CMS Signature - What is the difference Time Stamp

Question

I am trying to TimeStamp a Digital Siganture (with a local TimeStamp certificate) in C# with BouncyCastle. My understanding about TimeStamp is that it is to sign the current time. Not sure if it should be current time + original signature content? Please help on this also.

My main confusion is if the generated TimeStamp be added to Singed/Unsigned attributes of original signature. OR it will be added as a CounterSignature?

Answer 1

Time stamp's goal is to prove that signature was created before a given time, so with time stamp you must sign the digital signature and the current time. Time stamp must be added to CMS signature as unsigned attribute. Besides SignatureTimeStampToken is a signature itself.

To add a time stamp to CMS you can use a Signature time-stamp attribute which has 1.2.840.113549.1.9.16.2.14 object identifier and has ASN.1 Type (the information below is all extracted from CMS and TSP RFCs)

SignatureTimeStampToken ::= TimeStampToken

TimeStampToken ::= ContentInfo
 -- contentType is id-signedData ([CMS])
 -- content is SignedData ([CMS])

SignedData ::= SEQUENCE {
    version CMSVersion,
    digestAlgorithms DigestAlgorithmIdentifiers,
    encapContentInfo EncapsulatedContentInfo,
    certificates [0] IMPLICIT CertificateSet OPTIONAL,
    crls [1] IMPLICIT RevocationInfoChoices OPTIONAL,
    signerInfos SignerInfos }

In TimeStampToken the fields of type EncapsulatedContentInfo of the SignedData construct have the following meanings:

eContentType is an object identifier that uniquely specifies the content type. For a time-stamp token it is defined as:

    id-ct-TSTInfo  OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) ct(1) 4}

eContent is the content itself, carried as an octet string.The eContent SHALL be the DER-encoded value of TSTInfo.

The time-stamp token MUST NOT contain any signatures other than the signature of the TSA. The certificate identifier (ESSCertID) of the TSA certificate MUST be included as a signerInfo attribute inside a SigningCertificate attribute.

    TSTInfo ::= SEQUENCE  {
       version                      INTEGER  { v1(1) },
       policy                       TSAPolicyId,
       messageImprint               MessageImprint,
             -- MUST have the same value as the similar field in
             -- TimeStampReq
       serialNumber                 INTEGER,
            -- Time-Stamping users MUST be ready to accommodate integers
            -- up to 160 bits.
       genTime                      GeneralizedTime,
       accuracy                     Accuracy                 OPTIONAL,
       ordering                     BOOLEAN             DEFAULT FALSE,
       nonce                        INTEGER                  OPTIONAL,
             -- MUST be present if the similar field was present
             -- in TimeStampReq.  In that case it MUST have the same value.
       tsa                          [0] GeneralName          OPTIONAL,
       extensions                   [1] IMPLICIT Extensions   OPTIONAL  }

Hope this helps,