Implement personal message signing the same as in js sdk

[davidyuk]

https://github.com/aeternity/aepp-sdk-js/blob/b15c6feec8579c2e6e835a4ad78386a92979d30e/src/account/Memory.ts#L68

https://github.com/aeternity/aepp-sdk-js/blob/b15c6feec8579c2e6e835a4ad78386a92979d30e/src/utils/crypto.ts#L109

an old PR on this topic: LedgerHQ#13

[ghallak]

This is already implemented here

ledger-app/src/handlers/sign_msg.rs

Lines 41 to 61 in b057b0f

	let mut data_to_sign = Vec::new();
	data_to_sign.push(SIGN_MAGIC_LEN);
	data_to_sign.extend(SIGN_MAGIC.bytes());
	data_to_sign.push(
	message
	.len()
	.try_into()
	.expect("The size of message to be signed must fit in a u8"),
	);
	data_to_sign.extend(message);
	let hash: [u8; 32] = {
	let mut blake2b = Blake2b_256::new();
	let mut output = [0; 32];
	blake2b.update(&data_to_sign).map_err(|_| AppSW::MsgHashFail)?;
	blake2b.finalize(&mut output).map_err(|_| AppSW::MsgHashFail)?;
	output
	};
	utils::sign(account_number, &hash).ok_or(AppSW::MsgSignFail)

The code above signs a 32-bytes blake2b hash of a sequence of bytes consisting of:

1. SIGN_MAGIC_LEN (1 byte)
2. SIGN_MAGIC string (equivalent to Aeternity Signed Message:\n)
3. The length of the message (1 byte)
4. The actual message

You can read the code to confirm that and close the issue if there's nothing missing.

[davidyuk]

The length of the message (1 byte)

this works only for messages less than 253 chars, we need to support longer messages. For them the length can be multiple bytes, sdk uses varuint-bitcoin to encode it.

[ghallak]

Fixed in this commit fc7baeb