diff --git a/casper4/rlp_decoder.se b/casper4/rlp_decoder.se
new file mode 100644
index 0000000..2f2c65d
--- /dev/null
+++ b/casper4/rlp_decoder.se
@@ -0,0 +1,64 @@
+macro calldatachar($x):
+    div(calldataload($x), 2**248)
+
+macro calldatabytes_as_int($x, $b):
+    div(calldataload($x), 256**(32-$b))
+
+def any():
+    positions = array(32)
+    positionIndex = 0
+    data = string(1024)
+    dataPos = 0
+    # Can only parse lists
+    c = calldatachar(0)
+    if c < 192:
+        ~invalid()
+    if c < 248:
+        if ~calldatasize() != 1 + (c - 192):
+            ~invalid()
+        i = 1
+    else:
+        L = calldatabytes_as_int(i + 1, c - 247)
+        if ~calldatasize() != 1 + (c - 247) + L:
+            ~invalid()
+        i = 1 + (c - 247)
+    while i < ~calldatasize():
+        c = calldatachar(i)
+        positions[positionIndex] = dataPos
+        positionIndex += 1
+        if c < 128:
+            calldatacopy(data + dataPos, i, 1)
+            i += 1
+            dataPos += 1
+        elif c < 184:
+            calldatacopy(data + dataPos, i + 1, c - 128)
+            # Output could have been in single-byte format
+            if c == 129:
+                if calldatachar(i + 1) < 128:
+                    ~invalid()
+            i += c - 128 + 1
+            dataPos += (c - 128)
+        elif c < 192:
+            L = calldatabytes_as_int(i + 1, c - 183)
+            # Forbid leading zero byte
+            if calldatachar(i + 1) == 0:
+                ~invalid()
+            # Forbid too short values
+            if L < 56:
+                ~invalid()
+            calldatacopy(data + dataPos, i + 1 + c - 183, L)
+            i += (c - 183) + 1 + L
+            dataPos += L
+        else:
+            # Not handling nested arrays
+            ~invalid()
+        if dataPos > 1024 or positionIndex > 32:
+            ~invalid()
+    positions[positionIndex] = dataPos
+    output = string(2048)
+    i = 0
+    while i <= positionIndex:
+        output[i] = positions[i] + positionIndex * 32 + 32
+        i += 1
+    mcopy(output + positionIndex * 32 + 32, data, dataPos)
+    ~return(output, positionIndex * 32 + dataPos + 32)
diff --git a/casper4/remove_rlp_last.se b/casper4/sighash.se
similarity index 93%
rename from casper4/remove_rlp_last.se
rename to casper4/sighash.se
index 8ff70e3..b0798a6 100644
--- a/casper4/remove_rlp_last.se
+++ b/casper4/sighash.se
@@ -45,7 +45,7 @@ newmemindex = 1000
 if newlen < 56:
     ~mstore8(newmemindex, 192 + newlen)
     ~calldatacopy(newmemindex + 1, startpos, newlen)
-    ~return(newmemindex, 1 + newlen)
+    return(~sha3(newmemindex, 1 + newlen))
 else:
     _log = 0
     _newlen = newlen
@@ -55,4 +55,4 @@ else:
     ~mstore8(newmemindex, 247 + _log)
     ~mstore(newmemindex + 1, newlen * (256 ** (32 - _log)))
     ~calldatacopy(newmemindex + 1 + _log, startpos, newlen)
-    ~return(newmemindex, 1 + _log + newlen)
+    return(~sha3(newmemindex, 1 + _log + newlen))
diff --git a/casper4/simple_casper.v.py b/casper4/simple_casper.v.py
index bce6c1d..a8d3c7f 100644
--- a/casper4/simple_casper.v.py
+++ b/casper4/simple_casper.v.py
@@ -82,7 +82,8 @@ def __init__():
     self.interest_rate = 0.000001
     self.block_time = 7
     self.epoch_length = 256
-    self.withdrawal_delay = 2500000
+    # Only ~11.5 days, for testing purposes
+    self.withdrawal_delay = 1000000
     # Temporary backdoor for testing purposes (to allow recovering destroyed deposits)
     self.owner = 0x1db3439a222c519ab44bb1144fc28167b4fa6ee6
     # Add an initial validator
@@ -99,7 +100,7 @@ def __init__():
     # Initialize the epoch counter
     self.current_epoch = block.number / self.epoch_length
     # Set the RLP decoder address
-    self.rlp_decoder = 0x2dadb850a35439906b82be11dfe3e00d5f944213
+    self.rlp_decoder = 0x38920146f10f3956fc09970beededcb2d9638711
 
 # Called at the start of any epoch
 def initialize_epoch(epoch: num):
@@ -166,8 +167,20 @@ def withdraw(index: num):
 # Process a prepare message
 def prepare(index: num, epoch: num, hash: bytes32, ancestry_hash: bytes32,
             epoch_source: num, source_ancestry_hash: bytes32, sig: bytes <= 96):
-    # Signature check
-    sighash = sha3(concat("prepare", as_bytes32(epoch), hash, ancestry_hash, as_bytes32(epoch_source), source_ancestry_hash))
+    # Get hash for signature, and implicitly assert that it is an RLP list
+    # consisting solely of RLP elements
+    sighash = extract32(raw_call(self.sighasher, self.prepare_msg, gas=50000, outsize=32), 0)
+    values = raw_call(self.rlp_decoder, self.prepare_msg, gas=50000, outsize=2048)
+    # Extract parameters
+    epoch = extract32(values, extract32(value, 0, type=num128), type=num128)
+    hash = extract32(values, extract32(value, 32, type=num128), type=bytes32)
+    ancestry_hash = extract32(values, extract32(value, 64, type=num128), type=bytes32)
+    source_epoch = extract32(values, extract32(value, 96, type=num128), type=num128)
+    source_ancestry_hash = extract32(values, extract32(value, 128, type=num128), type=bytes32)
+    sig = slice(values, start=extract32(value, 160, type=num128), end=extract32(value, 192, type=num128))
+    # Assert that there are only 6 elements
+    assert as_num128(extract32(value, 0)) == 224
+    # For now, the sig is a simple ECDSA sig
     assert len(sig) == 96
     assert ecrecover(sighash,
                      as_num256(extract32(sig, 0)),