We recommend all wallets, exchanges, and clients that accept/support Zcash to follow these guidelines to prepare for the upcoming network upgrade. Network upgrades on a bi-annual basis to maintain the Zcash network.

Check that you are running the latest stable version of zcashd
Version verifiability
Clearly state the version of Zcash in a place users can find it. Somewhere inside the client’s user interface, state the protocol name and version number (available from the getblockchaininfo method). This allows users to check what version of Zcash their client is running.
Inform users that a network upgrade is happening before it happens. 4000 blocks (approximately a week) in advance, tell users a network upgrade is happening soon, and that transactions will be unavailable for about an hour at the activation block height.
Defensive transition
Disable the initiation of new transactions starting 24 blocks (approximately one hour) before the activation block-height. If a user sends a transaction right before the upgrade, it is likely to not make it onto the chain. This can cause user confusion and frustration.
Tell users when the upgrade has finished and re-enable initiation of transactions. Notify users with a message or at their next login after the network transition.

Sapling¶

Sapling is a network upgrade that introduces significant efficiency improvements for shielded transactions that will pave the way for broad mobile, exchange and vendor adoption of Zcash shielded addresses.

Transaction formatting
All transactions must use the new transaction format from Sapling onwards. Make sure that you can parse these v4 transactions. Previous formats will not be valid after the Sapling upgrade, so if you create transactions, the v4 format must be used after the upgrade has activated (but not until then). Hardware wallets and SPV clients are particularly affected here.

See ZIP 243. Test vectors for ZIP 243 have been pushed and are being reviewed.

Also see Sapling Protocol Specification.

General Guidelines¶

Using zcashd unmodified¶

If you use the RPC as provided in the zcashd client, which is true for most exchanges and general users of Zcash, you must update your zcashd node to at least version 2.0.0.

Additionally, Sapling introduces new parameters which must be downloaded by running the fetch-params.sh script. These new parameters are placed in the same directory as the older Sprout parameters.

We are still working on finishing Sapling support for our RPC, the changes to the RPC will be detailed in a list here as they become available.

Using custom code to create/sign/send transactions¶

If you manually create transactions, the following changes are critical. Reference section 7.1 of the Sapling specification for complete details:

• The transactions version number MUST be 4.
• The version group ID MUST be 0x892F2085.
• At least one of tx_in_count, nShieldedSpend, and nJoinSplit MUST be nonzero.
• If version ≥ 4 and nShieldedSpend + nShieldedOutput > 0 then:
• Let bvk and SigHash be as defined in §4.12 ‘Balance and Binding Signature (Sapling)’;
• bindingSig MUST represent a valid signature under the transaction binding verification key bvk of SigHash - i.e. BindingSig.Verifybvk(SigHash, bindingSig) = 1.
• If version ≥ 4 and nShieldedSpend + nShieldedOutput = 0, then valueBalance MUST be 0.
• A coinbase transaction MUST NOT have any JoinSplit descriptions, Spend description, or Output descriptions.
• valueBalance MUST be in the range {-MAX_MONEY .. MAX_MONEY}.

In addition, consensus rules associated with each JoinSplit description (§7.2 ‘Encoding of JoinSplit Descriptions’) each Spend description (§7.3 ‘Encoding of Spend Descriptions’) and each Output description (§7.4 ‘Encoding of Output Descriptions’) MUST be followed.

Mining Pools¶

Mining pools running the Stratum protocol will have to make some changes as well.

The hashReserved field in the Stratum Protocol will have to be replaced by the hashFinalSaplingRoot field from the block header (§7.5 ‘Block Header’).

Testing¶

Sapling is currently activated on testnet. To test transactions you’ll want to follow the Testnet Guide.

Until version 2.0.1 is released, currently supported Sapling RPCs are behind the experimental features flag. Developers must specify -experimentalfeatures and -developersapling when starting up their testnet node to interact with Sapling shielded addresses.

Alternatively, developers can use these features in regtest mode.

The next Sapling release, 2.0.1, will have wider range of support for Sapling addresses.

Overwinter¶

Overwinter is the first network upgrade for Zcash. Its purpose is strengthening the protocol for future network upgrades. It includes versioning, replay protection for network upgrades, performance improvements for transparent transactions, a new feature of transaction expiry, and more.

Overwinter activated successfully at block 347500, mined at June 25, 2018 20:42 UTC-04:00

Transaction formatting
All transactions must use the new transaction format from Overwinter and onwards. Make sure that you can parse these “v3” transactions (write a parser for them if you aren’t using our code). Previous formats will not be valid after the Overwinter upgrade, so if you create transactions, the “v3” format must be used after the upgrade has activated (but not until then). Hardware wallets and SPV clients are particularly affected here. See ZIPs 202 and 203 .
Transaction version number
The 4-byte transaction version will have its most significant bit set from Overwinter and onwards, for two-way replay protection of Overwinter and unambiguous transaction parsing of all current and future formats. For example, existing “v1” and “v2” transactions use version numbers “1” and “2”, but “v3” Overwinter transactions will use the unsigned version number “(1 << 31) | 3” in the transaction serialization format. See ZIP 202 .
Version group IDs
A transaction version will be uniquely paired with a version group ID to ensure unambiguous transaction parsing. For example, a “v3” transaction will always have the version group ID “0x03C48270” in its serialization format, even after future network upgrades. See ZIP 202 .
Branch IDs
Each network upgrade has an associated branch ID that identifies its consensus rules. For two-way replay protection, creating transactions will require the branch ID of the current chain tip when signing a transaction (in the BLAKE2b personalization field.) You can obtain the branch ID of any block height from the getblock API. See ZIP 200 .
Signature hashing
There are new SegWit-like features in this upgrade, such as transaction signatures committing to values of the inputs. We suggest reusing code from SegWit (e.g. for hashing transparent outputs) when implementing the new SignatureHash function. See ZIP 143 .
Transaction expiry
We recommend that you do use the default expiry height (20 blocks/~1 hours) and follow these UX guidelines so that Zcash users can develop a consistent expectation of when Zcash transactions expire and what happens. Zee ZIP 203 .

This isn’t an exhaustive list of the changes. Look at the Overwinter Zcash Improvement Proposals (ZIPs) below for complete details on the changes that will be made. The five ZIPs cover network handshaking, transaction format, transaction expiry, signature hashing, and network upgrade mechanisms.

The network upgrade is coordinated via an on-chain activation mechanism.

Zcashd v1.1.0 (and future releases) running protocol version 170005 will activate Overwinter at block 347500 at which point only v3 transactions are processed. Older versions of Zcashd <= 1.0.14, running protocol versions <= 170004, will partition themselves away from the main network into a legacy chain.

Wipeout protection is provided by the new transaction format and signature hashing scheme. Blocks from the legacy chain will not be accepted by the upgraded network. That is, the upgraded network is permanent, and Zcashd v1.1.0 (and future releases) can not reorganize back to the older non-upgraded chain.

Common Issues¶

tx-overwinter-active
This error is simply saying that Overwinter has been activated and your client must be upgraded to the latest version. Upgrade your client and try again. If the issue persists try restarting the client. If this error is appearing on a third party app like a mobile wallet, please file a support request with the developer of the product and let us know in the #user-support channel on the community chat - https://chat.zcashcommunity.com/
mandatory-script-verify-flag-failed

(Script evaluated without error but finished with a false/empty top stack element)

This error has been most commonly seen when using sendrawtransaction. This can be caused by a few things.

1. When creating raw transactions, the signrawtransaction step must be completed correctly. There is a field in signrawtransaction called prevtxs which can be seen here (https://zcash-rpc.github.io/signrawtransaction.html). The prevtxs parameter is optional, but if it is specified, the amount parameter must also be specified. This amount is the total amount of the previous output. Prior to Overwinter the amount parameter was not required, this is a change between Overwinter and the previous version.

2. This issue can also arise in an edge case where a user is signing the transaction from an offline node. If this is the case the offline node must be synced to above the Overwinter activation height, block 347500.

Node sync is stuck before Overwinter activation height

This bug occurs when you are starting a fresh node or restarting a node that is not synced to above the Overwinter activation height (block 347500) and causes the node to sync very slowly. The bug has to do with your node incorrectly banning peer nodes. The end result is your node will sync very slowly as it will not be able to maintain as many connections to other nodes as usual.

This issue has been fixed in 2.0.0. Please update your client to 2.0.0 or above.