(Objective-C) Extract TSTInfo from RFC3161 Timestamp Reply

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Sends an RFC 3161 timestamp request to a TSA (Timestamp Authority) server and converts the timestamp reply to XML, and then extracts the TSTInfo from the XML and converts it to XML.

For more information, see https://www.ietf.org/rfc/rfc3161.txt

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#import <CkoCrypt2.h>
#import <NSString.h>
#import <CkoHttp.h>
#import <CkoBinData.h>
#import <CkoHttpResponse.h>
#import <CkoAsn.h>
#import <CkoXml.h>
#import <CkoStringBuilder.h>

// Note: Requires Chilkat v9.5.0.75 or greater.

// This requires the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

// First sha-256 hash the data that is to be timestamped.
// In this example, the data is the string "Hello World"
BOOL success;
CkoCrypt2 *crypt = [[CkoCrypt2 alloc] init];
crypt.HashAlgorithm = @"sha256";
crypt.EncodingMode = @"base64";
NSString *base64Hash = [crypt HashFileENC: @"qa_data/hamlet.xml"];

CkoHttp *http = [[CkoHttp alloc] init];

CkoBinData *requestToken = [[CkoBinData alloc] init];
NSString *optionalPolicyOid = @"";
BOOL addNonce = NO;
BOOL requestTsaCert = YES;

// Create a time-stamp request token
success = [http CreateTimestampRequest: @"sha256" hashVal: base64Hash reqPolicyOid: optionalPolicyOid addNonce: addNonce reqTsaCert: requestTsaCert timestampToken: requestToken];
if (success == NO) {
    NSLog(@"%@",http.LastErrorText);
    return;
}

// Send the time-stamp request token to the TSA.
// This is the equivalent of the following CURL command:
// curl -H "Content-Type: application/timestamp-query" --data-binary '@file.tsq' http://timestamp.digicert.com > file.tsr
NSString *tsaUrl = @"http://timestamp.digicert.com";
CkoHttpResponse *resp = [http PBinaryBd: @"POST" url: tsaUrl data: requestToken contentType: @"application/timestamp-query" md5: NO gzip: NO];
if (http.LastMethodSuccess == NO) {
    NSLog(@"%@",http.LastErrorText);
    return;
}

// Get the timestamp reply from the HTTP response object.
CkoBinData *timestampReply = [[CkoBinData alloc] init];
[resp GetBodyBd: timestampReply];

// Convert the binary timestamp reply to XML
CkoAsn *asn = [[CkoAsn alloc] init];
success = [asn LoadBd: timestampReply];
if (success == NO) {
    NSLog(@"%@",asn.LastErrorText);
    return;
}

CkoXml *xml = [[CkoXml alloc] init];
success = [xml LoadXml: [asn AsnToXml]];

// Extract the TSTInfo from the XML.
// The TSTInfo is this base64 encoded ASN.1 

// 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  }

NSString *tstInfoBase64 = [xml GetChildContent: @"sequence[1]|contextSpecific|sequence|sequence|contextSpecific|octets"];

CkoBinData *bdTstInfo = [[CkoBinData alloc] init];
[bdTstInfo AppendEncoded: tstInfoBase64 encoding: @"base64"];

CkoAsn *asnTstInfo = [[CkoAsn alloc] init];
success = [asnTstInfo LoadBd: bdTstInfo];
if (success == NO) {
    NSLog(@"%@",asnTstInfo.LastErrorText);
    return;
}

CkoXml *xmlTstInfo = [[CkoXml alloc] init];
success = [xmlTstInfo LoadXml: [asnTstInfo AsnToXml]];
NSLog(@"%@",[xmlTstInfo GetXml]);

// Here's the TSTInfo XML:

// <?xml version="1.0" encoding="utf-8"?>
// <sequence>
//     <int>01</int>
//     <oid>2.16.840.1.114412.7.1</oid>
//     <sequence>
//         <sequence>
//             <oid>2.16.840.1.101.3.4.2.1</oid>
//             <null/>
//         </sequence>
//         <octets>4sRRyWOzC7EOic4fQ9+Op1pa10DbgoBGjBvkq09LZmE=</octets>
//     </sequence>
//     <int>00AD2C86E49872597B60F87D5C54BCFFAE</int>
//     <universal tag="24" constructed="0">MjAyMzAzMTYxMTQ5NTJa</universal>
// </sequence>

//    The genTime (GeneralizedTime) is contained in the final "universal" XML element and is 
//    in base64. It is the time at which the time-stamp token has been created by
//    the TSA. After decoding from base64, it is:
// 
//    The syntax is: YYYYMMDDhhmmss[.s...]Z
//    Example: 19990609001326.34352Z

CkoStringBuilder *sbGenTime = [[CkoStringBuilder alloc] init];
[sbGenTime DecodeAndAppend: [xmlTstInfo GetChildContent: @"universal"] encoding: @"base64" charset: @"utf-8"];
NSLog(@"%@",[sbGenTime GetAsString]);

// Result:
// 20230316115718Z