How do the following relate: ASN.1, pkcs1, pkcs8, pem, der.

BIG_NUM big_num = BN_set_word(key_bytes_num)

asn1_obj = ASN.1: (language that defines data structures)
  pkcs1: defines the RSA format
  pkcs8: defines the key format (RSA, EC, ..)

der = asn1_obj.to_bytes()
pem = der.to_base64() // contains labels

• Assuming we want to encode a private key, we start out with a secret which is possibly a large integer: key_bytes_num

• We can define that using the ASN.1, which is an interface description language (IDL).

• The key format, used for ASN.1 encoding can either be a pkcs1 or pkcs8.

• Finally, the raw bytes of the asn1_obj is the der representation, while the base64 representation is the pem format: pem = cat key.der | base64

Warm Welcome to ASN.1 and DER

There are some other languages that do the same things as ASN.1. For instance, Protocol Buffers offer both a language for defining types and a serialization format for encoding objects of the types you’ve defined.

link

Resources:

• https://letsencrypt.org/docs/a-warm-welcome-to-asn1-and-der/
• http://luca.ntop.org/Teaching/Appunti/asn1.html
• https://lapo.it/asn1js/
• Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile
• ASN.1 lang
• ASN.1 serialization format
• oid