Replace use of PNGs with converted SVGs (where available)

Signed-off-by: David Runge <dave@sleepmap.de>

book/source/04-certificates.md

@@ -49,7 +49,7 @@ An {term}`OpenPGP certificate` (or "{term}`OpenPGP key`") is a collection of an
 
 This documentation collectively refers to {term}`component keys<OpenPGP Component Key>` and {term}`identity components<Identity Component>` as "the {term}`components<Component>` of a {term}`certificate<OpenPGP Certificate>`."
 
-```{figure} diag/Components_of_an_OpenPGP_Certificate.svg
+```{figure} diag_converted/Components_of_an_OpenPGP_Certificate.svg
 :name: fig-openpgp-certificate-components
 :alt: Depicts a box with white background and the title "OpenPGP certificate". In the box several other boxes and accompanying texts, representing component keys and User IDs, are shown. There are three component keys boxes with a green frame, each with a dotted lower-left section, that shows the text "key creation time" and the green public key symbol in the lower right area. All three have a title, a unique fingerprint below the box and a unique capability keyword, perpendicular to the box on the right side. The top-most component key box has a light-green background, with the title "Component Key (primary)" and capability keyword "certification". The second-to-top component key box has a white background, with the title "Component Key" and capability keyword "encryption". The lowest component key box has a white background, with the title "Component Key" and capability keyword "signing". There are two User ID boxes, each with a black frame, open to top left and lower right corner. Both boxes have a user icon on the top left side, the title "User ID" on the top right side and a User ID string at the bottom. The top box has "Alice Adams <alice@example.org>" and the lower box has "Alice" as User ID string.
 
@@ -71,7 +71,7 @@ An {term}`OpenPGP certificate` usually contains multiple {term}`component keys<O
 
 [^ecdh-parameters]: For [ECDH](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-algorithm-specific-part-for-ecd) {term}`component keys<OpenPGP Component Key>`, two additional algorithm parameters are integral to the {term}`component key<OpenPGP Component Key>`'s constitutive and immutable properties. Those parameters specify a hash function and a {term}`symmetric<Symmetric Cryptography>` encryption algorithm.
 
-```{figure} diag/Component_Key.png
+```{figure} diag_converted/Component_Key.svg
 :name: fig-component-key
 :alt: Depicts a box with white background and no title. In the box one other box is shown. The inner box has a green frame, with a dotted lower-left section, that shows the text "key creation time" and the green public key symbol, as well as the red-dotted private key symbol in the lower right area. In the top left of the inner box the text reads "Component Key."
 
@@ -85,7 +85,7 @@ An {term}`OpenPGP component key`
 
 Each {term}`OpenPGP component key` possesses an *{term}`OpenPGP fingerprint`*. This {term}`fingerprint<OpenPGP Fingerprint>` is derived from the {term}`public key material<OpenPGP Certificate>`, the {term}`creation timestamp<Creation Time>`, and, when relevant, the ECDH parameters.
 
-```{figure} diag/Fingerprint.svg
+```{figure} diag_converted/Fingerprint.svg
 :name: fig-fingerprint
 :alt: Depicts a box with white background and the title "Fingerprint of an OpenPGP component key." Inside, another box with a green frame, the title "Component Key", the text "key creation time" on the lower left and a the green public key symbol on the lower right is shown. Below the component key box a fingerprint in a box with a light-yellow background and a yellow dotted line is depicted. The word "Fingerprint" is shown left of the box with the fingerprint and both are connected with a yellow dotted line.
 
@@ -125,7 +125,7 @@ Modern {term}`OpenPGP certificates<OpenPGP Certificate>` typically include sever
 
 While {term}`subkeys<OpenPGP Subkey>` have the same structural attributes as the {term}`primary key<OpenPGP Primary Key>`, they fulfill  different roles. {term}`Subkeys<OpenPGP Subkey>` are cryptographically linked with the {term}`primary key<OpenPGP Primary Key>`, a relationship further discussed in {numref}`binding_subkeys`.
 
-```{figure} diag/Binding_Subkeys.png
+```{figure} diag_converted/Binding_Subkeys.svg
 :name: fig-subkeys
 :alt: Diagram depicting three component keys. The primary key is positioned at the top, designated for certification. Below it, connected by arrows, are two subkeys labeled as "for encryption" and "for signing," respectively.
 
@@ -142,7 +142,7 @@ While {term}`subkeys<OpenPGP Subkey>` have the same structural attributes as the
 
 {term}`OpenPGP certificates<OpenPGP Certificate>` can contain multiple [User IDs](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-user-id-packet-tag-13). Each {term}`User ID` associates the {term}`certificate<OpenPGP Certificate>` with an {term}`identity`.
 
-```{figure} diag/Binding_a_UserID.png
+```{figure} diag_converted/Binding_a_UserID.svg
 :name: fig-user-ids
 :alt: Depicts a diagram with white background and the title "User IDs". Inside, a public primary component key for certification and a User ID is shown. A green arrow points from component key to User ID and is annotated with a signature.
 
@@ -204,7 +204,7 @@ Key attributes, such as {term}`capabilities<Capability>` (like *signing* or *enc
 
 It is crucial to note that the {term}`components<Component>` of an {term}`OpenPGP certificate` remain static after their creation. The use of {term}`signatures<OpenPGP Signature Packet>` to store {term}`metadata` allows for subsequent modifications without altering the original {term}`component<Component>`. For instance, a {term}`certificate holder` can update the {term}`expiration time` of a {term}`component` by issuing a new, superseding {term}`signature<OpenPGP Signature Packet>`.
 
-```{figure} diag/Primary_key_metadata.png
+```{figure} diag_converted/Primary_key_metadata.svg
 :name: fig-primary-metadata
 :alt: Depicts a direct key signature, associated with a primary component key.
 
@@ -261,7 +261,7 @@ Additionally, OpenPGP allows modeling {term}`User ID`-specific preferences. The
 
 Following our review of how {term}`keys<Component Key>` and {term}`identity components<Identity Component>` are linked, let's reexamine the {term}`OpenPGP certificate` from {numref}`fig-openpgp-certificate-components`. Our focus now extends to all of its binding signatures and the {term}`direct key signature` that contains {term}`metadata` for the full {term}`certificate<OpenPGP certificate>`:
 
-```{figure} diag/OpenPGP_Certificate.png
+```{figure} diag_converted/OpenPGP_Certificate.svg
 :name: fig-openpgp-certificate
 :alt: Depicts an OpenPGP certificate, including a set of components, binding signatures, and a direct key signature on the primary key.