Importing and exporting RSA/DSA/EC keys

The openssl package implements a modern interface to libssl and libcrypto for R. It builds on the new EVP api which was introduced in OpenSSL 1.0 and provides a unified API to the various methods and formats. OpenSSL supports three major public key crypto systems:

RSA : Most popular method. Supports both encryption and signatures.
DSA : Digital Signature Algorithm. Mostly for signatures, not very popular anymore.
ECDSA : Elliptic Curve DSA. Supports signatures and encryption via Diffie Hellman. Gaining popularity.

For each type there are several common formats for storing keys and certificates:

DER: binary format, serialized ASN.1 structure
PEM: base64 encoded DER + header wrapped in ===
SSH: single line of base64 with a header. Only for pubkeys.
JWK: JSON Web key. Stores key data in a JSON object

The openssl package automatically detects the format when possible. However being able to recognize the various formats can be useful.

The DER format

DER is the standard binary format using by protocols for storing and exchanging keys and certificates. It consists of a serialized ASN.1 structure which hold the key’s (very large) prime numbers.

key <- ec_keygen()
pubkey <- key$pubkey
bin <- write_der(pubkey)
print(bin)

 [1] 30 59 30 13 06 07 2a 86 48 ce 3d 02 01 06 08 2a 86 48 ce 3d 03 01 07 03 42
[26] 00 04 90 c0 35 7c d3 3c b3 29 d7 2f 8c 5c dc 29 1b 99 ff e1 e5 f9 92 69 92
[51] a3 c7 49 57 0f 3c fa f2 72 65 6a 71 b5 28 69 8a 14 03 d6 ed d7 e7 60 fd da
[76] e8 ea ba 8e 83 fe d2 0b fe 19 f4 42 55 20 27 8f

To read a DER key use read_key or read_pubkey with der = TRUE.

read_pubkey(bin, der = TRUE)

[256-bit ecdsa public key]
md5: fe25537c3f3e3c51db1a634e08b0f397

Users typically don’t need to worry about the key’s underlying primes, but have a look at key$data if you are curious.

The PEM format

In practice the user rarely encounters DER because it is mainly for internal use. When humans exchange keys and certificates they typically use the PEM format. PEM is simply base64 encoded DER data, plus a header. The header identifies the key (and possibly encryption) type.

cat(write_pem(pubkey))

-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEkMA1fNM8synXL4xc3Ckbmf/h5fmS
aZKjx0lXDzz68nJlanG1KGmKFAPW7dfnYP3a6Oq6joP+0gv+GfRCVSAnjw==
-----END PUBLIC KEY-----

cat(write_pem(key, password = NULL))

The PEM format allows for protecting private keys with a password. R will prompt you for the password when reading such a protected key.

cat(write_pem(key, password = "supersecret"))

The OpenSSH format

For better or worse, OpenSSH uses a custom format for public keys. The advantage of this format is that it fits on a single line which is nice for e.g. your ~/.ssh/known_hosts file. There is no special format for private keys, OpenSSH uses PEM as well.

str <- write_ssh(pubkey)
print(str)

[1] "ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBJDANXzTPLMp1y+MXNwpG5n/4eX5kmmSo8dJVw88+vJyZWpxtShpihQD1u3X52D92ujquo6D/tIL/hn0QlUgJ48="

The read_pubkey function will automatically detect if a file contains a PEM or SSH key.

read_pubkey(str)

[256-bit ecdsa public key]
md5: fe25537c3f3e3c51db1a634e08b0f397

The JSON Web Key (JWK) format

Yet another recent format to store RSA or EC keys are JSON Web Keys (JWK). JWK is part of the Javascript Object Signing and Encryption (JOSE) specification. The write_jwk and read_jwk functions are implemented in a separate package which uses the openssl package.

library(jose)
json <- write_jwk(pubkey)
jsonlite::prettify(json)

{
    "kty": "EC",
    "crv": "P-256",
    "x": "kMA1fNM8synXL4xc3Ckbmf_h5fmSaZKjx0lXDzz68nI",
    "y": "ZWpxtShpihQD1u3X52D92ujquo6D_tIL_hn0QlUgJ48"
}

Keys from jose and openssl are the same.

mykey <- read_jwk(json)
identical(mykey, pubkey)

[1] TRUE

print(mykey)

[256-bit ecdsa public key]
md5: fe25537c3f3e3c51db1a634e08b0f397

Importing and exporting RSA/DSA/EC keys

2020-09-18

The DER format

The PEM format

The OpenSSH format

The JSON Web Key (JWK) format