Factor out common key representation

Having this twice causes confusion about whether they are actually the
same. They are.

content/appendices.md

@@ -940,6 +940,30 @@ The acceptable character set matches the unreserved character set in [RFC
 3986](https://datatracker.ietf.org/doc/html/rfc3986#section-2.3).
 {{% /boxes/note %}}
 
+## Cryptographic key representation
+
+Sometimes it is necessary to present a private cryptographic key in the user
+interface.
+
+When this happens, the key should be presented as a string formatted as
+follows:
+
+1.  The raw key is prepended by the two bytes `0x8B` and `0x01`.
+2.  All the bytes in the array above, including the two header bytes,
+    are XORed together to form a parity byte. This parity byte is
+    appended to the byte array.
+3.  The byte array is encoded using base58, using the the alphabet
+    `123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz`.
+4.  A space is added after every 4th character.
+
+When reading in a key, clients should disregard whitespace, and
+perform the reverse of steps 1 through 4.
+
+{{% boxes/note %}}
+The base58 alphabet is the same as that used for [Bitcoin
+addresses](https://en.bitcoin.it/wiki/Base58Check_encoding#Base58_symbol_chart).
+{{% /boxes/note %}}
+
 ## 3PID Types
 
 Third-party Identifiers (3PIDs) represent identifiers on other

content/client-server-api/modules/end_to_end_encryption.md

@@ -674,7 +674,7 @@ The process between Alice and Bob verifying each other would be:
     their devices if they match or not.
 15. Assuming they match, Alice and Bob's devices each calculate Message
     Authentication Codes (MACs) for:
-    * Each of the keys that they wish the other user to verify (usually their 
+    * Each of the keys that they wish the other user to verify (usually their
       device ed25519 key and their master cross-signing key).
     * The complete list of key IDs that they wish the other user to verify.
 
@@ -1315,23 +1315,8 @@ replace it with the new key based on the key metadata as follows:
 ###### Recovery key
 
 If the recovery key (the private half of the backup encryption key) is
-presented to the user to save, it is presented as a string constructed
+given to the user, the key should be presented as a string using the common [cryptographic key
-as follows:
+representation](/appendices/#cryptographic-key-representation).
-
-1.  The 256-bit curve25519 private key is prepended by the bytes `0x8B`
-    and `0x01`
-2.  All the bytes in the string above, including the two header bytes,
-    are XORed together to form a parity byte. This parity byte is
-    appended to the byte string.
-3.  The byte string is encoded using base58, using the same [mapping as
-    is used for Bitcoin
-    addresses](https://en.bitcoin.it/wiki/Base58Check_encoding#Base58_symbol_chart),
-    that is, using the alphabet
-    `123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz`.
-4.  A space should be added after every 4th character.
-
-When reading in a recovery key, clients must disregard whitespace, and
-perform the reverse of steps 1 through 3.
 
 The recovery key can also be stored on the server or shared with other devices
 using the [Secrets](#secrets) module. When doing so, it is identified using the

content/client-server-api/modules/secrets.md

@@ -262,22 +262,8 @@ For example, data encrypted using this algorithm could look like this:
 ##### Key representation
 
 When a user is given a raw key for `m.secret_storage.v1.aes-hmac-sha2`,
-it will be presented as a string constructed as follows:
+the key should be presented as a string using the common [cryptographic key
-
+representation](/appendices/#cryptographic-key-representation).
-1.  The key is prepended by the two bytes `0x8b` and `0x01`
-2.  All the bytes in the string above, including the two header bytes,
-    are XORed together to form a parity byte. This parity byte is
-    appended to the byte string.
-3.  The byte string is encoded using base58, using the same [mapping as
-    is used for Bitcoin
-    addresses](https://en.bitcoin.it/wiki/Base58Check_encoding#Base58_symbol_chart),
-    that is, using the alphabet
-    `123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz`.
-4.  The string is formatted into groups of four characters separated by
-    spaces.
-
-When decoding a raw key, the process should be reversed, with the
-exception that whitespace is insignificant in the user's input.
 
 ##### Deriving keys from passphrases