Key Formats: PEM, DER, OpenSSH

RSA keys are stored in standardized formats:

PEM (Privacy Enhanced Mail): base64-encoded with header/footer:

-----BEGIN RSA PRIVATE KEY-----
MIIEowIBAAKCAQEAvgQNWYTJWNKGzaWlYGdLwl...
...
-----END RSA PRIVATE KEY-----

Used everywhere: TLS certs, SSH, AWS keypairs.

DER: binary (the bytes that PEM base64-encodes). Same content, different encoding.

PKCS#1: traditional RSA-only key format. PEM header: -----BEGIN RSA PRIVATE KEY-----.

PKCS#8: generic private key format (works for RSA, ECDSA, etc.). Header: -----BEGIN PRIVATE KEY-----.

OpenSSH: SSH-specific format. Public keys: ssh-rsa AAAAB3NzaC1y.... Private keys (newer): -----BEGIN OPENSSH PRIVATE KEY-----.

Conversion is a openssl one-liner:

openssl rsa -in key.pem -outform DER -out key.der          # PEM -> DER
openssl rsa -in key.pem -pubout -out pub.pem               # extract public from private
ssh-keygen -y -f key.pem > key.pub                         # PEM private -> SSH public

Inside the binary form: ASN.1 DER encoding of a SEQUENCE containing version, modulus, publicExponent, privateExponent, prime1, prime2, exponent1, exponent2, coefficient. Real keys have all this for fast Chinese Remainder Theorem-based decryption.