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 b9 a9 f2 af a2 73 ec f3 1a a9 75 09 b1 f0 48 68 01 10 41 74 51 35 3b
[51] 99 74 2c 60 e2 aa 99 1b 77 b5 53 30 2b 61 f3 98 3b 8c 93 08 d9 77 3a b9 bb
[76] af 86 4e c8 55 08 2b 27 5a c0 f1 b9 56 7e 5f 8e

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: 241f9914d97d70515baee92e5b703878
sha256: e55162b5bda08b75294b3db55156df87d36ac27dc21c52b4136e998865375b7c

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-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEuanyr6Jz7PMaqXUJsfBIaAEQQXRR
NTuZdCxg4qqZG3e1UzArYfOYO4yTCNl3Orm7r4ZOyFUIKydawPG5Vn5fjg==
-----END PUBLIC KEY-----

cat(write_pem(key, password = NULL))

-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgIF0fDQIVpnWxc8YC
Cs9Bgv3XffL1IXDei6b+LjWHs1+hRANCAAS5qfKvonPs8xqpdQmx8EhoARBBdFE1
O5l0LGDiqpkbd7VTMCth85g7jJMI2Xc6ubuvhk7IVQgrJ1rA8blWfl+O
-----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.

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

-----BEGIN ENCRYPTED PRIVATE KEY-----
MIHrMFYGCSqGSIb3DQEFDTBJMDEGCSqGSIb3DQEFDDAkBBBuiXXcm7qL3raoG6sG
6yr+AgIIADAMBggqhkiG9w0CCQUAMBQGCCqGSIb3DQMHBAh9DzMnypZ67wSBkJln
4/CskYNtIHNaeOhGpwDSZSGHkYlAxABVb3EKEeGAaGZxRY09udI5A6BTMAWtvHHC
Lbe6e1wU4LBjyiHtRj458K/J/VCSPExKrvRpgXXH914A8WqbpUIJU8zXiZxfmZ6S
Cjk8+90KvMZICIKwA1voqQPfHPmqYxLjB73xHmoyrwhfv6HbPzBggCf4Qy/NlA==
-----END ENCRYPTED PRIVATE KEY-----

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 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBLmp8q+ic+zzGql1CbHwSGgBEEF0UTU7mXQsYOKqmRt3tVMwK2HzmDuMkwjZdzq5u6+GTshVCCsnWsDxuVZ+X44="

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: 241f9914d97d70515baee92e5b703878
sha256: e55162b5bda08b75294b3db55156df87d36ac27dc21c52b4136e998865375b7c

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": "uanyr6Jz7PMaqXUJsfBIaAEQQXRRNTuZdCxg4qqZG3c",
    "y": "tVMwK2HzmDuMkwjZdzq5u6-GTshVCCsnWsDxuVZ-X44"
}
 

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: 241f9914d97d70515baee92e5b703878
sha256: e55162b5bda08b75294b3db55156df87d36ac27dc21c52b4136e998865375b7c