diff --git a/src/shamira/fft.py b/src/shamira/fft.py
--- a/src/shamira/fft.py
+++ b/src/shamira/fft.py
@@ -4,10 +4,22 @@ import itertools
 
 from .gf256 import gfmul, gfpow
 
-# values of n-th square roots
+# divisors of 255 and their factors in natural numbers
+DIVISORS = [3, 5, 15, 17, 51, 85, 255]
+FACTORS = {3: [3], 5: [5], 15: [3, 5], 17: [17], 51: [3, 17], 85: [5, 17], 255: [3, 5, 17]}
+# values of n-th square roots in GF256
 SQUARE_ROOTS = {3: 189, 5: 12, 15: 225, 17: 53, 51: 51, 85: 15, 255: 3}
 
 
+def ceil_size(n):
+	assert n <= DIVISORS[-1]
+	for (i, ni) in enumerate(DIVISORS):
+		if ni >= n:
+			break
+
+	return ni
+
+
 def precompute_x(n):
 	"""Return a geometric sequence [1, w, w**2, ..., w**(n-1)], where w**n==1.
 	This can be done only for certain values of n."""
@@ -73,3 +85,11 @@ def prime_fft(p, divisors, basic_dft=dft
 		for k2 in range(N2):
 			res[(k1*N2*N2_inv+k2*N1*N1_inv) % N] = ys[k1][k2]
 	return res
+
+
+def evaluate(coefs, n):
+	ni = ceil_size(n)
+	extended_coefs = coefs + [0]*(ni-len(coefs))
+	ys = prime_fft(extended_coefs, FACTORS[ni])
+
+	return ys[:n]