openpgp-notes/book/source/adv/signing_data.md

Advanced material: Signatures over data

(adv-inline-signature)=

Internals of inline signed messages

Inline-signed messages are one of the forms of OpenPGP data signatures. An {term}inline-signed message <inline signature> joins the signed data and its corresponding {term}data signature into a single {term}OpenPGP message.

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 can verify this format in one pass.
2. {term}Prefixed signed messages<Prefixed signed Message> This format predates¹ {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.

(one-pass-signature)=

One-pass signed message

This is the commonly used format for inline signed messages.

Structure

A {term}one-pass signed<One-pass signed Message> {term}OpenPGP message consists of three segments:

1. One-pass signature packets: These one or more {term}packets<Packet> precede the signed data and enable {term}signature<OpenPGP Signature Packet> computation (both creation and verification) in a single pass.

2. {term}OpenPGP message: This contains the original data (e.g., the body of a message), which is signed without additional interpretation or conversion. Internally, a signed message consists of one or more OpenPGP packets. The message that gets signed could, for example, consist of a {term}Literal Data Packet, or a {term}Compressed Data Packet.

3. {term}Data signature packets<OpenPGP Signature Packet>: These contain the {term}cryptographic signature corresponding to the original data.

:name: fig-ops-signed-message
:alt: Depicts the structure of a one-pass signed message. Two one-pass signatures lead a 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.

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 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 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 message, emitting it as a series of one or more {term}packets<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}OpenPGP message 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 a single pass.

Strictly speaking, knowing just the hash algorithm would be sufficient to begin the verification process. However, having efficient access to the signer's fingerprint or key ID upfront allows OpenPGP software to fetch the signer's certificate(s) before processing the entirety of the - potentially large - signed data. This may, for example, involve downloading the certificate from a keyserver. In case fetching the signer's certificate(s) fails, or requires additional input from the user, it is better to signal the user about this before processing the data.

Verification

{term}Inline-signed<Inline Signature> messages enable efficient {term}verification in one pass, structured as follows:

1. Initiation with {term}one-pass signature packets<One-pass Signature Packet>: These {term}packets<Packet> begin the {term}verification process. They include the {term}signer's {term}key ID/{term}fingerprint<OpenPGP Fingerprint>, essential for identifying the appropriate {term}public key<OpenPGP Certificate> for signature {term}validation.

2. Processing the {term}OpenPGP message: This step involves {term}hashing<Hash Digest> its data, preparing it for {term}signature<OpenPGP Signature Packet> {term}verification.

3. {term}Verifying<Verification> {term}signature packets<OpenPGP Signature Packet>: Located at the end of the message, these {term}packets<Packet> are checked against the previously calculated {term}hash digest.

Important to note, the {term}signer's {term}public key<OpenPGP Certificate>, critical for the final {term}verification step, is not embedded in the message. Verifiers must acquire this {term}key externally (e.g., from a {term}key server) to authenticate the {term}signature<OpenPGP Signature Packet> successfully.

(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.

Structure

In this format, the signature packets are stored ahead of the message itself:

1. {term}Data signature packets<OpenPGP Signature Packet>: These one or more packets contain the {term}cryptographic signature corresponding to the original data.

2. {term}OpenPGP message: This contains the original data (e.g., the body of a message), without additional interpretation or conversion.

: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.

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.

For verification, this is equally convenient as the one-pass signed message form.

However, when a signer creates a {term}prefixed signed message, the signed data must be processed twice:

• once reading it to calculate the cryptographic signature, and
• once more to store the data in the generated OpenPGP message, after the signature packet(s).

Nesting of one-pass signatures

Signing a message using the one-pass mechanism involves prepending a one-pass signature (OPS) packet to the message and appending the corresponding signature, sandwiching the signed content.

An OpenPGP message can contain multiple signatures added that way.

One-pass signatures are nested, meaning the outermost one-pass signature packet corresponds to the outermost signature packet.

When a message is signed, the signature is always calculated over the contents of the literal data packet, not the literal data packet itself. This means that if a message, which is compressed using a compressed data packet is wrapped using a one-pass signature, the signature is still being calculated over the plaintext inside the literal data packet.

There is one exception, though.

Of course there is.

The OPS packet has a "nested" flag², which can either be 1 or 0. If this flag is set to 0, it indicates that further OPSs will follow this packet, which are calculated over the same plaintext data as this OPS is. A value of 1 indicates, that either no further OPS packets will follow (this OPS is the last), or that this OPS is calculated over the the usual plaintext data, but wrapped inside any OPS+Signature combinations that follow this OPS.

This mechanism enables attested signatures, where the signer signs an already one-pass signed message including the already contained signature.

As a practical example, consider the following notation:

• LIT("Hello World") represents a literal data packet with the content Hello World.
• COMP(XYZ) represents a compressed data packet over some other packet XYZ.
• OPS₁ represents a one-pass signature packet with the nested flag set to 1. Analogous, OPS₀ has the nested flag set to 0.
• SIG represents a signature packet.

A normal, one-pass signed message looks like this: OPS₁ LIT("Hello World") SIG

Here, the signature is calculated over the plaintext Hello World, as is it in a message that has the following form: OPS₁ COMP(LIT("Hello World")) SIG.

A message, where multiple one-pass signatures are calculated over the same plaintext looks the following: OPS₀ OPS₀ OPS₁ LIT("Hello World") SIG SIG SIG

All three signatures are calculated over the same plaintext Hello World.

Now, a message, where the signer attests an already signed message has the following format: OPS₁ OPS₁ LIT("Hello World") SIG SIG

While the inner signature is calculated over the usual plaintext Hello World, the outer signature is instead calculated over OPS₁ Hello World SIG.

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1. One-pass signing was first specified in RFC 2440. The format was not supported in PGP 2.6.x. ↩︎

2. See description of the nested flag. ↩︎