diff --git a/puer.c b/puer.c
index 15b576c..4bb4f97 100644
--- a/puer.c
+++ b/puer.c
@@ -394,15 +394,15 @@ void ccm_encrypt(unsigned char key[16], uint32_t messageindex, unsigned char mes
 
 	// Xor full blocks
 	size_t index = 0;
+	uint32_t counter = 1;
 	for (; index + 16 <= length; index += 16) {
-		// Message blocks are numbered from index 1 onwards
-		ccm_xor_block(&message[index], key_words, messageindex, index + 1);
+		ccm_xor_block(&message[index], key_words, messageindex, counter++);
 	}
 	// Xor partial block, if any
 	if (index < length) {
 		unsigned char fullblock[16];
 		memcpy(fullblock, &message[index], length - index);
-		ccm_xor_block(fullblock, key_words, messageindex, index + 1);
+		ccm_xor_block(fullblock, key_words, messageindex, counter++);
 		memcpy(&message[index], fullblock, length - index);
 	}
 }
@@ -417,15 +417,17 @@ bool ccm_decrypt(unsigned char key[16], uint32_t messageindex, unsigned char mes
 
 	// Xor full blocks
 	size_t index = 0;
+	uint32_t counter = 1;
 	for (; index + 16 <= length; index += 16) {
 		// Message blocks are numbered from index 1 onwards
-		ccm_xor_block(&message[index], key_words, messageindex, index + 1);
+		ccm_xor_block(&message[index], key_words, messageindex, counter++);
 	}
 	// Xor partial block, if any
 	if (index < length) {
 		unsigned char fullblock[16];
+		memset(fullblock, 0, 16);
 		memcpy(fullblock, &message[index], length - index);
-		ccm_xor_block(fullblock, key_words, messageindex, index + 1);
+		ccm_xor_block(fullblock, key_words, messageindex, counter++);
 		memcpy(&message[index], fullblock, length - index);
 	}