Replace use of PNGs with converted SVGs (where available)

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

book/source/06-signatures.md

@@ -16,7 +16,7 @@ Within OpenPGP, the term *{term}`signature<OpenPGP Signature Packet>`* can have
 - **{term}`Cryptographic signature`**: a sequence of bytes created by {term}`cryptographic keys<Cryptographic Key>`, calculated according to a {term}`signature` scheme.
 - **{term}`OpenPGP signature packets<OpenPGP signature packet>`**: 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 {term}`packets<Packet>` combine a raw {term}`cryptographic signature` along with a *{term}`type<OpenPGP Signature Type>`* designation and additional {term}`metadata`.
 
-```{figure} diag/meaning_of_signatures.png
+```{figure} diag_converted/meaning_of_signatures.svg
 :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.
 
@@ -84,7 +84,7 @@ The {term}`signature packet<OpenPGP Signature Packet>` consists of two parts:
    - A {term}`hash digest` is calculated from the input data.
    - The {term}`cryptographic signature` is then calculated for this {term}`hash digest`.
 
-```{figure} diag/Signature_Creation.png
+```{figure} diag_converted/Signature_Creation.svg
 :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.
 
@@ -102,7 +102,7 @@ The main differences:
 - **Use of {term}`signature verification` mechanism**: 
 After calculating the {term}`hash digest` from the input data, a {term}`signature verification` mechanism is employed. This mechanism uses the {term}`hash digest`, the {term}`cryptographic signature` from the {term}`signature packet<OpenPGP Signature Packet>`, and the {term}`public key<OpenPGP Certificate>` of the {term}`signer`. Its purpose is to ascertain the cryptographic {term}`validity<Validation>` of the {term}`signature<OpenPGP Signature Packet>`.
 
-```{figure} diag/Signature_Verification.png
+```{figure} diag_converted/Signature_Verification.svg
 :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.