SignCore.CMS.SignedAttributes (sign_core v0.1.0)

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Build and encode the signedAttrs SET-OF Attribute that goes into a CMS SignerInfo (RFC 5652 §5.3) — and produce the to-be-signed bytes per RFC 5652 §5.4 ("the message digest calculation process").

What's in here

Three required PKCS#9 attributes per RFC 5652 §11:

  • contentType (1.2.840.113549.1.9.3) — the OID of the encapsulated content type, typically id-data for detached PAdES / CAdES.
  • messageDigest (1.2.840.113549.1.9.4) — the digest of the encapsulated content (or the to-be-signed bytes for detached).
  • signingTime (1.2.840.113549.1.9.5) — the time the signature was produced, as recorded by the signer (not authoritative — that's what RFC 3161 timestamping is for).

The §5.4 re-tag (no, OTP handles it)

CMS distinguishes two encodings of the same SET-OF Attribute:

  • As SET OF Attribute (universal SET tag 0x31) — the input to the signature digest. This is what to_be_signed/1 returns.
  • As [0] IMPLICIT Attributes (context-specific tag 0xA0) — the form embedded inside SignerInfo. The OTP codec emits this form automatically when you encode a SignerInfo.

Callers should compute the signature over to_be_signed/1's output and let the codec handle the IMPLICIT-tagged embed during the final SignerInfo assembly. We never re-tag bytes by hand.

OPEN-TYPE Attribute encoding

An ASN.1 Attribute is {type OID, values SET OF ANY}. The OTP CMS module ships an information-object-class table that maps known PKCS#9 attribute OIDs to typed value definitions:

  • id-contentType → OBJECT IDENTIFIER
  • id-messageDigest → OCTET STRING
  • id-signingTime → Time CHOICE (UTCTime or GeneralizedTime)

So when building the inner Attribute tuple, we pass the typed Erlang/Elixir value directly (an OID tuple, a binary, a tagged Time choice) rather than wrapping bytes as {:asn1_OPENTYPE, der}. OTP encodes them properly. This is the "OPEN-TYPE Attribute encoding gotcha" that the Phase 4 plan §8 walks through; in practice the OTP codec absorbs it.

Time choice cutover

Per RFC 5280 §4.1.2.5 (which CMS adopts via §11.3): UTCTime for years 1950..2049 (inclusive), GeneralizedTime otherwise. Phase 4 expects to ship in the UTCTime window for the foreseeable future, but the selection is automatic.

Summary

Types

attribute()

Erlang Attribute record-shaped tuple.

build_opts()

Required + optional input for build/1.

Functions

build(opts)

Build the three required signed attributes (contentType, messageDigest, signingTime) plus any extras.

to_be_signed(attrs)

Encode attrs as the universal SET OF Attribute (RFC 5652 §5.4 "to be signed" form). The returned bytes are the digest input — the signer calls Pkcs11ex.sign_bytes/2 over them, and the resulting raw signature is glued back into the SignerInfo.

Types

attribute()

@type attribute() :: {:Attribute, :public_key.oid(), [term()]}

Erlang Attribute record-shaped tuple.

build_opts()

@type build_opts() :: [
  digest: binary(),
  content_oid: :public_key.oid(),
  signing_time: DateTime.t()
]

Required + optional input for build/1.

Functions

build(opts)

@spec build(build_opts()) :: {:ok, [attribute()]} | {:error, term()}

Build the three required signed attributes (contentType, messageDigest, signingTime) plus any extras.

Required opts

  • :digest — binary(). The SHA-256 (or matching algorithm) digest over the encapsulated content. For detached signatures this is computed over the document bytes the signer commits to — for PAdES, the bytes covered by /ByteRange.

Optional opts

  • :content_oid — defaults to id-data (1.2.840.113549.1.7.1). Use a different content-type OID for non-detached payloads.
  • :signing_time — defaults to DateTime.utc_now/0. Truncated to seconds; sub-second precision is not encoded (UTCTime granularity).

Returns a list of Attribute tuples — sorted by the OTP codec into DER canonical SET order at encode time.

to_be_signed(attrs)

@spec to_be_signed([attribute()]) :: {:ok, binary()} | {:error, term()}

Encode attrs as the universal SET OF Attribute (RFC 5652 §5.4 "to be signed" form). The returned bytes are the digest input — the signer calls Pkcs11ex.sign_bytes/2 over them, and the resulting raw signature is glued back into the SignerInfo.

Equivalent to Codec.encode(:SignedAttributes, attrs); this name documents intent at the call site.