System.Security.Cryptography.CryptographicException: ASN1 corrupted data.

On Linux and macOS the type is `CryptographicException`; on Windows it can appear as `WindowsCryptographicException` with the same text. The inner exception reads, e.g., "The provided data is tagged with 'Universal' class value '2', but it should have been 'Universal' class value '16'."

.NET System.Security.Cryptography — ImportSubjectPublicKeyInfo / ImportPkcs8PrivateKey / ImportRSAPrivateKey / ImportECPrivateKey

What it means

The bytes you passed to one of the `Import…` methods are not the ASN.1 structure that method decodes. Each method reads exactly one structure — PKCS#8, PKCS#1, SEC 1 or SubjectPublicKeyInfo — and the outer DER does not match, so .NET reports the data as corrupted even though the key itself may be perfectly fine in another format. The inner `AsnContentException` names the tag mismatch.

Why it happens

• PKCS#1 vs PKCS#8 (or SEC 1)

  common

  `RSA PRIVATE KEY` bytes given to `ImportPkcs8PrivateKey`, or `PRIVATE KEY` bytes given to `ImportRSAPrivateKey`. The structures differ in their first element (INTEGER vs SEQUENCE).

• PEM text or base64 passed as DER

  common

  `Encoding.ASCII.GetBytes(pem)` or `Convert.FromBase64String` over the whole file including the armour lines does not produce DER.

• Public and private methods swapped

  occasional

  A private key passed to `ImportSubjectPublicKeyInfo` or a certificate (not a key) passed to any key import method.

How to fix it

1. 1.

   Use ImportFromPem for PEM files

   It dispatches on the PEM label and avoids picking the wrong DER decoder by hand. Certificates go through `X509Certificate2` instead.

   // .NET 5+: ImportFromPem reads the label and picks the right decoder
   // (PRIVATE KEY, RSA PRIVATE KEY, EC PRIVATE KEY, PUBLIC KEY, RSA PUBLIC KEY).
   using var key = ECDsa.Create();
   key.ImportFromPem(File.ReadAllText("key.pem"));

2. 2.

   Match the method to the DER structure

   If you must handle raw DER, choose the import method by what the data is; an ASN.1 viewer tells you which structure you have.

   // DER input: choose the method by structure
   rsa.ImportPkcs8PrivateKey(der, out _);        // "PRIVATE KEY"
   rsa.ImportRSAPrivateKey(der, out _);          // "RSA PRIVATE KEY"
   rsa.ImportSubjectPublicKeyInfo(der, out _);   // "PUBLIC KEY"
   rsa.ImportRSAPublicKey(der, out _);           // "RSA PUBLIC KEY"
   ecdsa.ImportECPrivateKey(der, out _);         // "EC PRIVATE KEY"

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