bip-0300: Add OP_DRIVECHAIN

Author: luke-jr

bip-0300.mediawiki

@@ -226,7 +226,7 @@ A sidechain fails to activate if:
 
 Otherwise, the sidechain activates (Active is set to TRUE).
 
-In the block in which the sidechain activates, the coinbase MUST include at least one 0-valued OP_TRUE. This output becomes the initial CTIP for the sidechain.
+In the block in which the sidechain activates, the coinbase MUST include at least one 0-valued OP_DRIVECHAIN output. This output becomes the initial CTIP for the sidechain.
 
 
 
@@ -394,7 +394,7 @@ Every time a deposit/withdrawal is made, the old CTIP is spent and a new one is
 
 Every M5 is valid, as long as:
 
-* It has at least one OP_TRUE output -- this becomes the new CTIP.
+* It has at least one OP_DRIVECHAIN output -- this becomes the new CTIP.
 * The new CTIP has '''more''' coins in it, than before.
 
 
@@ -405,16 +405,24 @@ We come, finally, to the critical matter: where users can take their money *out*
 M6 is invalid if:
 
 * The blinded hash of M6 does NOT match one of the approved Bundle-hashes.  (In other words: M6 must first be approved by 13,150 upvotes.)
-* The first output of M6 is NOT an OP_TRUE. (This OP_TRUE becomes the new CTIP. In other words: all non-withdrawn coins are paid back to the sidechain.)
+* The first output of M6 is NOT an OP_DRIVECHAIN. (This OP_DRIVECHAIN becomes the new CTIP. In other words: all non-withdrawn coins are paid back to the sidechain.)
 * The second output is NOT an OP_RETURN script of exactly 10 bytes, of which 8 bytes are a serialized Bitcoin amount.
 * The txn fee of M6 is NOT exactly equal to the amount of the previous bullet point.
 
 Else, M6 is valid. 
 
 (The point of the latter two bullet points, is to allow the bundle hash to cover the L1 transaction fee.)
 
+===OP_DRIVECHAIN===
 
+This proposal adds a single new opcode, OP_DRIVECHAIN, which has strict semantics for usage.
+OP_NOP5 (0xb4) is redefined as OP_DRIVECHAIN if and only if the entire script is OP_DRIVECHAIN followed by a single-byte push and OP_TRUE (exactly 4 bytes).
+The single-byte push contains the sidechain number.
+Note that this is not a "script number", and cannot be OP_1..OP_16 or any other kind of push; it is also unsigned, and must not be padded even if over sidechain number 127.
+The final OP_TRUE is to ensure this change remains a softfork:
+without it, sidechain numbers 0 and 128 would cause the legacy script interpreter to fail.
 
+If an OP_DRIVECHAIN input is spent, the additional rules for M5 or M6 (see above) must be enforced.
 
 ==Backward compatibility==