Multi-signature

Two different questions hide behind "multi-signature":

1. One signer, several boxes — how should one signature be laid out across pages? That's the Method.
2. Several signers, over time — how does a second party sign a document the first already signed? That's re-signing, which sign_pdf handles automatically.

Method — laying out one signing operation

Method decides how the field(s) created in a single sign_pdf call relate to the actual cryptographic signature value:

use mudrit_pdfsign::prelude::*;

let cfg = SignConfig::builder()
    .place("A", [40, 40, 200, 100])?     // a box on every page
    .method(Method::Single)              // one signature, shown on all of them
    .build();

let cfg = SignConfig::builder()
    .place("F", [40, 40, 200, 100])?
    .place("L", [350, 60, 560, 160])?    // two fields…
    .method(Method::MultiShared)         // …sharing ONE signature value (one revision)
    .build();

let cfg = SignConfig::builder()
    .place("F", [40, 40, 200, 100])?
    .place("L", [350, 60, 560, 160])?    // two fields…
    .method(Method::MultiChained)        // …two independent signatures, one revision each (approval only)
    .build();

Multi-party re-signing

You don't choose "sign" vs "add signature" — sign_pdf detects it. If the input is already signed, it appends a new signature as an incremental revision, so the existing signatures stay valid, instead of a fresh sign that would rebuild and break them.

let once  = sign_pdf(&pdf,  &signer_a, &cfg)?;   // signer A — first signature
let twice = sign_pdf(&once, &signer_b, &cfg)?;   // signer B — auto-appended; both valid

This works on plain and password-protected documents. When the already-signed input is also encrypted, pass the open password and the appended revision is encrypted with the document's own file key — so the output stays protected under the same password and cipher:

let resigned = sign_pdf(PdfReader::from(signed_protected).password("asd"), &signer_b, &cfg)?;

See the resign_multiparty example (signer A then B, plain and protected).

Certified variants

Certification composes with the layout methods. A certified Single signature shows one entry in Adobe's Signatures panel; a certified MultiShared shows N entries that share the one signature value. Both keep DocMDP validity, and either can be combined with output encryption (see the certified_variants example — certified Single, certified MultiShared, and encrypted).

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