CODEWITHSUNDEEP
opensslcryptographyrsa
Ajay kumar
Ajay kumar

Reputation: 739

What are Public key header and public key info?

I am trying to find what is public key header and public key info. I have dumped a certificate in C format using openssl, along with public key modulus. It listed public key info and public key header as public key. But I am not able to dump these fields using openssl.exe rsa -pubin -inform DER -text -noout < publickey.der for a public key generated on my server.

Could any please explain what is the significance of these fields in a digital certificate, are they same for all the public keys? Not able to get this info on openssl, it is just mentioned that DER encoding will have additional header and footer.

Upvotes: 3

Views: 6483

Answers (1)

emboss
emboss

Reputation: 39650

I've never heard of the public key header... could you give an example?

The Public Key info is probably the standard way how public keys in X.509 certificates are generally encoded, in the form of a SubjectPublicKeyInfo attribute. These SubjectPublicKeyInfos can be turned into a public key file (I assumed that's what you were trying to do? Please correct me if I'm wrong!) That section also tells you about where you can find information about particular algorithms, for RSA they reference RFC 3279. A SubjectPublicKeyInfo is defined as follows:

SubjectPublicKeyInfo  ::=  SEQUENCE  {
    algorithm            AlgorithmIdentifier,
    subjectPublicKey     BIT STRING  }

RFC 3279 says:

The rsaEncryption OID is intended to be used in the algorithm field of a value of type AlgorithmIdentifier. The parameters field MUST have ASN.1 type NULL for this algorithm identifier.

Further:

The RSA public key MUST be encoded using the ASN.1 type RSAPublicKey:

  RSAPublicKey ::= SEQUENCE {
     modulus            INTEGER,    -- n
     publicExponent     INTEGER  }  -- e

So it's the subjectPublicKey field that contains the relevant data - you can for example get to these values with the x509 command of OpenSSL:

openssl x509 -in certificate.cer -inform DER -noout -text

Prints out (for PEM certificates, drop the -inform DER):

Subject Public Key Info:
        Public Key Algorithm: rsaEncryption
            Public-Key: (1024 bit)
            Modulus:
                00:cb:c2:...
            Exponent: 65537 (0x10001)

There's also a neat trick how you can directly produce a PEM RSA public key file with x509:

openssl x509 -inform DER -in certificate.cer -pubkey -noout > pubkey.pem

and there's your public key exported in PEM encoding.

Upvotes: 3

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