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 <- as.list(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
[24] 03 42 00 04 a4 8c e4 2f ba 35 bb 51 fb dc 54 e4 23 2f ad 1e 33 1b d4
[47] 34 ec f6 e5 9a 01 c0 8a 71 a4 2b d7 26 9a f6 61 f7 45 16 bf 79 8f 49
[70] 4c b3 e6 fa 4d 60 16 79 79 e5 97 d6 48 d6 46 b8 d7 5c df 5f b5 ef

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: d5:51:98:98:c6:ef:22:93:cb:98:9e:02:77:a4:0f:33

Users typically don’t need to worry about the key’s underlying primes, but have a look at as.list(key) 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-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEpIzkL7o1u1H73FTkIy+tHjMb1DTs
9uWaAcCKcaQr1yaa9mH3RRa/eY9JTLPm+k1gFnl55ZfWSNZGuNdc31+17w==
-----END PUBLIC KEY-----

cat(write_pem(key, password = NULL))

-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQggUgDSn/6Gi27GfNi
SEvmG+TASp0GWBQm7Zp8QjkmMOahRANCAASkjOQvujW7UfvcVOQjL60eMxvUNOz2
5ZoBwIpxpCvXJpr2YfdFFr95j0lMs+b6TWAWeXnll9ZI1ka411zfX7Xv
-----END PRIVATE KEY-----

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

str <- write_pem(key, password = "supersecret")
cat(str)

-----BEGIN ENCRYPTED PRIVATE KEY-----
MIHVMEAGCSqGSIb3DQEFDTAzMBsGCSqGSIb3DQEFDDAOBAhR+BNhsPjI2QICCAAw
FAYIKoZIhvcNAwcECAa8lcpv7lk7BIGQ93hq0WEhS8zNNCfXyMBYcLf6kbqnSF2v
WWUGoR37gkmrm3xDiySG48VQAe2WD9M1z1DIxUQUrD38wumj7g3e8OWeDcpc+Rrl
SPpgKLUKy1tn9KmvRDLBGcsvtLKaB5YzmSNnWRTDkwZuSky/DJ8G+vuQOWoJCr4e
MxD8THFquBNTlvY2j1Z5OUFbmez+s9AX
-----END ENCRYPTED PRIVATE KEY-----

read_key(str, password = "supersecret")

[256-bit ecdsa private key]
md5: d5:51:98:98:c6:ef:22:93:cb:98:9e:02:77:a4:0f:33

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 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBKSM5C+6NbtR+9xU5CMvrR4zG9Q07PblmgHAinGkK9cmmvZh90UWv3mPSUyz5vpNYBZ5eeWX1kjWRrjXXN9fte8="

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: d5:51:98:98:c6:ef:22:93:cb:98:9e:02:77:a4:0f:33

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 jwk_write and jwk_read functions are implemented in a separate package which uses the openssl package.

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

{
    "kty": "EC",
    "crv": "P-256",
    "x": "pIzkL7o1u1H73FTkIy-tHjMb1DTs9uWaAcCKcaQr1yY",
    "y": "mvZh90UWv3mPSUyz5vpNYBZ5eeWX1kjWRrjXXN9fte8"
}

The jwk_read function parses the json blob back into the openssl key:

mykey <- jose::jwk_read(json)
print(mykey)

[256-bit ecdsa public key]
md5: d5:51:98:98:c6:ef:22:93:cb:98:9e:02:77:a4:0f:33