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Asked: 2026-05-12T10:58:44+00:00

I got a RSA pubkey.dat (almost obvious what it is) that has the following

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I got a RSA pubkey.dat (almost obvious what it is) that has the following structure on contents:

  • ASN1 Integer of around 1024 bits (Modulus)
  • ASN1 Integer (Exponent)
  • Blob of 256 bytes (Signature)

No tags like “—-begin—” or so. pure hex values in it.

There’s any way to identify its format like if it’s DER/PEM/etc , so i can open it with python crypto libraries or crypto++ on c++?

(Or if it matches a public standard structure name for me to check)

Seems like its not PEM as M2crypt can’t load it.

Thanks in advance.

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1 Answer

  1. Editorial Team

    PEM-encoding has mandatory format:

    -----BEGIN typeName-----
base64 of DER value
-----END typeName-----

    where, for public keys, typeName=”PUBLIC KEY” (AFAIR) so that’s very easy to check with a regular expression such as the following:

    /-----BEGIN [^-]+-----([A-Za-z0-9+\/=\s]+)-----END [^-]+-----/

    If it’s not PEM, it’s usually plain DER.

    The DER representation of an ASN.1 SEQUENCE always begins with 0x30 so usually when I have to decode a DER-or-PEM stream which I know for sure it’s an ASN.1 SEQUENCE (most complex values are SEQUENCEs, anyways) I check the first byte: if its’s 0x30, I decode as DER, else I decode as PEM.

    You can check your ASN.1 data quickly using my very own opensource ASN.1 parser (it’s all client-side Javascript, so I won’t see your data).

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