Shift away from "plaintext" term for the signed data

Talking about "plaintext" seems potentially confusing, especially when signing an encrypted message.

book/source/signing_data.md

@@ -54,7 +54,7 @@ This method is commonly used for signing or encrypting emails. Most email softwa
 
 OpenPGP defines two variant forms of inline-signed messages:
 
-1. **{term}`One-pass signed messages<One-pass signed Message>`** This is the commonly used format for inline-signed messages. A signer can produce and a verifier verify this format in one pass.
+1. **{term}`One-pass signed messages<One-pass signed Message>`** This is the commonly used format for inline-signed messages. A signer can produce and a verifier can verify this format in one pass.
 2. **{term}`Prefixed signed messages<Prefixed signed Message>`** This format predates[^inline-signature-formats] {term}`one-pass signed messages<One-pass signed Message>` and is conceptually slightly simpler. However, it has no strong benefits and is now rarely used.
 
 [^inline-signature-formats]: One-pass signing was first specified in RFC 2440. The format was not supported in PGP 2.6.x.
@@ -76,7 +76,7 @@ A {term}`one-pass signed<One-pass signed Message>` {term}`OpenPGP message` consi
 
 ```{figure} plain_svg/ops-signed-message.svg
 :name: fig-ops-signed-message
-:alt: Depicts the structure of a one-pass-signed message. Two one-pass-signatures lead the literal data packet, followed by two signature packets. Arrows show, how the hash-algorithm field of the one-pass-signatures is inspected in order to initiate the hashing procedure.
+:alt: Depicts the structure of a one-pass signed message. Two one-pass signatures lead the literal data packet, followed by two signature packets. Arrows show, how the hash-algorithm field of the one-pass signatures is inspected in order to initiate the hashing procedure.
 
 The structure of a one-pass signed message.
 ```
@@ -85,17 +85,31 @@ The structure of a one-pass signed message.
 Despite its name, a {term}`one-pass signature packet` is not a type of {term}`signature packet<OpenPGP Signature Packet>`.
 
 Instead, it's a type of auxiliary packet that can be used in conjunction with {term}`signature packets<OpenPGP Signature Packet>`. Its use allows storing the {term}`signature packets<OpenPGP Signature Packet>` after the message body.
-
-The position of the signature packet within the OpenPGP message is important for efficient data processing. The plaintext data might be large in size, and memory might be constrained.
-The producer of a signed OpenPGP message wants to streamline the signature calculation process in such a way that allows to emit the data to be signed while calculating the signature. The signature itself is therefore appended to the data.
-The verifier on the other hand needs to know the hash algorithm used to calculate the signature before starting the verification process. As a consequence, either the producer would need to pass the plaintext data twice, once to calculate the signature and a second time to emit the signed data (the result is a prefixed-signed message), or the verifier needs to process (and cache) the whole OpenPGP message to reach the signature packets at the end in order to determine the hash algorithm, to then re-process the whole message, verifying the signature.
-
-The one-pass signature solves this issue, by allowing both the creation and verification of a signed message in a single pass.
 ```
 
+#### The function of the one-pass signature packet
+
+To understand the purpose of this packet, consider that without it, the position of signature packets within an inline signed OpenPGP message constitutes a trade-off for efficient data processing. In particular when signed data is large and exceeds available memory in size.
+
+The producer of a signed OpenPGP message wants to streamline the signature calculation process in such a way that allows to emit a copy of the signed data while calculating the cryptographic signature. On the signer's side, the signature packet is therefore easy to store after the signed data.
+
+The verifier, on the other hand, needs some information from the signature packet in order to perform the signature verification process. In particular, the verifier needs to know which hash algorithm was used to calculate the signature, in order to perform the same hashing operation on the message data.
+
+As a consequence, without a {term}`one-pass signature packet`, either:
+- the producer would need to process the signed data twice:
+  - once to calculate the signature, and
+  - a second time to emit the signed data (the result is a prefixed signed message), or
+- the verifier would need to process the OpenPGP message twice:
+  - once to read the signature packets at the end in order to determine the hash algorithm, and
+  - a second time to process the body of the message, and calculate the hash verifying the signature.
+
+The one-pass signature packet solves this issue, by allowing both the creation and verification of a signed message in a single pass. It effectively contains a copy of the data in a signature packet, but without the cryptographic signature data.
+
+The signer can easily emit this metadata before processing the full message, and for the verifier, this metadata enables processing of the message body. Both signer and verifier can efficiently generate or check a one-pass signed message.
+
 #### Creation
 
-To produce an {term}`inline signature`, the {term}`signer` decides on a hash algorithm and emits a {term}`one-pass signature packet<One-pass Signature Packet>` into the destination {term}`OpenPGP message`. This contains essential information such as the {term}`fingerprint<OpenPGP Fingerprint>` of the {term}`signing key<OpenPGP Component Key>` and the {term}`hash<Hash Digest>` algorithm used for computing the {term}`signature<OpenPGP Signature Packet>`'s {term}`hash digest`. The signer then processes the entirety of the plaintext data, emitting it as a {term}`literal data<Literal Data Packet>` into the message as well. Once the data is processed, the {term}`signer` calculates a {term}`cryptographic signature` using the calculated hash value. Lastly, the result is emitted as a {term}`data signature packet` to the output message, and the whole packet sequence can be efficiently stored or transmitted.
+To produce a {term}`one-pass inline signature<One-pass signed Message>`, the {term}`signer` decides on a hash algorithm and emits a {term}`one-pass signature packet<One-pass Signature Packet>` into the destination {term}`OpenPGP message`. This contains essential information such as the {term}`fingerprint<OpenPGP Fingerprint>` of the {term}`signing key<OpenPGP Component Key>` and the {term}`hash<Hash Digest>` algorithm used for computing the {term}`signature<OpenPGP Signature Packet>`'s {term}`hash digest`. The signer then processes the entirety of the signed data, emitting it as a {term}`literal data<Literal Data Packet>` into the message as well. Once the data is processed, the {term}`signer` calculates a {term}`cryptographic signature` using the calculated hash value. Lastly, the result is emitted as a {term}`data signature packet` to the output message, and the whole packet sequence can be efficiently stored or transmitted.
 
 For efficient {term}`verification`, an application must understand how to handle the {term}`literal data<Literal Data Packet>` prior to reading from it. This requirement is addressed by the {term}`one-pass signature packets<One-pass Signature Packet>` located at the beginning of {term}`inline-signed<Inline Signature>` messages. This setup enables the verifier to process the data correctly and efficiently in only a single pass.
 
@@ -116,7 +130,7 @@ Important to note, the {term}`signer`'s {term}`public key<OpenPGP Certificate>`,
 (prefixed-signature)=
 ### Prefixed signed message
 
-A {term}`prefixed signed message` consists of {term}`signature packet(s)<signature packet>` followed by the message. For the verifier, processing one-pass-signed and prefixed-signed messages are equally convenient. However, on the signer's side, it takes more resources to generate a {term}`prefixed signed message`.
+A {term}`prefixed signed message` consists of {term}`signature packet(s)<signature packet>` followed by the message. For the verifier, processing one-pass signed and prefixed signed messages are equally convenient. However, on the signer's side, it takes more resources to generate a {term}`prefixed signed message`.
 
 #### Structure
 
@@ -128,9 +142,9 @@ In this format, the signature packets are stored ahead of the message itself:
 
 ```{figure} plain_svg/prefixed-signed-message.svg
 :name: fig-prefixed-signed-message
-:alt: Depicts the structure of a prefixed-signed message. As an example, two signature packets lead a literal data packet. Arrows show, how the signatures hash algorithm field is inspected to start the hashing procedure.
+:alt: Depicts the structure of a prefixed signed message. As an example, two signature packets lead a literal data packet. Arrows show, how the signatures hash algorithm field is inspected to start the hashing procedure.
 
-Structure of a prefixed-signed message.
+Structure of a prefixed signed message.
 ```
 
 Compared to a {term}`one-pass signed message`, there are no {term}`one-pass signature packets<One-pass Signature Packet>` in this format, and the (otherwise equivalent) {term}`signature packet(s)<signature packet>` are stored ahead of the signed data.