Existing Fields & Seed Values

Templates often ship with empty signature fields already placed — by Acrobat, iText, or an authoring tool — at a fixed page and rectangle. Rather than creating a new field, Mudrit can sign into a named empty field, and can also write seed-value (/SV) constraints onto a field it creates.

Sign into an existing field

.sign_existing_field(name) targets a pre-existing empty field by its fully-qualified name. The field's own page and rectangle are reused, so any .place(…) / .placement(…) entries are ignored.

use mudrit_pdfsign::prelude::*;

let cfg = SignConfig::builder()
    .sign_existing_field("Signature1")   // reuse the template field's page + rect
    .reason("Approved")
    .build();

let signed = sign_pdf(PdfReader::open("template.pdf")?, &signer, &cfg)?;

Discovering fields

Use list_signature_fields to enumerate the signature fields in a PDF — both empty slots and already-signed ones — to drive a UI or to pick which empty field to sign into.

use mudrit_pdfsign::list_signature_fields;

for f in list_signature_fields(PdfReader::open("template.pdf")?)? {
    println!("{} — page {} — {}", f.name, f.page, if f.signed { "signed" } else { "empty" });
}

Each result is a SignatureField:

Seed values (/SV)

A seed value is a set of constraints an author writes onto a signature field so a conforming signer or viewer enforces them — mandated digest(s), acceptable SubFilter(s), reason(s), a required timestamp URL, and whether revocation info (LTV) must be embedded. Attach one with .seed_value(SeedValue { .. }); Mudrit writes the /SV dictionary onto the field it creates.

// hints only — safe to co-embed a signature in the same pass
let cfg = SignConfig::builder()
    .place("F", [350, 60, 560, 160])?
    .seed_value(SeedValue {
        subfilters: vec![SvSubFilter::EtsiCadesDetached],   // advisory: prefer PAdES
        reasons: vec!["Approved".into(), "Reviewed".into()],
        timestamp_url: Some("http://timestamp.comodoca.com".into()),
        // require_* left false, no digest_methods → Adobe-safe
        ..SeedValue::default()
    })
    .build();

// hard requirements — reserve for a constrained EMPTY template a future signer fills
let cfg = SignConfig::builder()
    .place("F", [350, 60, 560, 160])?
    .seed_value(SeedValue {
        digest_methods: vec![SvDigest::Sha256],
        require_digest_method: true,
        subfilters: vec![SvSubFilter::EtsiCadesDetached],
        require_subfilter: true,
        require_rev_info: true,
        ..SeedValue::default()
    })
    .build();

SvDigest is Sha256 / Sha384 / Sha512; SvSubFilter is AdbePkcs7Detached (adbe.pkcs7.detached) or EtsiCadesDetached (ETSI.CAdES.detached). Mudrit writes the dictionary but does not itself refuse to sign against an incompatible configuration — the constraints are for the consuming viewer or re-signer to honour.

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