synta 0.2.2

ASN.1 parser, decoder, and encoder library with DER/BER support and C FFI
Documentation 
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# ContentInfo


`ContentInfo` is the outer CMS envelope — every CMS structure is wrapped in a `ContentInfo`
SEQUENCE. It is the standard entry point for parsing any CMS payload.

## Class

```python
class ContentInfo:
    @staticmethod
    def from_der(data: bytes) -> ContentInfo: ...
    # Parse a DER- or BER-encoded ContentInfo SEQUENCE.

    def to_der(self) -> bytes: ...

    content_type_oid: ObjectIdentifier
    # e.g. ID_SIGNED_DATA, ID_ENVELOPED_DATA, ID_ENCRYPTED_DATA, …

    content: bytes
    # Raw bytes of the content field (including the [0] EXPLICIT tag for most types).
```

## Usage

```python
from synta.cms import ContentInfo, ID_SIGNED_DATA, ID_ENVELOPED_DATA, ID_ENCRYPTED_DATA

data = open("message.p7m", "rb").read()
ci = ContentInfo.from_der(data)

print(ci.content_type_oid)  # e.g. ObjectIdentifier("1.2.840.113549.1.7.2")

if ci.content_type_oid == ID_SIGNED_DATA:
    # ci.content includes the [0] EXPLICIT wrapper; strip it to get the inner SEQUENCE.
    import synta
    from synta.cms import SignedData
    dec = synta.Decoder(ci.content, synta.Encoding.BER)
    inner = dec.decode_explicit_tag(0)
    sd = SignedData.from_der(inner.remaining_bytes())
    print(f"Signers: {len(sd.signer_infos)}")

elif ci.content_type_oid == ID_ENVELOPED_DATA:
    import synta
    from synta.cms import EnvelopedData
    dec = synta.Decoder(ci.content, synta.Encoding.BER)
    inner = dec.decode_explicit_tag(0)
    ed = EnvelopedData.from_der(inner.remaining_bytes())
    # decrypt with ed.decrypt(private_key)

elif ci.content_type_oid == ID_ENCRYPTED_DATA:
    import synta
    from synta.cms import EncryptedData
    dec = synta.Decoder(ci.content, synta.Encoding.BER)
    inner = dec.decode_explicit_tag(0)
    enc = EncryptedData.from_der(inner.remaining_bytes())
    # decrypt with enc.decrypt(key)
```

See also the [CMS Overview](overview.md) for a summary of all content types.