RSA pubkey file type detection
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12-09-2019 - |
Question
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.
Solution
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).