Kerberos Encryption Type By Number (enctype)

Luke Hebert

September 7, 2017 - 2 minutes read - 411 words

When you are troubleshooting Kerberos authentication, the enctype number in a ticket or in a config file is one of the first things you need to decode. Knowing which cipher a given number maps to saves you from guessing and helps you spot mismatches between a KDC and a client quickly.

Important

The content here is valid as of its posting but encryption types can and do change often. The tables included here may be incomplete be sure to reference the currently published [RFC 3961](http://www.iana.org/go/rfc3961 document or its related amendments.

What Is an Enctype?

In Kerberos, the enctype (encryption type) is a numeric identifier that tells the protocol which cryptographic algorithm to use for encrypting and signing tickets. The values are registered with IANA and defined in RFC 3961. Positive values are assigned for standard algorithms, negative values are reserved for private or experimental use, and zero is reserved and must not be assigned.

Common Enctype Numbers

Enctype	Algorithm
1	DES CBC with CRC
2	DES CBC with MD5
3	DES CBC with MD4
16	AES-128 CTS with HMAC-SHA1-96
17	AES-256 CTS with HMAC-SHA1-96
18	Camellia-128 CTS with CMAC
19	Camellia-256 CTS with CMAC
20	AES-128 CTS with HMAC-SHA384-192
21	AES-256 CTS with HMAC-SHA512-256
23	AES-128 GCM
24	AES-256 GCM
25	Camellia-128 GCM
26	Camellia-256 GCM

Important

Enctypes 1 through 3 (DES-based) are considered weak and should not be used in any modern deployment. Prefer AES-128 (16) or AES-256 (17) at minimum.

Where You Will See Enctypes

KDC configuration (krb5.conf): the supported_enctypes and default_tgs_enctypes directives list the numbers the KDC will offer.
Client configuration: the default_tkt_enctypes directive controls what the client requests.
Ticket inspection: tools like klist -e display the enctype number on each ticket so you can verify which cipher was actually negotiated.

Registration Details

The IANA registry for Kerberos parameters lists the following governance details:

Registration Procedure: Standards Action for standards-track RFCs; non-standards-track RFCs must be reviewed by an expert.
Expert: Ken Raeburn
Reference: RFC 3961
Value Range: Signed 32-bit integers from -2147483648 to 2147483647. Positive values are for standard algorithms, negative values for private use, and zero is reserved.

Full IANA Registry

For the complete and authoritative list, see the IANA Kerberos Parameters registry.

Quick Troubleshooting Tip

If a client and KDC disagree on enctypes, authentication fails with a KRB5_KT_SKEY or KRB5_PRINCIPAL_UNKNOWN error. Check both sides of the configuration, confirm the keytab was generated with a supported enctype, and verify that no legacy DES-only entries are still present.