The WalletId is a hexadecimal string derived from the wallet’s mnemonic.
It is used by the cardano-wallet server to refer to specific wallets.
For all wallet types, the WalletId is a blake2b-160 hash of something. This hash function produces a 20-byte digest, which becomes 40 hexadecimal digits.
Shelley Wallets
The WalletId is calculated the same way for shared (multi-sig) and non-shared Shelley wallets.
Therefore, each signer in a shared wallet will have a unique WalletId, because they have a different mnemonic.
Full Wallets (the default)
The extended private key is derived from the mnemonic, and then its public key is hashed to produce the WalletId.
$$WalletId = \mathrm{base16}(\mathrm{blake2b_{160}}(\mathrm{xpub}(rootXPrv)))$$
Single Account Wallets
These are wallets for which only an account-level XPrv or XPub is known.
$$WalletId = \mathrm{base16}(\mathrm{blake2b_{160}}(accountXPub))$$
Byron Wallets
The WalletId comes from the extended public key of the wallet root key.
$$WalletId = \mathrm{base16}(\mathrm{blake2b_{160}}(\mathrm{xpub}(rootXPrv)))$$
Example code
This shell script will produce a Shelley WalletId from the mnemonic words provided on stdin.
#!/usr/bin/env bash
xargs \
| cardano-address key from-recovery-phrase Shelley \
| cardano-address key public --with-chain-code \
| bech32 \
| xxd -r -p \
| b2sum -b -l 160 \
| cut -d' ' -f1