Supported Encryption Types

Kerberos uses a number of encryption systems for data protection. AES encryption type (AES128 and AES256) is available from JGSS 6.0. It is based on Advanced Encryption Standard (AES) in ciphertext stealing (CTS) mode with a Secure Hash Algorithm (HMAC SHA-1) checksum for integrity. AES encryption is defined in Kerberos RFC-3962.

IBM JGSS supports the use of DES and Triple-DES encryption systems as defined in Kerberos RFC-1510 and RFC-3961. IBM JGSS also supports the RC4 HMAC encryption system that is used in Microsoft Windows and Active Directory products. RC4 HMAC is defined in draft-brezak-win2k-krb-rc4-hmac-04.txt (Expired).

A client specifies a list of encryption systems as the value of the default_tkt_enctypes and default_tgs_enctypes fields in their Kerberos configuration file. From service refresh 3 fix pack 10, you can also specify the value of these fields by using the com.ibm.security.krb5.enctypes property. The possible values and their meanings are as follows.

aes128-cts-hmac-sha1-96

Represents AES in CTS mode with HMAC-SHA1 checksum. The key size is 128 bits.

aes256-cts-hmac-sha1-96

Represents AES in CTS mode with HMAC-SHA1 checksum. The key size is 256 bits.

des3-cbc-sha1

Represents Triple-DES in CBC mode with SHA1 checksum. This encryption system uses key derivation, that is, different keys are derived from a base key for specific uses.

des-cbc-md5

Represents DES in CBC mode with MD5 checksum.

des-cbc-crc

Represents DES in CBC mode with a 4-byte CRC-32 checksum.

rc4-hmac

Represents Rivest Cipher 4 (RC4) encryption with an MD5 HMAC checksum; uses key derivation with key usage numbers that sometimes differ from the ones that are used in the des3-cbc-sha1 encryption system.

rc4-hmac-exp

Identical to rc4-hmac with a reduced key length.

From service refresh 7 fix pack 20, the JGSS Kerberos encryption types des3-cbc-sha1, and rc4-hmac are disabled by default. These two types are considered weak, and were deprecated in 2018 with RFC 8429. Existing applications that must use these weak encryption types must set the property allow_weak_crypto to true in the Kerberos configuration file krb5.conf to continue usage, and to avoid encryption errors. For more information about the Kerberos configuration file, see Configuration and policy files.