See https://polarssl.org/kb/cryptography/asn1-key-structures-in-der-and-pem (search the page for “BEGIN RSA PRIVATE KEY”) (archive link for posterity, just in case).
BEGIN RSA PRIVATE KEY
is PKCS#1 and is just an RSA key. It is essentially just the key object from PKCS#8, but without the version or algorithm identifier in front. BEGIN PRIVATE KEY
is PKCS#8 and indicates that the key type is included in the key data itself. From the link:
The unencrypted PKCS#8 encoded data starts and ends with the tags:
-----BEGIN PRIVATE KEY----- BASE64 ENCODED DATA -----END PRIVATE KEY-----
Within the base64 encoded data the following DER structure is present:
PrivateKeyInfo ::= SEQUENCE { version Version, algorithm AlgorithmIdentifier, PrivateKey BIT STRING } AlgorithmIdentifier ::= SEQUENCE { algorithm OBJECT IDENTIFIER, parameters ANY DEFINED BY algorithm OPTIONAL }
So for an RSA private key, the OID is 1.2.840.113549.1.1.1 and there is a RSAPrivateKey as the PrivateKey key data bitstring.
As opposed to BEGIN RSA PRIVATE KEY
, which always specifies an RSA key and therefore doesn’t include a key type OID. BEGIN RSA PRIVATE KEY
is PKCS#1
:
RSA Private Key file (PKCS#1)
The RSA private key PEM file is specific for RSA keys.
It starts and ends with the tags:
-----BEGIN RSA PRIVATE KEY----- BASE64 ENCODED DATA -----END RSA PRIVATE KEY-----
Within the base64 encoded data the following DER structure is present:
RSAPrivateKey ::= SEQUENCE { version Version, modulus INTEGER, -- n publicExponent INTEGER, -- e privateExponent INTEGER, -- d prime1 INTEGER, -- p prime2 INTEGER, -- q exponent1 INTEGER, -- d mod (p-1) exponent2 INTEGER, -- d mod (q-1) coefficient INTEGER, -- (inverse of q) mod p otherPrimeInfos OtherPrimeInfos OPTIONAL }