C# convert certificate string into X509 certificate

Question

I am receiving a string and want to convert that into a certificate using C#. I tried following code and got "The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters. " error:

 byte[] bytes = Convert.FromBase64String(((string[])request.Headers.GetValues("MY-Cert"))[0]);
                var cert = new X509Certificate2(bytes);

What is the best way to read such string using C#.

Similar question was asked here but using C++

I am receiving string in following format:

Are you removing the "-----BEGIN CERTIFICATE-----" prefix (and the matching suffix)? — John Wu, Dec 17 '20 at 23:48
Yes, remove the first and last lines and it will work fine. You may also want to remove the cert from your post, because now we can decode it quite easily too... — DavidG, Dec 17 '20 at 23:50
@DavidG No, the post needs the certificate to make sense to other readers of the post. SharpCode just needs to generate a different certificate for in-production use. — Andrew Shepherd, Dec 18 '20 at 00:31
This is public certificate. It doesn't contain any sensitive information, so what is your concern, @DavidG, @AndrewShepherd? — Crypt32, Dec 18 '20 at 06:37
@Crypt32 I didn't try to decode it, so had no idea if it was public or not. — DavidG, Dec 18 '20 at 10:47
There is no need to decode. PEM header is self-explanatory, that it is public certificate without private key. — Crypt32, Dec 18 '20 at 10:51
@Crypt32 That might be something you have in your mental box of tricks, but us mere mortals don't... — DavidG, Dec 18 '20 at 10:55

score 11 · Accepted Answer · answered Dec 24 '20 at 19:42

Following code worked for me;

IEnumerable<string> headerValues;
var usercert = request.Headers.TryGetValues("MY-Cert", out headerValues);
var data = WebUtility.UrlDecode(((string[])headerValues)[0]);
byte[] bytes = Encoding.ASCII.GetBytes(data);
var cert = new X509Certificate2(bytes);

Crypt32 · Answer 2 · 2020-12-18T16:12:58.717

6

As it was pointed in comments, you need to remove first and last lines from PEM string and then convert Base64 string to X509Certificate2 object.

String certPemString = ((string[])request.Headers.GetValues("MY-Cert"))[0];
certPemString = certPemString
    .Replace("-----BEGIN CERTIFICATE-----",null)
    .Replace("-----END CERTIFICATE-----",null);
var cert = new X509Certificate2(Convert.FromBase64String(certPemString));

edited Dec 18 '20 at 16:12

answered Dec 18 '20 at 06:47

Crypt32

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What is `$null`? That is a syntax error. Perhaps you meant `string.Empty`? – DavidG Dec 18 '20 at 13:25
Sorry, it was a typo. Fixed. – Crypt32 Dec 18 '20 at 16:13
@Crypt32 Voting up for helpful answer. But to make this code work, I had to add `WebUtility.UrlDecode()` – SharpCoder Dec 24 '20 at 19:42

C# convert certificate string into X509 certificate

2 Answers2