Given RSA public key of size 2048, how do I get its modulus and exponent using built-in .net core function(s) only?

Question

I have RSA public key of size 2048 bits (as shown below):

Is there a way to get the key's modulus and exponent by using .net core (without writing any custom function)?

See this: https://github.com/dotnet/corefx/issues/32185#issuecomment-419658415 — Timothy Ghanem, Jun 06 '19 at 12:17

Topaco · Accepted Answer · 2019-06-06T12:11:52.330

3

The public key has a X.509-PEM-format. For .NET Core 3.0 there is a built-in support with the ImportSubjectPublicKeyInfo-method of the RSA-class:

// publicKeyX509PEM: String containig the key-data
byte[] publicKeyX509DER = ConvertX509PemToDer(publicKeyX509PEM);
RSA rsa = RSA.Create();
rsa.ImportSubjectPublicKeyInfo(publicKeyX509DER, out _);
RSAParameters parameters = rsa.ExportParameters(false);

// get parameters.Exponent and parameters.Modulus

Note, that the ImportSubjectPublicKeyInfo-method processes only the DER-format which is essentially the binary format of the PEM-format (for more details see here). Thus, the PEM-format must first be converted into the DER-format, e.g. with

private static byte[] ConvertX509PemToDer(string pemContents)
{
    return Convert.FromBase64String(pemContents
        .TrimStart("-----BEGIN PUBLIC KEY-----".ToCharArray())
        .TrimEnd("-----END PUBLIC KEY-----".ToCharArray())
        .Replace("\r\n", ""));
}

Alternatively, OpenSSL can also be used for the conversion:

openssl rsa -pubin -inform PEM -outform DER -in <path to pem-input-file> -out <path to der-output-file>

edited Jun 06 '19 at 12:11

answered Jun 06 '19 at 11:19

Topaco

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Maybe Microsoft can just copy your answer into its documentation ;) – President James K. Polk Jun 06 '19 at 13:33
Topaco: Once I get the file in DER format, can I use the built-in functions in .net core 2.2 to import? – Raghu Jun 06 '19 at 18:32
@Raghu This is a new method from Core 3.0 that does not exist in [Core 2.2](https://learn.microsoft.com/en-us/dotnet/api/system.security.cryptography.rsa?view=netcore-2.2). To my knowledge there is no built-in support in Core 2.2. This requires third-party libraries such as BouncyCastle, e.g. with the [`PublicKeyFactory.createKey`](https://stackoverflow.com/questions/17159147/inverse-operation-to-publickeyfactory-createkey/)-method, or custom code, e.g. something like the [`PemKeyUtils.DecodeX509PublicKey`](https://stackoverflow.com/a/49889680/9014097)-method. – Topaco Jun 07 '19 at 05:46

Given RSA public key of size 2048, how do I get its modulus and exponent using built-in .net core function(s) only?

1 Answers1