<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
   <SOAP-ENV:Body>
      <xenc:EncryptedData Type="http://www.w3.org/2001/04/xmlenc#Element" xmlns:xenc="http://www.w3.org/2001/04/xmlenc#">
         <xenc:EncryptionMethod Algorithm="urn:ietf:params:xml:ns:cpxmlsec:algorithms:gost28147"/>
         <ds:KeyInfo xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
            <xenc:EncryptedKey>
               <xenc:EncryptionMethod Algorithm="urn:ietf:params:xml:ns:cpxmlsec:algorithms:transport-gost2001"/>
               <ds:KeyInfo>
                  <ds:X509Data>
                     <ds:X509Certificate>...</ds:X509Certificate>
                  </ds:X509Data>
               </ds:KeyInfo>
               <xenc:CipherData>
                  <xenc:CipherValue>...</xenc:CipherValue>
               </xenc:CipherData>
            </xenc:EncryptedKey>
         </ds:KeyInfo>
         <xenc:CipherData>
            <xenc:CipherValue>...</xenc:CipherValue>
         </xenc:CipherData>
      </xenc:EncryptedData>
   </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

Offset|Length|LenByte|
======+======+=======+==========================================================================================
     0|   164|      2|   SEQUENCE : 
     3|    40|      1|      SEQUENCE : 
     5|    32|      1|         OCTET_STRING : 
      |      |       |            6cb4581597c8a1c6d8999d5fca15c4a731f671a6b5f4b1563c4c64d38b844780
    39|     4|      1|         OCTET_STRING : '3e45e24f'
    45|   120|      1|      CONTEXT SPECIFIC (0) : 
    47|     7|      1|         OBJECT_IDENTIFIER : 
      |      |       |            ГОСТ 28147-89, параметры по умолчанию (1.2.643.2.2.31.1)
    56|    99|      1|         CONTEXT SPECIFIC (0) : 
    58|    28|      1|            SEQUENCE : 
    60|     6|      1|               OBJECT_IDENTIFIER : 'ГОСТ Р 34.10-2001 (1.2.643.2.2.19)'
    68|    18|      1|               SEQUENCE : 
    70|     7|      1|                  OBJECT_IDENTIFIER : 
      |      |       |                     ГОСТ Р 34.10-2001, параметры обмена по умолчанию (1.2.643.2.2.
      |      |       |                     36.0)
    79|     7|      1|                  OBJECT_IDENTIFIER : 
      |      |       |                     ГОСТ Р 34.11-94, параметры по умолчанию (1.2.643.2.2.30.1)
    88|    67|      1|            BIT_STRING : 
    91|    64|      1|               OCTET_STRING : 
      |      |       |                  2a8f1fc006cb23d3324f2531dea3d086c73f2893c5e0749902f51e37f6a8d8627
      |      |       |                  98bb69129ade95f9e4fd9b050eacf569885db469e79dba549ff2b50608d01d1
   157|     8|      1|         OCTET_STRING : '679a1c5d27adfa04'

var DataStream : TBytesStream;
    publicBLOB : TBytesStream;
    simpleBLOB : TBytesStream;
    hProv  : HCRYPTPROV;
    akAlg  : ALG_ID;
    phKey,upKey,agKey : HCRYPTKEY;
begin
...  
  CryptAcquireContext(hProv, 'container', CP_GR3410_2001_PROV_A, PROV_GOST_2001_DH, 0);
  CryptGetUserKey(hProv, AT_KEYEXCHANGE, upKey);
  try
    CryptImportKey(hProv, publicBLOB.Memory, publicBLOB.Size, upKey, 0, agKey);
    akAlg := CALG_PRO_EXPORT; 
    CryptSetKeyParam(agKey, KP_ALGID, @akAlg, 0);
    CryptImportKey(hProv, simpleBLOB.Memory, simpleBLOB.Size, agKey, 0, phKey);
    DataStream := TBytesStream.Create(DecodeBase64('<текст сообщения>'));
    try
      dwData := DataStream.Size;
      DataStream.Position := 0;
      CryptDecrypt(phKey, 0, true, 0, DataStream.Memory, dwData);
      DataStream.Position := 0;
      ShowMessage(inttostr(DataStream.Size));
      mmoMessage.Lines.LoadFromStream(DataStream);
    except
      on E: Exception do ShowMessage('Ошибка расшифровки данных ' + E.Message);
    end;
...
end;