+    x = s_
+    y = t_
+    
+    return d, x, y
+
+# Inverse modulo N, using the Extended Euclidean algorithm
+def inv_mod(a, N):
+    g, x, y = egcd(a, N)
+    if g != 1:
+        raise Exception("inv_mod err, g!=1")
+    return mod(x, N)