Off-Chain Message Signing with the Openos CLI

Off-chain message signing is a method of signing non-transaction messages with a Openos wallet. This feature can be used to authenticate users or provide proof of wallet ownership.

Sign Off-Chain Message

To sign an arbitrary off-chain message, run the following command:

openos sign-offchain-message <MESSAGE>

The message will be encoded and signed with CLI's default private key and signature printed to the output. If you want to sign it with another key, just use the -k/--keypair option:

openos sign-offchain-message -k <KEYPAIR> <MESSAGE>

By default, the messages constructed are version 0, the only version currently supported. When other versions become available, you can override the default value with the --version option:

openos sign-offchain-message -k <KEYPAIR> --version <VERSION> <MESSAGE>

The message format is determined automatically based on the version and text of the message.

Version 0 headers specify three message formats allowing for trade-offs between compatibility and composition of messages:

ID	Encoding	Maximum Length	Hardware Wallet Support
0	Restricted ASCII *	1212	Yes
1	UTF-8	1212	Blind sign only
2	UTF-8	65515	No

* Those characters for which isprint(3) returns true. That is, 0x20..=0x7e.

Formats 0 and 1 are motivated by hardware wallet support where both RAM to store the payload and font character support are limited.

To sign an off-chain message with Ledger, ensure your Ledger is running latest firmware and Openos Ledger App version 1.3.0 or later. After Ledger is unlocked and Openos Ledger App is open, run:

openos sign-offchain-message -k usb://ledger <MESSAGE>

For more information on how to setup and work with the ledger device see this link.

Please note that UTF-8 encoded messages require Allow blind sign option enabled in Openos Ledger App. Also, due to the lack of UTF-8 support in Ledger devices, only the hash of the message will be displayed in such cases.

If Display mode is set to Expert, Ledger will display technical information about the message to be signed.

Verify Off-Chain Message Signature

To verify the off-chain message signature, run the following command:

openos verify-offchain-signature <MESSAGE> <SIGNATURE>

The public key of the default CLI signer will be used. You can specify another key with the --signer option:

openos verify-offchain-signature --signer <PUBKEY> <MESSAGE> <SIGNATURE>

If the signed message has a version different from the default, you need to specify the matching version explicitly:

openos verify-offchain-signature --version <VERSION> <MESSAGE> <SIGNATURE>

Protocol Specification

To ensure that off-chain messages are not valid transactions, they are encoded with a fixed prefix: \xffopenos offchain, where first byte is chosen such that it is implicitly illegal as the first byte in a transaction MessageHeader today. More details about the payload format and other considerations are available in the proposal.