ExtendedKey

/// An ExtendedKey is a secret key which can be used to derive new
/// secret keys to blind the commitment of a transaction output.
/// To be usable, a secret key should have an amount assigned to it,
/// but when the key is derived, the amount is not known and must be
/// given.
#[derive(Debug, Clone)]
pub struct ExtendedKey {
    /// Child number of the extended key
    pub n_child: u32,
    /// Root key id
    pub root_key_id: Identifier,
    /// Key id
    pub key_id: Identifier,
    /// The secret key
    pub key: SecretKey,
    /// The chain code for the key derivation chain
    pub chain_code: [u8; 32],
}

注释已经说得很清楚了。它的实质是 SecretKey，通过它可以派生出新的 SecretKey.

ChildKey

pub struct ChildKey {
    /// Child number of the key (n derivations)
    pub n_child: u32,
    /// Root key id
    pub root_key_id: Identifier,
    /// Key id
    pub key_id: Identifier,
    /// The private key
    pub key: SecretKey,
}