diff --git a/src/shamira/fft.py b/src/shamira/fft.py
--- a/src/shamira/fft.py
+++ b/src/shamira/fft.py
@@ -11,7 +11,7 @@ SQUARE_ROOTS = {3: 189, 5: 12, 15: 225, 
 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."""
-	assert n in SQUARE_ROOTS
+	assert n in SQUARE_ROOTS, n
 	w = SQUARE_ROOTS[n]  # primitive N-th square root of 1
 	return list(itertools.accumulate([1]+[w]*(n-1), gfmul))
 
@@ -46,10 +46,10 @@ def compute_inverse(N1, N2):
 	raise ValueError("Failed to find an inverse to {0} mod {1}.".format(N1, N2))
 
 
-def prime_fft(p, divisors):
+def prime_fft(p, divisors, basic_dft=dft):
 	"""https://en.wikipedia.org/wiki/Prime-factor_FFT_algorithm"""
 	if len(divisors) == 1:
-		return complex_dft(p)
+		return basic_dft(p)
 	N = len(p)
 	N1 = divisors[0]
 	N2 = N//N1
@@ -59,11 +59,11 @@ def prime_fft(p, divisors):
 	ys = []
 	for n1 in range(N1):  # compute rows
 		p_ = [p[(n2*N1+n1*N2) % N] for n2 in range(N2)]
-		ys.append(prime_fft(p_, divisors[1:]))
+		ys.append(prime_fft(p_, divisors[1:], basic_dft))
 
 	for k2 in range(N2):  # compute cols
 		p_ = [row[k2] for row in ys]
-		y_ = complex_dft(p_)
+		y_ = basic_dft(p_)
 		for (yi, row) in zip(y_, ys):  # update col
 			row[k2] = yi