CODEWITHSUNDEEP
c#sshcryptographyopensshed25519
mene
mene

Reputation: 382

C# Generate Ed25519 private and public key pair for SSH authentication


I need to generate a key pair for the authentication in a ssh tunnel with C#.
The only constraint is the cryptographic that should be Ed25519.
I'm able to generate a valid public key but not a valid private key (or maybe only the format).
I've tried with BouncyCastle and NSec libraries for generate them with no success.

Here some of my attempts:

#### NSec ####
var creationParameters = new KeyCreationParameters
{
    ExportPolicy = KeyExportPolicies.AllowPlaintextArchiving
};
using (var key = Key.Create(SignatureAlgorithm.Ed25519, creationParameters))
{
    var blob = key.Export(KeyBlobFormat.PkixPrivateKeyText);
    result.PrivateKey = Convert.ToBase64String(blob);

    blob = key.Export(KeyBlobFormat.PkixPublicKeyText);
    result.PublicKey = string.Format("ssh-ed25519 {0} generated-key", Convert.ToBase64String(blob));
}


#### BouncyCastle ####
IAsymmetricCipherKeyPairGenerator gen;
KeyGenerationParameters param;
gen = new Ed25519KeyPairGenerator();
param = new Ed25519KeyGenerationParameters(new SecureRandom());
gen.Init(param);
AsymmetricCipherKeyPair pair = gen.GenerateKeyPair();
PrivateKeyInfo pkInfo = PrivateKeyInfoFactory.CreatePrivateKeyInfo(pair.Private);
result.PrivateKey = Convert.ToBase64String(pkInfo.GetDerEncoded());
SubjectPublicKeyInfo info = SubjectPublicKeyInfoFactory.CreateSubjectPublicKeyInfo(pair.Public);
result.PublicKey = Convert.ToBase64String(info.GetDerEncoded());

For be more specific I wish obtain a result like the following generation from the cmd:

ssh-keygen -t ed25519 -f ssh-ed25519-private-key.pem

That generate the public key like:

ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICjTBKzEKElOtmjuYDaBsoF9UpsXUeLUmKuqiK86jv2A xxxxxx\xxxxxxx@xxxx

And the private like:

-----BEGIN OPENSSH PRIVATE KEY-----
b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAMwAAAAtzc2gtZW
QyNTUxOQAAACAo0wSsxChJTrZo7mA2gbKBfVKbF1Hi1JirqoivOo79gAAAAKgLape6C2qX
ugAAAAtzc2gtZWQyNTUxOQAAACAo0wSsxChJTrZo7mA2gbKBfVKbF1Hi1JirqoivOo79gA
AAAECRX7SDurbmPZzRMqYwONep7gk4ZCFp1Uj6hTpVbGWi0CjTBKzEKElOtmjuYDaBsoF9
UpsXUeLUmKuqiK86jv2AAAAAJWNnLWNvbnRyb2xzLXJkXGFtZW5lZ2F0QFcwNTItQU1lbm
VnYXQ=
-----END OPENSSH PRIVATE KEY-----

Thanks,
Andrea

Upvotes: 1

Views: 3242

Answers (1)

Woodstock
Woodstock

Reputation: 22926

I'm able to generate a valid public key but not a valid private key (or maybe only the format).

This doesn't make sense.

The public key is really a point (X,Y co-ordinate) on the Elliptic Curve defined by y^2 = x^3 + 486662*x^2 + x over Finite Field of size 57896044618658097711785492504343953926634992332820282019728792003956564819949

A public key is only able to be obtained by scalar multiplication of the generator point G (9 , 14781619447589544791020593568409986887264606134616475288964881837755586237401)

by a large number. Thus the private key is really just a 256-bit integer, a number.

In it's most raw form a private key is a 256-bit random value, the OpenSSH format above is a base64 encoded value including other components such as:

"openssh-key-v1"0x00    # NULL-terminated "Auth Magic" string
32-bit length, "none"   # ciphername length and string
32-bit length, "none"   # kdfname length and string
32-bit length, nil      # kdf (0 length, no kdf)
32-bit 0x01             # number of keys, hard-coded to 1 (no length)
32-bit length, sshpub   # public key in ssh format
    32-bit length, keytype
    32-bit length, pub0
    32-bit length, pub1
32-bit length for rnd+prv+comment+pad
    64-bit dummy checksum?  # a random 32-bit int, repeated
    32-bit length, keytype  # the private key (including public)
    32-bit length, pub0     # Public Key parts
    32-bit length, pub1
    32-bit length, prv0     # Private Key parts
    ...                     # (number varies by type)
    32-bit length, comment  # comment string
    padding bytes 0x010203  # pad to blocksize

I suggest strongly that you switch to libsodium, which is the standard when interacting with curve25519 or it's Twisted Edwards signature focused variant, ed25519.

Upvotes: 1

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