doubt regd X509 Certificate

[Sravan]

Hello all,
I have a doubt regd. the format of X509 Certificate. I know that this doubt is not at all related to OpenSSL but I can't find any other place where in I can get good replies for the doubt. So, here is it...

The syntax of an X509Certificate is as follows :

Certificate  ::=  SEQUENCE  {
       tbsCertificate       TBSCertificate,
       signatureAlgorithm   AlgorithmIdentifier,
       signatureValue       BIT STRING  }

And 'TBSCertificate' is defined as

TBSCertificate  ::=  SEQUENCE  {
       version         [0]  EXPLICIT Version DEFAULT v1,
       serialNumber         CertificateSerialNumber,
       signature            AlgorithmIdentifier,
       issuer               Name,
       validity             Validity,
       subject              Name,
       subjectPublicKeyInfo SubjectPublicKeyInfo,
       issuerUniqueID  [1]  IMPLICIT UniqueIdentifier OPTIONAL,
                            -- If present, version MUST be v2 or v3
       subjectUniqueID [2]  IMPLICIT UniqueIdentifier OPTIONAL,
                            -- If present, version MUST be v2 or v3
       extensions      [3]  EXPLICIT Extensions OPTIONAL
                            -- If present, version MUST be v3
       }

My doubt is, why the signatureAlgorithmIdentifier appears twice(both in TBSCertificate as 'signature' & Certificate as 'signatureAlgorithm')

Sravan

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