diff --git a/RTL/program3_tb.sv b/RTL/program3_tb.sv
index 7182cac..73743a7 100644
--- a/RTL/program3_tb.sv
+++ b/RTL/program3_tb.sv
@@ -126,7 +126,7 @@ module program3_tb ()        ;
       dut.DM.core[m+64] = msg_crypto1[m];
     for(int n=24; n<64; n++) begin	  	// load subsequent, possibly corrupt, encrypted message into data memory
 // set flipper = 8 or higher to disable bit corruption
-	  flipper = 8;//$random;                // value between 0 and 63, inclusive
+	  flipper = $random;//$random;                // value between 0 and 63, inclusive
       dut.DM.core[n+64] = msg_crypto1[n]^(1<<flipper);
       if(flipper<8) flipped[n]=1;
 	end
diff --git a/firmware/assembler.py b/firmware/assembler.py
index 26eb2a7..c59c432 100644
--- a/firmware/assembler.py
+++ b/firmware/assembler.py
@@ -119,6 +119,9 @@ for file in targets:
 			line = line.replace('\t', '') # remove leading tabs
 			line = line.replace('\n', '') # remove trailing newline
 			if ': ' in line:
+				if line.split(': ')[0] in labels:
+					print('dublicate label "' + line.split(': ')[0] + '" detected')
+					exit(1)
 				labels[line.split(': ')[0]] = index # ': ' must be used to end a label
 				no_comments.append(line.split(': ')[1])
 			else:
diff --git a/firmware/program3.asm b/firmware/program3.asm
index 2522ab1..347d87d 100644
--- a/firmware/program3.asm
+++ b/firmware/program3.asm
@@ -88,17 +88,35 @@ finish_preamble: LDI lfsr_routine
 	LDI #d32 // get value of space
 	SUB r1 // compare if r1 == 32
 	JEZ r2 // jump to finish preamble loop if this plaintext == space(32)
-	CLB r1 // clear leading bit of plaintext
-	GET r1 // get r1 to r0
-	STW r12 // store plaintext
-	NXT r12 // increment write only if we found the first non preamble char
+	LDI correct_pre
+	PUT r2 // put correct handler address in r2
+	CHK r1 // check r1 for errors
+	JEZ r2
+	error_pre: LDI #x80
+		STW r12
+		LDI common
+		JMP r0
+	correct_pre: CLB r1
+		GET r1
+		STW r12
+	common_pre: NXT r12 // increment write only if we found the first non preamble char
 main_loop: LDI lfsr_routine // load address for the lfsr_routine label
 	JAL r0 // jump to the lfsr_routine label
 	LDW r11 // load the next ciphertext byte
-	XOR r7 // bitwise XOR the current state with ciphertext space to generate plaintext
-	CLB r0 // clear the leading bit of the plaintext as in requirements
-	STW r12 // store plaintext to write pointer
-	NXT r11 // increment read pointer
+	PUT r1 // store ciphertext in r1
+	LDI correct
+	PUT r2 // load address of correct handler in r2
+	CHK r1 // check r1(ciphertext) for errors
+	JEZ r2 // if there are no errors, jump to correct handler, otherwise continue to error handler
+	error: LDI #x80
+		STW r12
+		LDI common
+		JMP r0
+	correct: GET r1 // retrieve ciphertext from r1
+		XOR r7 // bitwise XOR the current state with ciphertext space to generate plaintext
+		CLB r0 // clear the leading bit of the plaintext as in requirements
+		STW r12 // store plaintext to write pointer
+	common: NXT r11 // increment read pointer
 	NXT r12 // increment write pointer
 	LDI finish_post // load address of label done
 	NXT r9 // decrement number of remaining plaintext chars