In the Request Message sequence $UNIQUE_IDENTIFER_0 is for the first registered PRIVATE KEY
<RequestMessage>
<RequestHeader>
<ProtocolVersion>
<ProtocolVersionMajor type="Integer" value="3"/>
<ProtocolVersionMinor type="Integer" value="0"/>
</ProtocolVersion>
<ClientCorrelationValue type="TextString" value="TC-ECC-3-30 step=2"/>
</RequestHeader>
<BatchItem>
<Operation type="Enumeration" value="Register"/>
<RequestPayload>
<ObjectType type="Enumeration" value="Certificate"/>
<Attributes>
<CryptographicUsageMask type="Integer" value="Verify Sign"/>
<PublicKeyLink type="Reference" value="$UNIQUE_IDENTIFIER_0"/>
<Attribute>
<VendorIdentification type="TextString" value="x"/>
<AttributeName type="TextString" value="ID"/>
<AttributeValue type="TextString" value="TC-ECC-3-30-cert1"/>
</Attribute>
</Attributes>
<Certificate>
<CertificateType type="Enumeration" value="X_509"/>
<CertificateValue type="ByteString" value="308201d9cd ..... 6cc7641c76bb"/>
</Certificate>
</RequestPayload>
</BatchItem>
</RequestMessage>