Wrong error returned for unsupported hash algorithms

[hslatman]

In case MD5 is used as the digest algorithm the error returned is pkcs7: cannot decrypt data: only RSA, DES, DES-EDE3, AES-256-CBC and AES-128-GCM supported. This is an unexpected error message, and can lead to confusion. It's returned here:

pkcs7/pkcs7.go

Lines 94 to 108 in fbab67b

	func getHashForOID(oid asn1.ObjectIdentifier) (crypto.Hash, error) {
	switch {
	case oid.Equal(OIDDigestAlgorithmSHA1), oid.Equal(OIDDigestAlgorithmECDSASHA1),
	oid.Equal(OIDDigestAlgorithmDSA), oid.Equal(OIDDigestAlgorithmDSASHA1),
	oid.Equal(OIDEncryptionAlgorithmRSA):
	return crypto.SHA1, nil
	case oid.Equal(OIDDigestAlgorithmSHA256), oid.Equal(OIDDigestAlgorithmECDSASHA256):
	return crypto.SHA256, nil
	case oid.Equal(OIDDigestAlgorithmSHA384), oid.Equal(OIDDigestAlgorithmECDSASHA384):
	return crypto.SHA384, nil
	case oid.Equal(OIDDigestAlgorithmSHA512), oid.Equal(OIDDigestAlgorithmECDSASHA512):
	return crypto.SHA512, nil
	}
	return crypto.Hash(0), ErrUnsupportedAlgorithm
	}

. It should likely become a different sentinel error message, but we may need a new version for that, because it's currently exported, and could thus be a backwards incompatible change.

Also see smallstep/certificates#2100.

Another case: micromdm/scep#224.