CH04::encrypted private keys: drop

These were always a bad idea and they've been superceded both in
theory and in practice by HD wallets.

ch04.asciidoc

@@ -1207,68 +1207,6 @@ following sections we will look at advanced forms of keys and addresses,
 such as encrypted private keys, script and multisignature addresses,
 vanity addresses, and paper wallets.
 
-==== Encrypted Private Keys (BIP-38)
-
-((("bitcoin improvement proposals", "Encrypted Private Keys
-(BIP-38)")))((("keys and addresses", "advanced forms", "encrypted
-private keys")))((("public and private keys", "encrypted private
-keys")))((("passwords", "encrypted private keys")))((("security",
-"passwords")))Private keys must remain secret. The need for
-_confidentiality_ of the private keys is a truism that is quite
-difficult to achieve in practice, because it conflicts with the equally
-important security objective of _availability_. Keeping the private key
-private is much harder when you need to store backups of the private key
-to avoid losing it. A private key stored in a wallet that is encrypted
-by a password might be secure, but that wallet needs to be backed up. At
-times, users need to move keys from one wallet to another—to upgrade or
-replace the wallet software, for example. Private key backups might also
-be stored on paper (see <<paper_wallets>>) or on external storage media,
-such as a USB flash drive. But what if the backup itself is stolen or
-lost? These conflicting security goals led to the introduction of a
-portable and convenient standard for encrypting private keys in a way
-that can be understood by many different wallets and bitcoin clients,
-standardized by BIP-38 (see <<appdxbitcoinimpproposals>>).
-
-BIP-38 proposes a common standard for encrypting private keys with a
-passphrase and encoding them with Base58Check so that they can be stored
-securely on backup media, transported securely between wallets, or kept
-in any other conditions where the key might be exposed. The standard for
-encryption uses the Advanced Encryption Standard (AES), a standard
-established by the NIST and used broadly in data encryption
-implementations for commercial and military applications.
-
-A BIP-38 encryption scheme takes as input a bitcoin private key, usually
-encoded in the WIF, as a Base58Check string with the prefix of "5."
-Additionally, the BIP-38 encryption scheme takes a passphrase—a long
-password—usually composed of several words or a complex string of
-alphanumeric characters. The result of the BIP-38 encryption scheme is a
-Base58Check-encoded encrypted private key that begins with the prefix
-+6P+. If you see a key that starts with +6P+, it is encrypted and
-requires a passphrase in order to convert (decrypt) it back into a
-WIF-formatted private key (prefix +5+) that can be used in any wallet.
-Many wallet applications now recognize BIP-38-encrypted private keys and
-will prompt the user for a passphrase to decrypt and import the key.
-Third-party applications, such as the incredibly useful browser-based
-http://bitaddress.org[Bit Address] (Wallet Details tab), can be used to
-decrypt BIP-38 keys.
-
-The most common use case for BIP-38 encrypted keys is for paper wallets
-that can be used to back up private keys on a piece of paper. As long as
-the user selects a strong passphrase, a paper wallet with BIP-38
-encrypted private keys is incredibly secure and a great way to create
-offline bitcoin storage (also known as "cold storage").
-
-Test the encrypted keys in <<table_4-10>> using bitaddress.org to see
-how you can get the decrypted key by entering the passphrase.
-
-[[table_4-10]]
-.Example of BIP-38 encrypted private key
-|=======
-| *Private Key (WIF)* | 5J3mBbAH58CpQ3Y5RNJpUKPE62SQ5tfcvU2JpbnkeyhfsYB1Jcn
-| *Passphrase* | MyTestPassphrase
-| *Encrypted Key (BIP-38)* | 6PRTHL6mWa48xSopbU1cKrVjpKbBZxcLRRCdctLJ3z5yxE87MobKoXdTsJ
-|=======
-
 [[p2sh_addresses]]
 ==== Pay-to-Script Hash (P2SH) and Multisig Addresses