Add name and alt attributes for all figures

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

book/source/06-signatures.md

@@ -17,6 +17,8 @@ Within OpenPGP, the term *signature* can have two different meanings:
 - **OpenPGP signature packets**: Defined in the [OpenPGP standard](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-signature-packet-type-id-2), these packets combine a raw cryptographic signature along with a *type* designation and additional metadata.
 
 ```{figure} diag/meaning_of_signatures.png
+:name: fig-meaning-of-signatures
+:alt: Depicts a box on white background with the title "Meanings of signature in OpenPGP", "signature" in italics. The top half of the box shows a green seal symbol with the word "sig" in it on the left side. The symbol is connected to the text "Cryptographic signature" by a black dotted line. The bottom half of the box shows a diagram. On the left hand side a box with green dotted frame and white background provides the title "Signature type", while inside the box the text reads "Signature over Signature data, Signature metadata". The words "Signature metadata" serve as title for a yellow box at the lower half of the signature type box. The yellow box also contains a cryptographic signature symbol. Right of the signature type box, the text "OpenPGP signature packet" is shown, which is connected to the box by a green dotted line. Below the text a list is shown, which reads "signature type, signature over input data, additional metadata and cryptographic signature". The last item is connected to the cryptographic signature symbol in the yellow box by a black dotted line.
 
 Two meanings of the term "signature" in OpenPGP
 ```
@@ -39,6 +41,8 @@ Signatures on components are a complex topic, and we discuss them in depth in {r
 - the nature of the statement made by the signature, such as certifying an identity or binding component keys into a certificate
 
 ```{figure} mermaid/sig-types.png
+:name: fig-signature-types
+:alt: Depicts a diagram, describing different types of OpenPGP signatures. On the right hand side a long yellow box with the title "Signature Types and Targets" is shown, which contains signature type IDs and their names (in gray boxes) and further yellow boxes, grouping other types of signature type IDs. At the top the signature type ID "0x02 Standalone" is shown. Below, another yellow box groups the "Signature Packet"s "0x50 Third-Party Confirmation" and "0x40 Timestamp". Another box groups types of signatures, that apply to "Data" packets "0x00 Binary Data" and "0x01 Canonical Text". Below, a box groups types of signatures, that apply to "Primary Key + User ID/ Attr. Packet"s. The type IDs "0x10 Generic Certification", "0x11 Persona Certification", "0x12 Casual Certification" and "0x13 Positive Certification" are shown together in one gray box and "0x30 Certification Revocation" in another. Another yellow box groups types of signatures, that apply to "Primary Key" packets "0x1F Direct-Key Signature" and "0x20 Key Revocation". The last box groups types of signatures, that apply to "Primary + Subkey" packets. "0x18 Subkey Binding" and "0x19 Primary Key Binding" are shown together in one gray box, "0x28 Subkey Revocation" in another. On the left hand side of the diagram shows gray boxes identifying different types of signatures, with the most basic being "OpenPGP Signature" on the far left. With arrows it points to further signature types ("Signature on Data", "Signature on Component") and several signature type IDs ("0x02", "0x50" and "0x40"). The signature type "Signature on Data" points to "0x00" and "0x01". The signature type "Signature on Component" points to two more specific signature types, namely "Third-Party" and "Self-Signature". "Third-Party" points at the group of "0x10", "0x11", "0x12" and "0x13", as well as "0x30" and "0x1F". "Self-Signature" points at the group of "0x10", "0x11", "0x12" and "0x13", as well as "0x30", "0x1F", the group of "0x18" and "0x19" and finally "0x28".
 
 An overview of signature types in OpenPGP
 ```
@@ -56,6 +60,8 @@ As outlined above, an OpenPGP signature is a composite data structure, which com
 - **Raw cryptographic signature**
 
 ```{figure} diag/signature_packet.png
+:name: fig-signature-packet
+:alt: Depicts a diagram with the title "A signature packet". On the left hand side a box with green dotted frame and white background provides the title "Signature type", while inside the box the text reads "Signature over Signature data, Signature metadata". The words "Signature metadata" serve as title for a yellow box at the lower half of the signature type box. The yellow box also contains a cryptographic signature symbol. Right of the signature type box, the text "OpenPGP signature packet" is shown, which is connected to the box by a green dotted line. Below the text a list is shown, which reads "signature type, signature over input data, additional metadata and raw cryptographic signature". The last item is connected to the cryptographic signature symbol in the yellow box by a black dotted line.
 
 Structure of an OpenPGP signature packet
 ```
@@ -79,6 +85,8 @@ The signature packet consists of two parts:
    - The signature is then calculated for this hash digest.
 
 ```{figure} diag/Signature_Creation.png
+:name: fig-signature-creation
+:alt: Depicts a complex diagram with white background and the title "Signature creation". On the top left side a box with black frame and white background reads "Input Data packets, One or more packets". Below it the symbol of a signature packet is shown (however, instead of the green signature symbol, only a circle with white background and dotted frame is shown). Both are connected (via green dotted arrows) to a green, right pointing arrow symbol with green dotted frame and the title "Hash mechanism". Text above the green arrow symbol reads "A hash digest is calculated from the input data packets and the signature metadata". The "Hash mechanism" arrow points at a box with white background and green frame, which reads "hash digest". At the top right corner of the diagram the symbol for a component key with both public and private key and the title "Signer private key" is shown. Both hash digest and component key symbol point to a large green arrow symbol, with green dotted frame, at the lower right corner of the diagram, using green dotted arrow lines. The large arrow symbol has the title "Signing mechanism" and text overlaid across it reads "A cryptographic signature is calculated over the hash digest, using the private key material of the signer.". It points at a cryptographic signature symbol at the bottom of the diagram. The cryptographic signature symbol is connected (via a green dotted arrow line) to the circle with white background and dotted green frame in the signature packet symbol.
 
 Creating a signature in OpenPGP
 ```
@@ -95,6 +103,8 @@ The main differences:
 After calculating the hash digest from the input data, a signature verification mechanism is employed. This mechanism uses the hash digest, the cryptographic signature from the signature packet, and the public key of the signer. Its purpose is to ascertain the cryptographic validity of the signature.
 
 ```{figure} diag/Signature_Verification.png
+:name: fig-signature-verification
+:alt: Depicts a complex diagram with white background and the title "Signature verification". On the top left side a box with black frame and white background reads "Input Data packets, One or more packets". Below it the symbol of a signature packet is shown. Both are connected (via green dotted arrows) to a green, right pointing arrow symbol with green dotted frame and the title "Hash mechanism". Text above the green arrow symbol reads "A hash digest is calculated from the input data packates and the signature metadata". The "Hash mechanism" arrow points at a box with white background and green frame, which reads "hash digest". At the top right corner of the diagram the symbol for a component key with only public key and the title "Signer public key" is shown. Hash digest, component key symbol and the cryptographic signature symbol in the signature packet point to a large green arrow symbol, with green dotted frame, at the lower right corner of the diagram, using green dotted arrow lines. The large arrow symbol has the title "Signature verification mechanism" and text overlaid across it reads "A cryptographic signature is verified against the hash digest, using the public key of the signer.". It points at a success and fail symbol at the bottom of the diagram.
 
 Verifying a signature in OpenPGP
 ```