ch4: zooming in, edits

book/source/04-certificates.md

@@ -342,7 +342,7 @@ ECaYswAAAAoJEKqhjLslRoXFZ0cgouNjgeNr0E9W18g4gAIl6FM5SWuQxg12j0S0
 
 We'll now decode this OpenPGP data, and inspect the two packets in detail.
 
-To generate the output, we run the Sequoia-PGP tool `sq`, using the `packet dump` subcommand. The output of `sq` is one block of text, but we'll break the output up into sections here, to discuss the content of each packet:
+To inspect the internal structure of the OpenPGP data, we run the Sequoia-PGP tool `sq`, using the `packet dump` subcommand. The output of `sq` is one block of text, but to discuss the content of each packet we'll break the output up into sections here:
 
 ```text
 $ sq packet dump --hex alice_minimal.priv
@@ -394,11 +394,11 @@ This means that the next part of the packet follows the structure of [Version 6
 - `creation_time: 0x6516eaa6`: "The time that the key was created" (also see [Time Fields](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-time-fields))
 - `pk_algo: 0x1b`: "The public-key algorithm of this key" (decimal value 27, see the list of [Public-Key Algorithms](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-public-key-algorithms))
 - `public_len: 0x00000020`: "Octet count for the following public key material" (in this case, the length of the following `ed25519_public` field)
-- `ed25519_public`: [Algorithm-specific representation](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-algorithm-specific-part-for-ed2) of the public key material (the format is based on the value of `pk_algo`), in this case 32 bytes of the Ed25519 public key
+- `ed25519_public`: [Algorithm-specific representation](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-algorithm-specific-part-for-ed2) of the public key material (the format is based on the value of `pk_algo`), in this case 32 bytes of Ed25519 public key
 
 This concludes the Public Key section of the packet. The remaining data follows the [Secret-Key packet format](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-secret-key-packet-formats):
 
-- `s2k_usage: 0x00`: [This *S2K usage* value](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-secret-key-encryption-s2k-u) specifies that the secret-key data is not encrypted
+- `s2k_usage: 0x00`: This [*S2K usage* value](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-secret-key-encryption-s2k-u) specifies that the secret-key data is not encrypted
 - `ed25519_secret`: [Algorithm-specific representation](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-algorithm-specific-part-for-ed2) of the secret key data (the format is based on the value of `pk_algo`)
 
 [^CTB]: Sequoia uses the term CTB (Cipher Type Byte) to refer to the RFC's [Packet Tag](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-packet-headers)
@@ -493,7 +493,7 @@ The packet tag defines the semantics of the remaining data in the packet. We're
 
 - `version: 0x06`: This is a version 6 signature (some of the following packet format is specific to this signature version).
 - `type: 0x1f`: The [Signature Type](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-signature-types)
-- `pk_algo: 0x1b`: Public-key algorithm (decimal 27 corresponds, to [Ed25519](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-public-key-algorithms))
+- `pk_algo: 0x1b`: Public-key algorithm (decimal 27, corresponds to [Ed25519](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-public-key-algorithms))
 - `hash_algo: 0x0a`: Hash algorithm (decimal 10, corresponds to [SHA2-512](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-hash-algorithms))
 - `hashed_area_len: 0x0000003d`: Length of the following hashed subpacket data
 
@@ -503,19 +503,19 @@ There are two sets of subpacket data in a Signature: hashed, and unhashed. The d
 
 The following subpacket data consists of sets of "subpacket length, subpacket tag, data." We'll show the information for each subpacket as one line, starting with the [subpacket type description](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-signature-subpacket-specifi) (based on the subpacket tag). Note that bit 7 of the subpacket tag signals if that subpacket is ["critical"](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#section-5.2.3.7-10)[^critical].
 
-[^critical]: Critical here means: the receiver must be able to interpret the subpacket and is expected to fail, otherwise. non-critical subpackets may be ignored by the receiver
+[^critical]: Critical here means: the receiver must be able to interpret the subpacket and is expected to fail, otherwise. Non-critical subpackets may be ignored by the receiver
 
-- [Signature creation time](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#signature-creation-subpacket) (subpacket type 2) *critical*: `0x6516eaa6`
+- [Signature creation time](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#signature-creation-subpacket) (subpacket type 2) *critical*: `0x6516eaa6` (also see [Time Fields](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-time-fields))
-- [Key expiration time](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#key-expiration-subpacket) (subpacket type 9) *critical*: `0x05a48fbd`
+- [Key expiration time](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#key-expiration-subpacket) (subpacket type 9) *critical*: `0x05a48fbd` (also see [Time Fields](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-time-fields))
-- [Preferred symmetric ciphers for v1 SEIPD](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#preferred-v1-seipd) (type 11): `0x09 0x07`. (These values [correspond to](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#symmetric-algos): "AES with 256-bit key" and "AES with 128-bit key")
+- [Preferred symmetric ciphers for v1 SEIPD](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#preferred-v1-seipd) (type 11): `0x09 0x07`. (These values [correspond to](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#symmetric-algos): *AES with 256-bit key* and *AES with 128-bit key*)
-- [Preferred hash algorithms](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#preferred-hashes-subpacket) (subpacket type 21): `0x0a 0x08`. (These values [correspond to](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-hash-algorithms): "SHA2-512" and "SHA2-256")
+- [Preferred hash algorithms](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#preferred-hashes-subpacket) (subpacket type 21): `0x0a 0x08`. (These values [correspond to](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-hash-algorithms): *SHA2-512* and *SHA2-256*)
-- [Key flags](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#key-flags) (subpacket type 27) *critical*: `0x01`. (This value [corresponds](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-key-flags) to the "certifications" key flag)
+- [Key flags](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#key-flags) (subpacket type 27) *critical*: `0x01`. (This value [corresponds](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-key-flags) to the *certifications* key flag)
-- [Features](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#features-subpacket) (subpacket type 30): `0x01`. (This value [corresponds](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-features) to: "Symmetrically Encrypted Integrity Protected Data packet version 1")
+- [Features](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#features-subpacket) (subpacket type 30): `0x01`. (This value [corresponds](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-features) to: *Symmetrically Encrypted Integrity Protected Data packet version 1*)
 - [Issuer fingerprint](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#issuer-fingerprint-subpacket) (subpacket type 33): `aaa18cbb254685c58358320563fd37b67f3300f9fb0ec457378cd29f102698b3` (this is the fingerprint of the component key that issued the signature in this packet. Not that here, the value is the primary key fingerprint of the certificate we're looking at.)
 
 The next part of this packet contains "unhashed subpacket data":
 
-- `unhashed_area_len: 0x0000000a`: Length of the following unhashed subpacket data.
+- `unhashed_area_len: 0x0000000a`: Length of the following unhashed subpacket data (value: 10 bytes).
 
 As above, the following subpacket data consists of sets of "subpacket length, subpacket tag, data." In this case, only subpacket follows:
 
@@ -527,17 +527,21 @@ This concludes the unhashed subpacket data.
 - `salt_len, salt`: A random [salt value](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-advantages-of-salted-signat) (the size must be [matching for the hash algorithm](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#hash-algorithms-registry))
 - `ed25519_sig`: [Algorithm-specific](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-algorithm-specific-fields-for-ed2) representation of the signature (in this case: 64 bytes of Ed25519 signature)
 
+The signature is calculated over a hash. The hash, in this case, is calculated over the following data (for details, see [Computing Signatures](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-11.html#name-computing-signatures) in the RFC):
+
+- The signature's salt
+- A serialized form of the primary key's public data
+- A serialized form of this direct key signature packet (up to, but excluding the unhashed area)
+
 ```{figure} diag/key-minimal.png
 :width: 40%
 
 A minimal OpenPGP key, visualized
 ```
 
-### Seen as an OpenPGP certificate
+### Seen as a very minimal OpenPGP certificate
 
-Let's now look at a "public key" view of the (very minimal) OpenPGP key above. That is, the same data, but without the private key material parts.
+Let's now look at a "public key" view of the (very minimal) OpenPGP key above. That is, the same data, but without the private key material parts. An OpenPGP user might give such a certificate to a communication partner, or upload it to a key server:
-
-An OpenPGP user might give such a certificate to a communication partner, or upload it to a key server:
 
 ```text
 $ sq key extract-cert alice_minimal.priv
@@ -801,9 +805,9 @@ Signature Packet, new CTB, 3 header bytes + 325 bytes
     00000140  a6 73 c8 33 5a 9c d9 0a
 ```
 
-### User IDs
+### User ID
 
-User IDs are a mechanism for attaching *identities* to an OpenPGP certificate. Typically, a User ID will contain a name and an email address.
+User IDs are a mechanism for attaching *identities* to an OpenPGP certificate. Traditionally, User IDs contain a string that combines a name and an email address.
 
 To look into these, we'll make a certificate that has one [User ID](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#uid). User IDs are *"intended to represent the name and email address of the key holder"*. A certificate can have multiple User IDs associated with it.