diff --git a/src/shamira.py b/src/shamira.py
--- a/src/shamira.py
+++ b/src/shamira.py
@@ -1,12 +1,13 @@
 import os
 import re
 import base64
+import binascii
 
 import gf256
 
 
 def _shareByte(secretB,k,n):
-	assert n<255
+	assert k<=n<255
 	coefs=[int(secretB)]+[int(b) for b in os.urandom(k-1)]
 	points=[gf256.evaluate(coefs,i) for i in range(1,n+1)]
 	return points
@@ -31,8 +32,8 @@ def reconstructRaw(*shares):
 	secretLen=len(shares[0][1])
 	res=[None]*secretLen
 	for i in range(secretLen):
-		bs=[(x,s[i]) for (x,s) in shares]
-		res[i]=(gf256.getConstantCoef(*bs))
+		points=[(x,s[i]) for (x,s) in shares]
+		res[i]=(gf256.getConstantCoef(*points))
 	return bytes(res)
 
 
@@ -73,22 +74,25 @@ def encode(share,encoding="b32"):
 
 
 def decode(share,encoding="b32"):
-	(i,_,shareStr)=share.partition(".")
-	if not shareStr:
-		raise ValueError("bad share format")
-	i=int(i)
-	if encoding=="hex": f=base64.b16decode
-	elif encoding=="b32": f=base64.b32decode
-	else: f=base64.b64decode
-	shareBytes=f(shareStr)
-	return (i,shareBytes)
+	try:
+		(i,_,shareStr)=share.partition(".")
+		i=int(i)
+		if not 1<=i<=255:
+			raise ValueError()
+		if encoding=="hex": f=base64.b16decode
+		elif encoding=="b32": f=base64.b32decode
+		else: f=base64.b64decode
+		shareBytes=f(shareStr)
+		return (i,shareBytes)
+	except (ValueError,binascii.Error):
+		raise ValueError('bad share format: share="{0}", encoding="{1}"'.format(share,encoding))
 
 
 def detectEncoding(shares):
 	classes=[
-		(re.compile(r"\d+\.[0-9A-F]+=*"), "hex"),
-		(re.compile(r"\d+\.[A-Z2-7]+=*"), "b32"),
-		(re.compile(r"\d+\.[A-Za-z0-9+/]+=*"), "b64")
+		(re.compile(r"\d+\.([0-9A-F]{2})+"), "hex"),
+		(re.compile(r"\d+\.([A-Z2-7]{8})*([A-Z2-7]{8}|[A-Z2-7]{2}={6}|[A-Z2-7]{4}={4}|[A-Z2-7]{5}={3}|[A-Z2-7]{7}={1})"), "b32"),
+		(re.compile(r"\d+\.([A-Za-z0-9+/]{4})*([A-Za-z0-9+/]{4}|[A-Za-z0-9+/]{2}={2}|[A-Za-z0-9+/]{3}={1})"), "b64")
 	]
 	for (regexp, res) in classes:
 		if all(regexp.fullmatch(share) for share in shares):