Background and motivation

In PKIX / X.509, the SubjectKeyIdentifier and AuthorityKeyIdentifier are opaque identifiers, however traditionally they have been derived from a SHA-1 over the subjectPublicKey.

SHA-1 has largely been discouraged for a long time. Even in places that are not strictly security, such as SKI and AKI, the use of SHA-1 comes with scrutiny from a compliance perspective, and requires an ongoing "exception" process.

Today, we only support a few flavors of SHA-1 with X509SubjectKeyIdentifierHashAlgorithm. This proposal is to add other hash algorithms as defined by RFC 7093.

API Proposal

namespace System.Security.Cryptography.X509Certificates;

public enum X509SubjectKeyIdentifierHashAlgorithm {
    Sha1 = 0,
    ShortSha1 = 1,
    CapiSha1 = 2,
+   Rfc7093TruncatedSha256 = 3, //  leftmost 160-bits of the SHA-256 hash over subjectPublicKey
+   Rfc7093TruncatedSha384 = 4, //  leftmost 160-bits of the SHA-384 hash over subjectPublicKey
+   Rfc7093TruncatedSha512 = 5, //  leftmost 160-bits of the SHA-512 hash over subjectPublicKey
+   Rfc7093Sha256 = 6, // Full SHA-256 hash over SubjectPublicKeyInfo
+   Rfc7093Sha384 = 7, // Full SHA-384 hash over SubjectPublicKeyInfo
+   Rfc7093Sha512 = 8, // Full SHA-512 hash over SubjectPublicKeyInfo
}

API Usage

X509SubjectKeyIdentifierExtension mySki = new(
    myPublicKey,
    X509SubjectKeyIdentifierHashAlgorithm. Rfc7093TruncatedSha256,
    critical: false);

Alternative Designs

No response

Risks

No response